The role of the L-arginine-nitric oxide pathway in relaxation of the opossum lower oesophageal sphincter

Neuronal nitric oxide in the gut

Motility of the gastrointestinal tract is directly controlled by enteric inhibitory and excitatory motor neurons that innervate the layers of smooth muscle. Inhibitory motor neurons mediate receptive and accommodative relaxations and control the opening of sphincters, thus playing an important role in normal gut motility. Recent studies have demonstrated that nitric oxide (NO) is an important neurotransmitter released by inhibitory motor neurons in animal and human gut. Antagonists of nitric oxide synthase (NOS), the synthetic enzyme for NO, reduce the effectiveness of transmission from inhibitory motor neurons. Exogenous NO mimics inhibitory nerve activation, and a variety of compounds that affect the availability of endogenously produced NO modulate relaxations of gastrointestinal smooth muscle. It is clear, however, that NO is unlikely to be the only transmitter released by enteric inhibitory motor neurons: several other substances such as vasoactive intestinal polypeptide (VIP), or related peptides, and adenosine triphosphate (ATP) are also likely to contribute to nerve-mediated inhibition. The identification of NO as a major inhibitory neurotransmitter to gastrointestinal smooth muscle fills an important gap in our understanding of the physiological control of motility and opens up a wide range of new experimental possibilities. It may eventually lead to the development of new drugs for motility disorders. It should be noted, however, that NO is important in the brain, in cardiovascular control, in blood cell function and in many other organ systems, suggesting that it may be difficult to achieve specific pharmacological intervention targeted on inhibitory neurotransmission in the gut, without undesirable side effects.

Role of nitric oxide in sympathetic neurotransmission in opossum internal anal sphincter

BACKGROUND

The role of nitric oxide in the gut in response to sympathetic nerve stimulation has not been examined. The present study examined the influence of the NO synthase inhibitor L-NG-nitro-arginine (L-NNA) on responses to hypogastric nerve stimulation (HGNS) in the opossum internal anal sphincter (IAS).

METHODS

Resting pressures in the IAS (IASP) were monitored using low-compliance continuously perfused catheters.

RESULTS

The predominant response to HGNS was an elevation of the resting tone in the IAS. The other responses of infrequent nature were a decrease in the IASP and a biphasic response (an initial increase followed by a decrease in the IASP). beta-Adrenoceptor antagonist propranolol had no significant effect on the increase in the IASP by HGNS but almost abolished the decrease in the IASP caused by HGNS and unmasked the excitatory responses. The IAS responses to HGNS were frequency dependent and abolished by guanethidine (adrenergic blocker). L-NNA caused significant and stereoselective suppression of all IAS responses to HGNS. The suppressed HGNS responses were completely reversed by the NO precursor L-arginine stereoselectively. The NO synthase inhibitor and guanethidine had no effect on the increase in IASP by phenylephrine.

CONCLUSIONS

NO may play a significant role in the facilitatory modulation of sympathetic responses in the IAS.

Tonic inhibition of small intestinal motility by nitric oxide

The effects of blocking nitric oxide synthase with the arginine analog N omega-nitro-L-arginine (L-NNA) were investigated in anaesthetized cats, vagotomized and pretreated with guanethidine and atropine. Spontaneous NANC jejunal motility (recorded as the volume changes of an intraluminal balloon) was markedly increased in a dose-dependent and stereospecific manner. The effect of L-NNA was partly reversed by L-arginine, the substrate for nitric oxide (NO) synthesis. Thus, this study presents evidence for a tonic inhibitory influence, via the release of NO, on small intestinal motility in vivo. Furthermore, relaxations upon the L-NNA-induced hypermotility could be elicited by vagal nerve stimulation, which may suggest the existence of another NANC inhibitory transmitter. Hexamethonium abolished such relaxations but did not affect the tone or phasic activity after L-NNA.

Involvement of nitric oxide in the inhibitory innervation of the human isolated colon

BACKGROUND

The exact nature of the inhibitory nonadrenergic noncholinergic (NANC) neurotransmitter in the human colon is still unknown.

METHODS

The present study was designed to investigate the role of nitric oxide (NO) and adenosine 5'-triphosphate (ATP) in circular muscle strips of the human isolated colon.

RESULTS

NO and ATP induced tetrodoloxin-resistant relaxations that mimicked those evoked by nerve stimulation. Apamin inhibited the response to ATP, had a variable effect on the relaxations to transmural stimulation, and had no effect on those to NO or nitroglycerin. NG-nitro-L-arginine (L-NNA) concentration dependently reduced the NANC nerve-mediated relaxations, but had no effect on those to ATP, NO, or nitroglycerin; the L-NNA resistant part of the NANC relaxation to nerve stimulation was further reduced by apamin. The inhibitory effect of L-NNA or the combination of L-NNA and apamin was prevented by L-arginine but not by D-arginine.

CONCLUSIONS

These results suggest that NO and another substance, perhaps ATP, are involved in the inhibitory NANC neurotransmission in the circular muscle of the human colon.

Projections of nitric oxide synthesizing neurons in the guinea-pig colon

The neuronal form of the enzyme nitric oxide synthase, which is an obligatory constituent of neurons that utilise nitric oxide as a transmitter, was revealed histochemically in this study by its ability to transfer a proton from reduced nicotinamide adenine dinucleotide phosphate to nitro-blue tetrazolium. In the guinea-pig colon, nitric oxide synthase was located in numerous irregularly-shaped myenteric neurons with single axons. In the submucosa, a small number of neurons had strong enzyme activity, whereas many were weakly stained. Nerve fibres were found in the longitudinal muscle, circular muscle, muscularis mucosae and ganglia of the two plexuses. No nerve fibres were found in the lamina propria of the mucosa. The same distribution of nerve cells and fibres was revealed using immunohistochemistry for nitric oxide synthase. Lesion studies showed that the axons of myenteric neurons all projected anally. Myenteric cells were the source of nerve fibres in the circular muscle and in more anally located myenteric ganglia. The sparse innervation of submucous ganglia was intrinsic to the submucous plexus. It is suggested that nitric oxide synthase is one of the transmitters of inhibitory neurons to the muscle and is also utilized by descending interneurons of the myenteric plexus.

Motor effects of indomethacin, morphine or vagal nerve stimulation on the feline small intestine in vivo

Some factors known to affect jejunal motility (recorded as volume changes of an intraluminal balloon) were investigated in anaesthetized cats (ether-chloralose) pretreated with guanethidine and atropine. Indomethacin, morphine (both compounds administered systemically) or vagal nerve stimulation elicited jejunal excitatory motor responses. The effect of indomethacin seemed to be independent of cyclooxygenase inhibition and probably did not involve opioid receptors. It is suggested that the spasmogenic stimuli caused jejunal hypermotility by inhibiting tonically active, inhibitory motor neurons that are intrinsic to the gut. Furthermore, when the jenunal tone had been raised by indomethacin or morphine spontaneous relaxations were observed, and these could be mimicked by vagal stimulation. Hexamethonium antagonized these relaxations but did not attenuate the drug-induced jejunal hypermotility.

A possible role of the L-arginine-nitric oxide pathway in the modulation of cholinergic transmission in the guinea-pig taenia coli

1. The role of the L-arginine-nitric oxide (NO) pathway for non-adrenergic, non-cholinergic (NANC) relaxation of the guinea-pig taenia coli was studied by recording isometric tension in response to transmural field stimulation (TMS). 2. In preparations precontracted with prostaglandin F2 alpha (PGF2 alpha, 10(-6) M), TMS induced frequency-dependent responses of the muscle strips which could be abolished by tetrodotoxin (10(-6) M). NG-nitro-L-arginine (L-NNA, 10(-4) M), an L-arginine analogue, and potent inhibitor of NO synthesis, stereospecifically inhibited maximum relaxations, but did not shift the frequency-response curve. Pre-incubation with NG-nitro-D-arginine (D-NNA, 10(-4) M), atropine (10(-6) M) plus L-NNA (10(-4) M), or atropine (10(-6) M) alone, had no influence on the frequency-response characteristics. 3. L-NNA (10(-7)-10(-4) M) concentration-dependently inhibited relaxations in PGF2 alpha (10(-6) M) precontracted strips in response to TMS, but did not abolish relaxations. Preincubation with L-arginine (10(-4) M) inhibited these effects of L-NNA. L-NNA (10(-4) M) had no effect on the inhibitory response during TMS in strips preincubated with atropine (10(-6) M). 4. The relaxation induced by sodium nitroprusside and forskolin (10(-9)-10(-4) M) was not influenced by L-NNA (10(-4) M) preincubation as expressed by identical pD2 and Emax values. 5. Contractions induced by PGF2 alpha (10(-9)-10(-4) M) and carbachol (10(-9)-10(-4) M) were not affected by pretreatment with L-NNA (10(-4) M), was expressed by identical pD2 and Emax values. 5. Contractions induced by PGFA (10-1- 10-4M) and carbachol (10-1 0-4 M) were not affected by pretreatment with L-NNA (10-4 M), as expressed by identical pD2 and Em. values.6. In conclusion, the L-arginine-NO pathway seems to play a role in the NANC innervation of the guinea-pig taenia coli. The inhibitory effect of NO or a NO-like compound depends on the integrity of the cholinergic pathways and it is proposed that this compound exerts its effects prejunctionally on cholinergic nerves, by inhibiting the release of acetylcholine.

Role of nitric oxide in esophageal peristalsis in the opossum

To explore the involvement of NO in normal peristalsis, the effects of inhibitors of NO synthase, including N omega-nitro-L-arginine (L-NNA) and N omega-nitro-L-arginine methyl ester (L-NAME), on esophageal peristaltic contractions induced by diverse stimuli that may involve different neuronal circuits were studied. Studies were performed in opossums. Experimental conditions in vivo included primary peristalsis (P) induced by pharyngeal stroking, short-train (1 second) electrical stimulation of the vagus nerve which caused peristaltic (S) contractions, and long-train (10 second) electrical stimulation of the vagus nerves which caused contractions at the onset of (A contractions) and after (B contractions) the stimulation period. In vitro experiments were performed on strips of esophageal circular muscle using electrical field stimulation which caused contractions at the onset of (on contractions) and after (off contractions) the stimulation period. The administration of L-NAME significantly decreased the latency period and reduced the latency gradient for P contractions, thereby increasing the velocity of peristalsis. Concomitant administration of atropine prolonged the latency period but did not restore the latency gradient. L-NAME abolished B contractions in a dose-dependent fashion. In vitro, L-NAME caused dose-dependent inhibition of off contractions and augmentation of on contractions. These studies support the hypothesis that NO may be involved in (a) both the latency period and the latency gradient, as well as in the contraction amplitude of esophageal peristalsis; and (b) esophageal B and off contractions.

Nitrergic transmission: nitric oxide as a mediator of non-adrenergic, non-cholinergic neuro-effector transmission

1. The possibility that transmission at some non-adrenergic, non-cholinergic (NANC) neuro-effector junctions is mediated by nitric oxide (NO) arose from the discoveries that NO mediated the effects of nitrovasodilator drugs and that endothelium-derived relaxing factor (EDRF) was NO or a NO-yielding substance. 2. NO donated by nitrovasodilator drugs or formed by endothelial cells activates soluble guanylate cyclase in smooth muscle and the consequent increase in cyclic guanosine monophosphate (cGMP) results in relaxation. The relaxations produced by stimulation of some NANC nerves are also due to a rise in cGMP. 3. The biosynthesis of NO by oxidation of a terminal guanidino nitrogen of L-arginine is inhibited by some NG-substituted analogues of L-arginine. These substances block EDRF formation by NO synthase and endothelium-dependent vasodilatation, and the blockade is overcome by L-arginine 4. NANC relaxations in some tissues are blocked by NG-substituted analogues of L-arginine and restored by L-arginine. Other agents that affect endothelium-dependent vasodilator responses produce corresponding changes in responses to stimulation of these NANC nerves. Such observations indicate that transmission is mediated by NO: we have termed this mode of transmission nitrergic. 5. There is evidence for nitrergic innervation of smooth muscle in the gastrointestinal tract, genito-urinary system, trachea and some blood vessels (penile and cerebral arteries). 6. The recognition of a mediator role for NO in neurotransmission calls for reconsideration of previously accepted generalizations about mechanisms of transmission. 7. Studies on nitrergic transmission will provide new insights into physiological control mechanisms and pathophysiological processes and may lead to new therapeutic developments.

Role of nitric oxide in lower esophageal sphincter relaxation to swallowing

Studies were performed in the opossum to define the role of the L-arginine-nitric oxide (NO) pathway in lower esophageal sphincter (LES) relaxation to swallowing and vagal stimulation in viv and intramural nerve stimulation in vitro. In vivo, L-NAME, a water soluble NO synthase (NOS) inhibitor, caused antagonism of LES relaxation due to reflex-induced swallowing. L-NAME (20 mg/kg i.v.) reduced the amplitude of swallow induced relaxation from 88% to 28%. LES relaxation due to electrical stimulation of peripheral end of decentralized vagus nerve was also antagonized. The effects of L-NAME were reversed by L-arginine, but not by D-arginine. L-NAME treatment did not antagonize LES relaxation to intravenous administration of isoproterenol. In vitro, NO and sodium nitroprusside (SNP) caused a decrease in the sphincter tone. The relaxing effect caused by NO and SNP was not antagonized by tetrodotoxin or omega-conotoxin. Inhibitors of NO synthase, L-NMMA and L-NNA, caused slight increase in the spontaneous resting LES tone and concentration-dependent antagonism of electrical field stimulation (EFS) induced LES relaxation. L-NNA (10(-4)M) abolished EFS induced LES relaxation at low frequencies (less than 5 Hz) and antagonized the relaxation to a value 20% of the control at 20 Hz. The antagonistic action of L-NMMA and L-NNA was unaffected by D-arginine but was reversed by L-arginine. The inhibitory effect of NO, SNP, or two other putative inhibitory neurotransmitters (VIP and CGRP) on the LES was not antagonized by L-NNA. These studies show that inhibitors of NO synthase selectively antagonize LES relaxation to all three modes of intramural inhibitory nerve stimulation including physiological swallowing. These studies suggest that the L-arginine-nitric oxide pathway is involved in physiological relaxation of the LES.