Multiple P2Y receptors mediate contraction in guinea pig mesenteric vein

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.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

N-type and P/Q-type calcium channels regulate differentially the release of noradrenaline, ATP and beta-NAD in blood vessels

Using HPLC techniques we evaluated the electrical field stimulation-evoked overflow of noradrenaline (NA), adenosine 5'-triphosphate (ATP), and beta-nicotinamide adenine dinucleotide (beta-NAD) in the presence of low nanomolar concentrations of omega-conotoxin GVIA or omega-agatoxin IVA in the canine mesenteric arteries and veins. omega-conotoxin GVIA abolished the evoked overflow of NA and beta-NAD in artery and vein, whereas the evoked overflow of ATP remained unchanged in the presence of omega-conotoxin GVIA. omega-agatoxin IVA significantly reduced the evoked overflow of ATP and beta-NAD. The overflow of NA remained largely unaffected by omega-agatoxin IVA, except at 16Hz in the vein where the overflow of NA was reduced by about 50%. Artery and vein exhibited similar expression levels of the alpha(1B) (CaV2.2, N-type) subunit, whereas the vein showed greater levels of the alpha(1A) (CaV2.1, P/Q-type) subunit than artery. Therefore, there are at least two release sites for NA, beta-NAD and ATP in the canine mesenteric artery and vein: an N-type-associated site releasing primarily NA, beta-NAD and some ATP, and a P/Q-type-associated site releasing ATP, beta-NAD and some NA. The N-type-mediated mechanisms are equally expressed in artery and vein, whereas the P/Q-type-mediated mechanisms are more pronounced in the vein and may ensure additional neurotransmitter release at higher levels of neural activity. In artery, beta-NAD caused a dual effect consisting of vasodilatation or vasoconstriction depending on concentrations, whereas vein responded with vasodilatation only. In contrast, ATP caused vasoconstriction in both vessels. beta-NAD and ATP may mediate disparate functions in the canine mesenteric resistive and capacitative circulations.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

Evidence for the expression of multiple uracil nucleotide-stimulated P2 receptors coupled to smooth muscle contraction in porcine isolated arteries

BACKGROUND AND PURPOSE

The uracil nucleotides UDP and UTP have been reported to activate P2Y2, P2Y4 and P2Y6 receptors to cause vasoconstriction. We have performed a comparative analysis of these receptors in endothelium-denuded smooth muscle from porcine isolated coronary and ear arteries, using pharmacological and molecular tools.

EXPERIMENTAL APPROACH

Tissue segments were used to construct non-cumulative concentration response curves for UTP and UDP, in the absence and presence of the P2 receptor antagonists PPADS or suramin. RT-PCR and immunoblot analyses were employed to define gene expression and immunoreactivity for P2Y2, P2Y4 and P2Y6 receptors.

KEY RESULTS

In the coronary artery, UTP-evoked contractile responses were reduced in the presence of suramin, but not PPADS, while the smaller responses to UDP were unaffected by either antagonist. In the ear artery, contractile responses to UDP were much smaller than those to UTP; responses to UTP were inhibited by both PPADS and suramin. RT-PCR suggested predominant expression of P2Y2 receptors in the coronary artery, while P2Y4 and P2Y6 receptor gene expression appeared equivalent in both tissues. Immunoblot analyses provided evidence for P2Y6 receptors in both tissues, with equivocal evidence of P2Y2 and P2Y4 receptor immunoreactivities.

CONCLUSIONS AND IMPLICATIONS

We conclude that UTP-evoked contraction of porcine coronary artery smooth muscle appears to be predominantly P2Y2-mediated, while the ear artery appears to express a uracil nucleotide-sensitive P2 receptor(s) which fails to fit readily into the current classification.

Novel localization of CD38 in perivascular sympathetic nerve terminals

Using high performance liquid chromatography fraction analysis we have recently established that numerous smooth muscle preparations, including the canine mesenteric artery and vein, release beta-nicotinamide adenine dinucleotide upon short-pulse electrical field stimulation in tetrodotoxin- and omega-conotoxin GVIA-sensitive manners [ Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder. J Biol Chem 279:48893-48903.]. The beta-nicotinamide adenine dinucleotide metabolites ADP-ribose and cyclic ADP-ribose are also present in the tissue superfusates. CD38 is a multifunctional enzyme involved in the degradation of beta-nicotinamide adenine dinucleotide to ADP-ribose and cyclic ADP-ribose. Western immunoblot analysis revealed that CD38 is expressed in both artery and vein. Confocal laser scanning microscopy established colocalization of CD38 with tyrosine hydroxylase, synaptotagmin and synaptic vesicle protein in both blood vessels. High performance liquid chromatography with fluorescence detection demonstrated that whole tissue segments metabolize 1,N(6)-etheno-nicotinamide adenine dinucleotide to 1,N(6)-etheno-ADP-ribose and nicotinamide-guanine dinucleotide to cyclic GDP-ribose, suggesting the presence of both nicotinamide adenine dinucleotide-glycohydrolase and ADP-ribosyl cyclase activities in these blood vessels. Both enzymes appear to be associated with the membrane fraction, and therefore might be attributed to CD38. These data demonstrate a previously uncharacterized localization of CD38 in perivascular autonomic nerve terminals. Therefore, the beta-nicotinamide adenine dinucleotide/CD38 system may provide new mechanisms in autonomic neurovascular control.

Nicotinamide adenine dinucleotide is released from sympathetic nerve terminals via a botulinum neurotoxin A-mediated mechanism in canine mesenteric artery

Using high-performance liquid chromatography techniques with fluorescence and electrochemical detection, we found that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in response to electrical field stimulation (4-16 Hz, 0.3 ms, 15 V, 120 s) along with ATP and norepinephrine (NE) in the canine isolated mesenteric arteries. The release of beta-NAD increases with number of pulses/stimulation frequencies. Immunohistochemistry analysis showed dense distribution of tyrosine hydroxylase-like immunoreactivity (TH-LI) and sparse distribution of TH-LI-negative nerve processes, suggesting that these blood vessels are primarily under sympathetic nervous system control with some contribution of other (e.g., sensory) neurons. Exogenous NE (3 micromol/l), alpha,beta-methylene ATP (1 micromol/l), neuropeptide Y (NPY, 0.1 micromol/l), CGRP (0.1 micromol/l), vasoactive intestinal peptide (VIP, 0.1 micromol/l), and substance P (SP, 0.1 micromol/l) had no effect on the basal release of beta-NAD, suggesting that the overflow of beta-NAD is evoked by neither the sympathetic neurotransmitters NE, ATP, and NPY, nor the neuropeptides CGRP, VIP, and SP. Botulinum neurotoxin A (BoNTA, 0.1 micromol/l) abolished the evoked release of NE, ATP, and beta-NAD at 4 Hz, suggesting that at low levels of neural activity, release of these neurotransmitters results from N-ethylmaleimide-sensitive factor attachment protein receptor/synaptosomal-associated protein of 25 kDa-mediated exocytosis. At 16 Hz, however, the evoked release of NE, ATP, and beta-NAD was reduced by BoNTA by approximately 90, 60, and 80%, respectively, suggesting that at higher levels of neural activity, beta-NAD is likely to be released from different populations of synaptic vesicles or different populations of nerve terminals (i.e., sympathetic and sensory terminals).

An electrophysiological study of excitatory purinergic neuromuscular transmission in longitudinal smooth muscle of chicken anterior mesenteric artery

1. The object of the present study was to clarify the neurotransmitters controlling membrane responses to electrical field stimulation (EFS) in the longitudinal smooth muscle cells of the chicken anterior mesenteric artery. 2. EFS (5 pulses at 20 Hz) evoked a depolarization of amplitude 19.7+/-2.1 mV, total duration 29.6+/-3.1 s and latency 413.0+/-67.8 ms. This depolarization was tetrodotoxin (TTX)-sensitive and its amplitude was partially decreased by atropine (0.5 microM); however, its duration was shortened by further addition of prazosin (10 microM). 3. Atropine/prazosin-resistant component was blocked by the nonspecific purinergic antagonist, suramin, in a dose-dependent manner, indicating that this component is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). 4. Neither desensitization nor blocking of P2X receptor with its putative receptor agonist alpha,beta-methylene ATP (alpha,beta-MeATP, 1 microM) and its antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic (PPADS, up to 50 microM), had significant effect on the purinergic depolarization. In contrast, either desensitization or blocking of P2Y receptor with its putative agonist 2-methylthioATP (2-MeSATP, 1 microM) and its antagonist Cibacron blue F3GA (CBF3GA, 10 microM) abolished the purinergic depolarization, indicating that this response is mediated through P2Y but not P2X receptor. 5. The purinergic depolarization was inhibited by pertussis toxin (PTX, 600 ng ml(-1)). Furthermore, it was significantly inhibited by a phospholipase C (PLC) inhibitor, U-73122 (10 microM), indicating that the receptors involved in mediating the purinergic depolarization are linked to a PTX-sensitive G-protein, which is involved in a PLC-mediated signaling pathway. 6. Data of the present study suggest that the EFS-induced excitatory membrane response occurring in the longitudinal smooth muscle of the chicken anterior mesenteric artery is mainly purinergic in nature and is mediated via P2Y purinoceptors.

PI3K and PKC contribute to membrane depolarization mediated by alpha2-adrenoceptors in the canine isolated mesenteric vein

BACKGROUND

Norepinephrine (NE), a classic neurotransmitter in the sympathetic nervous system, induces vasoconstriction of canine isolated mesenteric vein that is accompanied by a sustained membrane depolarization. The mechanisms underlying the NE-elicited membrane depolarization remain undefined. In the present study we hypothesized that phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) are involved in the electrical field stimulation (EFS)-induced slow membrane depolarization (SMD) in canine isolated mesenteric vein. EFS (0.1-2 Hz, 0.1 ms, 15V, 10 s)-induced changes in the membrane potential were recorded with a conventional intracellular microelectrode technique and evaluated in the absence and presence of inhibitors of neuronal activity, alpha-adrenoceptors, membrane ion channels, PI3K, inositol 1,4,5-triphosphate (InsP3) receptors, and PKC. Activation of PI3Kgamma and PKCzeta in response to exogenous NE and clonidine in the absence and presence of receptor and kinase inhibitors were also determined.

RESULTS

Contractile responses to NE and clonidine (0.05 - 10 microM) were significantly diminished in the presence of yohimbine (0.1 microM). Exogenous NE (0.1 microM) and clonidine (1 microM) elicited SMD. The resting membrane potential of canine mesenteric vein smooth muscle cells was -68.8 +/- 0.8 mV. EFS elicited a biphasic depolarization comprised of excitatory junction potentials and SMD that are purinergic and adrenergic in nature, respectively. The magnitude of the SMD in response to EFS at 0.5 Hz was 9.4 +/- 0.7 mV. This response was reduced by 65-98% by the fast Na+ channel inhibitor tetrodotoxin (1 microM), by the inhibitor of N-type Ca2+ channels omega-conotoxin GVIA (5 nM), the non-selective alpha-adrenoceptor blocker phentolamine (1 microM), the selective alpha2-adrenoceptor blocker yohimbine (0.1 microM), the ion channel inhibitors niflumic acid (NFA, 100 microM), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 30 microM), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 200 microM), and Gd3+ (30 microM), and the PI3K inhibitors wortmannin (100 nM) and LY-294002 (10 microM). The SMD remained unchanged in the presence of the L-type Ca2+ channel blocker nicardipine (1 microM) and the InsP3 receptor blockers 2-aminoethoxydiphenylborate (2APB, 50 microM) and xestospongin C (3 microM). The inhibitor of PKC chelerythrine (1 microM), but not calphostin C (10 microM), diminished the SMD. Exogenous NE and clonidine (1 microM each) activated both PI3Kgamma and PKCzeta, and the activation of these kinases was abolished by preincubation of tissue with the alpha2-adrenoceptor blocker yohimbine.

CONCLUSION

Neuronally-released NE stimulates smooth muscle alpha2-adrenoceptors and activates PI3K and atypical PKC in the canine mesenteric vein. Events downstream of PKC lead to SMD and vasoconstriction. This represents a novel pathway for NE-induced membrane depolarization in a vascular smooth muscle preparation.

Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder

Chemical signaling in autonomic neuromuscular transmission involves agents that function as neurotransmitters and/or neuromodulators. Using high performance liquid chromatography techniques with fluorescence and electrochemical detection we observed that, in addition to ATP and norepinephrine (NE), electrical field stimulation (EFS, 4-16 Hz, 0.1-0.3 ms, 15 V, 60-120 s) of isolated vascular and non-vascular preparations co-releases a previously unidentified compound with apparent nucleotide or nucleoside structure. Extensive screening of more than 25 nucleotides and nucleosides followed by detailed peak identification revealed that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in tissue superfusates upon EFS of canine mesenteric artery (CMA), canine urinary bladder, and murine urinary bladder in the amounts of 7.1 +/- 0.7, 26.5 +/- 4.5, and 15.1 +/- 3.2 fmol/mg of tissue, respectively. Smaller amounts of the beta-NAD metabolites cyclic adenosine 5'-diphosphoribose (cADPR) and ADPR were also present in the superfusates collected during EFS of CMA (2.5 +/- 0.9 and 5.8 +/- 0.8 fmol/mg of tissue, respectively), canine urinary bladder (1.8 +/- 0.5 and 9.0 +/- 6.0 fmol/mg of tissue, respectively), and murine urinary bladder (1.4 +/- 0.1 and 6.2 +/- 2.4 fmol/mg of tissue, respectively). The three nucleotides were also detected in the samples collected before EFS (0.2-1.6 fmol/mg of tissue). Exogenous beta-NAD, cADPR, and ADPR (all 100 nm) reduced the release of NE in CMA at 16 Hz from 27.8 +/- 6.0 fmol/mg of tissue to 15.5 +/- 5.0, 12 +/- 3.0, and 10.0 +/- 4.0 fmol/mg of tissue, respectively. In conclusion, we detected constitutive and nerve-evoked overflow of beta-NAD, cADPR, and ADPR in vascular and non-vascular smooth muscles, beta-NAD being the prevailing compound. These substances modulate the release of NE, implicating novel nucleotide mechanisms of autonomic nervous system control of smooth muscle.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Membrane-bound and releasable nucleotidase activities: differences in canine mesenteric artery and vein

1. At least two enzymatic activities are proposed to degrade the extracellular ATP: (i) ubiquitously expressed membrane-bound enzymes (ecto-nucleotidases); and (ii) soluble (releasable) nucleotidases that are released during stimulation of sympathetic nerves and break down neuronal ATP. No quantitative data have placed the magnitude of these nucleotidase activities into a physiological perspective of neurovascular control. 2. We studied comparatively the membrane-bound and releasable nucleotidase activities in canine isolated inferior mesenteric arteries and veins using 1,N6-etheno(epsilon)-nucleotides (i.e. epsilon-ATP, epsilon-ADP, epsilon-AMP and epsilon-adenosine) as exogenous substrates. The enzymatic activities were estimated by measuring the disappearance of the epsilon-substrate and appearance of epsilon-products by means of HPLC-fluorescence detection during either stimulation of sympathetic perivascular nerves (releasable activity) or in the absence of nerve stimulation (ecto-nucleotidase activity). 3. Incubation of vascular segments with 50 nmol/L epsilon-ATP for 60 min resulted in a decrease of the epsilon-ATP substrate by 63.5 +/- 4.6 and 91.2 +/- 6.2% in the artery and vein, respectively. In contrast, the decrease of the epsilon-ATP during electrical field stimulation (EFS; 16 Hz, 0.3 msec, 2 min) was 39.8 +/- 4.2% in the artery and 13.1 +/- 7.3% in the vein. Therefore, the mesenteric arteries demonstrate a greater releasable ATPase activity and a weaker ecto-ATPase activity than mesenteric veins. 4. The degradation of epsilon-ADP and epsilon-AMP was similar in both blood vessels under either experimental protocol. The epsilon-adenosine was not significantly degraded in the absence or presence of EFS. 5. These data implicate a differential removal of extracellular ATP as a potential mechanism of serving resistance and capacitance in the splanchnic circulation.

Canine mesenteric artery and vein convey no difference in the content of major contractile proteins

BACKGROUND

Mesenteric arteries and veins are composed of tonic smooth muscles and serve distinct functions in the peripheral circulation. However, the basis for the functional disparity of the resistive and capacitative parts of the mesenteric circulation is poorly understood. We studied potential differences in the expression levels of six contractile proteins in secondary and tertiary branches of the inferior mesenteric artery and vein along with differences in the vessel wall morphology.

RESULTS

Bright field and electron microscopy showed that both vessel walls had the same major structural elements. The arterial walls, however, had greater number, and more tightly assembled, smooth muscle cell layers compared to vein walls. The content of actin, myosin heavy chain, myosin light chain, and calponin was similar in the two blood vessels. The artery expressed higher amount of the actin-binding protein caldesmon than the vein (41.86 +/- 2.33 and 30.13 +/- 3.37 microg/mg respectively, n = 12). Although the total tropomyosin content was almost identical in both blood vessels, the alpha isoform dominated in the artery, while the beta isoform prevailed in the vein.

CONCLUSIONS

Canine mesenteric artery and vein differ in vessel wall morphology but do not convey differences in the expression levels of actin, myosin light chain, myosin heavy chain and calponin. The two vascular networks express distinct amounts of caldesmon and tropomyosin, which might contribute to the fine tuning of the contractile machinery in a manner consistent with the physiological functions of the two vascular networks.

High-performance liquid chromatographic technique for detection of a fluorescent analogue of ADP-ribose in isolated blood vessel preparations

Analysis of endogenous nucleotides in biologic media is hampered by rapid degradation and low final concentrations that are difficult to detect. A reversed-phase high-performance liquid chromatographic (HPLC) technique is described that efficiently detects a stable fluorescence derivative of adenosine 5'-diphosphoribose (ADPR), 1,N6-etheno-ADPR (epsilon-ADPR), at low femtomolar concentration range in vascular tissue superfusates. epsilon-ADPR was formed by the reaction of ADPR with chloroacetaldehyde at 80 degrees C and pH 4.0. Gradient elution with 0.1 M KH2PO4 (pH 6.0), increasing methanol (0-35% over 18 min), and a 25-cm by 4.5-mm (5 microm) silica ODS-AM column were employed. epsilon-ADPR was detected by fluorescence at an excitation wavelength of 230 nm and an emission wavelength of 410 nm. The detection sensitivity for epsilon-ADPR was approximately 10 fmol. Linearity of the HPLC detection method was demonstrated in the range from 0.0125 to 1 pmol epsilon-ADPR. The method was validated in terms of within-day and between-day reproducibility of retention times and peak areas of standard nucleotide. Matrix-assisted laser desorption/ionization mass spectrometry measurements confirmed the presence of an etheno ring after reaction of ADPR with chloroacetaldehyde. The method was applied to quantitate the overflow of ADPR upon electrical field stimulation (8 Hz, 0.3 ms, 15 V, 1-2 min) of both canine and guinea-pig isolated mesenteric artery segments.

Multiple regulation by external ATP of nifedipine-insensitive, high voltage-activated Ca(2+) current in guinea-pig mesenteric terminal arteriole

We investigated the receptor-mediated regulation of nifedipine-insensitive, high voltage-activated Ca(2+) currents in guinea-pig terminal mesenteric arterioles (I(mVDCC)) using the whole-cell clamp technique. Screening of various vasoactive substances revealed that ATP, histamine and substance P exert modulatory effects on I(mVDCC). The effects of ATP on I(mVDCC) after complete P2X receptor desensitization exhibited a complex concentration dependence. With 5 mM Ba(2+), ATP potentiated I(mVDCC) at low concentrations (approximately 1-100 microM), but inhibited it at higher concentrations (>100 microM). The potentiating effects of ATP were abolished by suramin (100 microM) and PPADS (10 microM) and by intracellular application of GDPbetaS (500 microM), whereas a substantial part of I(mVDCC) inhibition by milimolar concentrations of ATP remained unaffected; due probably to its divalent cation chelating actions. In divalent cation-free solution, I(mVDCC) was enlarged and underwent biphasic effects by ATPgammaS and ADP, while 2-methylthio ATP (2MeSATP) exerted only inhibition, and pyrimidines such as UTP and UDP were ineffective. ATP-induced I(mVDCC) potentiation was selectively inhibited by anti-Galpha(s) antibodies or protein kinase A (PKA) inhibitory peptides and mimicked by dibutyryl cAMP. In contrast, ATP-induced inhibition was selectively inhibited by Galpha(q/11) antibodies or protein kinase C (PKC) inhibitory peptides and mimicked by PDBu. Pretreatment with pertussis toxin was ineffective. The apparent efficacy for I(mVDCC) potentiation with PKC inhibitors was: ATPgammaS > ATP>/=ADP and for inhibition with PKA inhibitors was: 2MeSATP > ATPgammaS > ATP > ADP. Neither I(mVDCC) potentiation nor inhibition showed voltage dependence. These results suggest that I(mVDCC) is multi-phasically regulated by external ATP via P2Y(11)-resembling receptor/G(s)/PKA pathway, P2Y(1)-like receptor/G(q/11)/PKC pathway, and metal chelation.

Co-release of endogenous ATP and noradrenaline from guinea-pig mesenteric veins exceeds co-release from mesenteric arteries

1. The present study was designed to compare the overflow of sympathetic neurotransmitters of guinea-pig inferior mesenteric artery and mesenteric vein evoked by electrical field stimulation (EFS) with special emphasis on the simultaneous release of ATP and noradrenaline (NA). The stimulation-evoked overflow of ADP, AMP and adenosine was also evaluated. 2. Endothelium-denuded segments of inferior mesenteric arteries or veins were superfused in a small volume (200 microL)-chamber for EFS and subsequent detection of NA (HPLC- electrochemical detection) and adenine nucleotides and adenosine (HPLC-fluorescence detection) in samples of the superfusate. 3. Both arteries and veins responded to EFS (15 V, 4-16 Hz, 0.3 msec for 60 s) with overflow of ATP and NA in a tetrodotoxin (1 micromol/L)- and guanethidine (10 micromol/L)-sensitive manner. The EFS-evoked overflow of NA in veins exceeded the overflow of NA in arteries at all frequencies of stimulation, whereas the EFS-evoked overflow of ATP, ADP and AMP in veins exceeded the overflow of adenine nucleotides in arteries at 4 and 8 Hz but not at 16 Hz stimulation. The EFS-evoked overflow of adenosine was similar in arteries and veins. 4. Activation of alpha1-adrenoceptors with methoxamine (10 micromol/L) did not produce overflow of ATP. 5. Blockade of alpha1/alpha2-adrenoceptors with phentolamine (1 micromol/L) did not affect EFS-evoked overflow of ATP, ADP, AMP and adenosine. 6. It is concluded that overflow of ATP and NA from sympathetic nerves may constitute an effective mechanism in the complex balance between capacitance and resistance in splanchnic circulation.

Presynaptic alpha2-adrenoceptor-mediated modulation of adenosine 5' triphosphate and noradrenaline corelease: differences in canine mesenteric artery and vein

1. The modulatory effects of agonists and antagonists of prejunctional alpha2-adrenoceptors on the electrical field stimulation (EFS, 0.3 ms, 12 V)-induced release of endogenous noradrenaline (NA) and the cotransmitter adenosine 5' triphosphate (ATP) were measured in endothelium-denuded segments of canine inferior mesenteric artery and compared with effects in mesenteric vein. The overflow of NA and ATP was evoked by long-duration (2 min) EFS at low frequency (4 Hz) and high frequency (16 Hz) of stimulation and was analysed using HPLC techniques with electrochemical detection and fluorescence detection, respectively. 2. The EFS-evoked overflow of both NA and ATP was significantly reduced by tetrodotoxin (1 microM) and guanethidine (10 microM) in the artery and vein. Desipramine (10 microM), a blocker of neuronal uptake of NA, increased the EFS (4 and 16 Hz)-evoked overflow of NA in both artery and vein. EFS-evoked overflow of NA in vein exceeded the NA overflow in artery at both 4 and 16 Hz in control preparations as well as in the presence of desipramine. However, the EFS-evoked overflow of ATP was equal in the artery and vein. 3. Stimulation of alpha2-adrenoceptors with clonidine (0.1 microM) and oxymethazoline (0.3 microM) reduced the EFS evoked overflow of NA in both artery and vein at 4 Hz, whereas the NA overflow at 16 Hz remained unchanged in both blood vessels. The overflow of ATP as well as of ADP (and hence ATP:ADP ratio) was unaffected by the alpha2-adrenoceptor agonists in the artery and vein. 4. In artery, blockade of alpha2-adrenoceptors with yohimbine at a concentration of 0.1 microM caused no effect on the NA overflow neither at 4 Hz nor at 16 Hz of EFS. Yohimbine at a concentration of 1 microM increased the overflow of NA at 4 Hz but not 16 Hz of EFS. In vein, however, yohimbine (0.1 and 1 microM) increased NA overflow at both 4 and 16 Hz of stimulation. Idazoxan (1 microM) increased the NA overflow in artery only at 4 Hz, whereas in vein idazoxan increased the NA overflow at both 4 and 16 Hz. No changes of EFS-evoked ATP overflow were observed in the presence of 0.1 microM yohimbine in both artery and vein. Greater concentration of yohimbine (i.e. 1 microM) increased the overflow of ATP in both the artery and vein only at 4 Hz EFS. Idazoxan (1 microM) enhanced the ATP overflow only at 16 Hz in vein. The overflow of ADP was affected by both yohimbine and idazoxan in a similar manner to the ATP overflow so that the ATP:ADP ratios were not changed. 5. In conclusion, sympathetic nerves in both mesenteric arteries and veins appear to release ATP along with NA. Release of NA in veins exceeds release of NA in arteries, whereas both the canine artery and vein release equal amount of ATP. At long-duration nerve stimulation (as might occur during stress) the alpha2-adrenoceptors appear to rather modulate release of NA than release of the cotransmitter ATP. The prejunctional autoinhibition of NA release is more effective at lower frequencies of nerve stimulation. The alpha2-adrenoceptor-mediated neuromodulation plays a greater role in veins than arteries. Quantitative differences in alpha2-adrenoceptor-mediated neuromodulation in the arteries and veins may participate to differing contributions of mesenteric blood vessels to the control of blood flow and volume distribution in splanchnic circulation.

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.

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.