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

Protein kinase A facilitates relaxation of mouse ileum via phosphorylation of neuronal nitric oxide synthase

BACKGROUND AND PURPOSE

The enteric neurotransmitter nitric oxide (NO) regulates gastrointestinal motility by relaxing smooth muscle. Pharmacological cAMP induction also relaxes gastrointestinal smooth muscle, but it is uncertain whether cAMP augments or suppresses enteric NO signalling. In other organ systems, cAMP can increase neuronal NO production by stimulating protein kinase A (PKA) to phosphorylate neuronal NOS (nNOS) Serine-1412 (S1412). We hypothesized that cAMP also increases nNOS S1412 phosphorylation by PKA in enteric neurons to augment nitrergic relaxation of mouse ileum.

EXPERIMENTAL APPROACH

We measured contractile force and nNOS S1412 phosphorylation in ileal rings suspended in an organ bath. We used forskolin to induce cAMP-dependent relaxation of wild type, nNOS^S1412A knock-in and nNOSα-null ileal rings in the presence or absence of PKA, protein kinase B (Akt) and NOS inhibitors.

KEY RESULTS

Forskolin stimulated phosphorylation of nNOS S1412 in mouse ileum. Forskolin relaxed nNOSα-null and nNOS^S1412A ileal rings less than wild-type ileal rings. PKA inhibition blocked forskolin-induced nNOS phosphorylation and attenuated relaxation of wild type but not nNOS^S1412A ileum. Akt inhibition did not alter nNOS phosphorylation with forskolin but did attenuate relaxation of wild type and nNOS^S1412A . NOS inhibition with L-NAME eliminated the effects of PKA and Akt inhibitors on relaxation.

CONCLUSION AND IMPLICATIONS

PKA phosphorylation of nNOS S1412 augments forskolin-induced nitrergic ileal relaxation. The relationship between cAMP/PKA and NO is therefore synergistic in enteric nitrergic neurons. Because NO regulates gut motility, selective modulation of enteric neuronal cAMP synthesis may be useful for the treatment of gastrointestinal motility disorders.

Contribution of Rho-kinase to membrane excitability of murine colonic smooth muscle

BACKGROUND AND PURPOSE

The Rho-kinase pathway regulates agonist-induced contractions in several smooth muscles, including the intestine, urinary bladder and uterus, via dynamic changes in the Ca(2+) sensitivity of the contractile apparatus. However, there is evidence that Rho-kinase also modulates other cellular effectors such as ion channels.

EXPERIMENTAL APPROACH

We examined the regulation of colonic smooth muscle excitability by Rho-kinase using conventional microelectrode recording, isometric force measurements and patch-clamp techniques.

KEY RESULTS

The Rho-kinase inhibitors, Y-27632 and H-1152, decreased nerve-evoked on- and off-contractions elicited at a range of frequencies and durations. The Rho-kinase inhibitors decreased the spontaneous contractions and the responses to carbachol and substance P independently of neuronal inputs, suggesting Y-27632 acts directly on smooth muscle. The Rho-kinase inhibitors significantly reduced the depolarization in response to carbachol, an effect that cannot be due to regulation of Ca(2+) sensitization. Patch-clamp experiments showed that Rho-kinase inhibitors reduce GTPγS-activated non-selective cation currents.

CONCLUSIONS AND IMPLICATIONS

The Rho-kinase inhibitors decreased contractions evoked by nerve stimulation, carbachol and substance P. These effects were not solely due to inhibition of the Ca(2+) sensitization pathway, as the Rho-kinase inhibitors also inhibited the non-selective cation conductances activated by excitatory transmitters. Thus, Rho-kinase may regulate smooth muscle excitability mechanisms by regulating non-selective cation channels as well as changing the Ca(2+) sensitivity of the contractile apparatus.

Nitric oxide-sensitive guanylyl cyclase is dispensable for nitrergic signaling and gut motility in mouse intestinal smooth muscle

BACKGROUND & AIMS

The nitric oxide-guanosine 3',5'-cyclic monophosphate (cGMP) signaling pathway has an important role in the control of smooth muscle tone. NO is produced by NO synthases and acts as a major inhibitory neurotransmitter in the gastrointestinal (GI) tract. The main target, NO-sensitive guanylyl cyclase (NO-GC), is stimulated by NO to produce the intracellular messenger cGMP. We investigated the role of NO-GC in nitrergic relaxation and GI motility.

METHODS

We tested relaxation of GI smooth muscle in mice that do not express NO-GC or mice with disruption of NO-GC specifically in smooth muscle cells. Different segments of the GI tract (fundus, lower esophageal sphincter, pyloric sphincter, and duodenum) were used in isometric force studies. NO donors and electrical field stimulation were used to assess nitrergic signaling. Whole-gut transit time was measured as an indicator of GI motility.

RESULTS

Mice that lack NO-GC do not have NO-induced relaxation of GI smooth muscle. Gut transit time was increased, resulting in GI dysfunction. Surprisingly, in mice that lack NO-GC specifically in smooth muscle, NO-induced relaxation was reduced only slightly, and whole-gut transit time was unchanged compared with wild-type mice.

CONCLUSIONS

Lack of NO-GC in smooth muscle cells does not impair NO-induced relaxation of GI tissues or GI motility. The NO receptor guanylyl cyclase in GI smooth muscle is therefore dispensable for nitrergic signaling in mice.

Intestinal nitric oxide synthase activity changes during experimental colon obstruction

OBJECTIVE

The experiments in this study were designed to follow the time course of nitric oxide (NO) synthesis in the large bowel during acute mechanical ileus.

MATERIAL AND METHODS

Occlusion of the mid-transverse colon was maintained for 420 min in anesthetized dogs. Strain-gauge transducers were used to analyze motility changes on the hepatic and lienal flexures, respectively. Constitutive NO synthase (cNOS) and inducible NOS (iNOS) activities were determined in tissue biopsies, and plasma nitrite/nitrate (NOx) level was measured in the portal blood. Following completion of the baseline studies, the animals were treated with either 7-nitroindazole (7-NI, selective neuronal NOS inhibitor), or N-nitro-L-arginine (NNA, non-selective NOS inhibitor).

RESULTS

In the sham-operated group the cNOS activities differed significantly in the oral and aboral tissue samples (oral: 102.9; versus aboral: 62.1 fmol/mg protein/min). The obstruction elicited a significant increase in portal NOx and elevated tissue inducible NO synthase (iNOS) activity. NNA treatment decreased the motility index in both intestinal segments for 60 min, but 120 min later the motility index was significantly elevated (2.5-fold increase in the oral part, and 1.8-fold enhancement in the aboral segment, respectively). Treatment with 7-NI decreased the cNOS activity in the oral and aboral parts by approximately 40% and 70%, respectively, and suppressed the motility increase in the aboral colon segment.

CONCLUSIONS

The motility of the colon was either significantly increased or decreased, depending on the type and selectivity of the NOS inhibitor compounds applied. NO of neuronal origin is a transmitter that stimulates peristaltic activity; but an increased iNOS/nNOS ratio significantly moderates the obstruction-induced motility increase.

Mechanisms involved in the loss of excitatory post-stimulus responses by inflammation

AIM

Electrical stimulation of colonic muscles elicits a response during the stimulation period, and a transient excitation after the stimulus. Post-stimulus or "rebound" excitation has been linked to pathways involving inhibitory neurotransmitters, prostaglandins and substance P but the mechanism is incompletely understood. Because rabbit colitis is characterized by a loss of inhibitory neurotransmission we hypothesized it might affect the rebound response. Therefore we characterized rebound responses in non-inflamed and inflamed tissue by comparing the effect of antagonists/blockers of putative (nitric oxide [NO], ATP, substance P, prostaglandins) and new (serotonin) neurotransmitters.

METHODS

Strips from rabbits with colitis induced by 2,4,6-trinitrobenzenesulfonic acid (TNBS) were subjected to electrical field stimulation. Because rebound responses are more prominent under nonadrenergic noncholinergic (NANC) conditions, the effect of specific antagonists (N(omega)-nitro-L-arginine methyl ester (L-NAME), indomethacin, SR140333, methiothepin) on the rebound response was compared under normal and NANC conditions.

RESULTS

NANC-conditions increased rebound responses in non-inflamed strips, but this effect was reduced or abolished in inflamed strips. Rebound responses were reduced by pretreatment with the NO-synthase inhibitor, L-NAME, under NANC conditions in non-inflamed strips but not affected in inflamed tissue. In contrast, the P(2) purine receptor antagonist, suramin, did not affect rebound responses in inflamed and non-inflamed strips. The effect of the cyclo-oxygenase inhibitor (COX), indomethacin, on rebound responses was reversed from excitatory to inhibitory by inflammation. Under NANC conditions rebound contractions were also reduced by the neurokinin-1 (NK(1)) antagonist, SR140333, both in normal and inflamed strips. The most pronounced reduction in rebound responses in inflamed and non-inflamed strips under normal conditions was observed with the 5-hydroxytryptamin (1,2) (5-HT(1,2)) antagonist, methiothepin.

CONCLUSION

Rebound responses are mainly non-cholinergic and involve NO, substance P, serotonin and inhibitory prostaglandins. In inflamed tissue the nitrergic pathway is absent, excitatory prostaglandins prevail and the cholinergic and tachykinergic components are relatively more important. However there remains an important serotonergic contribution. Our data suggest that inflammation damages different neural pathways to a different extent and is most selective for nitrergic pathways.

Excitatory motor innervation in the canine rectoanal region: role of changing receptor populations

1. Motor innervation in the canine rectoanal region was examined in isolated strips of the circular muscle layer. Contractile responses to electrical field stimulation began at lower frequencies and were more persistent in the internal anal sphincter (IAS) than in the rectum. 2. Motor innervation to the IAS was almost exclusively sympathetic, since it was blocked by guanethidine (Guan 3 microM) while the response in the proximal rectum was approximately 50% muscarinic, and sensitive to the M(3) selective antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 0.1 microM) and 50% tachykinergic, and sensitive to the neurokinin 2 (NK(2)) receptor antagonist GR 94800 (1 microM). From IAS to rectum there was a gradual shift in the relative contribution of intrinsic and extrinsic neural innervation. 3. Responses to exogenously applied transmitters exhibited a similar pattern to that observed with motor innervation. Norepinephrine (NE) was most potent in the IAS and acetylcholine (ACh) and NK-A were most potent in the proximal rectum. The responses were inhibited by prazosin, 4-DAMP and GR 94800 respectively. 4. A gradient in the density of adrenergic alpha(1), muscarinic and NK(2) receptors also existed from IAS to rectum as determined by measuring the binding of [(3)H]-prazosin, [(3)H]-quinuclidinyl benzilate ([(3)H]-QNB and [(3)H]-SR-48968 to smooth muscle membranes. 5. In summary, these data suggest that the shift in motor innervation in the rectoanal region is achieved in part by changes in receptor populations available for activation by sympathetic and enteric motor neurons.

Intestinal motor stimulation by the 5-HT4 receptor agonist ML10302: differential involvement of tachykininergic pathways in the canine small bowel and colon

5-Hydroxytryptamine (5-HT)4 receptor agonists stimulate gut motility through cholinergic pathways, although there are data suggesting that noncholinergic (tachykininergic) excitatory pathways may also be involved. Differences may exist between the small bowel and colon. Our aims were: (i) to compare the prokinetic effect exerted by the 5-HT4 receptor agonist ML10302 in the canine small bowel and colon in vivo; and (ii) to investigate the role of tachykininergic pathways in mediating this response. In fasting, conscious dogs, chronically fitted with electrodes and strain-gauge force transducers along the small bowel and colon, intravenous injection of ML10302 (35 microg kg-1) immediately stimulated spike activity and significantly increased propagated myoelectrical events at both intestinal levels. In the small bowel, the effects of ML10302 were unchanged by previous administration of the selective NK1 tachykinin receptor antagonist SR140333, the NK2 tachykinin receptor antagonist SR48968, or the NK3 tachykinin receptor antagonist SR142801. In the colon, all tachykinin receptor antagonists significantly inhibited stimulation of spike and mechanical activity by ML10302, without affecting ML10302-induced propagated myoelectrical events. Atropine (100 microg kg-1 i.v.) suppressed the stimulatory effect of ML10302 at both intestinal levels. In conclusion, the 5-HT4 receptor agonist ML10302 induced significant prokinesia both in the small bowel and colon through activation of cholinergic pathways. Tachykininergic pathways are not involved in the ML10302-induced prokinesia in the small bowel, but they play an important role in mediating the colonic motor response to ML10302.

Nitric oxide and neuronal and pancreatic beta cell death

Neural cells are found in all organs of the body and play an important role in the maintenance of the internal milieu. The pancreatic beta cell is the most numerous cell types in the endocrine pancreas. It is particularly important because of its role in insulin secretion, a crucial hormone in glucose metabolism. In view of this, the significance of the survival of neural and pancreatic beta cell cannot be over emphasised. Neural and pancreatic beta cell death occurs in a variety of ways. The destruction of neural cells can be induced with (1) free radicals (H(2)O(2), O(2)(-)(,) HO(-)) and nitric oxide; (2) Cytokines (tumour necrosis factor, interleukin-1 beta, interferon-gamma); (3) Glutamate; (4) Amphetamine analog (Ecstasy); (5) S100 protein; (6) Ammonia; (7) Iron ions; (8) Resins, e.g. methylmethycrylate. Pancreatic beta cell can be destroyed by (1) free radicals (H(2)O(2), O(2)(-)(,) HO(-)) and nitric oxide; (2) Cytokines (tumour necrosis factor, interleukin-1 beta, interferon-gamma); (3) alkylating agents (streptozotocin, alloxan, N-methyl-nitrosourea N-ethyl-N-nitrosourea, Methylmethanesulphonate and ethylmethanesulphonate); (4) hyperglycaemia; (5) islet amyloid poplypeptide; and (6) Inositol Monophosphate dehydrogenase inhibitors. There is enough evidence that most of these agents involved in neural and pancreatic beta cell death exert their toxic effects through the nitric oxide pathway. Neuroprotective agents include vitamin B12 analogs and alpha-tocopherol, NOS inhibitors, antioxidants (e.g. glutathione, superoxide dismutase), metals like cobalt, neurotrophic receptors (Akt kinase) and growth factors. The pancreatic beta cell death induced by these toxic agents can be prevented and or delayed by nicotinamide (vitamin B3), heat shock, copper, alpha-tocopherol (vitamin E), succinic acid, dihydroxylipoic acid, fusidic acid, glucocorticoids, cyclosporin A, growth factors and gene therapy.

Arginosuccinate synthetase, arginosuccinate lyase and NOS in canine gastrointestinal tract: immunocytochemical studies

Nitric oxide synthase (NOS) requires the substrate L-arginine for NO production to support multiple gastrointestinal functions. We asked, 'Where do enzymes to regenerate L-arginine from L-citrulline exist?'. We examined loci of immunoreactivities in the canine gastrointestinal tract for arginosuccinate synthetase and arginosuccinate lyase, enzymes that resynthesize L-arginine from L-citrulline, in relation to the distribution of nNOS immunoreactivity or NADPH-diaphorase histochemistry. Arginosuccinate synthetase and lyase were present in many neurones and nerve fibres in the myenteric plexus of the lower oesophageal sphincter (LOS), antrum, pylorus, ileum and colon; in the submucosal plexus of ileum and colon; in longitudinal muscle of ileum and colon; and in nerve bundles in circular muscle everywhere. LOS muscle was also immunoreactive for both enzymes. Circular and longitudinal muscle cells of the ileum and colon and cells resembling interstitial cells of Cajal in the deep muscular plexus of the ileum and the submuscular plexus of the colon also appeared immunoreactive. In neurones, arginosuccinate synthetase and nNOS were usually co-localized. NADPH diaphorase activity was present in LOS and likely in pylorus, but not in muscularis externa of ileum or colon. We conclude that resynthesis of L-arginine probably occurs in enteric nerves, interstitial cells of Cajal (ICC) and LOS muscle; also apparently in some cells without NOS to utilize it.

Rebound excitation and alternating slow wave patterns depend upon eicosanoid production in canine proximal colon

1. We tested the hypothesis that eicosanoid production could be related to the long-duration slow waves that occur after brief periods of inhibitory neurotransmission (rebound excitation) and the alternating patterns of long- and short-duration slow waves observed in the canine proximal colon. 2. Electrical field stimulation of colonic muscles inhibited slow waves during the stimulus and a long-duration slow wave occurred after the stimulus. Indomethacin reduced the post-stimulus response without affecting the inhibitory response. 3. ATP or 2-methylthio-ATP produced post-stimulus rebound responses similar to the response to field stimulation. Indomethacin inhibited the rebound response caused by ATP or 2-methylthio-ATP. 4. Alternating patterns consisting of long- and short-duration slow waves occurred spontaneously in some colonic muscles. These patterns could also be induced with acetylcholine. 5. Indomethacin, acetylsalicylic acid and ibuprofen abolished the alternating pattern and shifted the bimodal distribution of slow wave durations toward an intermediate duration. 6. Patch clamp experiments on isolated colonic myocytes showed that indomethacin blocked L-type Ca2+ currents. The effects of indomethacin on rebound excitation and alternating slow waves were accomplished at concentrations that blocked cyclooxygenase activity without significantly inhibiting L-type Ca2+ currents. 7. The results demonstrate that rebound excitation and alternating slow wave patterns in the canine colon have similar dependence on endogenous eicosanoid production. Rebound excitation may result from reduced production of an inhibitory eicosanoid during inhibitory nerve stimulation, and the alternating pattern may result from oscillations in eicosanoid production as a function of changes in cytoplasmic Ca2+ during long and short slow waves.

Effects of a novel guanylate cyclase inhibitor on nitric oxide-dependent inhibitory neurotransmission in canine proximal colon

1. Previous studies suggested that nitric oxide (NO) may cause hyperpolarization and relaxation of canine colonic smooth muscle by both cGMP-dependent and cGMP-independent mechanisms. This hypothesis was tested using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ), a novel inhibitor of NO-stimulated guanylate cyclase. 2. In the presence of histamine (30 microM), atropine and indomethacin (both at 1 microM), electrical field stimulation of intrinsic neurons (EFS; 5 Hz) produced inhibition of phasic contractile activity that is due to NO synthesis. ODQ caused a concentration-dependent block of this response (10 nM to 10 microM). 3. Inhibitory junction potentials (IJPs) due to NO synthesis were recorded from muscle cells located near the myenteric border of the circular muscle layer, using intracellular microelectrodes. IJPs were abolished by ODQ (1-10 microM). 4. EFS (10-20 Hz) produced frequency-dependent inhibition of electrical slow waves recorded from cells located near the submucosal surface of the circular muscle layer. This inhibition is due to NO synthesis, and it was abolished by ODQ (1-10 microM). 5. Hyperpolarization and relaxation produced by an NO donor, sodium nitroprusside, were abolished by ODQ pretreatment (1-10 microM). In contrast, inhibitory responses to 8-Br-cGMP (1 mM) were unaffected by ODQ. 6. ODQ alone (1-10 microM) had no significant effect on spontaneous electrical or phasic contractile activity. In tissues pre-treated with L-NAME (300 microM), ODQ decreased the amplitude of spontaneous or histamine-stimulated phasic contractile activity. 7. These results suggest that electrical and mechanical effects of endogenously released and exogenously applied NO in canine colon are largely due to cGMP synthesis by ODQ-sensitive soluble guanylate cyclase. No evidence to support a direct (cGMP-independent) mechanism of NO action was found. ODQ also appears to cause a non-specific inhibition of muscle contractile activity; however, this effect does not contribute to block of NO-dependent effects.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Tachykinin receptors mediate atropine-resistant rat duodenal reflex contractions in vivo

The study aimed to establish the possible role of tachykinins as mediators of atropine-resistant reflex contractions evoked by balloon distension in the proximal duodenum of urethane-anesthetized, guanethidine (34 mumol/kg s.c.)-pretreated rats. Distension of the balloon with a small amount (0.2-0.3 ml) of saline induced the appearance of phasic rhythmic contractions (about 11 mmHg in amplitude) which were promptly suppressed by either atropine (3 mumol/kg i.v.) or hexamethonium (28 mumol/kg i.v.). Despite the continuous i.v. infusion of atropine (2 mumol/h), low-amplitude rhythmic phasic contractions recovered, which were promptly suppressed by hexamethonium, to indicate the involvement of an atropine-resistant excitatory reflex. The amplitude of these atropine-resistant contractions was increased to about 4-5 mmHg by further distension of the balloon (0.4-0.6 ml) : under these conditions, the atropine-resistant contractions undergo a progressive fading. The fading was prevented by i.v. administration of the nitric oxide (NO) synthase inhibitor, L-nitroarginine methyl ester (L-NAME, 55 mumol/h), to provide a suitable baseline (amplitude of contractions was 7-8 mmHg) for studying the effect of tachykinin receptor antagonists. I.v. administration of the selective tachykinin NK2 receptor antagonists, MEN 10,627 (10-100 nmol/kg) and SR 48968 (100-300 nmol/kg) or of the selective NK1 antagonist SR 140333 (100 nmol/kg), at doses which do not affect the duodenal contractions induced by acetylcholine (5.5 mumol/kg i.v.), produced a prompt and long lasting suppression of the atropine-resistant reflex duodenal contractions produced by balloon distension in urethane-anesthetized rats, whilst SR-48965 (300 nmol/kg), the enantiomer of SR-48968 devoid, of NK2 receptor blocking activity, was without effect. I.v. administration of the selective NK1 receptor agonists [Sar9] substance P sulfone and septide or of the NK2 receptor selective agonist, [beta Ala8] neurokinin A(4-10) produced dose-dependent contractions of the duodenum. SR 140333 (100 nmol/kg i.v.) selectively antagonized the duodenal contractions produced by [Sar9] substance P sulfone and septide without affecting those produced by [beta Ala8] neurokinin A(4-10). On the other hand, MEN 10,627 (30-100 nmol/kg i.v.) and SR 48968 (100-300 nmol/kg i.v.) but not SR 48965 (300 nmol/kg i.v.) antagonized, at a comparable extent, duodenal contractions induced by both the selective NK2 and NK1 receptor agonists. We conclude that endogenous tachykinins are involved in mediating atropine-resistant reflex contractions evoked by distension of the rat duodenum in vivo: both NK1 and NK2 receptors are activated by endogenous ligands to produce NANC contractions of rat duodenum in vivo. However, the contractile response to i.v. administered NK1 receptor agonists, [Sar9] substance P sulfone and septide, may involve the release of mediators producing smooth muscle contraction via NK2 receptors.

Cholinergic-nitrergic interactions in the guinea-pig gastric fundus

The influence of nitric oxide (NO) on the spontaneous tone and on the contractile responses to electrical field stimulation or to exogenous acetylcholine (ACh) was studied. Circular strips from the guinea-pig gastric fundus were used. The NO-releasing compound sodium nitroprusside reduced the spontaneous tone while the NO-synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) increased it. The L-NAME-induced increase of the tone was antagonized by atropine or indomethacin, suggesting the involvement of cholinergic and prostaglandinergic pathways in this effect. L-NAME significantly potentiated the ACh (10(-8) to 10(-5) M)-induced contractions. L-NAME concentration-dependently potentiated the cholinergic contractions evoked by electrical field stimulation without affecting [3H]ACh overflow from [3H]choline-treated tissues. It is concluded that electrical field stimulation of gastric fundus muscle induces the release of endogenous nitrate which, in turn, functionally antagonizes cholinergic neurotransmission.

Modulation by nitric oxide of spontaneous motility of the rat isolated duodenum: role of tachykinins

1. Incubation of proximal segments of the rat isolated duodenum with NG-nitro-L-arginine (L-NOARG; 3-100 microM) produced a concentration-dependent increase in both resting tone and the amplitude of the spontaneous contractions. These effects were attenuated by concurrent incubation with L-arginine (1 mM) but not D-arginine (1 mM). 2. These changes in resting tone and motility induced by L-NOARG (30 microM) were substantially reduced by concurrent incubation with tetrodotoxin (1 microM) or hexamethonium (10 microM), implicating the involvement of a local neuronal response. 3. The L-NOARG-induced increase in duodenal motility was not, however, inhibited by atropine (1 microM), guanethidine (6.4 microM) phentolamine (1 microM), or indomethacin (10 microM), indicating a non-cholinergic, non-adrenergic and non-prostanoid-mediated contractile response. 4. The NK1/NK2 tachykinin receptor antagonist, (D-Pro2, D-Trp7.9 substance P, 1-10 microM), and the NK2-receptor antagonists, MEN 10,207 and MEN 10,376 (1-5 microM), concentration-dependently reduced the effect of L-NOARG (30 microM) on spontaneous duodenal motility. 5. The resting tone and amplitude of the spontaneous contractions was likewise increased by incubation with NG-monomethyl-L-arginine (L-NMMA; 100-1000 microM). However, incubation with L-NMMA (100 microM) attenuated the actions of more potent L-NOARG (30 microM) on resting motility. 6. Administration of E.coli endotoxin (3 mg kg-1, i.v.) to the rat 5 h prior to tissue removal, at a time of known induction of NO synthase, reduced the amplitude of spontaneous contractions of the isolated duodenum, an effect inhibited by pretreatment of the rats with dexamethasone (1 mg kg-1) 2 h prior to endotoxin challenge. 7. These findings indicate a role of endogenous NO in the modulation of spontaneous tone and motility in the rat duodenum. Induction of NO synthase may result in a reduction in spontaneous motility of the tissue. By contrast, inhibition of constitutive NO biosynthesis unmasks a contractile response that is neuronally mediated and involves tachykinin NK2 receptors.

Detection of nitric oxide release from canine enteric neurons

Previous pharmacological and immunohistochemical studies have suggested that nitric oxide (NO) is a mediator of enteric neuromuscular transmission in the canine proximal colon. The present study demonstrates the release of nitric oxide (NO) and/or its oxidation products (collectively termed NOx) following stimulation of intrinsic neurons with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP). Strips of muscularis externa, including the ganglionated myenteric and submucosal plexuses were suspended under tension in modified Krebs solution. DMPP (1-100 microM) produced concentration-dependent inhibition of circular muscle contractile activity and this effect was antagonized by L-nitroarginine methyl ester or L-nitroarginine (100 microM). Tetrodotoxin (TTX, 1 microM) significantly reduced mechanical responses to DMPP (10 microM) but had no effect on responses to high concentrations of DMPP (100 microM). Aliquots of the bathing medium were assayed for NOx after regeneration of NO from NO2- and/or NO3-. NO was measured as chemiluminescence produced by reaction with ozone. Detection was linear over the range 6-25,000 pmol of added NO2- or NO3-. In the absence of drugs, a basal release of 1113.5 +/- 100.4 pmol NOx/g tissue (n = 27) was detected after a 30-min incubation period. DMPP (1-100 microM) stimulated a concentration-dependent increase in NOx to 5099.9 +/- 430 pmol/g per 30 min (n = 17) at 100 microM. NOx release was inhibited by an inhibitor of nitric oxide synthase activity (N(G)-monomethyl-L-arginine) or by reduction in extracellular Ca2+ concentration. DMPP-stimulated NOx accumulation was significantly reduced, but not abolished by TTX (1 microM). These results provide further evidence that NO is released following stimulation of intrinsic neurons in canine proximal colon and support previous suggestions that nicotinic agonists may directly stimulate terminals of NO-releasing neurons.

Recycling of L-citrulline to sustain nitric oxide-dependent enteric neurotransmission

Neurons that synthesize nitric oxide from arginine produce stoichiometric amounts of citrulline. We investigated whether nitric oxide-releasing enteric neurons have the capacity to recycle citrulline to arginine and thereby sustain nitrergic neurotransmission. Argininosuccinate synthetase-like immunoreactivity and argininosuccinate lyase-like immunoreactivity, enzymes capable of citrulline to arginine conversion, were both localized in discrete populations of myenteric and submucosal neurons in the canine proximal colon. Argininosuccinate synthetase-like immunoreactivity and argininosuccinate lyase-like immunoreactivity co-localized with neuronal beta-nicotinamide adenine dinucleotide phosphate diaphorase staining, a marker for nitric oxide synthase. The functional significance of argininosuccinate synthetase-like immunoreactivity and argininosuccinate lyase-like immunoreactivity was shown by testing the effects of exogenous citrulline on responses to enteric inhibitory nerve stimulation, which were assessed by measuring contractions, inhibitory junction potentials and electrical slow waves. As shown previously, arginine analogues (L-nitroarginine methyl ester or L-nitroarginine; 100 microM) inhibited nitric oxide-dependent responses, and excess L-arginine restored inhibitory responses. Citrulline alone (0.1-2 mM) had no effect on nitrergic transmission under control conditions, but in the presence of L-nitroarginine methyl ester or L-nitroarginine, citrulline (0.1-2 mM) restored nitrergic transmission in a concentration-dependent manner. Other neutral amino acids (L-serine, L-leucine) did not mimic the effects of citrulline. Taken together, these data suggest that enteric nitrergic neurons have the enzymatic apparatus and functional capability of recycling citrulline to arginine.

Effects of nitric oxide on human and canine prostates

OBJECTIVES

To determine whether nitric oxide (NO) is a mediator of prostatic smooth muscle activity.

METHODS

Pharmacologic experiments using electrical field stimulation (EFS) were performed on strips of human and canine prostate.

RESULTS

EFS alone elicited frequency-dependent contractions in preparations of human and canine prostates. The greatest contractile activity was achieved at 30 Hz. In the presence of 10(-5) M guanethidine (GUA) and 2 x 10(-6) M atropine (ATR), EFS elicited relaxation of canine prostate strips relative to baseline tension. A weak biphasic response consisting of initial relaxation and subsequent contraction relative to baseline tension was observed in the human prostate strips exposed to similar conditions. The smooth muscle activity observed in the presence of GUA plus ATR was attributed to nonadrenergic, noncholinergic (NANC) nerve transmission. 10(-4) M L-NG-nitroarginine methylester (NAME) significantly increased EFS-elicited NANC smooth muscle activity both in human and canine prostates. L-arginine, 10(-2) M, reversed the effect of L-NAME in human and canine prostates. Sodium nitroprusside, 10(-4) M, a donor of NO, caused relaxation of both human and canine prostates. The mean magnitude of the relaxant response/cross-sectional area in human prostate (2.64 +/- 0.4 g/cm2) was significantly greater than in the canine prostate (1.09 +/- 0.17 g/cm2) (P < 0.005).

CONCLUSIONS

These results provide compelling evidence that NO plays a role in mediating contractile function of human and canine prostates.

Sensitization of the contractile system of canine colonic smooth muscle by agonists and phorbol ester

1. Sensitization of the contractile system in response to combinations of excitatory agonists acetylcholine (ACh), methacholine, histamine and neurokinin A (NKA) was investigated in colonic circular smooth muscle of dog, NKA (1 nM) potentiated the contractile response to 1 microM ACh, but did not increase the fura-2 fluorescence ratio (R340/380). Contraction in response to low concentrations of either methacholine or histamine was potentiated significantly by 0.1 microM 4-phorbol 12,13-dibutyrate (PDBu), suggesting that activation of protein kinase C can potentiate contraction at threshold concentrations of agonists. 2. Variability in the sensitivity of the contractile system to Ca2+ was demonstrated over a range of agonist concentrations. KCl, ACh, histamine and NKA each produced a concentration-dependent increase in the amplitude of phasic contractions and R340/380. However, ACh, histamine and NKA each induced maximal increases in R340/380 at concentrations less than that needed to induce maximum force. 3. In depolarized muscles, NKA (50 nM) and PDBu (1 microM) each increased the magnitude of tonic contraction with no change or a decrease in both R340/380 and myosin light chain phosphorylation. In alpha-toxin-permeabilized fibres, 0.1 microM PDBu and 1 microM NKA shifted the Ca(2+)-force response to the left. Ca(2+)-induced contractions were also potentiated by 100 microM GTP-gamma-S or 1 microM NKA plus 10 microM GTP. Potentiation of contraction by NKA and GTP was antagonized by 10 microM GDP-beta-S. 4. The results suggest that endogenous agonists acting via G-proteins sensitize the contractile element of colonic smooth muscle in part by activation of protein kinase C. In some cases, sensitization may be secondary to increased myosin phosphorylation (ACh), but in other cases it appears to be independent of increased myosin light chain phosphorylation (NKA and PDBu). Therefore regulatory mechanisms in addition to myosin phosphorylation contribute to the apparent sensitization of the contractile system to Ca2+.

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

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