Differential effects of nitric oxide donors on basal and electrically evoked release of acetylcholine from guinea-pig myenteric neurones

Contractile dysfunction and nitrergic dysregulation in small intestine of a primate model of Parkinson's disease

Bowel dysfunction is a common non-motor symptom in Parkinson’s disease (PD). The main contractile neurotransmitter in the GI tract is acetylcholine (ACh), while nitric oxide (NO) causes the relaxation of smooth muscle in addition to modulating ACh release. The aim of this study was to characterise functional and neurochemical changes in the isolated ileum of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated marmoset, an established model of PD motor dysfunction. While NO-synthase inhibitor L-NAME concentration dependently augmented the neurogenically-evoked contractions and inhibited the relaxations in normal tissues, it had no effects on the MPTP ileum. Immunohistochemical analyses of the myenteric plexus showed that ChAT-immunoreactivity (-ir) was significantly reduced and the density of the enteric glial cells as shown by SOX-10-ir was increased. However, no change in TH-, 5-HT-, VIP- or nNOS-ir was observed in the MPTP tissues. The enhancement of the neurogenically-evoked contractions and the inhibition of the relaxation phase by L-NAME in the control tissues is in line with NO’s direct relaxing effect on smooth muscle and its indirect inhibitory effect on ACh release. The absence of the relaxation and the inefficacy of L-NAME in the MPTP tissues suggests that central dopaminergic loss dopamine may eventually lead to the impairment of NO signal coupling that affects bowel function, and this may be the result of a complex dysregulation at the level of the neuroeffector junction.

Adenosine negatively regulates duodenal motility in mice: role of A(1) and A(2A) receptors

BACKGROUND AND PURPOSE

Adenosine is considered to be an important modulator of intestinal motility. This study was undertaken to investigate the role of adenosine in the modulation of contractility in the mouse duodenum and to characterize the adenosine receptor subtypes involved.

EXPERIMENTAL APPROACH

RT-PCR was used to investigate the expression of mRNA encoding for A(1), A(2A), A(2B) and A(3) receptors. Contractile activity was examined in vitro as changes in isometric tension.

KEY RESULTS

In mouse duodenum, all four classes of adenosine receptors were expressed, with the A(2B) receptor subtype being confined to the mucosal layer. Adenosine caused relaxation of mouse longitudinal duodenal muscle; this was antagonized by the A(1) receptor antagonist and mimicked by N(6) -cyclopentyladenosine (CPA), selective A(1) agonist. The relaxation induced by A(1) receptor activation was insensitive to tetrodotoxin (TTX) or N(ω) -nitro-l-arginine methyl ester (l-NAME). Adenosine also inhibited cholinergic contractions evoked by neural stimulation, effect reversed by the A(1) receptor antagonist, but not myogenic contractions induced by carbachol. CPA and 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680), A(2A) receptor agonist, both inhibited the nerve-evoked cholinergic contractions. l-NAME prevented only the CGS-21680-induced effects. S-(4-Nitrobenzyl)-6-thioinosine, a nucleoside uptake inhibitor, reduced the amplitude of nerve-evoked cholinergic contractions, an effect reversed by an A(2A) receptor antagonist or l-NAME.

CONCLUSIONS AND IMPLICATIONS

Adenosine can negatively regulate mouse duodenal motility either by activating A(1) inhibitory receptors located post-junctionally or controlling neurotransmitter release via A(1) or A(2A) receptors. Both receptors are available for pharmacological recruitment, even if only A(2A) receptors appear to be preferentially stimulated by endogenous adenosine.

Kynurenines and intestinal neurotransmission: the role of N-methyl-D-aspartate receptors

Gastrointestinal neuroprotection involves the net effect of many mechanisms which protect the enteral nervous system and its cells from death, dysfunction or degeneration. Neuroprotection is also a therapeutic strategy, aimed at slowing or halting the progression of primary neuronal loss following acute or chronic diseases. The neuroprotective properties of a compound clearly have implications for an understanding of the mechanism of dysfunctions and for therapeutic approaches in a number of gastrointestinal diseases.This paper focused on the roles of glutamate and N-methyl-D-aspartate (NMDA) receptors in the intrinsic neuronal control of gastrointestinal motility; the consequences of inflammation on gastrointestinal motility changes; and the involvement of tryptophan metabolites (especially kynurenic acid) in the regulatory function of the enteral nervous system and the modulation of the inflammatory response. Common features in the mechanisms of action, illustrative evidence from animal models, and experimental neuroprotective therapies making use of the currently available possibilities are also discussed.Overall, the evidence suggests that gastrointestinal neuroprotection against inflammation and glutamate-induced neurotoxicity may be mediated synergistically through the blockade of NMDA receptors and the inhibition of neuronal nitric oxide synthase activity and xanthine oxidoreductase-dependent superoxide production. These components are likewise significant factors in the pathomechanism of gastrointestinal inflammatory diseases and inflammation-linked motility alterations. Inhibition of the enteric NMDA receptors by kynurenic acid or its analogues may provide a novel option via which to influence intestinal hypermotility and inflammatory processes simultaneously.

Review article: the pathogenesis of diverticular disease--current perspectives on motility and neurotransmitters

BACKGROUND

Low-fibre diet, structural abnormalities and ageing are traditional aetiological factors implicated in the development of diverticular disease. More recently, motility disorders are implicated in its causation leading to speculation that neurotransmitters play a role in mediating these disturbances.

AIMS

To draw together studies on the role of neurotransmitters in the development of diverticular disease and its symptoms.

METHODS

Medline, GoogleScholar and Pubmed were searched for evidence on this subject using the terms neurotransmitters, motility, diverticular disease and pathogenesis. Articles relevant to the subject were cited and linked references were also reviewed.

RESULTS

Serotonin, which has been found to be an excitatory colonic neurotransmitter, has been found in early studies to be increased in colonic enterochromaffin cells. Acetylcholine, which is thought to be an excitatory neurotransmitter and cholinergic activity, has also seen to be increased in diverticular disease. These findings may suggest that an increase in excitatory neurotransmitters may result in the hypersegmentation thought to cause pulsion diverticula. Similarly, a decrease in nitric oxide which is inhibitory is found.

CONCLUSIONS

There is some evidence that neurotransmitters may play a role in the motility disturbances seen in diverticular disease; however, a clear role is yet to be ascertained.

Lack of evidence in vivo for nitrergic inhibition by Escherichia coli (STa) enterotoxin of fluid absorption from rat proximal jejunum

Fluid absorption from the proximal jejunum of the anaesthetised rat was measured in vivo by fluid recovery. As expected, heat stable (STa) enterotoxin from E. coli reduced fluid absorption. Neither intraperitoneal L-NAME, thought to inhibit a putative neurally mediated action of STa, nor similar doses of D-NAME, ameliorated the inhibitory effect on jejunal fluid absorption of STa. Luminally perfused 10 mM sodium nitroprusside (SNP) had no effect on fluid absorption when expressed per gram dry weight per hour but reduced fluid absorption when expressed per cm length per hour. Similarly, 80 but not 40 mg/Kg of L-NAME reduced fluid absorption when expressed per cm length per hour, while the same dose of D-NAME did not. L-NAME and SNP significantly increased the wet weight to dry weight and the length to dry weight ratio of perfused loops. We conjecture that smooth muscle relaxation caused by these compounds increases interstitial fluid volumes that can be misconstrued as changes in absorption when this is expressed per cm length or per tissue wet weight. When fluid absorption is expressed per gram dry weight of tissue, there is no evidence for a role of nitric oxide in normal or STa inhibited fluid absorption.

Longitudinal muscle shows abnormal relaxation responses to nitric oxide and contains altered levels of NOS1 and elastin in uncomplicated diverticular disease

OBJECTIVE

Recent evidence challenges the 'low-fibre/high-colonic intraluminal pressure' hypothesis of diverticular disease (DD) and raises the possibility that other mechanisms are involved. Although bowel wall smooth muscle is known to be hypercontractile in DD, the nature of its relaxation is unknown. The present study investigated colonic smooth muscle responses to nitric oxide, as well as the smooth muscle contents of neural nitric oxide and elastin associated with the disease.

METHOD

Immunohistochemical/image analysis of antibodies to nitric oxide synthase (NOS1), co-localized with protein gene product (PGP) and to elastin, was performed on three histological sections of sigmoid colons from 20 patients (10 DD, 10 controls) following resections for rectal tumours. Organ bath experiments examined smooth muscle responsiveness to nitroprusside, a nitric oxide donor.

RESULTS

Uncomplicated diverticular longitudinal muscle showed lower nitric oxide immunoreactivity compared with controls: median percentage surface area of NOS1 over PGP was 26.0% (range 0.5-58.3), controls 45.0% (35.0-70.1; P = 0.018). Median percentage surface area of elastin was elevated, 21.3% (10.6-45.6), controls 8.2% (1.7-13.5; P = 0.0002), together with a low sensitivity to nitroprusside [mean - log EC(50) 5.3 (SD 0.5), controls 6.6 (SD 0.5), difference 1.3, 95% CI 0.8-1.7; P < 0.0001] and there were lower maximum relaxation responses to nitroprusside compared with controls: median percentage (relaxation induced by nitroprussside/contraction induced by bethanecol) was 52.0%, range (20.0-92.0), controls 100.0% (71.0-125.0), P < 0.0001. No statistically significant differences were found in circular muscle, at the sample size studied.

CONCLUSION

This study established, for the first time, specific abnormalities in longitudinal muscle relaxation and contents of neural nitric oxide and elastin in uncomplicated DD. These findings may have important implications for both colon structure and function in the disease.

The intestinal trophic response to enteral food is reduced in parenterally fed preterm pigs and is associated with more nitrergic neurons

In term neonates, total parenteral nutrition (TPN) induces mucosal atrophy, whereas the first intake of milk is followed by intestinal growth. This may be explained in part by an NO-mediated increased blood flow. We hypothesized that the immature gut has an altered response to TPN and enteral nutrition. In Expt. 1, preterm caesarean-delivered pigs were administered elemental nutrients for 3 d, infused parenterally (TPN, n = 7) or enterally (TENT, n = 7). In Expt. 2, preterm pigs were fed sow's colostrum, cow's colostrum, or infant formula for 2 d after a 3-d TPN period (TPN-SOW, TPN-COW, TPN-FORM, n = 8-11). Intestinal morphology and the number of enteric neurons containing nitric oxide synthase-1 (NOS-1) were quantified. Both the TPN and TENT groups had increases in intestinal mass, circumference, and mucosal mass, volume, and surface density, relative to values at birth (+30-50%, P < 0.05). In Expt. 2, the magnitudes of the intestinal trophic responses to feeding were similar to those in Expt. 1, but were also associated with an increased number of nitrergic myenteric neurons and some mucosal damage, most frequently observed for the formula group. We conclude that 1) a short period of TPN does not induce mucosal atrophy in preterm pigs, whereas elemental nutrients infused luminally do not mimic the trophic response seen with milk diets, 2) enteral feeding of preterm pigs after a short period of TPN is associated with a modest, diet-dependent trophic response that may be related in part to the actions of an increased population of enteric NOS-1 neurons.

Glutamate stimulation of acetylcholine release from myenteric plexus is mediated by endogenous nitric oxide

Glutamate was found to be an excitatory neurotransmitter in the enteric nervous system. Although several lines of evidence indicate a role of glutamate in the regulation of gut motility and secretion the physiological significance of glutamatergic transmission is not clear yet. We studied the effect of glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining in longitudinal muscle strips with attached myenteric plexus of guinea pig ileum. L-glutamate (100 microM) significantly enhanced both the evoked [3H]acetylcholine release and the optical density of nicotinamide adenine dinucleotide phosphate-diaphorase positive neurones, i.e. the intensity of staining. The non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (3 microM) abolished the stimulatory effect of L-glutamate on acetylcholine efflux. Similarly, the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (100 microM) significantly reduced the effect of L-glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining. Our data suggest that endogenous nitric oxide seems to mediate the stimulatory effect of glutamate on acetylcholine release from guinea pig myenteric neurons.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Inhibition of nitric oxide synthesis blocks the inhibitory response to capsaicin in intestinal circular muscle preparations from different species

Moderate concentrations of the sensory stimulant drug capsaicin caused relaxation in human and animal intestinal circular muscle preparations (guinea-pig proximal, mouse distal colon, human small intestine and appendix) in vitro. With the exception of the guinea-pig colon, the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NOARG; 10(-4) M) strongly inhibited the relaxant effect of capsaicin. Tetrodotoxin, an inhibitor of voltage-sensitive Na+ channels failed to significantly reduce the inhibitory effect of capsaicin in the guinea-pig colon, human ileum and appendix; it caused an approximately 50% reduction in the mouse colon. The relaxant effect of capsaicin was strongly reduced in colonic preparations from transient receptor potential vanilloid type (TRPV1) receptor knockout mice as compared to their wildtype controls. It is concluded that nitric oxide, possibly of sensory origin, is involved in the relaxant action of capsaicin in the circular muscle of the mouse and human intestine.

Modulation by NO of acetylcholine release in the ileum of wild-type and NOS gene knockout mice

Nitric oxide (NO) inhibits the release of acetylcholine and cholinergic contractions in the small intestine of several species, but no information is available about the mouse ileum. This study examines the effects of NO on the electrically evoked release of [3H]acetylcholine and smooth muscle contraction in myenteric plexus-longitudinal muscle preparations of wild-type mice and of neuronal NO synthase (nNOS) and endothelial NOS (eNOS) knockout mice. The NOS inhibitor N(G)-nitro-L-arginine (L-NNA) and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ) concentration dependently increased the evoked [3H]acetylcholine release and cholinergic contractions in preparations from wild-type mice and from eNOS knockout mice. Effects of L-NNA were specifically antagonized by L-arginine. In contrast, L-NNA and ODQ did not modify the release and contractions in preparations from nNOS knockout mice. The NO donor S-nitroso-N-acetyl-DL-penicillamine inhibited the electrically evoked release of [3H]acetylcholine and longitudinal muscle contractions in a quantitatively similar manner in wild-type preparations as well as in nNOS and eNOS knockout preparations. We conclude that endogenous NO released by electrical field stimulation tonically inhibits the release of acetylcholine. Furthermore, data suggest that nNOS and not eNOS is the enzymatic source of NO-mediating inhibition of cholinergic neurotransmission in mouse ileum.

Effect of sodium nitroprusside and 8-bromo cyclic GMP on nerve-mediated and acetylcholine-evoked secretory responses in the rat pancreas

The effects of sodium nitroprusside (SNP) and 8-bromo-guanosine 3'5' cyclic monophosphate (8-Br-cyclic GMP) on nerve-mediated and acetylcholine (ACh)-evoked amylase secretion, tritiated choline ([3H]-choline) release and on intracellular free calcium concentration ([Ca2+]i) in the isolated rat pancreas were investigated. Electrical field stimulation (EFS; 10 Hz) and ACh (1 x 10(-5) M) caused large increases in amylase output from pancreatic segments. The response to ACh was blocked by atropine (1 x 10(-5) M) whereas the EFS-evoked response was markedly reduced but not abolished. In contrast, pretreatment with tetrodotoxin (1 x 10(-6) M) abolished the secretory effect of EFS. Either SNP (1 x 10(-3) M) or 8-Br-cyclic GMP (1 x 10(-4) M) inhibited amylase secretion compared to basal. Combining either SNP or 8-Br-cyclic GMP with EFS resulted in a marked decrease in amylase output compared to EFS alone. In contrast, either SNP or 8-Br-cyclic GMP had no significant effect on the amylase response to ACh. When extracellular Ca2+ concentration ([Ca2+]o) was elevated from 2.56 mM to 5.12 mM, SNP failed to inhibit the response to EFS. EFS stimulated the release of 3H from pancreatic segments preloaded with [3H]-choline. Either SNP or 8-Br-cyclic GMP had no effect on basal 3H release but significantly reduced the EFS-evoked response. In fura-2 loaded acinar cells, SNP elicited a small decrease in [Ca2+]i compared to basal and had no effect on the ACh-induced [Ca2+]i peak response. Nitric oxide may modulate the release of endogenous neural ACh in response to EFS in the rat pancreas.

Presynaptic modulation of cholinergic neurotransmission in the human proximal stomach

1. This study investigates whether the cholinergic neurones, innervating the human proximal stomach, can be modulated by nitric oxide (NO) or vasoactive intestinal polypeptide (VIP), or via presynaptic muscarinic, alpha(2)- or 5-hydroxytryptamine(4) (5-HT(4)-) receptors. 2. Circular muscle strips, without mucosa, were incubated with [(3)H]-choline to incorporate [(3)H]-acetylcholine into the cholinergic transmitter stores. The basal and electrically-induced release of tritium and [(3)H]-acetylcholine were analysed in a medium containing guanethidine (4 x 10(-6) M), hemicholinium-3 (10(-5) M), physostigmine (10(-5) M) and atropine (10(-6) M). Tissues were stimulated twice for 2 min (S(1) and S(2): 40 V, 1 ms, 4 Hz) and drugs were added before S(2). 3. The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3 x 10(-4) M) and the NO donor sodium nitroprusside (10(-5) M), as well as VIP (10(-7) M) did not influence the basal release nor the electrically-evoked release. 4. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically-evoked release of [(3)H]-acetylcholine, and this was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2 x 10(-6) M). 5. The 5-HT(4)-receptor agonist prucalopride (3 x 10(-7) M) significantly enhanced the electrically-evoked release of [(3)H]-acetylcholine, and the 5-HT(4)-receptor antagonist SB204070 (10(-9) M) prevented this. 6. When atropine (10(-6) M) was omitted from the medium and added before the second stimulation, it significantly increased the release of [(3)H]-acetylcholine. 7. These results suggest that the release of acetylcholine from the cholinergic neurones, innervating the circular muscle in the human proximal stomach, can be inhibited via presynaptic muscarinic auto-receptors and alpha(2)-adrenoceptors, and stimulated via presynaptic 5-HT(4)-receptors. No evidence for modulation by NO or VIP was obtained.

Participation of endogenous nitric oxide in the effect of hypoxia in vitro on neuro-effector transmission in guinea-pig ileum

The implication of endogenous nitric oxide in the effect of hypoxia on the neurotransmission in the enteric nervous system of guinea-pig ileum was studied in vitro. Three methodological approaches have been used: (i) Stretch-induced phases of peristaltic reflex in ileal segments; (ii) twitch contractions of longitudinal segments, evoked by electrical field stimulation; and (iii) release of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations, measured by liquid spectrophotometry. The effect of nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (L-NNA, 100 microM) was studied under normoxic conditions. L-NNA did not change significantly the ascending contraction phase of peristaltic reflex and the amplitude of twitch contractions. However, the same concentration of L-NNA increased the stimulation-evoked acetylcholine release. The descending relaxation phase decreased in the presence of L-NNA. In another set of experiments, hypoxia was mimicked by replacement of oxygen from the perfusion medium with nitrogen for a period of 30 min. Hypoxia significantly decreased the ascending contraction phase, the twitch contractions, and the release of acetylcholine from the myenteric plexus. Under hypoxic conditions, pretreatment with L-NNA did not change either the contractile responses, nor the release of acetylcholine. Our results suggest that under conditions of oxygen deprivation, endogenous nitric oxide seems to be inefficient in modulating the cholinergic neurotransmission in guinea-pig ileum.

Nitric oxide-induced cytostasis and cell cycle arrest of a human breast cancer cell line (MDA-MB-231): potential role of cyclin D1

DETA-NONOate, a nitric oxide (NO) donor, induced cytostasis in the human breast cancer cells MDA-MB-231, and the cells were arrested in the G(1) phase of the cell cycle. This cytostatic effect of the NO donor was associated with the down-regulation of cyclin D1 and hypophosphorylation of the retinoblastoma protein. No changes in the levels of cyclin E or the catalytic partners of these cyclins, CDK2, CDK4, or CDK6, were observed. This NO-induced cytostasis and decrease in cyclin D1 was reversible for up to 48 h of DETA-NONOate (1 mM) treatment. DETA-NONOate (1 mM) produced a steady-state concentration of 0.5 microM of NO over a 24-h period. Synchronized population of the cells exposed to DETA-NONOate remained arrested at the G(1) phase of the cell cycle whereas untreated control cells progressed through the cell cycle after serum stimulation. The cells arrested at the G(1) phase after exposure to the NO donor had low cyclin D1 levels compared with the control cells. The levels of cyclin E and CDK4, however, were similar to the control cells. The decline in cyclin D1 protein preceded the decrease of its mRNA. This decline of cyclin D1 was due to a decrease in its synthesis induced by the NO donor and not due to an increase in its degradation. We conclude that down-regulation of cyclin D1 protein by DETA-NONOate played an important role in the cytostasis and arrest of these tumor cells in the G(1) phase of the cell cycle.

Influence of nitric oxide donors and of the alpha(2)-agonist UK-14,304 on acetylcholine release in the pig gastric fundus

This study in circular muscle strips of the pig gastric fundus aimed to measure the release of acetylcholine directly and to investigate whether NO and alpha(2)-adrenoceptor agonists can modulate acetylcholine release from cholinergic neurones. After incubation of the tissues with [(3)H]-choline, basal and electrically induced release of tritium and [(3)H]-acetylcholine were analyzed in a medium containing physostigmine (10(-5) M) as well as atropine (10(-6) M). The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3x10(-4) M), and the NO donors sodium nitroprusside (10(-5) M) and 3-morpholino-sydnonimine (10(-5) M) did not influence the basal release nor the electrically evoked release, indicating that NO does not modify [(3)H]-acetylcholine release. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically evoked release of [(3)H]-acetylcholine, and this effect was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2x10(-6) M), suggesting that presynaptic alpha(2)-adrenoceptors are present on cholinergic neurones of the pig gastric fundus.

The control of gut motility

Gut motility in non-mammalian vertebrates as in mammals is controlled by the presence of food, by autonomic nerves and by hormones. Feeding and the presence of food initiates contractions of the stomach wall and subsequently gastric emptying, peristalsis, migrating motor complexes and other patterns of motility follow. This overview will give examples of similarities and differences in control systems between species. Gastric receptive relaxation occurs in fish and is an enteric reflex. Cholecystokinin reduces the rate of gastric emptying in fish as in mammals. Inhibitory control of peristalsis is exerted, e.g. by VIP, PACAP, NO in fish and amphibians, while excitatory stimuli arise from nerves releasing tachykinins, acetylcholine or serotonin (5-HT). In crocodiles, we have found the presence of the same nerve types, although the effects on peristalsis have not been studied. Recent studies on signal transduction in the gut smooth muscle of fish and amphibians suggest that external Ca2+ is of great importance, but not the only source of Ca2+ recruitment in tachykinin-, acetylcholine- or serotonin-induced contractions of rainbow trout and Xenopus gastrointestinal smooth muscle. The effect of acetylcholine involves reduction of cAMP-levels in the smooth muscle cells. It is concluded that, in general, the control systems in non-mammalian vertebrates are amazingly similar between species and animal groups and in comparison with mammals.

Correolide, a nor-triterpenoid blocker of Shaker-type Kv1 channels elicits twitches in guinea-pig ileum by stimulating the enteric nervous system and enhancing neurotransmitter release

Correolide (1 - 10 microM), a nortriterpene purified from Spachea correae and a selective blocker of Kv1 potassium channels, elicits repetitive twitching in guinea-pig ileum. This effect is not seen in guinea-pig duodenum, portal vein, urinary bladder or uterine strips, nor in rat or mouse ileum. The time course and amplitude of the correolide-induced twitches in guinea-pig ileum are similar to those elicited by electrical stimulation of the enteric nervous system. The correolide-induced twitching is not affected by pre-treatment with capsaicin (1 microM), but is facilitated by the NO synthase inhibitor, N(G)-nitro-L-arginine methyl esther (L-NAME, 200 microM). The correolide-induced twitching is abolished by tetrodotoxin (1 microM) or hexamethonium (100 microM), and is markedly inhibited by nifedipine (0.3 microM) or atropine (0.2 microM). The atropine-resistant component is inhibited by selective antagonists of NK1 and NK2 tachykinin receptors, namely GR 82334 and GR 94800 (1 microM each). The former compound is more effective in inhibiting the correolide-induced, atropine-resistant activity. Correolide intensified the twitching of ileum segments exposed to saturating concentrations of margatoxin (MgTX), which suggests that Kv1 sub-types other than Kv1.1 (Kv1.4 or Kv1.5) are involved in the relatively greater degree of stimulation of the enteric nervous system by correolide, as compared to MgTX. We propose that blockade of Kv1 channels by correolide increases the excitability of intramural nerve plexuses promoting release of acetylcholine and tachykinins from excitatory motor neurons. This, in turn, leads to Ca(2+)-dependent action potentials and twitching of the muscle fibres.

Evidence for the involvement of ATP, but not of VIP/PACAP or nitric oxide, in the excitatory effect of capsaicin in the small intestine

The contractile effect of capsaicin in the guinea-pig small intestine involves an activation of enteric cholinergic neurons. Our present data show that the P(2) purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM) significantly reduces the contractile response to capsaicin (2 microM) in the presence, but not in the absence, of the tachykinin receptor antagonists [O-Pro(9), (Spiro-gamma-lactam)Leu(10), Trp(11)]physalaemin (1-11) (GR 82334; 3 microM) and (S)-(N)-(1-(3-(1-benzoyl-3-(3, 4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenylpiperidine-4-yl)-N -methylacetamide (SR 142804: 100 nM) (for blocking tachykinin NK1 and NK3 receptors, respectively). PPADS (30 microM) fails to influence submaximal cholinergic contractions evoked by cholecystokinin octapeptide (CCK-8; 2-3 nM) or senktide (1 nM), or the direct smooth muscle-contracting effect of histamine (100-200 nM). A higher concentration (300 microM) of PPADS is also without effect against the stimulatory action of cholecystokinin octapeptide. This means that PPADS can probably be safely used as a purinoceptor antagonist in intestinal preparations. The putative pituitary adenylate cyclase activating peptide (PACAP) receptor antagonist PACAP-(6-38) (3 microM) significantly reduces the contractile effect of PACAP-(1-38) (10 nM) and abolishes that of vasoactive intestinal polypeptide (VIP; 10 nM). PACAP-(6-38) (3 microM) fails to influence the effect of capsaicin (2 microM) both in the absence and in the presence of tachykinin receptor antagonists. The nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NOARG; 100 microM) also fails to inhibit the capsaicin-induced motor response. We conclude that an endogenous ligand of PPADS-sensitive P(2) purinoceptors (possibly ATP), but not a VIP/PACAP-like peptide or NO, is involved in the nontachykininergic activation of cholinergic neurons in the course of the capsaicin-induced contraction.

Nitric oxide modulates release of noradrenaline in guinea-pig gastric fundus

The interaction between nitric oxide (NO) and the release of [(3)H]noradrenaline ([(3)H]NA) in conditions of non-activated and activated nicotinic receptors in guinea-pig gastric fundus preincubated with [(3)H]NA was studied. Nicotinic receptor agonist, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) (100 microM) significantly increased the resting release of [(3)H]NA. NO-synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA) (100 microM) significantly decreased DMPP-induced release of [(3)H]NA. Field electrical stimulation (FES) (2Hz; 1 ms; 360 st) significantly increased the release of [(3)H]NA above the basal levels. L-NNA significantly decreased the stimulation-evoked release of [(3)H]NA. DMPP increased the stimulation-evoked release of [(3)H]NA, effect which was significantly decreased by L-NNA. The data suggests that endogenous NO increases the release of [(3)H]NA, evoked either by activation of the nicotinic receptors or by electrical stimulation in guinea-pig gastric fundus.

PACAP and nitric oxide inhibit contractions in the proximal intestine of the atlantic cod, Gadus morhua

The possible inhibitory roles of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and nitric oxide in the control of intestinal motility were investigated in the Atlantic cod, Gadus morhua. Circular and longitudinal smooth muscle preparations developed spontaneous contractions that were inhibited by atropine (10(−)(5)mol l(−)(1)). PACAP 27 and PACAP 38 (10(−)(7)mol l(−)(1)) reduced the amplitude of the contractions but did not usually affect the resting tension. In the circular preparations, the mean active force developed (above resting level; +/− s.e.m.) was reduced from 0. 62+/−0.18 mN to 0.03+/−0.03 mN (N=10) by PACAP 27 and from 0.53+/−0. 20 mN to 0.31+/−0.13 mN (N=7) by PACAP 38, while neither cod nor mammalian VIP (10(−)(10)-10(−)(6)mol l(−)(1)) had any effect. In the longitudinal preparations, PACAP 27 reduced the force developed from 1.58+/−0.22 mN to 0.44+/−0.25 mN (N=8) and PACAP 38 reduced it from 1.61+/−0.47 mN to 0.75+/−0.28 mN (N=5). The nitric oxide donor sodium nitroprusside (NaNP) almost abolished the contractions in the circular preparations, reducing the mean force developed from 0. 47+/−0.05 mN to 0.02+/−0.06 mN (10(−)(6)mol l(−)(1); N=9) and 0+/−0. 07 mN (10(−)(5)mol l(−)(1); N=8). In the longitudinal preparations, NaNP reduced the force developed from 2.03+/−0.36 mN to 0.33+/−0.22 mN (10(−)(6)mol l(−)(1); N=8) and 0.19+/−0.30 mN (10(−)(5)mol l(−)(1); N=8). The L-arginine analogue N(G)-nitro-L-arginine methyl ester (L-NAME; 3×10(−)(4)mol l(−)(1)) enhanced the contractions in both circular and longitudinal preparations, increasing the mean force developed from 0.51+/−0.12 mN to 0.94+/−0.21 mN (N=8) and from 1.49+/−0.36 mN to 3.34+/−0.67 mN (N=7), respectively. However, preincubation with L-NAME before a second addition of PACAP 27 (10(−)(7)mol l(−)(1)) did not affect the response to PACAP, neither did preincubation with the guanylate cyclase inhibitor 6-anilinoquinoline-5,8-quinone (LY83583; 10(−)(5)mol l(−)(1)), while the inhibitory response to NaNP (3×10(−)(7)mol l(−)(1)) was abolished by LY83583. The PACAP analogue PACAP 6–27 (3×10(−)(7)mol l(−)(1)) had no effect on the response to either NaNP (3×10(−)(7)mol l(−)(1)) or PACAP 27 (10(−)(8)mol l(−)(1)) in the circular preparations. These findings indicate the presence of both a cholinergic and a nitrergic tonus in the smooth muscle preparations of the cod. Although PACAP and NaNP both inhibit contractions, there is no evidence of any interactions between the two substances. In addition, NaNP, but not PACAP, probably acts via stimulating the production of cyclic GMP. In conclusion, both PACAP and nitric oxide may act as inhibitory transmitters, using distinct signalling pathways, in the control of intestinal motility in the Atlantic cod.

Release of nitric oxide within the coeliac plexus is involved in the organization of a gastroduodenal inhibitory reflex in the rabbit

1. The coeliac plexus can organize a gastroduodenal inhibitory reflex without action potentials. The involvement of the nitric oxide-cGMP pathway in this reflex was investigated in the rabbit on an in vitro preparation of the coeliac plexus connected to the stomach and duodenum. Intraluminal duodenal pressures were measured with water-filled balloons. Gastric distension inhibited duodenal motility, thus characterizing a gastroduodenal inhibitory reflex organized by the coeliac plexus. 2. L-Arginine, superfused at the coeliac plexus level, enhanced this reflex, whereas Nomega-nitro-L-arginine (L-NOARG) or 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (carboxy PTIO) reduced or abolished it. Moreover, diethylamine/nitric oxide complex superfused at the coeliac plexus level inhibited duodenal motility in the absence of gastric distension. 3. The effects of nitric oxide were mediated through the activation of guanylyl cyclase, as 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ) reduced or abolished the gastroduodenal inhibitory reflex, whereas zaprinast enhanced it. Moreover, 8-bromo-cGMP and cGMP, superfused at the coeliac plexus level, inhibited duodenal motility in the absence of gastric distension. 4. On the other hand, when perfused at the visceral level, L-NOARG, propranolol plus phentolamine, and guanethidine did not affect the reflex. Thus, neither nitric oxide nor noradrenaline could be the transmitters released at the muscular level to induce this reflex. 5. Our study demonstrates that the gastroduodenal inhibitory reflex, which is organized by the coeliac plexus without action potentials, is induced by the release within the plexus of nitric oxide acting on the cGMP pathway. These results provide new insights into the control of digestive motility by the prevertebral ganglia.

Nitric oxide (NO) increases acetylcholine release from and inhibits smooth muscle contraction of guinea-pig gastric fundus

Experiments were carried out to investigate the interaction between nitric oxide (NO) and cholinergic neurotransmission in smooth muscle strips of guinea-pig gastric fundus. Electrical field stimulation (2 Hz, 1 ms, 360 shocks) evoked atropine-sensitive contractions. Dimethylphenylpiperazinium (DMPP) (100 microM), a nicotinic receptor agonist, reversed the stimulation-evoked contraction and resulted in relaxation. Nomega-nitro-L-arginine (L-NNA) (100 microM), an NO synthase inhibitor, significantly increased the amplitude of stimulation-evoked contraction and abolished the effect of DMPP. Electrical stimulation increased the release of [3H]acetylcholine ([3H]ACh) from the tissue strips above the basal levels. Neither L-NNA (100 microM) nor DMPP (100 microM) alone influenced the basal release of [3H]ACh. Nomega-nitro-L-arginine (100 microM) decreased the electrical stimulation-evoked release of [3H]ACh. Dimethylphenylpiperazinium increased the stimulation-evoked release of [3H]ACh but had no effect in the presence of L-NNA. It is suggested that in guinea-pig gastric fundus, endogenous NO released in response to field stimulation has an opposite effect at the pre- and postsynaptic sites: it increases the release of ACh from cholinergic nerve terminals but reduces smooth muscle responses to ACh.

NO releases bombesin-like immunoreactivity from enteric synaptosomes by cross-activation of protein kinase A

The effect of nitric oxide (NO) on the release of bombesin-like immunoreactivity (BLI) was examined in synaptosomes of rat small intestine. The NO donor S-nitroso-N-acetylpenicillamine (SNAP; 10(-7) to 10(-4) M) significantly stimulated BLI release. In the presence of the NO scavenger oxyhemoglobin (10(-3) M) or the guanylate cyclase inhibitor ODQ (10(-5) M), SNAP-induced BLI release was antagonized. In addition, SNAP increased the synaptosomal cGMP content and elevation of cGMP levels by zaprinast (3 x 10(-5) M), an inhibitor of the cGMP-specific phosphodiesterase (PDE) type 5, and increased basal and SNAP-induced BLI release. NO-induced BLI release was blocked by Rp-adenosine 3',5'-cyclic monophosphorothioate (3 x 10(-5) M and 10(-4) M), an inhibitor of the cAMP-dependent protein kinase A, whereas KT-5823 (3 x 10(-6) M) and Rp-8-(4-chlorophenylthio)-cGMP (5 x 10(-5) M), inhibitors of the cGMP-dependent protein kinase G, had no effect. Because cGMP inhibits the cAMP-specific PDE3, thereby increasing cAMP levels, the role of PDE3 was investigated. Trequinsin (10(-8) M), a specific blocker of PDE3, stimulated basal BLI release but had no additive effect on NO-induced release, suggesting a similar mechanism of action. These data demonstrate that because of a cross-activation of cAMP-dependent protein kinase A by endogenous cGMP BLI can be released by NO from enteric synaptosomes.

Failure of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) to inhibit soluble guanylyl cyclase in rat ventricular cardiomyocytes

1. The effects of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase (sGC), were investigated in aortic rings and ventricular cardiomyocytes from rats. The production of cyclic GMP was stimulated by NO.-donors or carbachol. Additionally, the effects of ODQ were studied in cytosolic extracts from both tissues in which the cyclic GMP production was stimulated by S-nitroso-N-acetylpenicillamine (SNAP). 2. In endothelium-intact aortic rings, SNAP (100 microM), 2,2'-(hydroxynitrosohydrazino)bis-ethana-mine (DETA NONOate; 100 microM), or carbachol (10 microM) increased cyclic GMP levels about 4 fold. These effects were abolished by ODQ (50 microM). 3. In cardiomyocytes, SNAP (100 microM), DETA NONOate (100 microM), or carbachol (10 microM) increased cyclic GMP levels about 2 fold. These effects were not affected by ODQ (50 microM). 4. In cytosolic extracts from aortic rings and cardiomyocytes, SNAP (100 microM) induced about 50 fold increases in cyclic GMP levels. ODQ (50 microM) reduced these effects by about 50%. 5. In extracts from cardiomyocytes, increases by SNAP (100 microM) of cyclic GMP levels were attenuated by myoglobin dependent on concentration: at 300 microM myoglobin, SNAP (100 microM) increased cyclic GMP levels only 3 fold. Inhibitory effects of ODQ (50 microM) were abolished by 300 microM myoglobin. 6. It is suggested that both NO. and ODQ can bind to myoglobin which, at high concentrations. can diminish their effects on sGC. Such a scavenger function of myoglobin could explain why NO. and ODQ exert only minor effects in cardiomyocytes (with high myoglobin content) but strong effects in aortic tissue (virtually devoid of myoglobin).

Comparison of gamma-aminobutyric acid effects in different parts of the cat ileum

The effects of gamma-aminobutyric acid (GABA) and those of a GABA(A) (muscimol) and a GABA(B) (baclofen) receptor agonists were determined on the spontaneous activity of longitudinally or circularly oriented preparations (segments) isolated from terminal, proximal and distal parts of the cat ileum. GABA applied at 1 microM to 2 mM caused dose-dependent biphasic changes (relaxation and contraction) in spontaneous activity of the longitudinal and circular layers in the terminal and distal parts of the cat ileum and monophasic changes (contraction) in the proximal part. The potency of GABA to elicit relaxant and/or contractile effects in different parts of the ileum showed a proximal-to-terminal increasing pattern. In the longitudinal layer of the distal and terminal ileum, muscimol (100 microM) mimicked the relaxation phase of the GABA effect, while baclofen (100 microM) simulated the contractile phase. Bicuculline, atropine and tetrodotoxin abolished GABA- and muscimol-induced relaxation and suppressed, but failed to prevent GABA- and baclofen-induced contractions. In addition, 2-hydroxysaclofen antagonized the baclofen-induced contractile effect, reduced the GABA-induced contractile phase but failed to prevent GABA- and muscimol-induced relaxation. In the circular layer of the same regions, muscimol mimicked the biphasic GABA effects, while baclofen was without effect. Bicuculline, atropine and tetrodotoxin completely prevented the GABA- and muscimol effects, while 2-hydroxysaclofen failed to antagonize them. In the longitudinal and circular layers of the proximal ileum, muscimol (100 microM) exerted a 'GABA-like' transient contractile effect, while baclofen (100 microM) did not elicit any response. Bicuculline, atropine and tetrodotoxin antagonized the GABA- and muscimol-induced contractile responses of longitudinal and circular layers, while 2-hydroxysaclofen was ineffective. The results suggested that the inhibitory and/or excitatory action of GABA on cholinergic transmission in different regions of cat ileum varies along an increasing gradient towards the terminal ileum and is mediated by GABA(A) and GABA(B) receptors in the terminal and distal ileum and by GABA(A) receptors in the proximal ileum.

Investigation of the interaction between cholinergic and nitrergic neurotransmission in the pig gastric fundus

1. The interaction between the cholinergic and nitrergic innervation was investigated in circular muscle strips of the pig gastric fundus. 2. In physiological salt solution containing 4 x 10(-6) M guanethidine, electrical field stimulation (EFS; 40 V, 0.5 ms, 0.5-32 Hz, 10 s at 4 min intervals) induced small transient relaxations at 0.5-4 Hz, and large frequency-dependent contractions, sometimes followed by off-relaxations, at 8-32 Hz. 3. In the presence of L-NG-nitroarginine methyl ester (L-NAME; 3 x 10(-4) M) or physostigmine (10(-6) M), relaxations were reversed into contractions and contractions were enhanced. Physostigmine added to L-NAME further enhanced contractions, while addition of L-NAME to physostigmine had no additional effect. Off-relaxations were enhanced in the presence of L-NAME and physostigmine. L-NAME and physostigmine consistently increased basal tone. 4. Tissues contracted by 5-hydroxytryptamine or by acetylcholine responded to EFS in a similar way as in basal conditions and L-NAME reversed the relaxations at the lower stimulation frequencies into contractions and enhanced the contractions at the higher stimulation frequencies. 5. Off-relaxations in the presence of L-NAME were partially reduced by alpha-chymotrypsin (10 U ml(-1)). 6. In the absence of physostigmine, the concentration-response curve to exogenous acetylcholine was not influenced by L-NAME. 7. Contractions of the same amplitude induced by EFS at 4 Hz and by exogenous acetylcholine were either decreased or enhanced to the same extent by sodium nitroprusside (SNP; 10(-5) M), depending upon the degree of relaxation by SNP. 8. These experiments suggest that endogenous nitric oxide interferes with cholinergic neurotransmission in the pig gastric fundus by functional antagonism at the postjunctional level. The interaction is independent of the degree of contraction.

Involvement of nitric oxide in extrinsic nervous control of ileal contractile activity

The experiments were carried out on guinea pig mesenteric nerve-ileal preparations (ileal segments with mesenteric nerves originating from the superior mesenteric ganglion) isolated at various distances from the ileocecal junction (ICJ). Contractile activity was recorded in the presence of hexamethonium (50 microM). On the background of electrical field stimulation (EFS; 0.1 Hz, 0.5 ms, supramaximal current intensity)-induced twitch contractions, the mesenteric nerve stimulation (MNS; frequency of 2-30 Hz, 0.5 ms, supramaximal current intensity, 20-s trains) exerted two types of effects, depending on the distance from ICJ at which the preparations were isolated and on the pulse frequency. In preparations isolated from the ileum at a distance of 20 cm from ICJ, MNS at all the frequencies studied inhibited the EFS-induced twitch contractions, reaching the maximum at 30 Hz. In preparations isolated from the terminal ileum at a distance of 10 cm from ICJ, MNS at 20 Hz and 30 Hz decreased the twitch contraction amplitude, whereas MNS at 2-10 Hz produced an increase in the tone on which twitch contractions with reduced amplitude were superimposed. The finding that guanethidine (5 microM) eliminated the MNS twitch inhibition provides evidence for the adrenergic origin of the latter. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine (100 microM) was efficient in reducing the MNS twitch inhibition but only at low-frequency (5 Hz) MNS (p < 0.05). Our results suggest the participation of nitric oxide in the nervous control exerted by the superior mesenteric ganglion over the ileal contractile activity.

Inhibition by nitric oxide and cyclic GMP of 5-hydroxytryptamine release from the vascularly perfused guinea-pig small intestine

The effects of nitric oxide (NO) on the spontaneous release of 5-hydroxytryptamine (5-HT) were studied in the in vitro vascularly perfused guinea-pig small intestine. The NO donor SIN-1 concentration-dependently decreased 5-HT release with an EC50 of 1.34 microM, whereas the NO synthase inhibitor N(G)-nitro-L-arginine (100 microM) was without effect. The inhibition by SIN-1 of 5-HT release was enhanced by superoxide dismutase (150 U/ml) and antagonized by the selective inhibitor of soluble guanylyl cyclase, ODQ (1 microM). Tetrodotoxin (1 microM) prevented the inhibition by SIN-1 of 5-HT release, which suggests that the effect of SIN-1 is indirectly mediated via release of an inhibitory neurotransmitter. Substance P could be excluded as inhibitory transmitter because the effect of SIN-1 remained unchanged in the presence of the NK1 receptor antagonist CP 99994 (100 nM). The cyclic GMP analogue, 8-bromo cyclic GMP (300 microM), also decreased basal release of 5-HT, but this decrease was not tetrodotoxin-sensitive. It is concluded that NO inhibits the release of 5-HT from enterochromaffin cells via release of an enteric neurotransmitter. Acetylcholine (via nicotinic receptors) and substance P (via NK1 receptors) are not involved in the NO-mediated inhibition. The inhibition of 5-HT outflow by NO is due to the activation of soluble guanylyl cyclase. 8-Bromo cyclic GMP inhibited 5-HT release by a direct effect on the enterochromaffin cells.

Nitric oxide-sensitive guanylyl cyclase inhibits acetylcholine release and excitatory motor transmission in the guinea-pig ileum

This study examined the mechanism through which nitric oxide inhibits the release of acetylcholine and excitatory motor neurotransmission in the guinea-pig ileum. The selective inhibitor of nitric oxide-sensitive guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), concentration-dependently enhanced both basal release (-log EC50: 6.8) and electrically (10 Hz)-evoked release (-log EC50: 6.0) of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations preincubated with [3H]choline. The increase by ODQ of basal release appeared to be exocytotic since it was prevented by tetrodotoxin (300 nM) and absence of calcium from the superfusion medium. In addition, ODQ (1 microM) increased the electrically-evoked tachykininergic and cholinergic muscle contractions as measured in the presence of scopolamine (100 nM) or of the neurokinin-1 receptor antagonist CP 99994 (100 nM), respectively. The nitric oxide synthase inhibitor L-N(G)-nitro-arginine (100 microM) behaved similar to ODQ and increased cholinergic and tachykininergic motor neurotransmission. The nitric oxide-independent activator of soluble guanylyl cyclase, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole, concentration-dependently inhibited the electrically evoked acetylcholine release (-log EC50: 6.0) and longitudinal muscle contractions (-log EC50: 5.7). ODQ (10 microM) antagonized the effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole. The results suggest that endogenous nitric oxide tonically activates soluble guanylyl cyclase in myenteric neurons which leads to inhibition of the release of the excitatory transmitters acetylcholine and substance P. ODQ prevents the effects of nitric oxide and thus facilitates cholinergic and tachykininergic motor neurotransmission in the guinea-pig ileum.

Blockade of nitrergic neuroeffector transmission in guinea-pig colon by a selective inhibitor of soluble guanylyl cyclase

The role of soluble guanylyl cyclase in nitrergic inhibitory neuroeffector transmission was investigated in the longitudinal muscle from guinea-pig colon, by using an inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ). In preparations precontracted with histamine, electrical field stimulation (EFS) or exogenous nitric oxide (NO) induced relaxations. The relaxation induced by NO-application was abolished by ODQ. Both ODQ and the NO-synthase inhibitor N omega-nitro-L-arginine (L-NOARG) partially inhibited the EFS-evoked relaxation to a similar extent. These effects were dose-dependent. The inhibition was more pronounced in the late phase of the EFS-induced relaxation. The inhibitory effect of ODQ on EFS-induced relaxation was not affected by additional application of L-NOARG. When NO-formation was blocked by L-NOARG, a subsequent addition of ODQ gave no further inhibition of the relaxation. These findings suggest that inhibitory non-adrenergic, non-cholinergic neurotransmission in guinea-pig colon is dependent on endogenous formation of NO, and that the NO-effect is exclusively mediated via the soluble guanylyl cyclase pathway. The existence of an NO-independent inhibitory transmission, which is not mediated through the cyclic GMP pathway, is also indicated. Furthermore, it is demonstrated that the NO/soluble guanylyl cyclase-independent transmission has an earlier onset as compared with the NO/soluble guanylyl cyclase-dependent pathway.

Activation of soluble guanylyl cyclase by YC-1 in aortic smooth muscle but not in ventricular myocardium from rat

1. The effects of YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole), an activator of soluble guanylyl cyclase, on tension, levels of cyclic GMP and cyclic AMP, and cardiac L-type Ca2+-current (I[Ca(L)]) were investigated in aortic smooth muscle and ventricular heart muscle from rat. 2. YC-1 (0.1-30 microM) induced a concentration-dependent relaxation in aortic rings precontracted with phenylephrine (3 microM). The relaxant effects of YC-1 were reversed by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (30 microM; ODQ), potentiated by zaprinast (10 microM) and antagonized by Rp-8-Br-cGMPS (100 microM). 3. In ventricular heart muscle strips, YC-1 (30 microM) exhibited no effects on force of contraction (Fc) in the absence or presence of either zaprinast (10 microM) or 3-isobutyl-1-methylxanthine (30 microM). Fc was slightly increased by YC-1 (30 microM) in the presence of isoprenaline (100 nM), but this effect was not influenced by ODQ (30 microM). 4. Cardiac I[Ca(L)] was not significantly affected by YC-1 (30 microM), either in the absence or presence of isoprenaline (30 nM). 5. In aortic rings, cyclic GMP levels were increased almost 3 fold by YC-1 (30 microM); this effect was abolished by ODQ (30 microM). In isolated ventricular cardiomyocytes, cyclic GMP levels were not affected by YC-1 (30 microM) but almost doubled by activation of particular guanylyl cyclase with atriopeptin II (100 nM). 6. YC-1 (30 microM) did not increase cyclic AMP levels either in aortic rings or in ventricular cardiomyocytes. In contrast, isoprenaline (3 microM) increased cyclic AMP levels about two fold in both tissues. In cardiomyocytes, the effect of isoprenaline (3 microM) was slightly enhanced by YC-1 (30 microM). 7. It is concluded that relaxation of smooth muscle preparations by YC-1 is mediated mainly by activation of soluble guanylyl cyclase and subsequent increase in cyclic GMP levels. The failure of YC-1 to affect cardiac Fc, levels of cyclic GMP, and I[Ca(L)] suggests that soluble guanylyl cyclase is not influenced by YC-1 in rat heart muscle or only barely present in this tissue.

Blockade of nitric oxide-evoked smooth muscle contractions by an inhibitor of guanylyl cyclase

Nitric oxide-induced contractile responses of smooth muscle were studied in vitro in guinea-pig small intestine. Application of nitric oxide (NO; 0.3-30 microM) evoked a small initial relaxation followed by a marked contractile response in plexus-containing longitudinal smooth muscle preparations from small intestine. The extent of the NO-evoked contractile response was dose-dependent and the response was blocked by tetrodotoxin. Atropine significantly reduced the NO-evoked contraction and the remaining part was abolished by the NK1-receptor antagonist CP 96,345. An inhibitor of soluble guanylyl cyclase, ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one), abolished the NO-evoked contractile response. The results suggest that NO, in addition to the classical direct smooth muscle relaxing effect, causes activation of excitatory neurones, via a pathway utilizing soluble guanylyl cyclase, which leads to a smooth muscle contraction.