Comparison of inhibitory neuromuscular transmission in the Cynomolgus monkey IAS and rectum: special emphasis on differences in purinergic transmission

KEY POINTS

Inhibitory neuromuscular transmission (NMT) was compared in the internal anal sphincter (IAS) and rectum of the Cynomolgus monkey, an animal with high gene sequence identity to humans. Nitrergic NMT was present in both muscles while purinergic NMT was limited to the rectum and VIPergic NMT to the IAS. The profile for monkey IAS more closely resembles humans than rodents. In both muscles, SK3 channels were localized to PDGFRα⁺ cells that were closely associated with nNOS⁺ /VIP⁺ nerves. Gene expression levels of P2RY subtypes were the same in IAS and rectum while KCNN expression levels were very similar. SK3 channel activation and inhibition caused faster/greater changes in contractile activity in rectum than IAS. P2Y1 receptor activation inhibited contraction in rectum while increasing contraction in IAS. The absence of purinergic NMT in the IAS may be due to poor coupling between P2Y1 receptors and SK3 channels on PDGFRα⁺ cells.

ABSTRACT

Inhibitory neuromuscular transmission (NMT) was compared in the internal anal sphincter (IAS) and rectum of the Cynomolgus monkey, an animal with a high gene sequence identity to humans. Electrical field stimulation produced nitric oxide synthase (NOS)-dependent contractile inhibition in both muscles whereas P2Y1-dependent purinergic NMT was restricted to rectum. An additional NOS-independent, α-chymotrypsin-sensitive component was identified in the IAS consistent with vasoactive intestinal peptide-ergic (VIPergic) NMT. Microelectrode recordings revealed slow NOS-dependent inhibitory junction potentials (IJPs) in both muscles and fast P2Y1-dependent IJPs in rectum. The basis for the difference in purinergic NMT was investigated. PDGFRα⁺ /SK3⁺ cells were closely aligned with nNOS⁺ /VIP⁺ neurons in both muscles. Gene expression of P2RY was the same in IAS and rectum (P2RY1>>P2RY2-14) while KCNN3 expression was 32% greater in rectum. The SK channel inhibitor apamin doubled contractile activity in rectum while having minimal effect in the IAS. Contractile inhibition elicited with the SK channel agonist CyPPA was five times faster in rectum than in the IAS. The P2Y1 receptor agonist MRS2365 inhibited contraction in rectum but increased contraction in the IAS. In conclusion, both the IAS and the rectum have nitrergic NMT whereas purinergic NMT is limited to rectum and VIPergic NMT to the IAS. The profile in monkey IAS more closely resembles that of humans than rodents. The lack of purinergic NMT in the IAS cannot be attributed to the absence of PDGFRα⁺ cells, P2Y1 receptors or SK3 channels. Rather, it appears to be due to poor coupling between P2Y1 receptors and SK3 channels on PDGFRα⁺ cells.

ANO1 in intramuscular interstitial cells of Cajal plays a key role in the generation of slow waves and tone in the internal anal sphincter

KEY POINTS

The internal anal sphincter develops tone important for maintaining high anal pressure and continence. Controversy exists regarding the mechanisms underlying tone development. We examined the hypothesis that tone depends upon electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (ANO1, encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (Cav_L , encoded by Cacna1c). Measurement of membrane potential and contraction indicated that ANO1 and Cav_L have a central role in SW generation, phasic contractions and tone, independent of stretch. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Ano1 and Cacna1c expression levels were examined by quantitative PCR in fluorescence-activated cell sorting. ICC-IM were the predominant cell type expressing ANO1 and the most likely candidate for SW generation. SWs in ICC-IM are proposed to conduct to smooth muscle where Ca²⁺ entry via Cav_L results in phasic activity that sums to produce tone.

ABSTRACT

The mechanism underlying tone generation in the internal anal sphincter (IAS) is controversial. We examined the hypothesis that tone depends upon generation of electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (encoded by Cacna1c). Phasic contractions and tone in the IAS were nearly abolished by ANO1 and Cav_L antagonists. ANO1 antagonists also abolished SWs as well as transient depolarizations that persisted after addition of Cav_L antagonists. Tone development in the IAS did not require stretch of muscles, and the sensitivity of contraction to ANO1 antagonists was the same in stretched versus un-stretched muscles. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Dual labelling revealed that ANO1 expression could be resolved in ICC but not smooth muscle cells (SMCs) in the IAS and rectum. Ano1, Cacna1c and Kit gene expression were the same in extracts of IAS and rectum muscles. In IAS cells isolated with fluorescence-activated cell sorting, Ano1 expression was 26.5-fold greater in ICC than in SMCs while Cacna1c expression was only 2-fold greater in SMCs than in ICC. These data support a central role for ANO1 and Cav_L in the generation of SWs and tone in the IAS. ICC-IM are the probable cellular candidate for ANO1 currents and SW generation. We propose that ANO1 and Cav_L collaborate to generate SWs in ICC-IM followed by conduction to adjacent SMCs where phasic calcium entry through Cav_L sums to produce tone.

Nitrergic neuromuscular transmission in the mouse internal anal sphincter is accomplished by multiple pathways and postjunctional effector cells

The effector cells and second messengers participating in nitrergic neuromuscular transmission (NMT) were investigated in the mouse internal anal sphincter (IAS). Protein expression of guanylate cyclase (GCα, GCβ) and cyclic GMP-dependent protein kinase I (cGKI) were examined in cryostat sections with dual-labeling immunohistochemical techniques in PDGFRα(+) cells, interstitial cells of Cajal (ICC), and smooth muscle cells (SMC). Gene expression levels were determined with quantitative PCR of dispersed cells from Pdgfrα(egfp/+), Kit(copGFP/+), and smMHC(Cre-egfp) mice sorted with FACS. The relative gene and protein expression levels of GCα and GCβ were PDGFRα(+) cells > ICC ≫ SMC. In contrast, cGKI gene expression sequence was SMC = ICC > PDGFRα(+) cells whereas cGKI protein expression sequence was neurons > SMC ≫ ICC = PDGFRα(+) cells. The functional role of cGKI was investigated in cGKI(-/-) mice. Relaxation with 8-bromo (8-Br)-cGMP was greatly reduced in cGKI(-/-) mice whereas responses to sodium nitroprusside (SNP) were partially reduced and forskolin responses were unchanged. A nitrergic relaxation occurred with nerve stimulation (NS, 5 Hz, 60 s) in cGKI(+/+) and cGKI(-/-) mice although there was a small reduction in the cGKI(-/-) mouse. N(ω)-nitro-l-arginine (l-NNA) abolished responses during the first 20-30 s of NS in both animals. The GC inhibitor ODQ greatly reduced or abolished SNP and nitrergic NS responses in both animals. These data confirm an essential role for GC in NO-induced relaxation in the IAS. However, the expression of GC and cGKI by all three cell types suggests that each may participate in coordinating muscular responses to NO. The persistence of nitrergic NMT in the cGKI(-/-) mouse suggests the presence of a significant GC-dependent, cGKI-independent pathway.

Spatial organization and coordination of slow waves in the mouse anorectum

The internal anal sphincter (IAS) develops tone and is important for maintaining a high anal pressure while tone in the rectum is less. The mechanisms responsible for tone generation in the IAS are still uncertain. The present study addressed this question by comparing the electrical properties and morphology of the mouse IAS and distal rectum. The amplitude of tone and the frequency of phasic contractions was greater in the IAS than in rectum while membrane potential (Em) was less negative in the IAS than in rectum. Slow waves (SWs) were of greatest amplitude and frequency at the distal end of the IAS, declining in the oral direction. Dual microelectrode recordings revealed that SWs were coordinated over a much greater distance in the circumferential direction than in the oral direction. The circular muscle layer of the IAS was divided into five to eight 'minibundles' separated by connective tissue septa whereas few septa were present in the rectum. The limited coordination of SWs in the oral direction suggests that the activity in adjacent minibundles is not coordinated. Intramuscular interstitial cells of Cajal and platelet-derived growth factor receptor alpha-positive cells were present in each minibundle suggesting a role for one or both of these cells in SW generation. In summary, three important properties distinguish the IAS from the distal rectum: (1) a more depolarized Em; (2) larger and higher frequency SWs; and (3) the multiunit configuration of the muscle. All of these characteristics may contribute to greater tone generation in the IAS than in the distal rectum.

Functional role of vasoactive intestinal polypeptide in inhibitory motor innervation in the mouse internal anal sphincter

There is evidence that vasoactive intestinal polypeptide (VIP) participates in inhibitory neuromuscular transmission (NMT) in the internal anal sphincter (IAS). However, specific details concerning VIP-ergic NMT are limited, largely because of difficulties in selectively blocking other inhibitory neural pathways. The present study used the selective P2Y1 receptor antagonist MRS2500 (1 μm) and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine (l-NNA; 100 μm) to block purinergic and nitrergic NMT to characterize non-purinergic, non-nitrergic (NNNP) inhibitory NMT and the role of VIP in this response. Nerves were stimulated with electrical field stimulation (0.1-20 Hz, 4-60 s) and the associated changes in contractile and electrical activity measured in non-adrenergic, non-cholinergic conditions in the IAS of wild-type and VIP(-/-) mice. Electrical field stimulation gave rise to frequency-dependent relaxation and hyperpolarization that was blocked by tetrodotoxin. Responses during brief trains of stimuli (4 s) were mediated by purinergic and nitrergic NMT. During longer stimulus trains, an NNNP relaxation and hyperpolarization developed slowly and persisted for several minutes beyond the end of the stimulus train. The NNNP NMT was abolished by VIP6-28 (30 μm), absent in the VIP(-/-) mouse and mimicked by exogenous VIP (1-100 nm). Immunoreactivity for VIP was co-localized with neuronal nitric oxide synthase in varicose intramuscular fibres but was not detected in the VIP(-/-) mouse IAS. In conclusion, this study identified an ultraslow component of inhibitory NMT in the IAS mediated by VIP. In vivo, this pathway may be activated with larger rectal distensions, leading to a more prolonged period of anal relaxation.

Changes in neuromuscular transmission in the W/W(v) mouse internal anal sphincter

BACKGROUND

Intramuscular interstitial cells of Cajal (ICC-IM) have been shown to participate in nitrergic neuromuscular transmission (NMT) in various regions of the gastrointestinal (GI) tract, but their role in the internal anal sphincter (IAS) is still uncertain. Contractile studies of the IAS in the W/W(v) mouse (a model in which ICC-IM numbers are markedly reduced) have reported that nitrergic NMT persists and that ICC-IM are not required. However, neither the changes in electrical events underlying NMT nor the contributions of other non-nitrergic neural pathways have been examined in this model.

METHODS

The role of ICC-IM in NMT was examined by recording the contractile and electrical events associated with electrical field stimulation (EFS) of motor neurons in the IAS of wildtype and W/W(v) mice. Nitrergic, purinergic, and cholinergic components were identified using inhibitors of these pathways.

KEY RESULTS

Under NANC conditions, purinergic and nitrergic pathways both contribute to EFS-induced inhibitory junction potentials (IJPs) and relaxation. Purinergic IJPs and relaxation were intact in the W/W(v) mouse IAS, whereas nitrergic IJPs were reduced by 50-60% while relaxation persisted. In the presence of L-NNA (NOS inhibitor) and MRS2500 (P2Y1 receptor antagonist), EFS gave rise to cholinergic depolarization and contractions that were abolished by atropine. Cholinergic depolarization was absent in the W/W(v) mouse IAS while contraction persisted.

CONCLUSIONS & INFERENCES

ICC-IM significantly contributes to the electrical events underlying nitrergic and cholinergic NMT, whereas contractile events persist in the absence of ICC-IM. The purinergic inhibitory neural pathway appears to be independent of ICC-IM.

Interstitial cells of Cajal in the cynomolgus monkey rectoanal region and their relationship to sympathetic and nitrergic nerves

The morphology of interstitial cells of Cajal (ICC) in the circular muscle layer of the cynomolgus monkey internal anal sphincter (IAS) and rectum and their relationship to sympathetic and nitrergic nerves were compared by dual-labeling immunohistochemistry. Contractile studies confirmed that nitrergic nerves participate in neural inhibition in both regions whereas sympathetic nerves serve as excitatory motor nerves only in the IAS. Muscle bundles extended from myenteric to submucosal edge in rectum but in the IAS bundles were further divided into "minibundles" each surrounded by connective tissue. Dual labeling of KIT and smooth muscle myosin revealed KIT-positive stellate-shaped ICC (ICC-IAS) within each minibundle. In the rectum intramuscular ICC (ICC-IM) were spindle shaped whereas stellate-shaped ICC were located at the myenteric surface (ICC-MY). ICC were absent from both the myenteric and submucosal surfaces of the IAS. Nitrergic nerves (identified with anti-neuronal nitric oxide synthase antibodies or NADPH diaphorase activity) and sympathetic nerves (identified with anti-tyrosine hydroxylase antibody) each formed a plexus at the myenteric surface of the rectum but not the IAS. Intramuscular neuronal nitric oxide synthase- and tyrosine hydroxylase-positive fibers were present in both regions but were only closely associated with ICC-IM in rectum. Minimal association was also noted between ICC-IAS and cells expressing the nonspecific neuronal marker PGP9.5. In conclusion, the morphology of rectal ICC-IM and ICC-MY is similar to that described elsewhere in the gastrointestinal tract whereas ICC-IAS are unique. The distribution of stellate-shaped ICC-IAS throughout the musculature and their absence from both the myenteric and submucosal surfaces suggest that ICC-IAS may serve as pacemaker cells in this muscle whereas their limited relationship to nerves suggests that they are not involved in neuromuscular transmission. Additionally, the presence of numerous minibundles, each containing both ICC-IAS and nerves, suggests that this muscle functions as a multiunit type muscle.

Species dependent differences in the actions of sympathetic nerves and noradrenaline in the internal anal sphincter

Excitatory motor innervation to the internal anal sphincter (IAS) of the monkey, the rabbit and mouse were compared. Contractile responses to electrical field stimulation of nerves (EFS, atropine 1 micromol L(-1) and N(omega)-nitro-L-arginine 100 micromol L(-1) present throughout) were examined in isolated strips of IAS. In the monkey IAS, EFS caused frequency dependent (1-30 Hz) contractions which were abolished by guanethidine (10 micromol L(-1)) or phentolamine (3 micromol L(-1)). The sympathetic neurotransmitter noradrenaline (NA) also caused concentration-dependent (10 nmol L(-1)-100 micromol L(-1)) contractions which were abolished by phentolamine revealing a small relaxation that was abolished by propranolol (3 micromol L(-1)). In contrast, EFS caused only relaxation of the mouse and rabbit IAS which was not affected by guanethidine. Furthermore, NA relaxed these muscles and relaxation was nearly abolished by combined addition of phentolamine and propranolol. In conclusion, the monkey IAS is functionally innervated by sympathetic nerves that contract the muscle via excitatory alpha-adrenergic receptors. In contrast, no significant motor function could be identified for sympathetic nerves in the rabbit or mouse IAS although adrenergic receptors linked to muscle inhibition are present. These data reveal species dependent differences in sympathetic motor innervation and suggest that some species are more appropriate than others as models for motor innervation to the human IAS.

Template switching within exons 3 and 4 of KV11.1 (HERG) gives rise to a 5' truncated cDNA

K(V)11.1 (HERG) channels contribute to membrane potential in a number of excitable cell types. We cloned a variant of K(V)11.1 from human jejunum containing a 171 bp deletion spanning exons 3 and 4. Expression of a full-length cDNA clone containing this deletion gave rise to protein that trafficked to the cell membrane and generated robust currents. The deletion occurred in a G/C-rich region and identical sequence elements of UGGUGG were located at the deletion boundaries. In recent studies these features have been implicated to cause deletions via template switching during cDNA synthesis. To examine this possibility we compared cDNAs from human brain, heart, and jejunum synthesized at lower (42 degrees C) and higher temperatures (70 degrees C). The 171 bp deletion was absent at the higher temperature. Our results suggest that the sequence and secondary structure of mRNA in the G/C rich region leads to template switching producing a cDNA product with a 171 bp deletion.

Differential expression of P2X-purinoceptor subtypes in circular and longitudinal muscle of canine colon

Adenosine triphosphate (ATP) mediates excitatory junction potentials through P2X receptors in many smooth muscles. However, relatively little is known about postjunctional intestinal P2X receptors. We examined the effect of exogenous ATP on circular and longitudinal myocytes of canine colon using the patch clamp technique at 32 degrees C. In both cell types, ATP induced inward currents (I(ATP)) at -70 mV in a concentration-dependent manner. The potency profile of ATP analogues in circular myocytes was: ATP approximately 2-methylthio-ATP > alpha,beta-methylene ATP, and that in longitudinal myocytes was: alpha,beta-methylene ATP approximately ATP > 2-methylthio-ATP. Pretreatment of circular myocytes with alpha,beta-methylene ATP inhibited the response to subsequent ATP, suggesting receptor desensitization. I-V relationships of I(ATP) were linear with inward rectification and E(rev) of -13 mV. I(ATP) at -70 mV was carried predominantly by Na+ as determined by shifts in E(rev) when extracellular Na+ was lowered. In RT-PCR, circular myocytes expressed mRNAs encoding P2X2, 3 and 4, while longitudinal myocytes expressed mRNAs for P2X3 and 5. P2X7 was absent in both cells. Fragments of each subtype were cloned and sequenced. We failed to clone P2X1 and P2X6 genes. Overall, different P2X receptor subtypes are expressed in circular and longitudinal canine colonic myocytes. Their activation produces non-selective cation currents that can depolarize and excite muscles of both layers.

Muscarinic M2 Receptor Stimulation of Cav1.2b Requires Phosphatidylinositol 3-Kinase, Protein Kinase C, and c-Src

This study investigated regulation of L-type calcium channels (Cav1.2b) by acetylcholine (ACh) in rabbit portal vein myocytes. Whole-cell currents were recorded using 5 mmol/L barium as charge carrier. ACh (10 μmol/L) increased peak currents by 40%. This effect was not reversed by the selective muscarinic M3 receptor antagonist 4-DAMP (100 nmol/L) but was blocked by the M2 receptor antagonist methoctramine (5 μmol/L). The classical and novel protein kinase C (PKC) antagonist calphostin C (50 nmol/L) abolished ACh responses, whereas the classical PKC antagonist Gö6976 (200 nmol/L) had no effect. ACh responses were also abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (20 μmol/L), by the c-Src inhibitor PP2 (10 μmol/L) (but not the inactive analogue PP3), and by dialyzing cells with an antibody to the G-protein subunit Gβγ. Cells dialyzed with c-Src had significantly greater currents than control cells. Current enhancement persisted in the presence of LY294002, suggesting that c-Src is downstream of PI3K. Phorbol 12,13-dibutyrate (PDBu, 0.1 μmol/L) increased currents by 74%. This effect was abolished by calphostin C and reduced by Gö6976. The PDBu response was also reduced by PP2, and the PP2-insensitive component was blocked by Gö6976. In summary, these data suggest that ACh enhances Cav1.2b currents via M2 receptors that couple sequentially to Gβγ, PI3K, a novel PKC, and c-Src. PDBu stimulates the novel PKC/c-Src pathway along with a second pathway that is independent of c-Src and involves a classical PKC.

Expression and function of KCNH2 (HERG) in the human jejunum

Previous studies suggest that ether-a-go-go related gene (ERG) KCNH2 potassium channels contribute to the control of motility patterns in the gastrointestinal tract of animal models. The present study examines whether these results can be translated into a role in human gastrointestinal muscles. Messages for two different variants of the KCNH2 gene were detected: KCNH2 V1 human ERG (HERG) (28) and KCNH2 V2 (HERG(USO)) (13). The amount of V2 message was greater than V1 in both human jejunum and brain. The base-pair sequence that gives rise to domains S3-S5 of the channel was identical to that previously published for human KCNH2 V1 and V2. KCNH2 protein was detected immunohistochemically in circular and longitudinal smooth muscle and enteric neurons but not in interstitial cells of Cajal. In the presence of TTX (10(-6) M), atropine (10(-6) M). and l-nitroarginine (10(-4) M) human jejunal circular muscle strips contracted phasically (9 cycles/min) and generated slow waves with superimposed spikes. Low concentrations of the KCNH2 blockers E-4031 (10(-8) M) and MK-499 (3 x 10(-8) M) increased phasic contractile amplitude and the number of spikes per slow wave. The highest concentration of E-4031 (10(-6) M) produced a 10-20 mV depolarization, eliminated slow waves, and replaced phasic contractions with a small tonic contracture. E-4031 (10(-6) M) did not affect [(14)C]ACh release from enteric neurons. We conclude that KCNH2 channels play a fundamental role in the control of motility patterns in human jejunum through their ability to modulate the electrical behavior of smooth muscle cells.

Spatial localization and properties of pacemaker potentials in the canine rectoanal region

The present study investigated the spatial organization of electrical activity in the canine rectoanal region and its relationship to motility patterns. Contraction and resting membrane potential (E(m)) were measured from strips of circular muscle isolated 0.5-8 cm from the anal verge. Rapid frequency [25 cycles/min (cpm)] E(m) oscillations (MPOs, 12 mV amplitude) were present across the thickness of the internal anal sphincter (IAS; 0.5 cm) and E(m) was constant (-52 mV). Between the IAS and the proximal rectum an 18 mV gradient in E(m) developed across the muscle thickness with the submucosal edge at -70 mV and MPOs were replaced with slow waves (20 mV amplitude, 6 cpm). Slow waves were of greatest amplitude at the submucosal edge. Nifedipine (1 micro M) abolished MPOs but not slow waves. Contractile frequency changes were commensurate with the changes in pacemaker frequency. Our results suggest that changing motility patterns in the rectoanal region are associated with differences in the characteristics of pacemaker potentials as well as differences in the sites from which these potentials emanate.

Electrical activity induced by nitric oxide in canine colonic circular muscle

Nitric oxide generates slow electrical oscillations (SEOs) in cells near the myenteric edge of the circular muscle layer, which resemble slow waves generated by interstitial cells of Cajal (ICCs) at the submucosal edge of this muscle. The properties of SEOs were studied to determine whether these events are similar to slow waves. Rapid frequency membrane potential oscillations (MPOs; 16 +/- 1 cycles/min and 9.6 +/- 0.2 mV) were recorded from control muscles near the myenteric edge. Sodium nitroprusside (0.3 microM) reduced MPOs and initiated SEOs (1.3 +/- 0.3 cycles/min and 13.4 +/- 1.4 mV amplitude). SEOs were abolished by the guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (10 microM). MPOs were abolished by nifedipine (1 microM), whereas SEO frequency increased and the amount of depolarization decreased. BAY K 8644 (1 microM) prolonged SEOs and reduced their frequency. SEOs were abolished by Ni(2+) (0.5 mM), low Ca(2+) solution (0.1 mM Ca(2+)), cyclopiazonic acid (10 microM), and the mitochondrial uncouplers antimycin (10 microM) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM). Oligomycin (10 microM) was without effect. These effects are similar to those described for colonic slow waves. Our results suggest that nitric oxide-induced SEOs are similar in mechanism to slow waves, an activity not previously thought to be generated by myenteric pacemakers.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

beta-Adrenergic receptor stimulation of L-type Ca2+ channels in rabbit portal vein myocytes involves both alphas and betagamma G protein subunits

1. Previous studies have shown that purified G protein alphas and betagamma subunits stimulate vascular L-type Ca2+ channels through protein kinase A and C (PKA and PKC), respectively. The present study tested whether activation of endogenous G proteins via beta-adrenergic receptor binding also stimulates vascular Ca2+ channels through both Galphas and Gbetagamma and the subsequent activation of PKA and PKC. 2. Peak Ba2+ current (IBa) in freshly isolated rabbit portal vein smooth muscle cells was significantly increased by bath application of 0.5 microM isoproterenol (isoprenaline; ISO) when measured using the whole-cell patch clamp method (53 +/- 3 % increase, n = 15). Stimulation of IBa by ISO was partially reversed by a PKA inhibitor, KT 5720, or a PKC inhibitor, calphostin C, and completely blocked when cells were pretreated with both KT 5720 and calphostin C. 3. Dialysis of cells with polyclonal antibody to Galphas significantly reduced but did not completely eliminate ISO-induced stimulation of IBa. The remaining stimulation was abolished by calphostin C. Dialysis of cells with a polyclonal antibody to Gbeta also significantly reduced ISO-induced stimulation and the remaining stimulation was abolished by KT 5720. Dialysis of cells with both antibodies completely prevented the stimulation of IBa by ISO. 4. ISO-induced stimulation of IBa was reversed by ICI-118,551, a specific beta2-adrenoceptor antagonist, but not by CGP 20712A, a specific beta1-adrenoceptor antagonist. In addition, the beta2-adrenoceptor agonist zinterol significantly increased peak IBa while the beta1-adrenoceptor agonist dobutamine and beta3-adrenoceptor agonist BRL 37344A had little effect on peak IBa. 5. These data suggest that beta-adrenergic receptor stimulation of vascular L-type Ca2+ channels involves both alphas and betagamma G-protein subunits, which exert their effects through PKA and PKC, respectively.

Frequency dependent alpha(2)-adrenoceptor mediated modulation of excitatory junction potentials in guinea-pig mesenteric artery

Excitatory junctional potentials (EJPs) elicited with brief duration (10 s) electrical field stimulation of guinea-pig mesenteric arteries were nearly abolished at all frequencies by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 30 microM) but persisted following reserpinization. Suramin (100 microM) enhanced EJPs at 0.2-0.5 Hz responses and reduced them at 2-32 Hz. Phentolamine (1 microM) and yohimbine (0.1 microM) enhanced EJPs at 0.2-8 Hz but not at 16-32 Hz. Oxymetazoline (0.3 microM) reduced EJPs at 0.2-0.5 Hz but not at 1-32 Hz. Following reserpinization, EJPs were enhanced at 0.2-2 Hz but not at 4-32 Hz. Clonidine (0.1 microM) was without effect at all frequencies in control arteries but reduced EJPs at 0.2-2 Hz in reserpine-treated arteries. In conclusion, pre-junctional alpha(2)-adrenoceptors modulate ATP release during low frequency, brief duration sympathetic nerve stimulation.

Regulation of BK(Ca) channels expressed in human embryonic kidney 293 cells by epoxyeicosatrienoic acid

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites of cytochrome P450 monooxygenase, which are released from endothelial cells and dilate arteries. Dilation seems to be caused by activation of large-conductance Ca2+ activated K+ channels (BK(Ca)) leading to membrane hyperpolarization. Previous studies suggest that EETs activate BK(Ca) channels via ADP-ribosylation of the G protein Galphas with a subsequent membrane-delimited action on the channel [Circ Res 78:415-423, 1996; 80:877-884, 1997; 85:349-356, 1999]. The present study examined whether this pathway is present in human embryonic kidney (HEK) 293 cells when the BK(Ca) alpha-subunit (cslo-alpha) is expressed without the beta-subunit. 11,12-EET increased outward K+ current in whole-cell recordings of HEK293 cells. In cell-attached patches, 11,12-EET also increased the activity of cslo-alpha channels without affecting unitary conductance. This action was mimicked by cholera toxin. The ADP-ribosyltransferase inhibitors 3-aminobenzamide and m-iodobenxylguanidine blocked the stimulatory effect of 11,12-EET. In inside-out patches 11,12-EET was without effect on channel activity unless GTP was included in the bathing solution. GTP and GTPgammaS alone also activated cslo-alpha channels. Dialysis of cells with anti-Galphas antibody completely blocked the activation of cslo-alpha channels by 11,12-EET, whereas anti-Galphai/o and anti-Gbetagamma antibodies were without effect. The protein kinase A inhibitor KT5720 and the adenylate cyclase inhibitor SQ22536 did not reduce the stimulatory effect of 11,12-EET on cslo-alpha channels in cell-attached patches. These data suggest that EET leads to Galphas-dependent activation of the cslo-alpha subunits expressed in HEK293 cells and that the cslo-beta subunit is not required.

Cotransmission from sympathetic vasoconstrictor neurons: differences in guinea-pig mesenteric artery and vein

Vasoconstrictor responses to electrical field stimulation (EFS, 0.2-32 Hz, 0.1 ms, 12 V, for 1 min) were measured in endothelium-denuded segments of guinea-pig mesenteric vein and compared to responses in mesenteric artery. The distribution of both tyrosine-hydroxylase-like immunoreactivity (TH-LI) and neuropeptide Y-like immunoreactivity (NPY-LI) was also studied using anti-TH and anti-NPY antibodies. The effect of exogenous NPY (10 nM) on EFS (8 Hz, 0.3 ms, 12 V, for 1 min)-evoked overflow of noradrenaline (NA) was also studied using an HPLC technique with electrochemical detection. Veins responded with contractions at lower frequencies of stimulation than arteries. Prazosin (0.1 microM) abolished the EFS-evoked contractions in artery at 0.5-32 Hz and in vein at 0.2-1 Hz of stimulation. However, in vein, the contractile responses to EFS at 2-32 Hz of stimulation were only reduced by prazosin. Phentolamine (1 microM) abolished the responses to 0.5-4 Hz and reduced the responses to 8-32 Hz of EFS in artery. In vein, phentolamine (1 microM) abolished the responses to 0.2-1 Hz and facilitated the contractions elicited by 16-32 Hz. The NPY-receptor antagonist BIBP3226 (1 microM), in combination with phentolamine, abolished contractions in vein. Yohimbine (0.1 microM) abolished the responses to lower frequencies of stimulation in both artery (0.5-2 Hz) and vein (0.2-1 Hz). The responses to greater frequency stimulation were not affected by yohimbine in artery, and were facilitated in vein. Pre-treatment of animals for 24 h with reserpine abolished contractile responses to EFS in artery, whereas in vein, responses to 0.2-2 Hz were abolished while responses to 4-32 Hz were unchanged. Suramin (100 microM) or alpha,beta-methylene ATP (alpha,beta MeATP; 10-100 microM) treatment did not affect the contractile responses to EFS in either artery or vein. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS; 30 microM), even potentiated the responses to 2-16 Hz in vein. However, following resperine-treatment, both PPADS and suramin reduced the nerve-evoked contractions of vein. Either BIBP3226 (1 microM) alone or BIBP3226 in combination with PPADS or suramin abolished the contractile response to EFS in reserpine-treated veins. NPY (100 nM) produced significantly more contraction in vein than in artery (i.e., 93 +/- 2.5 versus 7 +/- 4% of the response to 70 mM KCl, respectively). NPY (10 nM) significantly reduced the NA overflow evoked by EFS at 8 Hz. Flat mount preparations and cryostat sections of both mesenteric artery and vein revealed that TH-LI and NPY-LI were co-localized in a dense network of fibers within the adventitial layer. In conclusion, NA exclusively mediates the contractile response to sympathetic nerve stimulation in guinea-pig mesenteric artery, whereas at least three neurotransmitters [i.e., NA, adenosine 5'-triphosphate (ATP) and NPY] are involved in the neural response of mesenteric vein.

Multiple P2Y receptors mediate contraction in guinea pig mesenteric vein

Vasoconstrictor responses to exogenous adenine and pyrimidine nucleotides were measured in endothelium-denuded segments of guinea pig mesenteric vein and compared with responses in mesenteric artery. The rank order of potency for nucleotides in veins was: 2-MeSADP = 2-MeSATP > UTP > ATPgammaS = alpha,betaMeATP > UDP = ATP > ADP >> beta,gamma-D-MeATP = beta,gamma-L-MeATP. In contrast 2-MeSADP, UTP, and UDP were inactive in arteries, and the rank order of potency of other nucleotides differed; that is, alpha,betaMeATP > beta, gamma-D-MeATP > beta,gamma-L-MeATP = ATPgammaS = 2-MeSATP > ATP > ADP. In veins, UTP, ATP, and 2-MeSATP were more efficacious contractile agents than alpha,beta MeATP. In addition, the ability to desensitize responses to these nucleotides and inhibit them with various blockers differed. The response to alpha,betaMeATP in veins exhibited rapid desensitization and was inhibited by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS) and suramin. The response to 2-MeSATP in veins did not desensitize; nor was it inhibited by prior alpha,betaMeATP desensitization, but it was inhibited by PPADS, suramin, and the selective P2Y(1) receptor antagonist adenosine 3',5'-bisphosphate (ABP, 10-100 microM). Responses to ATP and UTP in veins did not desensitize and were not inhibited by PPADS, suramin, ABP, or alpha, betaMeATP desensitization. In conclusion, our results suggest that venous contraction to a variety of nucleotides is mediated in large part by P2Y receptors including P2Y(1) receptors and an UTP-preferring P2Y receptor. A small component of contraction also appears to be mediated by P2X(1) receptors. This receptor profile differs markedly from that of mesenteric arteries in which P2X(1) receptors predominate.

An unusual Ca(2+) entry pathway activated by protein kinase C in dog splenic artery

The characteristics of the Ca(2+) entry pathways that are activated by protein kinase C (PKC) in canine splenic artery were investigated. Phorbol 12, 13-dibutyrate (PDB) contracted tissues and increased Ca(2+) influx. PDB-induced contraction was reduced by preincubation of tissues in Ca(2+)-free Krebs' solution (1 mM EGTA) but was unaffected when Ca(2+)-free solution was applied after contraction was initiated with PDB. In contrast, (45)Ca influx and contraction induced by PDB were resistant to nifedipine, Cd(2+), Gd(3+), La(3+), or Ni(2+) whether added before or during exposure to PDB. Indeed, Cd(2+) reduced (45)Ca(2+) efflux and potentiated Ca(2+) influx, but not PDB-induced contraction. Norepinephrine (NE)-induced contractions were inhibited by preincubation in Ca(2+)-free Krebs' solution (1 mM EGTA). Nifedipine (10 microM) led to a small reduction in the NE-induced contraction but was without effect on (45)Ca(2+) influx. Pretreatment for 16 min with Cd(2+), Gd(3+), or La(3+) (each 1 mM) reduced or abolished NE-induced contraction and Ca(2+) influx. Application of these cations after exposure to NE did not affect (45)Ca(2+) influx but reduced tension. The Q(10) for the increase in (45)Ca(2+) influx was approximately 2 for high K(+) and NE, but 4 for PDB. The results suggest that stimulation of PKC in dog splenic artery activates a Ca(2+) entry pathway that is resistant to di- and trivalent cations. The inhibition of Ca(2+) influx by preincubating with cations during short-term exposure to NE may represent an action on Ca(2+) turnover that precedes activation of PKC.

Anchoring protein is required for cAMP-dependent stimulation of L-type Ca(2+) channels in rabbit portal vein

Stimulation of cardiac L-type Ca(2+) channels by cAMP-dependent protein kinase (PKA) requires anchoring of PKA to a specific subcellular environment by A-kinase anchoring proteins (AKAP). This study evaluated the possible requirement of AKAP in PKA-dependent regulation of L-type Ca(2+) channels in vascular smooth muscle cells using the conventional whole cell patch-clamp technique. Peak Ba(2+) current in freshly isolated rabbit portal vein myocytes was significantly increased by superfusion with either 0.5 microM isoproterenol (131 +/- 3% of the control value, n = 11) or 10 microM 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP; 114 +/- 1%, n = 8). The PKA-induced stimulatory effects of both isoproterenol and 8-BrcAMP were completely abolished by a specific PKA inhibitor KT-5720 (0.2 microM) or by dialyzing cells with Ht 31 (100 microM), a peptide that inhibits the binding of PKA to AKAP. In contrast, Ht 31 did not block the excitatory effect of the catalytic subunit of PKA when dialyzed into the cells. These data suggest that stimulation of Ca(2+) channels in vascular myocytes by endogenous PKA requires localization of PKA through binding to AKAP.

Regulation of L-type Ca2+ channels in rabbit portal vein by G protein alphas and betagamma subunits

1. The effect of purified G protein subunits alphas and betagamma on L-type Ca2+ channels in vascular smooth muscle and the possible pathways involved were investigated using freshly isolated smooth muscle cells from rabbit portal vein and the whole-cell patch clamp technique. 2. Cells dialysed with either Galphas or Gbetagamma exhibited significant increases in peak Ba2+ current (IBa) density (148 % and 131 %, respectively) compared with control cells. The combination of Galphas and Gbetagamma further increased peak IBa density (181 %). Inactive Galphas and Gbetagamma did not have any effect on Ca2+ channels. 3. The stimulatory effect of Galphas on peak IBa was entirely abolished by the protein kinase A inhibitor Rp-8-Br-cAMPS, or the adenylyl cyclase inhibitor SQ 22536. On the other hand, the stimulatory response of Ca2+ channels to Gbetagamma was not affected by the protein kinase A inhibitors Rp-8-Br-cAMPS and KT 5720, or by the Ca2+-dependent protein kinase C inhibitor bisindolylmaleimide 1, but was completely blocked by the protein kinase C inhibitor calphostin C. Pretreatment of cells with phorbol 12-myristate 13-acetate for over 18 h prevented the stimulatory effect of Gbetagamma on peak IBa. In addition, acute application of phorbol 12,13-dibutyrate enhanced peak IBa density in control cells, which could be entirely blocked by calphostin C. 4. These data indicate that enhancement of Ba2+ currents by Galphas and Gbetagamma can be attributed to increased activity of protein kinase A and protein kinase C, respectively. No direct membrane-delimited pathway for Ca2+ channel regulation by activated Gs proteins could be detected in vascular smooth muscle cells.

Cyclic GMP-dependent protein kinase activates cloned BKCa channels expressed in mammalian cells by direct phosphorylation at serine 1072

NO-induced activation of cGMP-dependent protein kinase (PKG) increases the open probability of large conductance Ca2+-activated K+ channels and results in smooth muscle relaxation. However, the molecular mechanism of channel regulation by the NO-PKG pathway has not been determined on cloned channels. The present study was designed to clarify PKG-mediated modulation of channels at the molecular level. The cDNA encoding the alpha-subunit of the large conductance Ca2+-activated K+ channel, cslo-alpha, was expressed in HEK293 cells. Whole cell and single channel characteristics of cslo-alpha exhibited functional features of native large conductance Ca2+-activated K+ channels in smooth muscle cells. The NO-donor sodium nitroprusside increased outward current 2.3-fold in whole cell recordings. In cell-attached patches, sodium nitroprusside increased the channel open probability (NPo) of cslo-alpha channels 3.3-fold without affecting unitary conductance. The stimulatory effect of sodium nitroprusside was inhibited by the PKG-inhibitor KT5823. Direct application of PKG-Ialpha to the cytosolic surface of inside-out patches increased NPo 3.2-fold only in the presence of ATP and cGMP without affecting unitary conductance. A point mutation of cslo-alpha in which Ser-1072 (the only optimal consensus sequence for PKG phosphorylation) was replaced by Ala abolished the PKG effect on NPo in inside-out patches and the effect of SNP in cell attached patches. These results indicate that PKG activates cslo-alpha by direct phosphorylation at serine 1072.

Modulation of cholinergic neuromuscular transmission by nitric oxide in canine colonic circular smooth muscle

This study examines the effect of nitric oxide (NO) on cholinergic transmission in strips of canine colonic circular muscle in which neural plexus-pacemaker regions had been removed. Electrical field stimulation gave rise to atropine- and TTX-sensitive excitatory junction potentials (EJPs), the amplitude of which were frequency dependent. In 47% of control muscles, the EJP was followed by an inhibitory junction potential (IJP), whereas in the presence of atropine all preparations exhibited only IJPs. The NO synthase inhibitor Nomega-nitro-L-arginine (L-NNA), the guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (ODQ), and the protein kinase G (PKG) antagonist Rp-8-bromo-PET-cGMPS all significantly increased EJP amplitude and reduced or abolished IJPs. The potentiation of EJPs by L-NNA was reversed by the NO donors sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine in a manner blocked by ODQ. [14C]ACh overflow was also measured to evaluate the possible prejunctional effects of NO. Both norepinephrine and TTX significantly decreased [14C]ACh overflow; however, L-NNA, ODQ, and SNP were without effect. These data suggest that both cholinergic and nitrergic motoneurons functionally innervate the interior of the circular muscle layer. The inhibitory actions of NO on cholinergic transmission appear to be post- rather than prejunctional and to involve guanylyl cyclase as well as possibly PKG.

Basal activation of ATP-sensitive potassium channels in murine colonic smooth muscle cell

The function and molecular expression of ATP-sensitive potassium (KATP) channels in murine colonic smooth muscle was investigated by intracellular electrical recording from intact muscles, patch-clamp techniques on isolated smooth muscle myocytes, and reverse transcription polymerase chain reaction (RT-PCR) on isolated cells. Lemakalim (1 microM) caused hyperpolarization of intact muscles (17. 2 +/- 3 mV). The hyperpolarization was blocked by glibenclamide (1-10 microM). Addition of glibenclamide (10 microM) alone resulted in membrane depolarization (9.3 +/- 1.7 mV). Lemakalim induced an outward current of 15 +/- 3 pA in isolated myocytes bathed in 5 mM external K+ solution. Application of lemakalim to cells in symmetrical K+ solutions (140/140 mM) resulted in a 97 +/- 5 pA inward current. Both currents were blocked by glibenclamide (1 microM). Pinacidil (1 microM) also activated an inwardly rectifying current that was insensitive to 4-aminopyridine and barium. In single-channel studies, lemakalim (1 microM) and diazoxide (300 microM) increased the open probability of a 27-pS K+ channel. Openings of these channels decreased with time after patch excision. Application of ADP (1 mM) or ATP (0.1 mM) to the inner surface of the patches reactivated channel openings. The conductance and characteristics of the channels activated by lemakalim were consistent with the properties of KATP. RT-PCR demonstrated the presence of Kir 6.2 and SUR2B transcripts in colonic smooth muscle cells; transcripts for Kir 6.1, SUR1, and SUR2A were not detected. These molecular studies are the first to identify the molecular components of KATP in colonic smooth muscle cells. Together with the electrophysiological experiments, we conclude that KATP channels are expressed in murine colonic smooth muscle cells and suggest that these channels may be involved in dual regulation of resting membrane potential, excitability, and contractility.

Control of motility patterns in the human colonic circular muscle layer by pacemaker activity

1. This study characterized the electrical and mechanical activities of human colonic muscle strips obtained from either the ascending, descending or sigmoid colon of patient volunteers during elective colon resections. 2. Rhythmic contractile activity was observed in colonic circular muscle strips in the absence of external stimuli. This activity persisted in the presence of atropine, phentolamine, propranolol, tetrodotoxin and Nomega-nitro-L-arginine but was abolished by nifedipine. 3. The activity of whole circular muscle (WCM) was compared with that of the myenteric half (MCM), the submucosal half (SCM) and the interior (ICM) of the circular muscle layer. WCM exhibited a prominent 2-4 contractions min-1 contractile pattern which was also present in strips of SCM. In contrast, MCM and ICM exhibited slow (0.3-0.6 contractions min-1), long duration contractions with superimposed higher frequency contractions (17-18 contractions min-1). 4. Resting membrane potential (Vm), recorded at various positions through the thickness of WCM strips did not differ and averaged -50 mV. 5. Slow waves were observed in 83 % of muscles. They averaged 12 mV in amplitude, 9.4 s in duration and had a frequency of 2-4 contractions min-1. Slow waves were greatest in amplitude near the submucosal edge and decreased with distance away from this edge. Each slow wave was associated with a transient contraction. 6. Near the myenteric edge, rapid fluctuations of Vm with a mean frequency of 18 contractions min-1 were recorded in 67 % of muscles. Spiking activity was common and was superimposed upon slow waves and rapid Vm fluctuations. 7. In summary, slow waves were identified in the human colonic circular muscle layer which arise at or near the submucosal edge. These electrical events give rise to a 2-4 contractions min-1 contractile rhythm which is characteristic of the intact muscle layer. Thus, the nature and spatial organization of pacemaker activity in the human colon bears significant resemblance to other animal models, such as the dog and pig.

Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

1. Acetylcholine (ACh) elicits an endothelium-dependent relaxation and hyperpolarization in the absence of nitric oxide (NO) and prostaglandin synthesis in the guinea-pig coronary artery (GPCA). This response has been attributed to a factor termed endothelial-derived hyperpolarizing factor (EDHF). Recently it has been suggested that EDHF may be a cytochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatrienoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the presence of 100 microM N(omega)-nitro-L-arginine and 10 microM indomethacin. 2. ACh, AA and 11,12-EET each produced concentration-dependent relaxations in arteries contracted with the H1-receptor agonist AEP (2,2-aminoethylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatetranynoic acid (30 microM). 3. The cytochrome P450 inhibitors proadifen (10 microM) and clotrimazole (10 microM) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 microM) and 7-ethoxyresorufin (10 microM) were without effect on all three vasodilators. Proadifen and clotrimazole also inhibited ACh (1 microM) and LEM (1 microM)-induced hyperpolarization. 4. The ability of various potassium channel blockers to inhibit relaxation responses elicited with ACh, AA and 11,12-EET was also determined. Iberiotoxin (IBTX; 100 nM) was without effect on responses to ACh but significantly reduced responses to both AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) significantly reduced response to ACh but not responses to AA and 11,12-EET. Combined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a greater extent than 4-AP alone. Apamin (1 microM), glibenclamide (10 microM) and BaCl2 (50 microM) had no significant effect on responses to ACh, AA and 11,12-EET. 5. IBTX (100 nM) significantly reduced both 11,12-EET (33 microM) and AA (30 microM) hyperpolarization without affecting the ACh (1 microM)-induced hyperpolarization. In contrast, 4-AP significantly reduced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations. 6. In summary, our results suggest that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacology between AA and 11,12-EET suggests that this factor is an EET. However, the disparity of pharmacology between responses to ACh versus responses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that leads to NO- and prostaglandin-independent hyperpolarization and relaxation in the GPCA.

Modulation of Ca2+ channels by cyclic nucleotide cross activation of opposing protein kinases in rabbit portal vein

Cyclic nucleotides are known to modify voltage-gated (L-type) Ca2+ channel activity in vascular smooth muscle cells, but the exact mechanism(s) underlying these effects is not well defined. The purpose of the present study was to investigate the modulatory role of the cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) pathways in Ca2+ channel function by using both conventional and perforated-patch-clamp techniques in rabbit portal vein myocytes. The membrane-permeable cAMP derivative, 8-bromo cAMP (0.1 to 10 micromol/L), significantly increased (14% to 16%) peak Ba2+ currents, whereas higher concentrations (0.05 to 0.1 mmol/L) decreased Ba2+ currents (23% to 31%). In contrast, 8-bromo cGMP inhibited Ba2+ currents at all concentrations tested (0.01 to 1 mmol/L). Basal Ca2+ channel currents were significantly inhibited by the PKA blocker 8-Bromo-2'-O-monobutyryladenosine-3',5'-monophosphorothioate, Rp-isomer (Rp 8-Br-MP cAMPS, 30 micromol/L) and enhanced by the PKG inhibitor beta-Phenyl-1,N2-etheno-8-bromoguanosine-3',5'-monophosphorothioate, Rp-isomer (Rp-8-Br PET cGMPS, 10 nmol/L). In the presence of Rp 8-bromo PET cGMPS (10 to 100 nmol/L), both 8-bromo cAMP (0.1 mmol/L) and 8-bromo cGMP (0.1 mmol/L) enhanced Ba2+ currents (13% to 39%). The excitatory effect of 8-bromo cGMP was blocked by Rp 8-bromo MB-cAMPS. Both 8-bromo cAMP (0.05 mmol/L) and forskolin (10 micromol/L) elicited time-dependent effects, including an initial enhancement followed by suppression of Ba2+ currents. Ba2+ currents were also enhanced when cells were dialyzed with the catalytic subunit of PKA. This effect was reversed by the PKA blocker KT 5720 (200 nmol/L). Our results suggest that cAMP/PKA stimulation enhances and cGMP/PKG stimulation inhibits L-type Ca2+ channel activity in rabbit portal vein myocytes. Our results further suggest that both cAMP and cGMP have a primary action mediated by their own kinase as well as a secondary action mediated by the opposing kinase.

Adenosine-induced hyperpolarization in guinea pig coronary artery involves A2b receptors and KATP channels

The role of P1 purinoceptor subtypes, the adenylyl cyclase (AC) pathway, and ATP-sensitive K+ (KATP) channels in adenosine (Ado)-induced membrane hyperpolarization was investigated in isolated segments of the guinea pig coronary artery using conventional microelectrode techniques. Ado (1-100 microM) elicited concentration-dependent hyperpolarization (half-maximal effective concentration 7.5 +/- 0.5 microM) that averaged 28.6 +/- 2.9 mV at 100 microM Ado. The A1 selective agonist N6-cyclopentyladenosine (CPA), the A1/A2 agonist 2-chloroadenosine, and the A2a/A2b agonist 5-(N-ethylcarboxamido)adenosine (NECA) each induced glibenclamide (3 microM)-sensitive hyperpolarization at 10 microM. However, the selective A2a-receptor agonists CGS-21680 and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methoxyphenyl]ethyladenosine (10 microM each) were without effect. Responses to CPA and NECA were significantly reduced by the AC inhibitor SQ-22,536 (100 microM). Activation of the AC-adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) pathway by four additional methods, i.e., 1) forskolin (0.3-1 microM), 2) isoproterenol (0.1-1 microM), 3) combined milrinone (0.4 microM) and rolipram (30 microM), and 4) combined N6-phenyladenosine 3',5'-monophosphate, 8-(6-aminohexyl)aminoadenosine 3',5'-cyclic monophosphate, and the Sp-isomer of 5,6-dichloro-1-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphothioate (100 microM each), also gave rise to glibenclamide-sensitive hyperpolarization. These results suggest that stimulation of A2b receptors coupled to AC represents the predominant mechanism by which Ado elicits hyperpolarization in this vessel. The ensuing increase in cAMP activates PKA, which then increases the activity of KATP channels. Our results further suggest that KATP channels are an important target for numerous pathways that raise cAMP levels in the coronary artery.

Characterization of delayed rectifier K+ currents in rabbit coronary artery cells near resting membrane potential

Previous studies have suggested that delayed rectifier K+ (Kdr) channels contribute to the control of membrane potential in vascular smooth muscle. To explore this hypothesis further, we investigated the characteristics of Kdr channels in the negative voltage range in the rabbit coronary artery. The Kdr channel blocker 4-aminopyridine (1 mM) contracted intact vessels and depolarized them from -52 to -37 mV, suggesting that these channels significantly contribute to the maintenance of resting membrane potential. In contrast, the ATP-sensitive K+ channel blocker glybenclamide (3 microM) had little effect on resting tone and did not alter the contraction elicited with 4-aminopyridine. K+ currents in isolated cells were then investigated by using whole-cell patch-clamp techniques. Increasing extracellular K+ concentration ([K+]o) from 5 to 135 mM resulted in the appearance of large inward currents at potentials between -60 and 0 mV. The voltage dependence of conductance for inward K+ currents was steeper and shifted toward more negative potentials when compared with outward K+ currents in 5 mM [K+]o solution. Various blockers of Kdr channels, i.e., 4-aminopyridine (3 mM), phencyclidine (0.1 mM), and intracellular tetraethylammonium (10 mM), nearly abolished currents in high [K+]o solution. In contrast, Ba2+ (0.1 mM) was without effect. These results suggest that the inward currents detected at potentials between -60 and 0 mV in high [K+]o solution are Kdr currents. Our results suggest that Kdr channels physiologically contribute to the control of membrane potential in the rabbit coronary artery.

Basal release of nitric oxide induces an oscillatory motor pattern in canine colon

1. The consequences of intrinsic, basal nitric oxide release on electrical and contractile activity of canine proximal colon were examined. Membrane potential and contraction were simultaneously recorded from the circular muscle in the presence of drugs to block adrenergic and cholinergic responses. 2. Electrical slow waves were recorded from muscle cells near the submucosal surface of the circular layer. Spontaneous contractions were initiated by each slow wave. Contractile amplitude increased 1.9-fold when nerves were blocked with tetrodotoxin (TTX, 1 microM). 3. Muscle cells near the myenteric surface displayed myenteric potential oscillations (MPOs) averaging 16 cycles per minute (c.p.m.) in frequency and 10 mV in amplitude. Twenty-five per cent of muscles displayed an additional slow, neurogenic oscillation (mean frequency, 1 c.p.m.; amplitude, 14 mV) superimposed upon the MPO rhythm. 4. The nitric oxide (NO) synthase inhibitor N omega -nitro-L-arginine (L-NA, 100 microM; n = 16) abolished neurogenic oscillations, depolarized cells, and increased MPO upstroke velocity, amplitude and frequency. The actions of L-NA were mimicked by N omega-nitro-L-arginine methylester (L-NAME, 100 microM) and oxyhaemoglobin (3%). 5. Spontaneous contractions were increased 2.3-fold by L-NA, and TTX had no effect on contractions after addition of L-NA. 6. The NO-donor sodium nitroprusside (SNP, 1 microM) reversed the electrical and mechanical effects of L-NA and initiated slow oscillations similar to the neurogenic oscillations. Slow oscillations were also evoked with S-nitroso-N-acetylpenicillamine (SNAP, 1 microM). The effects of NO donors were blocked by oxyhaemoglobin. 7. Slow electrical oscillations could not be elicited by SNP after removal of a thin strip of circular muscle along the myenteric edge. 8. These data suggest that the spontaneous electrical and contractile activity of the proximal colon is tonically suppressed by basal release of NO. Basal NO causes an oscillatory pattern of electrical and mechanical activity. This activity does not require patterned firing of nerves; rather a continuous, low level release of NO would be capable of producing the neurogenic oscillatory behaviour. The slow oscillatory activity depends upon the presence of the myenteric region of the circular muscle layer, which contains cell bodies of enteric neurons and interstitial cells of Cajal.

Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens

The present study investigates how changes in intracellular Ca2+ concentration modulate the influx of 45Ca2+ in isolated rat vasa deferentia. Raising extracellular K+ concentration ([K+]0) to > or = 32 mM increased 45Ca2+ influx during the 1st min in solutions containing 0.03-1.5 mM extracellular Ca2+ concentration ([Ca2+]0). During the 6th min in [K+]0 > or = 50 mM, 45Ca2+ influx was less than during the 1st min. This decline in 45Ca2+ influx occurred for [Ca2+]0 > or = 0.4 mM. Procaine potentiated K(+)-stimulated 45Ca2+ influx in 1.5 mM [Ca2+]0 and eliminated the decline of 45Ca2+ influx in low [Ca2-]0. Ryanodine and norepinephrine reduced K(+)-stimulated 45Ca2+ influx. 45Ca2+ content changed with time in accordance with the changes observed in 45Ca2+ influx. In isolated cells, voltage-dependent inward currents inactivated more rapidly with 1.5 mM Ca2+ as the charge carrier than with 1.5 mM Ba2+, and the steady-state inactivation relationship was shifted in the hyperpolarizing direction. Inward current was reduced with either caffeine, ryanodine, or norepinephrine. The inhibitory effects of norepinephrine were abolished by depletion of intracellular Ca2+ stores. These results are compatible with the hypothesis that K(+)-stimulated 45Ca2+ influx declines with time due to Ca(2+)-induced inhibition of Ca2- channels. Ca(2+)- and inositol 1,4,5-trisphosphate-induced releases of Ca2+ from the sarcoplasmic reticulum appear to play an important role in this process.

Relationship between nitric oxide and vasoactive intestinal polypeptide in enteric inhibitory neurotransmission

Although considerable evidence suggests that NO serves as a neurotransmitter in gastrointestinal muscles, it is unlikely to be the only substance involved in enteric inhibitory neurotransmission. Vasoactive intestinal polypeptide (VIP) is known to be expressed by inhibitory motor neurons in the gut, and it appears to be co-localized with nitric oxide synthase (NOS) in a subpopulation of enteric neurons. These data suggest that NO and VIP may be parallel neurotransmitters. Others have suggested that VIP is the primary inhibitory transmitter, and it stimulates production of NO in smooth muscle cells. In this "serial cascade" model NO is a paracrine substance. We performed experiments on circular muscles and cells from the canine proximal colon to further test the idea that NO and VIP are parallel neurotransmitters and to determine the validity of the serial cascade model in these muscles. We found that NO-independent inhibitory effects were unmasked when excitatory and NO-dependent inhibitory responses were blocked. NO-independent inhibitory effects were reduced by alpha-chymotrypsin and blocked by tetrodotoxin. NOS- and VIP-like immunoreactivities were co-localized in enteric neurons and varicose fibers in the circular muscle layer. Similar to several other reports we found no evidence for a constitutive NOS in smooth muscle cells. Several aspects of the serial cascade model were not supported by our results: (i) the electrical and mechanical effects of VIP did not depend upon NO synthesis; (ii) VIP-induced changes in [Ca2+]i did not depend upon NO synthesis; and (iii) VIP did not cause the release of NO from canine colonic muscles. These results are consistent with the hypothesis that NO and VIP are co-transmitters, released in parallel from enteric inhibitory nerves.

Cyclic GMP-independent relaxation and hyperpolarization with acetylcholine in guinea-pig coronary artery

1. The effects of acetylcholine (ACh) on membrane potential, relaxation and cyclic GMP levels were compared to the NO donor L-nitrosocysteine (Cys-NO) in segments of guinea-pig coronary artery. 2. ACh and Cys-NO produced concentration-dependent relaxations of muscles contracted with the H1 receptor agonist, 2-(2-aminoethyl)pyridine (AEP, 0.35 mM). The relaxation to ACh was unchanged in the presence of NG-monomethyl-L-arginine (L-NMMA; 350 microM) or indomethacin (3 microM). 3. Oxyhaemoglobin (HbO; 20 microM) alone or in combination with L-NMMA increased the EC50 for ACh-induced relaxation whereas relaxation with Cys-NO was almost completely abolished with HbO. 4. Scorpion venom (SV; 8.7 micrograms ml-1) increased the EC50 for relaxation with ACh but not Cys-NO. Combined L-NMMA, HbO and SV produced nearly complete abolition of ACh-induced relaxations. 5. Basal cyclic GMP levels (i.e., 20 pmol mg-1 protein) were significantly increased following addition of either ACh (190 pmol mg-1 protein) or Cys-NO (240 pmol mg-1 protein). L-NMMA significantly reduced the rise of cyclic GMP with ACh but not Cys-NO. In contrast, SV did not significantly reduce the rise in cyclic GMP produced with ACh. In the combined presence of L-NMMA and HbO neither ACh nor Cys-NO produced a significant increase in cyclic GMP levels. 6. ACh gave rise to significantly greater membrane hyperpolarization than Cys-NO both in the presence and absence of AEP. Combined L-NMMA and HbO did not reduce the amplitude of hyperpolarization with ACh. 7. These data indicate that some but not all of the actions of ACh in the coronary artery can be mimicked by the NO donor, Cys-NO, suggesting that ACh releases NO as well as a second hyperpolarizing factor (i.e., EDHF). Release of NO results in a large increase in tissue cyclic GMP levels and minimal change in membrane potential whereas release of EDHF results in a large membrane hyperpolarization which is independent of changes in tissue cyclic GMP levels. Both of these pathways appear to contribute to relaxation throughout the entire ACh concentration-relaxation relationship.

Mechanism of cyclic AMP-induced hyperpolarization in canine colon

The mechanism of forskolin (FSK)-induced hyperpolarization was investigated in strips of canine colonic circular muscle. FSK responses were compared to those of the K+ channel opener lemakalim (LEM). Both FSK (10 microM) and LEM (10 microM) hyperpolarized cells near the myenteric border by 10 to 20 mV. Responses to both agents were abolished by 35 mM external K+, indicating a probable mediation by K+ channels. FSK increased the open probability of Ca(++)-activated K+ channels in isolated colonic myocytes. However, in muscle strips charybdotoxin (100 nM) and tetraethylammonium (10 mM) failed to reduce FSK- and LEM-induced hyperpolarizations whereas tetrapentylammonium (50 microM) and 4-aminopyridine (10 mM) blocked both responses. Phencyclidine (100 microM), Ba++ (1 mM) and the antagonist of ATP-sensitive K+ currents glybenclamide (10 microM) blocked LEM- but not FSK-induced hyperpolarizations. Delayed rectifier current in isolated myocytes was activated near -20 mV and was blocked by (order of potency): nifedipine > tetrapentylammonium > phencyclidine > 4-aminopyridine > tetraethylammonium. Charybdotoxin (100 nM), Ba++ (1 mM) and glybenclamide (10 microM) were without effect. Ca(++)-activated K+ current was activated near +30 mV and was blocked by: charybdotoxin > tetraethylammonium > tetrapentylammonium >> phencyclidine = 4-amino-pyridine. These data suggest that LEM induces membrane hyperpolarization by activation of a K+ current with a pharmacology similar to ATP-sensitive K+ current whereas cyclic AMP-induced hyperpolarization appears to involve activation of a current other than delayed rectifier current, Ca(++)-activated K+ current or ATP-sensitive K+ current.

Regulation of Ca2+ channels by cAMP and cGMP in vascular smooth muscle cells

Whole-cell Ca2+ channel currents in rabbit portal vein cells were recorded using the amphotericin B-perforated patch-clamp technique at 35 degrees C. This technique allowed recording of stable inward currents in the absence of run-down for more than 30 minutes. Depolarizing voltage steps from a holding potential of -70 mV elicited voltage-dependent inward currents. The voltage dependence of inward currents measured in either 2.5 mmol/L Ba(2+)- or 2.5 mmol/L Ca(2+)-containing solution were very similar. However, maximum Ba2+ current (obtained at around +10 mV) was approximately 1.5-fold larger than maximum Ca2+ current. Changing the holding potential from -70 to -40 mV decreased inward currents but did not shift the voltage dependence significantly. Inward currents were also completely blocked by the dihydropyridine Ca2+ channel blocker, nicardipine (10 mumol/L), suggesting the presence of predominantly L-type Ca2+ channels in rabbit portal vein cells. Isoproterenol caused small increases in the amplitude of Ba2+ currents in a concentration-dependent manner (10 nmol/L to 1 mumol/L), which were reversed with propranolol. Forskolin (1 mumol/L) or 8-bromo-cAMP (0.1 mmol/L) also caused small increases in the amplitude of Ba2+ currents, suggesting that the stimulatory actions of isoproterenol are importantly linked to the production of cAMP. Higher concentrations of of isoproterenol (10 mumol/L) or forskolin (10 mumol/L) caused a transient increase in Ba2+ currents followed by f decrease in current amplitude. Higher doses of 8-bromo-cAMP (1 mmol/L) and low doses of 8-bromo-cGMP (0.1 mmol/L) inhibited Ba2+ currents, increased the rate of current inactivation, and produced a negative voltage shift in steady-state availability. These results indicate that low concentrations of intracellular cAMP produce modest increases in Ca2+ channel activity, whereas cGMP and higher concentrations of cAMP result in inhibition of Ca2+ channel activity in vascular smooth muscle cells. The observed similarities of cGMP and high concentrations of cAMP on Ba2+ current amplitude, kinetics, and steady-state inactivation suggest mediation by a common mechanism, possibly involving activation of cGMP-dependent protein kinase.

Enteric inhibitory neural regulation of human colonic circular muscle: role of nitric oxide

BACKGROUND

Nitric oxide and an apamin-sensitive transmitter may both contribute to neural inhibition in the human colon. The present study investigated the role of NO in regulating spontaneous rhythmic contractions and examined NO-dependent and independent components of neurally evoked hyperpolarization in the human colon.

METHODS

Mechanical and electrical activity were recorded from isolated circular muscle strips.

RESULTS

Rhythmic contractions were inhibited by nerve stimulation. This response was reduced by apamin, oxyhemoglobin, and L-NG-nitro arginine methyl ester (L-NAME). Electrical recording revealed two components of neurally evoked hyperpolarization: a fast hyperpolarization resulting from a single stimulus and a sustained hyperpolarization that developed with repetitive stimulation. Fast hyperpolarization was not affected by L-NAME or oxyhemoglobin but was significantly reduced by apamin. The sustained hyperpolarization was reduced by L-NAME or apamin. Exogenous NO and the P2y receptor agonist 2-methylthio adenosine 5'-triphosphate (2-MATP) inhibited spontaneous contractions and produced hyperpolarization. Apamin reduced the effects of 2-MATP but not those of NO.

CONCLUSIONS

The results support the concept that the inhibitory neurotransmission in the human colon involves two transmitters. A single stimulus results in an apamin-sensitive response. With multiple stimuli, a NO-dependent response develops and sums with the apamin-sensitive mechanism, producing sustained hyperpolarization and inhibition of contractions.

Modulation of K+ and Ca2+ channels by histamine H1-receptor stimulation in rabbit coronary artery cells

1. The modulation of whole-cell K+ and Ca2+ currents by stimulation of histamine H1-receptors in freshly isolated single smooth muscle cells from the rabbit coronary artery was characterized using the patch-clamp technique at 35 degrees C. Single-channel K+ currents were also analysed using the cell-attached patch configuration. 2. The histamine H1-receptor agonist, 2-(2-aminoethyl)pyridine (AEP) (0.1 mM), increased the amplitude of voltage-activated inward Ba2+ currents, recorded using the perforated-patch recording technique, which could be completely blocked by the dihydropyridine antagonist, nicardipine (1 microM). 3. Whole-cell outward K+ currents in rabbit coronary artery cells could be classified into at least two components: (a) a slowly inactivating, 4-aminopyridine (4-AP)-sensitive low-noise current, and (b) a non-inactivating, tetraethylammonium (TEA)-sensitive high-noise current. 4. AEP (0.1 mM) caused changes in whole-cell outward K+ currents which depended upon membrane voltage. Specifically: (a) AEP enhanced the amplitude of outward currents at voltages between -30 and 0 mV, and (b) AEP decreased the outward currents at more positive potentials. 5. The removal of extracellular Ca2+ caused little inhibition of the effects of AEP on K+ currents, whereas the depletion of intracellular Ca2+ stores by pretreatment with ryanodine and caffeine prevented the effects of AEP on K+ channels. Moreover, acute exposure to ryanodine (10 microM) or thapsigargin (1 microM), a Ca(2+)-ATPase inhibitor, caused voltage-dependent changes in the outward currents similar to those observed with AEP. These results suggest that the voltage-dependent effects of AEP on K+ currents are mainly mediated by release of Ca2+ from intracellular stores. 6. The dual stimulatory and inhibitory effect of AEP on whole-cell K+ currents was shown to be due to a differential effect on two distinct types of K+ channels. The stimulatory effect observed over the voltage range -30 to 0 mV was prevented by pretreatment of cells with low concentrations of TEA (1 mM), whereas the inhibitory effect observed at positive potentials was prevented by pretreatment of cells with 4-AP (3 mM). 7. Single-channel recordings revealed two types of unitary K+ currents with conductances of 225 and 70 pS in the cell-attached configuration with symmetrical K+ solutions (150 mM K+ in pipette-150 mM K+ in bath). Bath application of AEP (0.1 mM) caused a marked increase in the open probability of the large conductance channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of nitric oxide synthesis reveals non-cholinergic excitatory neurotransmission in the canine proximal colon

1. Neuromuscular transmission in the circular muscle of the canine proximal colon was examined, in the presence and absence of nitric oxide synthase inhibitors, by use of mechanical and intracellular microelectrode recording techniques. 2. Electrical field stimulation (EFS; 0.1-20 HZ) produced frequency-dependent contractions of circular muscle strips which reached a maximum at 15 Hz. These responses were enhanced by NG-monomethyl-L-arginine (L-NMMA; 300 microM) and reduced by atropine (1 microM). The effects of L-NMMA were reversed by L-arginine (3 mM). All responses to EFS were abolished by tetrodotoxin (1 microM). 3. In the presence of atropine, phentolamine and propranolol (all at 1 microM; 'non-adrenergic, non-cholingergic (NANC) conditions'), EFS evoked frequency-dependent inhibition of phasic contractions which reached a maximum at 5 Hz. At higher frequencies of EFS, inhibition diminished, and these responses were followed by post-stimulus excitation. 4. Under NANC conditions and in the presence of L-NG-nitroarginine methyl ester (L-NAME; 200 microM), EFS evoked contractions at frequencies of 5 Hz or greater. These contractions were reduced by co-incubation with L-arginine (2 mM) and abolished by tetrodotoxin (1 microM). 5. In the presence of atropine (1 microM), EFS (5-20 Hz) caused frequency-dependent inhibition of electrical slow waves. In the presence of L-NAME (100 microM) and atropine, the inhibitory response to EFS was abolished and an increase in slow wave duration was seen at stimulation frequencies greater than 5 Hz. The effects of EFS on slow wave duration were abolished by tetrodotoxin (1 microM). 6. Atropine-resistant contractions to EFS were enhanced by indomethacin (10 microM) and reduced or abolished by the non-selective NK1/NK2 tachykinin receptor antagonist D-Pro2, D-Trp7,9 SP, and by the selective NK2 receptor antagonist MEN 10,376 (10 microM).7. Exogenous tachykinins mimicked non-cholinergic excitatory electrical and mechanical responses. The rank order of potency for contraction was neurokinin A>neurokinin B>substance P, suggesting a predominance of the NK2 sub-type of tachykinin receptors on colonic smooth muscle cells. Low concentrations of neurokinin A also increased the amplitude and duration of electrical slow waves.8. These results suggest that: (i) in previous studies, non-cholinergic excitatory responses were masked by the simultaneous release of NO; (ii) non-cholinergic excitatory responses occur throughout the period of stimulation and are not manifest only as 'rebound' excitation; (iii) one or more tachykinins, possibly,acting via NK2 receptors, may mediate non-cholinergic excitatory responses.

Intracellular divalent cations block smooth muscle K+ channels

The patch-clamp technique was used to examine the sensitivity of delayed rectifier K+ channels to changes in intracellular divalent cations (Mg2+ and Ca2+). During voltage-step and ramp depolarizations, a delayed rectifier K+ current (IK(dr)) was identified in renal, pulmonary, coronary, and colonic smooth muscle cells as a low-noise outward current that activated near -40 mV, was sensitive to 4-aminopyridine (4-AP), and was insensitive to charybdotoxin. During whole-cell voltage-clamp experiments in each of the cell types, the 4-AP-sensitive IK(dr) was significantly less in cells dialyzed with 10 mM Mg2+ as compared with cells in which no Mg2+ was added to the internal dialysis solution (P < or = .05, n > or = 4). In coronary artery cells, 100 microM 2-(2-aminoethyl)pyridine (an H1 receptor agonist) or 10 microM ryanodine, agents that cause an increase in [Ca2+]i, also caused a significant reduction of the 4-AP-sensitive IK(dr) similar to that produced by Mg2+. 4-AP (5 mM) significantly depolarized single renal arterial cells that were dialyzed with Mg(2+)-free solution but not those dialyzed with 10 mM Mg2+ (P < .01, n = 4). In inside-out patches of renal arterial smooth muscle cells, with 200 nM charybdotoxin in the patch pipette to block large conductance Ca(2+)-activated K+ channels, a 59 +/- 10-picosiemen K+ channel that was sensitive to cytoplasmic Mg2+ was identified. In Mg(2+)-free solution, channel open probability was 0.028 +/- 0.012 (n = 8) and 0.095 +/- 0.011 (n = 8) at +40 and +80 mV, respectively. When the bath solution was changed to one containing 5 or 15 mM Mg2+, channel open probability was significantly reduced by 66% and 68% (+40 mV) or 93% and 96% (+80 mV), respectively. This decrease in the open probability of the delayed rectifier K+ channel resulted from a concentration- and voltage-dependent decrease in mean open time. At +40 mV, time constants for the open time distribution were significantly decreased from 5.5 +/- 0.52 to 1.2 +/- 0.14 milliseconds, whereas the closed time constant was significantly increased from 634 +/- 11.1 to 820 +/- 14.4 milliseconds (P < .01, n = 4). It is concluded that a 4-AP-sensitive delayed rectifier K+ channel in both vascular and visceral smooth muscle cells is modulated by changes in intracellular Ca2+ and Mg2+ that may alter membrane potential and the contractile state of smooth muscle.

Beta-adrenergic inhibition of electrical and mechanical activity in canine colon: role of cAMP

The effects of beta-adrenergic receptor stimulation on the electrical and mechanical activity of canine colonic circular muscles were compared with forskolin (Fsk), a known stimulator of adenylate cyclase. The actions of isoproterenol (Iso) were mediated by beta 2-receptors. Iso and Fsk increased intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in both the presence and absence of acetylcholine (ACh), whereas ACh (0.3 microM) alone reduced cAMP levels. These agents caused inhibition of spontaneous and ACh-induced contractions. Inhibition was associated with a reduction in the amplitude and duration of electrical slow waves recorded near the submucosal border. Near the myenteric border, Iso and Fsk hyperpolarized the membrane by up to 30 mV and changed the pattern of electrical rhythmicity. These effects were mimicked by 8-bromo-cAMP (1-3 mM). Contractile inhibition with Fsk and Iso was associated with a decrease in the amplitude and duration of Ca2+ transients measured with fura-2 fluorescence. cAMP (10-300 microM) reduced the Ca2+ sensitivity of the contractile apparatus in muscles permeabilized with staphylococcal alpha-toxin. The actions of Iso appear linked to cAMP. We hypothesize that cAMP produces relaxation both by modulation of membrane ionic channels with a consequent decline in the entry of Ca2+ as well as through a decrease in the sensitivity of the contractile apparatus to Ca2+.

NG-nitro L-arginine methyl ester and other alkyl esters of arginine are muscarinic receptor antagonists

Analogues of L-arginine with modifications at the terminal guanidino nitrogen and/or the carboxyl terminus of the molecule have been widely used for their ability to inhibit the production of nitric oxide and are thought to be competitive antagonists of nitric oxide synthase. The present studies were designed to test the possibility that these agents are also muscarinic receptor antagonists. Acetylcholine produced concentration-dependent contraction of endothelium-denuded rabbit coronary artery as well as isolated strips of canine colonic smooth muscle. The arginine analogue NG-nitro L-arginine methyl ester (L-NAME, 100 microM) but not NG-monomethyl L-arginine (L-NMMA, 100 microM) significantly shifted these contractile relations to the right, an effect that was not reversed by addition of 1 mM L-arginine. In radioligand binding studies using the muscarinic radioligand [3H]quinuclidinyl benzilate and tissues known to contain differing contributions of M1, M2, and M3 muscarinic receptors, addition of increasing concentrations of L-NAME resulted in a monophasic competition of binding with affinities (Ki) ranging from 68 microM in endothelium to 317 microM in whole aorta. Addition of the hydrolysis-resistant guanosine 5'-triphosphate analogue GTP gamma S (100 microM) had no effect on L-NAME competition of [3H]quinuclidinyl benzilate binding. Addition of L-NAME in radioligand binding competition studies using the agonist carbachol did not result in an alteration of the receptor's affinity for agonist, confirming the competitive nature of the interaction of L-NAME with the muscarinic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the actions of acetylcholine and BRL 38227 in the guinea-pig coronary artery

1. The contractile and electrical responses to acetylcholine (ACh) in isolated segments of guinea-pig and rabbit coronary arteries were compared to those of the putative adenosine 5'-triphosphate (ATP)-dependent K+ channel opener, BRL 38227. 2. Both ACh and BRL 38227 produced concentration-dependent relaxation of vessel segments contracted with the H1-receptor agonist, 2-(2-aminoethyl)pyridine. 3. An IC90 of either vasodilator also produced 17-20 mV of hyperpolarization of the guinea-pig coronary artery. 4. Glibenclamide (1-35 microM) depolarized the guinea-pig coronary artery by 8-12 mV and antagonized BRL 38227- but not ACh-induced relaxation and hyperpolarization. 5. In the guinea-pig coronary artery, the K+ channel blockers phencyclidine (PCP, 100 microM), tetraethylammonium (TEA, 10 mM) and scorpion venom (8.7 micrograms ml-1) all significantly reduced ACh-induced relaxation and hyperpolarization whereas only PCP was an effective antagonist of both relaxation and hyperpolarization with BRL 38227. 6. Similar effects of glibenclamide and scorpion venom on ACh- and BRL 38227-induced relaxation were observed in the rabbit coronary artery. 7. Apamin (3.5 microM) was without effect on either the ACh- or BRL 38227-induced relaxation in the guinea-pig coronary artery. 8. In conclusion, the actions of BRL 38227 in coronary artery are compatible with its proposed effects on ATP-dependent K+ channels. In contrast, the results with ACh suggest that some step between the initial binding of ACh to endothelial muscarinic receptors and the final relaxation of the smooth muscle depends upon the opening of Ca(2+)-activated K+ channels.

Action of alpha 2A-adrenergic receptors in circular smooth muscle of canine proximal colon

Addition of the alpha 2-adrenergic receptor agonist clonidine (1 microM) to tetrodotoxin-treated strips of canine colonic circular smooth muscle resulted in a significant increase in contractile force that was blocked by addition of the alpha 2-antagonist yohimbine (0.1 microM). The alpha 2-receptor antagonist radioligand [3H]rauwolsine bound rapidly and reversibly to a single class of saturable sites (Bmax, 38.4 +/- 6.2 fmol/mg protein) in colonic circular smooth muscle membranes with an affinity (KD = 5.1 +/- 0.9 nM) characteristic of alpha 2-adrenergic receptors in smooth muscle. Studies in cells freshly isolated from circular muscle of canine proximal colon verified the smooth muscle origin of these receptors. Rank order of potency of alpha 2-adrenergic receptor antagonists in competition for [3H]rauwolsine binding was yohimbine greater than oxymetazoline much greater than prazosin. Affinity of alpha 2-receptors for yohimbine was indistinguishable from that of its optical isomer, rauwolsine, in both competition studies (KI = 3.4 nM) and in saturation-binding experiments employing [3H]yohimbine directly (KD = 4.2 nM). The alpha-receptor agonist epinephrine, in competition studies employing [3H]rauwolsine, revealed high-affinity binding sites that were converted to low-affinity binding sites for agonist by addition of 100 microM GTP gamma S. Addition of the alpha 2 more-selective agonist clonidine (100 microM) resulted in inhibition of adenylate cyclase activity that was abolished by pretreatment of tissue strips with pertussis toxin suggesting coupling of the alpha 2-receptor in colon to adenylate cyclase via the GTP-binding protein Gi. Our data demonstrate a physiological role for adenylate cyclase-coupled receptors of the alpha 2A-subtype in canine colon circular smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Purinergic relaxation and hyperpolarization in guinea pig and rabbit coronary artery: role of the endothelium

Purinergic relaxation and hyperpolarization was investigated in the isolated guinea pig coronary artery (GPCA) and compared to purinergic relaxation in the rabbit coronary artery (RCA). Vessels were contracted with the histamine H1 receptor agonist 2-(2-aminoethyl)pyridine (AEP) and the actions of adenosine, AMP, ATP, 2-chloroadenosine (CIAD),2-methylthioATP (MeATP) and beta,gamma-methylene ATP compared. The order of potency with P1 agonists in GPCA was CIAD greater than adenosine = AMP and for P2y agonists it was MeATP greater than ATP greater than beta,gamma-methylene ATP. Both ATP and CIAD produced less relaxation of GPCA segments contracted with K+ (36-100 mM) than with AEP. The IC50 for CIAD, but not ATP, was increased in the GPCA with 8(p-sulfophenyl)theophylline (100 microM). Relaxations with ATP and CIAD were not different in endothelium-denuded GPCA. Methylene blue (50 microM) increased AEP contractile amplitude by 33% and increased the IC50 values for both CIAD and ATP. A similar shift in potency to purines was seen by varying AEP concentration. ATP, CIAD and MeATP also relaxed RCA. These responses were not different in endothelium-denuded vessels. Pressure application of ATP, MeATP, adenylylimidodiphosphate and to a lesser extent beta,gamma methATP transiently hyperpolarized cells in the GPCA, whereas CIAD did not. ATP hyperpolarization was enhanced in the presence of AEP and abolished after endothelium removal. The results indicate that purinergic relaxation in the GPCA and RCA is mediated by both P1 and P2y receptors located on the smooth muscle cells. In the GPCA, P1 receptors appear limited to the muscle, whereas P2y receptors are present on both muscle and endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical and mechanical effects of acetylcholine and substance P in subregions of canine colon

Effects of acetylcholine (ACh) and substance P on the electrical and mechanical activities of the circular muscle layer of the canine proximal colon were studied. Because this muscle layer is bordered by two different pacemaker regions, responses from segments containing either a single pacemaker region or no pacemaker region were compared with responses of the complete muscle layer. Concentration-response relationships for ACh and substance P were similar between the various segments, suggesting that receptors for these agonists are expressed throughout the layer. The dominant contractile pattern induced by ACh and substance P in each segment was a 1- to 3-cycle/min rhythm. In a like manner, these agonists also elicited an electrical pattern in which a long-duration slow wave occurred one to three times per minute between short-duration slow waves. Low concentrations of nifedipine (0.01 microM) selectively antagonized the 1- to 3-cycle/min rhythm. In circular muscles with no pacemaker region, ACh (1 microM) caused depolarization, induced oscillations in membrane potential averaging 24 +/- 5 mV in amplitude and 2.9 +/- 0.9 cycles/min in frequency, and generated rhythmic contractions at the same frequency. This "interior" circular muscle was functionally innervated by cholinergic excitatory nerves. Exposure to ACh (1 microM) did not alter the conduction of slow waves through the thickness of the circular layer. In summary, the excitatory neurotransmitters, ACh and substance P, induce a dominant electrical and contractile rhythm throughout the circular muscle layer that is different from the spontaneous rhythms produced at either the myenteric or submucosal border.

Comparison of neurotransmission with nerve trunk and transmural field stimulation in guinea-pig mesenteric artery

1. Intracellular electrical and contractile responses to sympathetic nerve trunk stimulation (NTS) and transmural electrical field stimulation (TMS) were compared in the guinea-pig mesenteric artery in vitro. 2. Step increases in voltage with NTS gave rise to excitatory junctional potentials (EJPs) which reached a plateau amplitude of 5-10 mV, whereas with TMS larger amplitude EJPs and sometimes action potentials were obtained. 3. EJPs of equal amplitude (1-7 mV) elicited with TMS and NTS had the same rise time, duration and decay half-time. 4. Slow depolarization obtained with repetitive stimulation was significantly greater in amplitude with TMS than with NTS. 5. Equal amplitude EJPs were obtained throughout the preparation with NTS. With TMS, the amplitude of responses declined substantially with distance from the stimulating electrodes. 6. Tetrodotoxin (TTX) completely blocked EJPs, slow depolarization and contraction with NTS; however, with TMS a TTX-resistant component was observed. The TTX-resistant response to TMS was abolished in the presence of a low-Ca2+ superfusion solution but was not affected by endothelium removal. 7. Phentolamine or prazosin abolished slow depolarization but not EJPs with NTS or TMS. Prazosin abolished contraction with NTS and reduced but did not abolish contraction with TMS. 8. alpha, beta-Methylene ATP abolished EJPs with NTS, whereas with TMS only EJPs obtained with low stimulus intensities were abolished. alpha, beta-Methylene ATP did not block contraction with either NTS or TMS. 9. Combined TTX, prazosin and alpha, beta-methylene ATP abolished EJPs initiated with TMS at all but the highest stimulus intensities (12-20 V, 0.3 ms duration). 10. It is concluded that responses obtained with NTS can be reliably attributed to the release of transmitter by the conduction of action potentials in paravascular nerves, whereas activation by TMS is a more complex phenomenon dependent upon stimulus strength and probably involving multiple forms of activation.