Nitric oxide modulates a calcium-activated potassium current in muscle cells from opossum esophagus

Nitric oxide activation of a potassium channel (BKCa) in feline lower esophageal sphincter

AIM: To assess the effect of nitric oxide (NO) on the large conductance potassium channel (BK_Ca) in isolated circular (CM) and sling (SM) muscle cells and muscle strips from the cat lower esophageal sphincter (LES) to determine its regulation of resting tone and relaxation.

METHODS: Freshly enzymatically-digested and isolated circular smooth muscle cells were prepared from each LES region. To study outward K⁺ currents, the perforated patch clamp technique was employed. To assess LES resting tone and relaxation, muscle strips were mounted in perfused organ baths.

RESULTS: (1) Electrophysiological recordings from isolated cells: (a) CM was more depolarized than SM (-39.7 ± 0.8mV vs -48.1 ± 1.6 mV, P < 0.001), and maximal outward current was similar (27.1 ± 1.5 pA/pF vs 25.7 ± 2.0 pA/pF, P > 0.05); (b) The NO donor sodium nitroprusside (SNP) increased outward currents only in CM (25.9 ± 1.9 to 46.7 ± 4.2 pA/pF, P < 0.001) but not SM (23.2 ± 3.1 to 27.0 ± 3.4 pA/pF, P > 0.05); (c) SNP added in the presence of the BK_Ca antagonist iberiotoxin (IbTX) produced no increase in the outward current in CM (17.0 ± 2.8 vs 13.7 ± 2.2, P > 0.05); and (d) L-NNA caused a small insignificant inhibition of outward K⁺ currents in both muscles; and (2) Muscle strip studies: (a) Blockade of the nerves with tetrodotoxin (TTX), or BK_Ca with IbTX had no significant effect on resting tone of either muscle; and (b) SNP reduced tone in both muscles, and was unaffected by the presence of TTX or IbTX.

CONCLUSION: Exogenous NO activates BK_Ca only in CM of the cat. However, as opposed to other species, exogenous NO-induced relaxation is predominantly by a non-BK_Ca mechanism, and endogenous NO has minimal effect on resting tone.

The influences of g proteins, ca, and k channels on electrical field stimulation in cat esophageal smooth muscle

NO released by myenteric neurons controls the off contraction induced by electrical field stimulation (EFS) in distal esophageal smooth muscle, but in the presence of nitric oxide synthase (NOS) inhibitor, L-NAME, contraction by EFS occurs at the same time. The authors investigated the intracellular signaling pathways related with G protein and ionic channel EFS-induced contraction using cat esophageal muscles. EFS-induced contractions were significantly suppressed by tetrodotoxin (1 microM) and atropine (1 microM). Furthermore, nimodipine inhibited both on and off contractions by EFS in a concentration dependent meaner. The characteristics of 'on' and 'off' contraction and the effects of G-proteins, phospholipase, and K(+) channel on EFS-induced contraction in smooth muscle were also investigated. Pertussis toxin (PTX, a G(i) inactivator) attenuated both EFS-induced contractions. Cholera toxin (CTX, G(s) inactivator) also decreased the amplitudes of EFS-induced off and on contractions. However, phospholipase inhibitors did not affect these contractions. Pinacidil (a K(+) channel opener) decreased these contractions, and tetraethylammonium (TEA, K(+) (Ca) channel blocker) increased them. These results suggest that EFS-induced on and off contractions can be mediated by the activations Gi or Gs proteins, and that L-type Ca(2+) channel may be activated by G-protein alpha subunits. Furthermore, K(+) (Ca)-channel involve in the depolarization of esophageal smooth muscle. Further studies are required to characterize the physiological regulation of Ca(2+) channel and to investigate the effects of other K(+) channels on EFS-induced on and off contractions.

Regional differences in cholinergic regulation of potassium current in feline esophageal circular smooth muscle

Potassium channels are important contributors to membrane excitability in smooth muscles. There are regional differences in resting membrane potential and K(+)-channel density along the length of the feline circular smooth muscle esophagus. The aim of this study was to assess responses of K(+)-channel currents to cholinergic (ACh) stimulation along the length of the feline circular smooth muscle esophageal body. Perforated patch-clamp technique assessed K(+)-channel responses to ACh stimulation in isolated smooth muscle cells from the circular muscle layer of the esophageal body at 2 (distal)- and 4-cm (proximal) sites above the lower esophageal sphincter. Western immunoblots assessed ion channel and receptor expression. ACh stimulation produced a transient increase in outward current followed by inhibition of spontaneous transient outward currents. These ACh-induced currents were abolished by blockers of large-conductance Ca(2+)-dependent K(+) channels (BK(Ca)). Distal cells demonstrated a greater peak current density in outward current than cells from the proximal region and a longer-lasting outward current increase. These responses were abolished by atropine and the specific M(3) receptor antagonist 4-DAMP but not the M(1) receptor antagonist pirenzipine or the M(2) receptor antagonist methoctramine. BK(Ca) expression along the smooth muscle esophagus was similar, but M(3) receptor expression was greater in the distal region. Therefore, ACh can differentially activate a potassium channel (BK(Ca)) current along the smooth muscle esophagus. This activation probably occurs through release of intracellular calcium via an M(3) pathway and has the potential to modulate the timing and amplitude of peristaltic contraction along the esophagus.

Role of Ca2+-activated Cl- channels and MLCK in slow IJP in opossum esophageal smooth muscle

The possible contribution of Ca2+-activated Cl- channel [I(Cl(Ca))] and myosin light-chain kinase (MLCK) to nonadrenergic, noncholinergic slow inhibitory junction potentials (sIJP) was studied using conventional intracellular microelectrode recordings in circular smooth muscle of opossum esophageal body and guinea pig ileum perfused with Krebs solution containing atropine (3 microM), guanethidine (3 microM), and substance P (1 microM). In opossum esophageal circular smooth muscle, resting membrane potential (MP) was -51.9 +/- 0.7 mV (n = 89) with MP fluctuations of 1-3 mV. A single square-wave nerve stimulation of 0.5 ms duration and 80 V induced a sIJP with amplitude of 6.3 +/- 0.2 mV, half-amplitude duration of 635 +/- 19 ms, and rebound depolarization amplitude of 2.4 +/- 0.1 mV (n = 89). 9-Anthroic acid (A-9-C), niflumic acid (NFA), wortmannin, and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) abolished MP fluctuations, sIJP, and rebound depolarization in a concentration-dependent manner. A-9-C and NFA but not wortmannin and ML-9 hyperpolarized MP. In guinea pig ileal circular smooth muscle, nerve stimulation elicited an IJP composed of both fast (fIJP) and slow (sIJP) components, followed by rebound depolarization. NFA (200 microM) abolished sIJP and rebound depolarization but left the fIJP intact. These data suggest that in the tissues studied, activation of I(Cl(Ca)), which requires MLCK, contributes to resting MP, and that closing of I(Cl(Ca)) is responsible for sIJP.

Ion channel diversity in the feline smooth muscle esophagus

We have characterized ion-channel identity and density differences along the feline smooth muscle esophagus using patch-clamp recording. Current clamp recording revealed that the resting membrane potential (RMP) of esophageal smooth muscle cells (SMC) from the circular layer at 4 cm above the lower esophageal sphincter (EBC4; LES) were more depolarized than at 2 cm above LES. Higher distal Na(+) permeability (but not Cl(-) permeability) contributes to this RMP difference. K(+) channels but not large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels contribute to RMP at both levels, because nonspecific K(+)-channel blockers depolarize all SMC. Depolarization of SMC under voltage clamp revealed that the density of voltage-dependent K(+) channels (K(V)) was greatest at EBC4 due to increased BK(Ca.) Delayed rectifier K(+) channels (K(DR)), compatible with subtype K(V)1.2, were present at both levels. Differences in K(Ca)-to-K(DR) channel ratios were also manifest by predictable shifts in voltage-dependent inactivation at EBC4 when BK(Ca) channels were blocked. We provide the first evidence for regional electrophysiological differences along the esophageal body resulting from SMC ion channel diversity, which could allow for differential muscular responses to innervation and varied muscular contribution to peristaltic contractions along the esophagus.

Nitric oxide and thiol reagent modulation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeci

The modulation of large conductance Ca2+-activated K+ (BKCa) channels by the nitric oxide (NO) donors S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) and agents which oxidize or reduce reactive thiol groups were compared in excised inside-out membrane patches of the guinea-pig taenia caeci. When the cytosolic side of excised patches was bathed in a physiological salt solution (PSS) containing 130 mM K+ and 15 nM Ca2+, few BKCa channel openings were recorded at potentials negative to 0 mV. However, the current amplitude and open probability (NPo) of these BKCa channels increased with patch depolarization. A plot of ln(NPo) against the membrane potential (V) fitted with a straight line revealed a voltage at half-maximal activation (V0.5) of 9.4 mV and a slope (K) indicating an e-fold increase in NPo with 12.9 mV depolarization. As the cytosolic Ca2+ was raised to 150 nM, V0.5 shifted 11.5 mV in the negative direction, with little change in K (13.1 mV). NOCys (10 microM) and SNP (100 microM) transiently increased NPo 16- and 3. 7-fold, respectively, after a delay of 2-5 min. This increase in NPo was associated with an increase in the number of BKCa channel openings evoked at positive potentials by ramped depolarizations (between -60 and +60 mV). Moreover, this NOCys-induced increase in NPo was still evident in the presence of 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), the specific blocker of soluble guanylyl cyclase. The sulfhydryl reducing agents dithiothreitol (DTT; 10 and 100 microM) and reduced glutathione (GSH; 1 mM) also significantly increased NPo (at 0 mV) 7- to 9-fold, as well as increasing the number of BKCa channel openings evoked during ramped depolarizations. Sulfhydryl oxidizing agents thimerosal (10 microM) and 4,4'-dithiodipyridine (4,4DTDP; 10 microM) and the thiol-specific alkylating agent N-ethylmaleimide (NEM; 1 mM) significantly decreased NPo (at 0 mV) to 40-50% of control values after 5-10 min. Ramped depolarizations to +100 mV evoked relatively few BKCa channel openings. The effects of thimerosal on NPo were readily reversed by DTT, while the effects of NOCys were prevented by NEM. It was concluded that both redox modulation and nitrothiosylation of cysteine groups on the cytosolic surface of the alpha subunit of the BKCa channel protein can alter channel gating.

Neuromuscular control of esophageal peristalsis

The esophagus is a muscular conduit connecting the pharynx and the stomach. Its function is controlled by an intrinsic nervous system and by input from the central nervous system through the vagus nerve. Peristalsis in its striated muscle is directed by sequential vagal excitation arising in the brain stem, whereas peristalsis in its smooth muscle involves complex interactions among the central and peripheral neural systems and the smooth muscle elements of the esophagus. The peripheral neuronal elements responsible for producing esophageal off-response, relaxation of the lower esophageal sphincter, and hyperpolarization of the circular esophageal muscle cells reside in the myenteric plexus of the esophagus. For many years these nerves were considered nonadrenergic and noncholinergic because the inhibitory neurotransmitter released on their activation was unknown. We now know that nitric oxide or a related compound is that inhibitory neurotransmitter. The primary excitatory neurotransmitter controlling esophageal motor function is acetylcholine. Some disorders of esophageal motor function, including diffuse esophageal spasm and achalasia, may result from defects in or an imbalance between these excitatory and inhibitory neuromuscular systems.

Tonic inhibitory action by nitric oxide on spontaneous mechanical activity in rat proximal colon: involvement of cyclic GMP and apamin-sensitive K+ channels

1. The cellular mechanisms by which endogenous nitric oxide (NO) modulates spontaneous motility were investigated in rat isolated proximal colon. The mechanical activity was detected as changes in intraluminal pressure. 2. Apamin (1-100 nM) produced a concentration-dependent increase in the amplitude of the spontaneous pressure waves. The maximal contractile effect was of the same degree as that produced by Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM) and the joint application of apamin plus L-NAME had no additive effects. Apamin (0.1 microM) reduced the inhibitory effects (i.e. reduction in the amplitude of the pressure waves) induced by sodium nitroprusside (SNP) (1 nM - 10 microM) or 8-Br-cyclic GMP (1-100 microM). 3. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (0.1-5 microM), inhibitor of NO-stimulated guanylate cyclase, produced a concentration-dependent increase of the spontaneous contractions. ODQ (1 microM) in the presence of apamin (0.1 microM) did not produce any further increase in the contraction amplitude, whereas after L-NAME (100 microM) it decreased the spontaneous contractions. ODQ (1 microM) reduced the SNP inhibitory effects. 4. Zaprinast (1-50 microM), inhibitor of cyclic GMP phosphodiesterase, produced a concentration-dependent decrease of the spontaneous contractions. The effects of zaprinast were significantly reduced in the presence of apamin (0.1 microM) or L-NAME (100 microM). 5. These results suggest that small conductance Ca2+-dependent K+ channels and cyclic GMP are involved in the modulation of the spontaneous contractile activity in rat proximal colon. Cyclic GMP production system and opening of apamin-sensitive K+ channels appear to work sequentially in transducing an endogenous NO signal.

Activation by methylene blue of large Ca(2+)-activated K+ channels

Using the patch-clamp method, we found that methylene blue (MB), a free radical inhibitor of guanylyl cyclase, activated large, Ca(2+)-activated K+ channels (BKCa) in either cell attached or excised, inside-out patches of human mesangial cells in culture. Since BKCa are important feedback regulators of contraction of smooth muscle and mesangial cells, these results indicate that MB may be an important opener of BKCa channels and a regulator of vascular volume and resistance.

Role of sarcoplasmic reticulum in inhibitory junction potentials and hyperpolarizations by nitric oxide donors in opossum oesophagus

1. Previous patch clamp studies of oesophageal circular muscle cells showed that nitric oxide (NO) modulated the opening of Ca2(+)-activated K+ channels involved in mediating the inhibitory junction potentials (i.j.ps). This study clarified the role of Ca2+ release from the superficial sarcoplasmic reticulum (SR) in the mechanism of i.j.ps or hyperpolarizing responses to NO-releasing compounds. Electrical and mechanical activities were simultaneously recorded by intracellular microelectrode or double sucrose gap techniques. 2. The NO-donors, sydnonimine (SIN-1) and sodium nitroprusside, each at 500 microM, hyperpolarized oesophageal circular muscle cells by 15-20 mV, like i.j.ps. 3. The selective inhibitors of SR Ca2(+)-ATPase (cyclopiazonic acid 10-30 microM and thapsigargin 5 microM) and the SR Ca2+ release channel activator (ryanodine 30 microM) caused depolarization and spontaneous contractions which were diminished after prolonged (> 30 min) incubation with these agents in Ca2(+)-containing medium. Moreover, these agents inhibited both the i.j.p. and NO-donor hyperpolarizations, suggesting that a functional SR Ca2+ uptake is necessary for the response to endogenous or exogenous NO. 4. These results, along with our previous findings of the dependence of i.j.ps and NO-donor hyperpolarizations on K+ channel activation and cyclic GMP elevation, support the hypothesis that subplasmalemmal (Ca2+)i elevation, via vectorial Ca2+ release from superficial SR toward the plasmalemma, may be an important mechanism by which NO, from NO-liberating compounds or released from inhibitory neurones induces relaxation and i.j.ps in opossum oesophagus.