Modulation of neurotransmission in the guinea-pig vas deferens by capsaicin: involvement of calcitonin gene-related peptide and substance P

Purinergic Signaling in the Cardiovascular System

There is nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory-motor nerves, as well as in intracardiac neurons. Centers in the brain control heart activities and vagal cardiovascular reflexes involve purines. Adenine nucleotides and nucleosides act on purinoceptors on cardiomyocytes, AV and SA nodes, cardiac fibroblasts, and coronary blood vessels. Vascular tone is controlled by a dual mechanism. ATP, released from perivascular sympathetic nerves, causes vasoconstriction largely via P2X1 receptors. Endothelial cells release ATP in response to changes in blood flow (via shear stress) or hypoxia, to act on P2 receptors on endothelial cells to produce nitric oxide, endothelium-derived hyperpolarizing factor, or prostaglandins to cause vasodilation. ATP is also released from sensory-motor nerves during antidromic reflex activity, to produce relaxation of some blood vessels. Purinergic signaling is involved in the physiology of erythrocytes, platelets, and leukocytes. ATP is released from erythrocytes and platelets, and purinoceptors and ectonucleotidases are expressed by these cells. P1, P2Y₁, P2Y₁₂, and P2X1 receptors are expressed on platelets, which mediate platelet aggregation and shape change. Long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides promote migration and proliferation of vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis, vessel remodeling during restenosis after angioplasty and atherosclerosis. The involvement of purinergic signaling in cardiovascular pathophysiology and its therapeutic potential are discussed, including heart failure, infarction, arrhythmias, syncope, cardiomyopathy, angina, heart transplantation and coronary bypass grafts, coronary artery disease, diabetic cardiomyopathy, hypertension, ischemia, thrombosis, diabetes mellitus, and migraine.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Autonomic nerves and perivascular fat: interactive mechanisms

The evidence describing the autonomic innervation of body fat is reviewed with a particular focus on the role of the sympathetic neurotransmitters. In compiling the evidence, a strong case emerges for the interaction between autonomic nerves and perivascular adipose tissue (PVAT). Adipocytes have been shown to express receptors for neurotransmitters released from nearby sympathetic varicosities such as adrenoceptors (ARs), purinoceptors and receptors for neuropeptide Y (NPY). Noradrenaline can modulate both lipolysis (via α2- and β3-ARs) and lipogenesis (via α1- and β3-ARs). ATP can inhibit lipolysis (via P1 purinoceptors) or stimulate lipolysis (via P2y purinoceptors). NPY, which can be produced by adipocytes and sympathetic nerves, inhibits lipolysis. Thus the sympathetic triad of transmitters can influence adipocyte free fatty acid (FFA) content. Substance P (SP) released from sensory nerves has also been shown to promote lipolysis. Therefore, we propose a mechanism whereby sympathetic neurotransmission can simultaneously activate smooth muscle cells in the tunica media to cause vasoconstriction and alter FFA content and release from adjacent adipocytes in PVAT. The released FFA can influence endothelial function. Adipocytes also release a range of vasoactive substances, both relaxing and contractile factors, including adiponectin and reactive oxygen species. The action of adipokines (such as adiponectin) and reactive oxygen species (ROS) on cells of the vascular adventitia and nerves has yet to be fully elucidated. We hypothesise a strong link between PVAT and autonomic fibres and suggest that this poorly understood relationship is extremely important for normal vascular function and warrants a detailed study.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Cannabinoids inhibit noradrenergic and purinergic sympathetic cotransmission in the rat isolated mesenteric arterial bed

BACKGROUND AND PURPOSE

Noradrenaline and ATP are sympathetic co-transmitters. In the rat perfused mesenteric bed cannabinoids have been shown to modify the overall response to sympathetic nerve stimulation. This study has assessed whether cannabinoid receptor activation modulates differentially the noradrenergic and purinergic components of sympathetic vasoconstriction.

EXPERIMENTAL APPROACH

Rat mesenteric beds were perfused with physiological salt solution and the effects of cannabinoids on responses to nerve stimulation, or exogenous noradrenaline or alpha,beta-methylene ATP (alpha,beta-meATP; P2X receptor agonist) were determined after raising tone with U46619. The effects of cannabinoids on the noradrenaline and ATP components of sympathetic neurotransmission were assessed using the alpha 1-adrenoceptor antagonist, prazosin, or after P2X receptor desensitization with alpha,beta-meATP.

KEY RESULTS

Anandamide, WIN 55,212-2 and CP55,940 attenuated sympathetic neurogenic vasoconstrictor responses. The inhibitory actions of anandamide and WIN 55,212-2 were blocked by LY320135, a CB1 receptor antagonist, but not by SR144528, a CB2 receptor antagonist. The inhibitory actions of CP55,940 were unaffected by LY320135 and SR144528. WIN 55,212-3, the inactive S(-) enantiomer of WIN 55,212-2, had no effect on sympathetic neurogenic responses. None of the cannabinoids affected contractile responses to exogenous noradrenaline or alpha,beta-meATP. Anandamide and WIN 55,212-2 inhibited both the noradrenaline and ATP components of the sympathetic neurogenic contractile responses, with effects on the ATP component being most marked.

CONCLUSIONS AND IMPLICATIONS

These results indicate that prejunctional CB1-like receptors mediate the sympathoinhibitory action of anandamide and WIN 55,212-2, but not CP55,940, in the rat mesenteric bed. Cannabinoids inhibit both the noradrenergic and purinergic components of sympathetic neurotransmission.

Potentiation of sympathetic neuromuscular transmission mediated by muscarinic receptors in guinea pig isolated vas deferens

In guinea-pig isolated vasa deferentia, purinergic neurogenic contractions and responses to applied adenosine 5'-triphosphate (ATP) were potentiated by carbachol; responses to adrenergic transmission and applied noradrenaline were not. Following blockade of P2 receptors and alpha-adrenoceptors, the residual neurogenic response was massively potentiated by carbachol, suggesting the presence of a non-purinergic, non-adrenergic component. In the presence of guanethidine, carbachol had no significant effect, indicating that sympathetic transmission was the only element involved. Use of oxotremorine and selective muscarinic receptor antagonists suggested that the potentiating effect of carbachol and oxotremorine was mediated via M3 muscarinic receptors without involvement of nicotinic receptors.

Tachykinins and tachykinin receptors: effects in the genitourinary tract

Tachykinins (TKs) are a family of peptides involved in the central and peripheral regulation of urogenital functions through the stimulation of TK NK1, NK2 and NK3 receptors. At the urinary system level, TKs locally stimulate smooth muscle tone, ureteric peristalsis and bladder contractions, initiate neurogenic inflammation and trigger local and spinal reflexes aimed to maintain organ functions in emergency conditions. At the genital level, TKs are involved in smooth muscle contraction, in inflammation and in the modulation of steroid secretion by the testes and ovaries. TKs produce vasodilatation of maternal and fetal placental vascular beds and appear to be involved in reproductive function, stress-induced abortion, and pre-eclampsia. The current data suggest that the genitourinary tract is a primary site of action of the tachykininergic system.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Origin and neurochemical characteristics of nerve fibres supplying the mammalian vas deferens

The present paper deals with the origin and neurochemical characteristics of autonomic postganglionic and sensory nerve fibres supplying the mammalian vas deferens. The vas deferens is innervated by postganglionic nerve fibres originating primarily from neurons in pelvic ganglia and, to a lesser extent, from neurons in the inferior mesenteric ganglion and sympathetic chain ganglia as well as by sensory nerve fibres arising from dorsal root ganglia. Three major populations of nerve terminals innervating the organ can be distinguished: (1) noradrenergic fibres; (2) cholinergic fibres containing vasoactive intestinal polypeptide, neuropeptide Y, nitric oxide synthase, and (in the pig) somatostatin, supplying particularly the lamina propria; and (3) non-noradrenergic, presumably sensory fibres, containing calcitonin gene-related peptide and/or substance P. The population of noradrenergic nerves is the most common. In the pig, it can be divided into three subpopulations: a somatostatin-containing, a Leu-enkephalin-containing and a subpopulation immunonegative to these peptides, in descending order of magnitude. In the rat, guinea-pig, and man, NPY seems to be the most common peptide occurring in the noradrenergic axons. In the pig, coexistence patterns of the substances existing within nerve fibres supplying the vas deferens blood vessels are clearly different from those found in nerve fibres innervating the organ wall. The majority of the noradrenergic fibres associated with blood vessels contain neuropeptide Y only, while non-noradrenergic perivascular nerves contain predominantly vasoactive intestinal polypeptide. The possibility of different sources of origin of the particular nerve fibre subpopulations supplying the mammalian vas deferens and its blood vessels is discussed.

Role of N-, P- and Q-type voltage-gated calcium channels in transmitter release from sympathetic neurones in the mouse isolated vas deferens

1. N-type voltage-gated calcium channels are known to play an important role in transmitter release from autonomic neurones, and recent studies have demonstrated that non-N-type calcium channels are also involved. The calcium channels coupled to transmitter release from sympathetic neurones in the mouse isolated vas deferens were investigated in the present study. 2. Contractions of the mouse vas deferens were evoked by electrical stimulation at 1-50 Hz. The contractions were entirely nerve-mediated, since they were abolished by tetrodotoxin, and were used as an indirect measure of transmitter release. 3. The N-type calcium channel blocker, omega-conotoxin GVIA, inhibited contractions in a concentration-dependent manner, with a maximal effect at 30 nM. Contractions evoked by stimulation frequencies less than 10 Hz were abolished, and those evoked by 20 and by 50 Hz stimulation were decreased in amplitude by 51.3 +/- 13.9% and 9.3 +/- 2.6%, respectively. 4. The N-, P- and Q-type channel blocker, omega-conotoxin MVIIC, inhibited contractions in a concentration-dependent manner and caused greater maximum inhibition than omega-conotoxin GVIA, suggesting an action on P- and/or Q-type channels, in addition to N-type. 5. The P-type channel blocker, omega-agatoxin IVA, alone did not have a significant effect at concentrations up to 300 nM, but inhibited contractions in the presence of omega-conotoxin GVIA. Subsequent addition of omega-conotoxin MVIIC abolished the remaining contractions. Identical results were obtained when the three toxins were tested cumulatively on the purinergic and noradrenergic components of the contraction in the presence of (1.3 microM prazosin and following desensitization to 10 microM alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-NeATP), respectively. 6. The results suggest that N-, P- and Q-type channels are involved in the release of noradrenaline and ATP from sympathetic neurones in the mouse vas deferens.

[Cys(O2NH2)2]enkephalin analogues and dalargin: selectivity for delta-opioid receptors

To investigate the structure-activity relationships for potent and selective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)2,Leu5]enkephalin and [Cys(O2NH2)2, Met5] enkephalin and the hexapeptide [D-Ala2,Leu5]enkephalyl-Arg (dalargin) were tested and compared with [Leu5]enkephalin and [Met5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea pig ileum (mu- and kappa-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)2,Leu5] enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O2NH2)2,Met5]enkephalin > [Leu5]enkephalin > dalargin > [Met5]enkephalin. The highest IC50 values in the guinea pig ileum assays, indicating the lowest affinity for mu-/kappa-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met5]enkephalin. The order of potency in this tissue was: dalargin > [Met5]enkephalin > [Leu5]enkephalin > [Cys(O2NH2)2,Met5]enkephalin > [Cys(O2NH2)2,Leu5]enkephalin. The ratio, IC50 in guinea pig ileum: IC50 in mouse vas deferens, indicating selectivity of the respective peptide for delta-opioid receptors, was extremely high for [Cys(O2NH2)2,Leu5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O2NH2)2,Met5]enkephalin was higher than the ratios for dalargin, [Leu5]enkephalin and [Met5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at delta-opioid receptors, while both D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of [Leu5]enkephalin decrease them.

Multiple receptors for calcitonin gene-related peptide and amylin on guinea-pig ileum and vas deferens

1. The responses of the electrically stimulated guinea-pig ileum and vas deferens to human and rat calcitonin gene-related peptide (CGRP) and amylin were investigated. 2. The inhibition of contraction of the ileum produced by human alpha CGRP was antagonized by human alpha CGRP8-37 (apparent pA2 estimated at 7.15 +/- 0.23) > human alpha CGRP19-37 (apparent pA2 estimated as 6.67 +/- 0.33) > [Tyr0]-human alpha CGRP28-37. The amylin antagonist, AC187, was three fold less potent than CGRP8-37 in antagonizing human alpha CGRP. 3. Both human beta- and rat alpha CGRP inhibited contractions of the ileum, but this was less sensitive to inhibition by CGRP8-37 than the effect of human alpha CGRP. However, CGRP19-37 was twenty times more effective in inhibiting the response to rat alpha CGRP (apparent pA2 estimated as 8.0 +/- 0.1) compared to human alpha CGRP. 4. Rat amylin inhibited contractions in about 10% of ileal preparations; this effect was not antagonized by any CGRP fragment. Human amylin had no action on this preparation. 5. Both human and rat alpha CGRP inhibited electrically stimulated contractions of the vas deferens, which were not antagonized by 3 microM CGRP8-37 or 10 microM AC187. 6. Rat amylin inhibited the stimulated contractions of the vas deferens (EC50 = 77 +/- 9 nM); human amylin was less potent (EC50 = 213 +/- 22 nM). The response to rat amylin was antagonized by 10 microM CGRP8-37 (EC50 = 242 +/- 25 nM) and 10 microM AC187 (EC50 = 610 +/- 22 nM). 7. It is concluded that human alpha CGRP relaxes the guinea-pig ileum via CGRP1-like receptors, but that human beta CGRP and rat alpha CGRP may use additional receptors. These are distinct CGRP2-like and amylin receptors on guinea-pig vas deferens.

Opposite modulation by tachykinin (NK1) and CGRP receptors of sympathetic control of mouse vas deferens motility

Electrical field stimulation of isolated mouse vas deferens elicited sympathetic twitch whose amplitude was transiently enhanced by the selective tachykinin NK1 receptor agonist, [Sar9,Met(O2)11]substance P (0.3-30 nM), but not by selective NK2 and NK3 receptor agonists. Potentiation by [Sar9,Met(O2)11]substance P was antagonized by (+/-)-CP 96,345 [(2S,3S)-cis-2-(diphenylmethyl)-N- [(2-methoxyphenyl)-methyl]-1-azabicyclo[2.2.2]octan-3-amine] (IC50 = 0.1 microM). On the other hand, electrical field stimulation-induced contractions were inhibited by calcitonin gene-related peptide, CGRP (0.1-30 nM), and this action was reduced by its antagonist, human CGRP-(8-37) (3 microM). [Sar9,Met(O2)11]substance P (3 nM) did not affect either high-K+ or noradrenaline-induced contraction, while CGRP (3 nM) significantly reduced the noradrenaline-induced motor response. Capsaicin (1 microM) inhibited sympathetic twitches, and this effect was partially antagonized by human CGRP-(8-37). In the presence of this antagonist, capsaicin induced a short-living and (+/-)-CP 96,345-sensitive twitch enhancement. These data suggest that the sympathetic control of mouse vas deferens motility can be modulated in an opposite manner by tachykinin NK1 (prejunctionally located) and by CGRP (pre- and/or postjunctionally located) receptors.

The fade of the purinergic neurogenic contraction of the guinea-pig vas deferens: analysis of possible mechanisms

The purinergic response of the guinea-pig vas deferens to long trains of pulses at high frequency consists of an initial twitch followed by a much lower plateau. Mechanical, neurochemical and electrophysiological techniques were used to examine the reason for the fade. Mechanical measurements. In tissues stimulated by trains of 180 pulses/10 Hz and treated with prazosin to suppress the noradrenergic contraction component, the response to alpha, beta-methylene ATP and to exogenous ATP was as high during the secondary plateau of the purinergic neurogenic contraction as it was outside electrical stimulation periods; the response to 50 pulses/100 Hz was also unchanged during the low plateau. The plateau was not increased by reactive blue 2,8-(p-sulphophenyl)theophylline, propranolol or capsaicin. Neurochemical measurements. In tissues preincubated with [3H]-noradrenaline, electrical stimulation elicited an overflow of tritium and of ATP. In the absence of drugs as well as in the presence of prazosin and suramin to suppress contractions, the overflow of tritium per pulse decreased slightly in the course of trains of 90 pulses/10 Hz; the overflow of ATP per pulse decreased to a greater extent on average, but the decrease was not statistically significant. In the presence of prazosin and nifedipine, also to suppress contractions, the overflow of tritium per pulse again decreased slightly in the course of trains of 105 pulses/10 Hz, but the overflow of ATP per pulse if anything tended to increase. Electrophysiological measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of capsaicin on acetylcholine-evoked responses in rat phaeochromocytoma cells

1. The effects of capsaicin on cellular responses evoked by acetylcholine (ACh) and those by adenosine 5'-triphosphate (ATP) were investigated in rat phaeochromocytoma PC12 cells. 2. Capsaicin (1 to 30 microM) suppressed dopamine release and the intracellular Ca2+ increase evoked by 100 microM ACh. The concentration-dependence of the ACh-evoked release of dopamine was not shifted but the maximal response was reduced by capsaicin. Dopamine release evoked by 80 mM KCl was also suppressed by capsaicin (3 and 30 microM), but the extent of suppression was smaller than that of the ACh-evoked release. 3. Under whole-cell voltage-clamp, capsaicin (1 to 30 microM) reversibly inhibited the inward current activated by ACh (30 to 300 microM). The inhibition exhibited dependence on the concentration of ACh, and the current activated by a higher concentration of ACh was less inhibited. Voltage-dependence of block by capsaicin was not observed when it was tested either by applying a ramp pulse during the current activation by ACh or by eliciting the current in cells held at various potentials. 4. High concentrations of capsaicin (30 to 100 microM) enhanced the inward current as well as dopamine release evoked by 30 microM ATP. 5. The results suggest that low concentrations of capsaicin selectively antagonize responses mediated by nicotinic receptor-channels without affecting those mediated by purinoceptor-coupled channels. As the antagonism by capsaicin of the ACh-evoked responses was observed at concentrations as low as 1 microM, the influence on nicotinic receptors should be taken into account when this compound is used as a pharmacological tool to deplete neuropeptides.

Evoked noradrenaline release in the rabbit ear artery: enhancement by purines, attenuation by neuropeptide Y and lack of effect of calcitonin gene-related peptide

1. Adenosine (30 microM) and its analogues 5'-N-ethylcarboxaminoadenosine (5 and 30 microM) and L-phenylisopropyladenosine (5 and 30 microM), potentiated the evoked but not spontaneous release of tritiated noradrenaline in the rabbit central ear artery. 2. Prejunctional inhibition of the evoked but not spontaneous release of tritiated noradrenaline by 100 nM neuropeptide Y is greater at 2 min than at 10 min after superfusion of the peptide. 3. Calcitonin gene-related peptide (2.63 to 263 nM) did not affect the evoked or spontaneous release of tritiated noradrenaline in this preparation. 4. These results are discussed in terms of prejunctional modulation of sympathetic transmission in the rabbit central ear artery.

Inhibition of hypoxia-induced relaxation of rabbit isolated coronary arteries by NG-monomethyl-L-arginine but not glibenclamide

1. The effects of NG-monomethyl-L-arginine, tetrodotoxin and glibenclamide on hypoxia-induced coronary artery relaxation, induced by bubbling Krebs solution with 95% N2 and 5% CO2 instead of 95% O2 and 5% CO2, were assessed by measuring the changes in isometric tension in isolated epicardial coronary artery rings of the rabbit. In addition, the effects of glibenclamide on the relaxation induced by adenosine were investigated. 2. Hypoxia caused a transient relaxation of 38 +/- 3% (P < 0.01) and 17 +/- 2% (P < 0.01) in endothelium-intact or -denuded arteries respectively. NG-monomethyl-L-arginine (30 and 100 microM) inhibited the relaxation in endothelium-intact rings to 31 +/- 2% (P < 0.05) and 16 +/- 2% (P < 0.01) respectively and slightly but significantly attenuated the relaxation in endothelium-denuded rings to 15 +/- 1% and 13 +/- 1% (P < 0.05) respectively. 3. Glibenclamide, a potassium channel inhibitor, did not significantly after the hypoxia-induced relaxation. 4. Incubation with tetrodotoxin (3 and 10 microM) for 30 min reduced the relaxation to 31 +/- 3% (P < 0.05) and 14 +/- 2% (P < 0.01), and 14 +/- 2% (P < 0.05) and 11 +/- 1% (P < 0.05) in endothelium-intact and -denuded rings respectively. However, indomethacin (10 microM), atropine (1 microM), propranolol (10 microM) and phentolamine (10 microM) did not significantly affect the relaxation. 5. Adenosine (1, 10 and 100 MicroM) caused relaxation of 6 +/- 1%, 52 +/-3% and 97 +/-2% respectively in endothelium-denuded rings precontracted with prostaglandin F2alpha (PGF2 alpha, 3 MicroM) and the relaxation was markedly inhibited by 8-phenyltheophylline. Furthermore, glibenclamide (1 and 10 MicroM) reduced the relaxation induced by adenosine (1, 10 and 100 MicroM) to 2 +/-1% (P<0.05), 38 =/-3% (P<0.05) and 85 +/-2%(P<0.05), and 0.6 +/- 0.4% (P<0.05), 27 +/- 4% (P<0.05) and 72 +/- 4% (P<0.01) respectively, in these endothelium-denuded preparations.6. These data suggest that hypoxia-induced relaxation is mediated by the release of nitric oxide rather than by the activation of glibenclamide-sensitive potassium channels in rabbit isolated coronary arteries. A neurogenic mechanism partially modulates the relaxation, possibly by activating non-adrenergic and noncholinergic nerve endings. The inhibition by glibenclamide on adenosine-induced relaxation in isolated coronary arteries may help to explain the fact that glibenclamide inhibits hypoxic coronary relaxation in perfused hearts but not in isolated coronary preparations.

Origin of neurons supplying the vas deferens of the rat

Retrograde axonal tracing methods using Fluoro-Gold were used to examine the neuronal input to the vas deferens in the adult Wistar rat. The greatest number of labelled efferent neurons were found in the ipsilateral pelvic accessory ganglion (PAG) (68%) and the major pelvic ganglion (MPG) (15%). Fewer than 3% of labelled neurons were localized to the inferior mesenteric and sympathetic chain ganglia. Labelled neurons were also located in the ipsilateral L1, L2, and L6, S1 dorsal root ganglia (DRG), corresponding to afferents that travel in the hypogastric and pelvic nerves, respectively. Contributions from contralateral neurons in the PAG, MPG and L1 DRG were also documented. The role of afferents supplying the vas deferens is not known but they may relay nociceptive or mechanoreceptive input. Efferent input from peripheral ganglia probably contributes to contractility of the vas deferens based on previous investigations.

Effect of opioid receptor subtype-selective agonists on purinergic and adrenergic components of neurogenic contractions of mouse vas deferens

1. Effects of opioid agonists on the purinergic and adrenergic components of neurogenic contractions and in some experiments on transmitter overflow were studied in the mouse isolated vas deferens. 2. When the vas deferens was stimulated every 2 min by pairs of pulses 2 s apart in the presence of prazosin 0.3 microM (to isolate the purinergic component) or alpha,beta-methylene-ATP 3 microM (to isolate the adrenergic component), each pulse elicited a separate twitch. The opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, mu-receptor-selective), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-selective) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide (U-50488, kappa-selective) concentration-dependently reduced both purinergic and adrenergic contractions. For each agonist, maximal effects and concentrations causing half-maximal effects were very similar for inhibition of the purinergic component on the one hand and for inhibition of the adrenergic component on the other hand, although the adrenergic component was inhibited with a slight preference. Moreover, effects on contractions elicited by the first and the second pulse of the pairs were very similar. 3. When vasa deferentia preincubated with [3H]-noradrenaline were stimulated with trains of 100 pulses delivered at 20 Hz, morphine 10 microM reduced significantly both evoked tritium overflow and evoked contractions. Its effect was antagonized by naloxone. 4. It is concluded that, in contrast to drugs acting at some other presynaptic receptors, opioid mu-, delta- and kappa-agonists inhibit purinergic and adrenergic neurogenic contractions of the mouse vas deferens in a similar manner. In contrast to a previous report, no enhancement by morphine of the release of noradrenaline elicited by high frequency pulse trains was observed.

Is the neuronal ATP release from guinea-pig vas deferens subject to α2-adrenoceptor-mediated modulation?

The effects of a variety of alpha 2-adrenoceptor agonists and antagonists were studied on stimulation-evoked release of endogenous ATP, measured by the luciferin-luciferase assay, and on the release of [3H]noradrenaline from the guinea-pig vas deferens. The biphasic mechanical contraction of the guinea-pig smooth muscle was recorded concomitantly. The alpha 2-adrenoceptor agonist, xylazine (1 microM) inhibited the field stimulation-evoked (8 Hz, 0.1 ms, 480 shocks) release of ATP and [3H]noradrenaline, and both phases of the contraction. The inhibitory effect of xylazine on the release of ATP, noradrenaline and muscle contraction was prevented by the selective alpha 2-adrenoceptor antagonist, CH 38083 [7,8-(methylenedioxi)-14 alpha-alloberbanol, 1 microM]. In the presence of prazosin (0.1-1 microM) or WB 4101 [2-(2,6-dimethoxyphenoxyethyl)aminomethyl- 1,4-benzodioxane hydrochloride, 0.1-1 microM], i.e. under the condition when the effect of noradrenaline on postjunctional alpha 1-adrenoceptors was excluded, the stimulation-evoked release of [3H]noradrenaline was significantly enhanced, however, the release of endogenous ATP and also both phases of contraction were reduced. In the presence of prazosin, xylazine was able to inhibit the stimulation-evoked release of ATP. In vas deferens dissected from reserpine pretreated (2 x 5 mg/kg, i.p.) guinea-pigs, the content of noradrenaline was 0.5% of control and there was no detectable evoked release of noradrenaline. Under this condition, the release of ATP evoked by electrical stimulation was still detectable, but the amount of ATP was much smaller than that measured from control animals. Xylazine did not reduce the release of ATP. Oxymetazoline, a relatively selective alpha 2-adrenoceptor agonist failed to inhibit the release of [3H]noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

ATP overflow from the mouse isolated vas deferens

1. A modified form of the highly specific luciferin-luciferase assay was used to measure the overflow of adenosine 5'-triphosphate (ATP) from the field-stimulated mouse vas deferens in vitro. Precise timing of the stimulation, superfusate collection and assay minimized ATP degradation before assay, offering the opportunity for quantitative studies. 2. Stimulation with between 50 and 200 supramaximal pulses at 5 Hz increased ATP overflow by between 7 and 28 times over basal. 3. ATP overflow increased steadily with increasing numbers of stimuli from 50 to 200 pulses. Increasing the frequency of stimulation initially had no effect, but above 10 Hz there was increased overflow, suggesting an interaction between facilitated release and the rate of degradation by extracellular ATPases. 4. Contractions of the vas induced by exogenous (-)-phenylephrine produced only a small increase in ATP overflow, suggesting that the stimulation induced increase in ATP overflow is mostly pre-junctional in origin, though tetrodotoxin eliminated stimulation-induced overflow only in a proportion of preparations.

Novel selective agonists and antagonists confirm neurokinin NK1 receptors in guinea-pig vas deferens

1. This study investigated the recognition characteristics of neurokinin receptors mediating potentiation of the contractile response to field stimulation in the guinea-pig vas deferens. 2. A predominant NK1 receptor population is strongly suggested by the relative activities of the common naturally-occurring tachykinin agonists, which fall within less than one order of magnitude. This conclusion is supported by the relative activities of the synthetic NK1 selective agonists substance P methyl ester, [Glp6,L-Pro9]-SP(6-11) and delta-aminovaleryl-[L-Pro9,N-MeLeu10]- SP(7-11) (GR73632) which were 0.78, 9.3 and 120 as active as substance P, respectively. Furthermore, the NK2 selective agonist [Lys3, Gly8,-R-gamma-lactam-Leu9]-NKA(3-10) (GR64349) was active only at the highest concentrations tested (greater than 10 microM), and the NK3 selective agonist, succ-[Asp6,N-MePhe8]-SP(6-11) (senktide) was essentially inactive (10 nM-32 microM). 3. [D-Arg1,D-Pro2,D-Trp7,9,Leu11]-SP(1-11) antagonized responses to neurokinin A, neurokinin B, physalaemin, eledoisin, [Glp6,D-Pro9]-SP(6-11), GR73632 and GR64349 (apparent pKB s 5.6-6.2), but was less potent in antagonizing responses to substance P, substance P methyl ester and [Glp6,L-Pro9]-SP(6-11) (apparent pKB s less than or equal to 5.0-5.0). 4. In contrast, the recently developed NK1-selective receptor antagonist [D-Pro9[Spiro-gamma-lactam]Leu10,Trp11]-SP(1-11) (GR71251) did not produce agonist-dependent pKB estimates. Schild plot analysis indicated a competitive interaction with a single receptor population where the antagonist had an estimated overall pKB of 7.58 +/- 0.13 for the four agonists of differing subtype selectivity tested (GR73632, GR64349, substance P methyl ester and neurokinin B). This estimate is similar to that we obtained for NK1-mediated (substance P methyl ester) contraction in the guinea-pig ileum preparation (pKB= 7.86+ 0.05). 5. Tachykinin action appears not to depend on release of a number of intermediary mediators including acetylcholine, histamine or cyclo-oxygenase products, nor to involve interaction with neuronal mechanisms including alpha 2-adrenoceptor feedback, noradrenergic Uptake-I or opioid-release, since antagonism or inhibition of these mechanisms did not modify responses to tachykinins. 6. We conclude that tachykinin action in the field-stimulated guinea-pig vas deferens preparation is mediated through interaction with a predominant neurokinin NK, receptor population and this preparation can therefore be used to study NK, modulation of sympathetic neurotransmission.

Overview. Purinergic mechanisms

An overview of the history of studies of the biological actions of extracellular ATP is presented. The basis of the subdivision of receptors for purines into P1-purinoceptors for adenosine and P2-purinoceptors for ATP and ADP are considered, as well as the recent proposal for subdivision of the ATP receptors into P2X-, P2Y-, P2Z-, and P2T- purinoceptor subtypes. These purinoceptor subtypes are discussed with respect to their transduction mechanisms, their distribution, and their physiological roles, including their roles in cotransmission and neuromodulation.

Inhibition of sympathetic neurotransmitter release by modulators of cyclic GMP in canine vascular smooth muscle

The contractile response to neurally released norepinephrine (NE) from sympathetic nerve endings innervating vascular smooth muscle are inhibited by substances which raise either cyclic AMP and cyclic GMP concentrations in smooth muscle. However, cyclic AMP is believed to facilitate NE release from sympathetic nerves whereas the role of cyclic GMP in this process is undefined. We examined the effects of presumed modulation of the intraneuronal concentration of cyclic AMP and cyclic GMP on sympathetic neurotransmission to isolated canine mesenteric artery by measurement of the efflux of [2-14C]NE during transmural nerve stimulation (calcium dependent release of NE) and administration of tyramine (calcium independent release of NE) and measurement of the contractions to exogenous NE and tyramine. Stimulation of adenylate cyclase with forskolin, prostacyclin and iloprost, a stable prostacyclin analog, and inhibition of Type III cyclic AMP phosphodiesterase with neural specific rolipram, 'non-specific pelrinone and milrinone and isobutylmethylxanthine did not enhance the efflux of [2-14C]NE from sympathetic nerves innervating the blood vessels. Isoproterenol enhanced the efflux of [2-14C]NE. The effect was inhibited by propranolol but not affected by milrinone, amrinone or rolipram. Activators of guanylate cyclase (SIN-1a an active metabolic of molsidomine, nitroglycerin and sodium nitroprusside) and inhibitors of Type II cyclic GMP phosphodiesterase (M&B-22948 and verofyllin) inhibited the efflux of NE released by transmural nerve stimulation but not by tyramine. These data support the conclusion that cyclic GMP may be an inhibitory modulator of calcium and depolarization dependent NE release from sympathetic nerves, whereas neuronal cyclic AMP may not be a primary modulator of neurotransmission to vascular smooth muscle.

Sensory-motor neuromodulation of sympathetic vasoconstriction in the rabbit central ear artery

Histochemical and pharmacological studies were performed on the rabbit central ear artery. In perivascular nerves, positive immunoreactivity for calcitonin gene-related peptide and substance P was demonstrated. Calcitonin gene-related peptide-like immunoreactivity was also found to be colocalised with substance P-like immunoreactivity in a subpopulation of perivascular nerves. In vitro incubation with 6-hydroxydopamine did not alter the intensity and/or density of either the calcitonin gene-related peptide- or substance P-like immunoreactive fibres, whereas incubation with capsaicin significantly reduced both. In pharmacological studies, calcitonin gene-related peptide reduced the vasoconstrictor responses to exogenous noradrenaline and alpha, beta-methylene ATP and to electrical field stimulation in a concentration-dependent manner. In segments of the central ear artery preconstricted with noradrenaline, relaxation mediated by calcitonin gene-related peptide was endothelium-independent. These results shed new light on the innervation and nervous control of the rabbit central ear artery previously thought to be exclusively under sympathetic (adrenergic and purinergic) control. Further, the results suggest that calcitonin gene-related peptide localised in sensory nerves in the rabbit central ear artery may act as an inhibitory modulator of excitatory sympathetic vascular neurotransmission.