Modulation of autonomic neuroeffector transmission by nitric oxide in guinea pig ileum

Patients with achalasia lack nitric oxide synthase in the gastro-oesophageal junction

The abnormal function of the lower oesophageal sphincter in achalasia is likely to be due to impaired nonadrenergic, noncholinergic (NANC) inhibitory input. Since recent studies in animals suggest that nitric oxide (NO) is implicated physiologically in the inhibitory responses of the lower oesophageal sphincter, we have investigated whether the synthesis of NO is altered in the gastro-oesophageal junction of patients with achalasia. NO synthase activity was investigated in samples of tissue from the gastro-oesophageal junction obtained during surgery in eight patients with typical achalasia and six non-achalasic controls who underwent oesophagectomy for reasons other than sphincter dysfunction. The NO synthase activity was determined by the transformation of 14C-L-arginine into 14C-L-citrulline in tissue homogenates. In addition, immunohistochemical staining of the tissues was performed using a polyclonal antibody raised against a peptide sequence of rat brain NO synthase. Furthermore, the relaxant response to an exogenous NO donor (sodium nitroprusside, SNP) was measured in vitro in muscle strips obtained from two patients with achalasia and in two non-achalasic controls. NO synthase activity was detected in each of the samples obtained from six control patients (0.59 +/- 0.21 pmol mg-1 min-1; mean +/- SE). By contrast, none of the samples obtained from the eight patients with achalasia had any detectable NO synthase activity. Immunohistochemical studies confirmed the presence of NO synthase in the myenteric plexus of the gastro-oesophageal junction of control patients and its absence in achalasia. SNP relaxed muscle strips precontracted with bethanechol in both control samples and those from patients with achalasia.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Modulation of cholinergic and substance P-like neurotransmission by nitric oxide in the guinea-pig ileum

1. The role of endogenous nitric oxide (NO) as a modulator of enteric neurotransmission was investigated in longitudinal muscle myenteric plexus (LMMP) preparations of guinea-pig isolated ileum. 2. In tissues previously incubated with [3H]-choline, exogenous NO inhibited electrically-evoked [3H]-choline overflow as well as responses to exogenous agonists, indicating that NO has the potential of neuromodulation both pre- and postjunctionally. 3. A series of NO synthase inhibitors enhanced contractile responses to nerve stimulation indicating inhibitory neuromodulation by endogenous NO. 4. The potency order of the NO synthase inhibitors and their consistent effects after dexamethasone, on responses to nerve stimulation, indicate action on a constitutive NO synthase. 5. Responses enhanced by NO synthase inhibitors were inhibited by the substance P receptor antagonist, spantide, suggesting a neuromodulatory influence on substance P-like neurotransmission by the endogenous NO. 6. NO synthase inhibition did not modify contractile responses to application of acetylcholine or substance P, or [3H]-choline overflow, indicating that endogenous NO mainly has a prejunctional inhibitory action on substance P-like neurotransmission. Nor did it modify responses to direct electrical muscle stimulation in the presence of tetrodotoxin. This suggests a prejunctional enhancing effect by NO synthesis inhibition. 7. Evidence for endogenous NO modulation of acetylcholine release was obtained when NO synthase inhibition modified atropine-sensitive, nerve-mediated contractile responses. However, [3H]-choline overflow was unaltered by NO synthase inhibition. 8. NO synthase inhibition did not modify responses to inhibitory neurotransmission. 9. The findings suggest that endogenous NO inhibits substance P-like motor neurotransmission, probably via prejunctional mechanisms. Cholinergic transmission may also be reduced by endogenous NO, acting prejunctionally.

Differential effects of nitric oxide synthesis inhibition on basal blood flow and antidromic vasodilation in rat oral tissues

The role of nitric oxide in the mediation of (a) antidromic and (b) substance P-induced vasodilation in the pulp, lip, oral mucosa and submandibular gland was investigated in anaesthetized rats by means of laser Doppler flowmetry. Bolus or continuous infusion of N omega-nitro-L-arginine methyl ester (L-NAME) increased mean arterial blood pressure and reduced basal blood flow in the pulp but not in the lip. Electrical stimulation of the inferior alveolar nerve, in the presence of phenoxybenzamine, resulted in a long lasting vasodilation in lower lip and incisor pulp. Infusion of L-NAME enhanced the antidromic vasodilation in both lip and pulp. Pretreatment with L-arginine prevented these effects. Administration of the enantiomer (D-NAME) did not exert any effect on basal blood flow and on antidromic vasodilation. Infusion of substance P resulted in a transient vasodilation in all of the oral tissues studied. L-NAME reduced this vasodilation in the submandibular gland (only the lower doses) but it potentiated the responses in the pulp and oral mucosa. Pretreatment with L-arginine prevented the potentiated responses in the pulp and those induced by the lower doses of substance P in the oral mucosa. Thus, nitric oxide appears to differentially regulate the basal blood flow and the antidromic or substance P-induced vasodilation in the microvasculature of the lip and dental pulp.

The regulatory effect of bradykinin on the actions of sensory nerves in the perfused rat mesentery is mediated by nitric oxide

In the perfused rat mesentery, when adrenergic nerves were blocked with guanethidine and vascular smooth muscle tone was increased with methoxamine, transmural field stimulation caused a dilator response. Bradykinin significantly suppressed vasodilator responses to a transmural field stimulation in a concentration-dependent manner. After pretreatment with saponin to damage endothelial cells, bradykinin still suppressed vasodilator responses to transmural field stimulation. The effect of bradykinin was unaltered by indomethacin. N omega-Nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis, abolished the inhibition of vasodilator responses to transmural field stimulation elicited by bradykinin. However, in the presence of L-arginine and L-NAME the inhibitory effect of bradykinin reappeared. Furthermore, methylene blue itself caused potentiation of vasodilator responses to transmural field stimulation and reversed the effect of bradykinin. These findings suggest that bradykinin can produce an inhibitory modulation of the actions of sensory nerves in the perfused rat mesentery and that the effect of bradykinin may be mediated by nitric oxide released from non-adrenergic, non-cholinergic nerves.

Modulation of neuroeffector transmission by endogenous nitric oxide: a role for acetylcholine receptor-activated nitric oxide formation, as indicated by measurements of nitric oxide/nitrite release

Nitric oxide (NO) synthase inhibitors enhanced nerve-mediated contractile responses in guinea pig ileum longitudinal muscle, likely via a prejunctional effect on substance P-like neuroeffector transmission. Supporting a modulatory role for NO, application of NO through administration of acid sodium nitrite evoked marked inhibitory effects on responses to transmural nerve stimulation. Substance P-like responses to nerve stimulation were abolished by substance P receptor antagonists and were enhanced by atropine, indicating a cholinergic influence on substance P-like neuroeffector transmission. Since acetylcholine can evoke release of NO from endothelium, the possible role of acetylcholine in NO release in ileum was examined. The release of NO/nitrite, determined by chemiluminescence, was inhibited by NG-monomethyl-L-arginine (L-NMMA), by calcium removal, by tetrodotoxin or by atropine, indicating a nerve-mediated control of NO production. A basis for the NO release is likely to be spontaneous neuronal activity, where release of acetylcholine, with subsequent muscarinic receptor activation, contributes to stimulation of NO formation.

Nitric oxide is the mediator of tachykinin NK3 receptor-induced relaxation in the circular muscle of the guinea-pig ileum

The tachykinin NK3 receptor agonist, senktide, produces concentration-dependent contraction of the circular muscle of the guinea-pig ileum (EC50 2.59 nM). In the presence of the blocker of neuronal type of voltage-sensitive calcium channels, omega-conotoxin (0.1 microM), the contractile response to a low concentration of senktide was converted to an inhibitory effect on spontaneous activity of the ileum. This inhibitory effect was further enhanced in the presence of atropine (1 microM) and was abolished by tetrodotoxin (1 microM), indicating its neural origin. In the presence of atropine and omega-conotoxin, the inhibitory response to senktide (1 nM) was greatly inhibited or even abolished by L-nitroarginine (30 microM), its effect being prevented by L-arginine but not by D-arginine (300 microM in each case). Apamin (0.1 microM) failed to significantly affect the inhibitory response to senktide. Apamin enhanced spontaneous activity of the preparation while L-nitroarginine had no effect. Neither apamin nor L-nitroarginine affected the inhibitory response to isoprenaline. These findings indicate that inhibition of circular muscle activity produced through NK3 receptor stimulation in the guinea-pig ileum is mediated through a neuronal pathway involving nitric oxide or a nitric oxide-like substance(s) generation.

Nitric oxide modulates responses to sensory nerve activation of the perfused rat mesentery

The modulatory actions of nitric oxide on sensory nerves were investigated on dilator responses of the perfused rat mesentery to transmural nerve stimulation. N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis, caused a significant augmentation of vasodilator responses to transmural nerve stimulation, an effect which was abolished by L-arginine. L-NAME had no effect on vasodilator responses to exogenous calcitonin gene-related peptide. In preparations without endothelium L-NAME still caused potentiation of vasodilator responses to transmural nerve stimulation. Methylene blue, an inhibitor of guanylate cyclase, also significantly enhanced vasodilator responses to transmural nerve stimulation. After pretreatment with diethyldithiocarbamate to inhibit superoxide dismutase, vasodilator responses to transmural nerve stimulation were also potentiated. This response was abolished by exogenous superoxide dismutase. These findings suggest that endogenous nitric oxide modulates, in an inhibitory fashion, the actions of sensory nerves in the rat mesentery. The results also suggest that endogenous superoxide dismutase may participate in the regulation of the actions of sensory nerves via control of cellular superoxide anion level.

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.

Nitric oxide is not involved in the effects induced by non-adrenergic non-cholinergic stimulation and calcitonin gene-related peptide in the rat mesenteric vascular bed

The mechanism involved in the effects induced by the activation of perineural non-adrenergic non-cholinergic (NANC) nerves or by exogenous calcitonin gene-related peptide (CGRP) was investigated in the rat mesenteric vascular bed (MVB) perfused with Kreb's solution containing methoxamine and guanethidine. The activation of NANC terminals of the tissue was carried out by means of electrical field stimulation (EFS). An increase in the perfusion pressure of the preparations was observed in the presence of two inhibitors of nitric oxide synthase: NG-monomethyl-L-arginine (L-NMMA) (100 microM) and NG-nitro-L-arginine methyl ester (L-NAME) (100 microM). However L-NMMA and L-NAME did not modify the relaxant effect induced by EFS and exogenous CGRP. Furthermore the relaxant effect induced by EFS and exogenous CGRP was not affected by the removal of endothelium from the preparations. These results provide evidence that the vasodilation induced by NANC stimulation or by exogenous CGRP in MVB does not involve NO production.

Nitric oxide targets in the guinea-pig intestine identified by induction of cyclic GMP immunoreactivity

The immunohistochemical localization of cyclic GMP was used to determine potential physiological sites of action of nitric oxide in the guinea-pig small intestine and colon. In control tissue, cyclic GMP-immunoreactivity was observed only in macrophages, whose identity was confirmed by double-label experiments using either F4/80, a macrophage-specific antibody, or fluorescein isothiocyanate-labelled dextran injected intravenously. Following exposure to the nitric oxide donor, sodium nitroprusside, cyclic GMP-immunoreactivity was induced in subpopulations of neurons in the myenteric and submucosal plexuses of the ileum and colon. In the colon, cyclic GMP-immunoreactivity was induced in 5-10% of myenteric neurons. The cyclic GMP-immunoreactive neurons did not contain nitric oxide synthase. In the ileum, cyclic GMP-immunoreactive neurons comprised about 2% of myenteric neurons and 40% of submucosal neurons; these cyclic GMP-immunoreactive neurons were also immunoreactive for vasoactive intestinal peptide, but they did not contain nitric oxide synthase. Interstitial cells between the mesothelium and the longitudinal muscle layer, vascular smooth muscle and vascular pericytes also showed sodium nitroprusside-induced cyclic GMP-immunoreactivity. The interstitial cells of Cajal at the inner surface of the circular muscle layer and the smooth muscle cells of the circular and longitudinal muscle layers showed increases in cyclic GMP-immunoreactivity that varied in extent from animal to animal. The results suggest that nitric oxide could act at several sites in the intestine through the stimulation of guanylyl cyclase.

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

1. Neuromuscular transmission in the circular muscle of the canine proximal colon was examined, in the presence and absence of nitric oxide synthase inhibitors, by use of mechanical and intracellular microelectrode recording techniques. 2. Electrical field stimulation (EFS; 0.1-20 HZ) produced frequency-dependent contractions of circular muscle strips which reached a maximum at 15 Hz. These responses were enhanced by NG-monomethyl-L-arginine (L-NMMA; 300 microM) and reduced by atropine (1 microM). The effects of L-NMMA were reversed by L-arginine (3 mM). All responses to EFS were abolished by tetrodotoxin (1 microM). 3. In the presence of atropine, phentolamine and propranolol (all at 1 microM; 'non-adrenergic, non-cholingergic (NANC) conditions'), EFS evoked frequency-dependent inhibition of phasic contractions which reached a maximum at 5 Hz. At higher frequencies of EFS, inhibition diminished, and these responses were followed by post-stimulus excitation. 4. Under NANC conditions and in the presence of L-NG-nitroarginine methyl ester (L-NAME; 200 microM), EFS evoked contractions at frequencies of 5 Hz or greater. These contractions were reduced by co-incubation with L-arginine (2 mM) and abolished by tetrodotoxin (1 microM). 5. In the presence of atropine (1 microM), EFS (5-20 Hz) caused frequency-dependent inhibition of electrical slow waves. In the presence of L-NAME (100 microM) and atropine, the inhibitory response to EFS was abolished and an increase in slow wave duration was seen at stimulation frequencies greater than 5 Hz. The effects of EFS on slow wave duration were abolished by tetrodotoxin (1 microM). 6. Atropine-resistant contractions to EFS were enhanced by indomethacin (10 microM) and reduced or abolished by the non-selective NK1/NK2 tachykinin receptor antagonist D-Pro2, D-Trp7,9 SP, and by the selective NK2 receptor antagonist MEN 10,376 (10 microM).7. Exogenous tachykinins mimicked non-cholinergic excitatory electrical and mechanical responses. The rank order of potency for contraction was neurokinin A>neurokinin B>substance P, suggesting a predominance of the NK2 sub-type of tachykinin receptors on colonic smooth muscle cells. Low concentrations of neurokinin A also increased the amplitude and duration of electrical slow waves.8. These results suggest that: (i) in previous studies, non-cholinergic excitatory responses were masked by the simultaneous release of NO; (ii) non-cholinergic excitatory responses occur throughout the period of stimulation and are not manifest only as 'rebound' excitation; (iii) one or more tachykinins, possibly,acting via NK2 receptors, may mediate non-cholinergic excitatory responses.

Inhibitors of nitric oxide synthetase prevent castor-oil-induced diarrhoea in the rat

1. Castor oil (2 ml orally) produced copious diarrhoea in rats 3 h after its administration. 2. Pretreatment (intraperitoneal, i.p.) of rats with the NO synthesis inhibitors NG-nitro-L-arginine methyl ester (L-NAME, 1-25 mg kg-1) and NG-monomethyl-L-arginine (L-NMMA, 2.5-100 mg kg-1) inhibited or prevented castor-oil-induced diarrhoea. L-Arginine (150-600 mg kg-1, i.p.) administered to rats pretreated with L-NAME 10 mg kg-1, drastically reduced the antidiarrhoeal activity of L-NAME in a dose-related manner. D-Arginine (900 mg kg-1) did not modify the protection by L-NAME. 3. Pretreatment (i.p.) of rats with L-NAME (2.5-25 mg kg-1) decreased the intestinal fluid accumulation and Na+ secretion induced by castor oil. L-Arginine (600 mg kg-1) but not D-arginine (900 mg kg-1) counteracted the inhibitory effect of L-NAME (10 mg kg-1). 4. L-NAME (10 and 25 mg kg-1) had no significant effect on the intestinal transit in normal rats or those given castor oil. 5. These results provide evidence that nitric oxide (NO) could play an important role in castor-oil-induced diarrhoea.

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.

Nitric oxide inhibits resting sphincter of Oddi activity

The sphincter of Oddi has basal myogenic phasic activity that is modulated by neural and hormonal pathways. Stimulatory innervation to this organ is cholinergic, whereas the inhibitory pathways are unknown. Nitric oxide (NO), generated from L-arginine, relaxes gastrointestinal smooth muscle in vitro. We, therefore, hypothesized that resting sphincter of Oddi and duodenal motilities are regulated by a NO-mediated inhibitory pathway. In 23 anesthetized prairie dogs, systemic blood pressure and sphincter of Oddi and duodenal motilities were monitored during systemic infusion of N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase. L-NAME was infused alone and simultaneously with excess D- and L-arginine. L-NAME alone and L-NAME with D-arginine produced hypertension and increased sphincter of Oddi and duodenal motilities. L-arginine blocked these increases, suggesting that baseline sphincter of Oddi and duodenal motility regulation involves the generation of NO from L-arginine. We conclude that baseline sphincter of Oddi phasic activity is regulated by cholinergic stimulatory and NO-mediated inhibitory neural pathways.

Involvement of endogenous nitric oxide in the regulation of rat intestinal motility in vivo

The effect of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) on the motility of the small intestine in an acute model in the anaesthetised rat was determined by changes in jejunal intraluminal pressure. L-NAME (0.5-10 mg kg-1 i.v.) caused a dose-dependent increase in intraluminal pressure and initiated phasic intestinal contractions. These responses were inhibited by concurrent administration of L-arginine (200 mg kg-1 i.v.) but not by D-arginine (200 mg kg-1). The increase in jejunal motility induced by L-NAME was attenuated by atropine (4 mg kg-1), although even high doses of atropine (16 mg kg-1) did not abolish these responses. This indicates that although there are interactions between NO and muscarinic cholinergic mechanisms, other processes are also involved in these contractile events following administration of L-NAME. These observations in the rat suggest that endogenous NO plays a role in the modulation of intestinal motility in vivo.

Influence of vasoactive intestinal polypeptide and NG-nitro-L-arginine methyl ester on cholinergic neurotransmission in the rat gastric fundus

The possible modulating effect of vasoactive intestinal polypeptide (VIP) and nitric oxide (NO), on cholinergic neurotransmission was assessed in longitudinal muscle strips of rat gastric fundus. VIP and NO are the putative co-transmitters of the inhibitory non-adrenergic non-cholinergic (NANC) neurones in this tissue. VIP concentration dependently inhibited cholinergic contractions induced by 2-min transmural stimulation, relaxed tissues, the tone of which was continuously raised by transmural stimulation, and shifted to the right the frequency-response curves for contraction induced by transmural stimulation with a cumulative increase of frequency. The same effect was found when contractions were induced with methacholine, suggesting that only functional antagonism at the postsynaptic smooth muscle cell level is involved. On 30-min incubations, 3 x 10(-4) M NG-nitro-L-arginine methyl ester (L-NAME) potentiated cholinergic responses to 20-s transmural stimulation, while not influencing contractions of similar amplitude evoked by methacholine; the cholinergic responses to 2-min transmural stimulation were also not influenced. The potentiating effect of L-NAME was prevented by L-arginine but not D-arginine. These results suggest that endogenous NO released from the inhibitory NANC neurones during short trains of transmural stimulation interferes with cholinergic neurotransmission either by functional antagonism of acetylcholine at the postsynaptic level or by presynaptic inhibition of acetylcholine release.

Nitric oxide synthase activity in infantile hypertrophic pyloric stenosis

BACKGROUND

Hypertrophic pyloric stenosis is a common infantile disorder characterized by enlarged pyloric musculature and gastric-outlet obstruction. Its physiopathologic mechanism is not known, but a defect in pyloric relaxation (pylorospasm) has been postulated. Nitric oxide is a mediator of relaxation in the mammalian digestive tract, raising the possibility that pylorospasm could be caused by a defect in nitric oxide production. Since neuronal nitric oxide synthase and NADPH diaphorase are identical, we used the NADPH diaphorase histochemical reaction to study the distribution of nitric oxide synthase in pyloric tissue from patients with infantile hypertrophic pyloric stenosis.

METHODS

We studied pyloric tissue from nine infants with infantile hypertrophic pyloric stenosis and seven control infants and children. Cryostat sections were processed for NADPH diaphorase histochemical analysis. A polyclonal tau antiserum was used to identify the enteric nervous system by immunohistochemical methods.

RESULTS

NADPH diaphorase activity was restricted to the enteric nervous system and blood vessels. In the pyloric tissues from the control patients, intense diaphorase activity was present in the nerve fibers of the circular musculature, in the neurons and nerve bundles of the myenteric plexus, and in some nerve fibers of the longitudinal musculature. In the pyloric tissues from patients with infantile hypertrophic pyloric stenosis, the enteric nerve fibers in the hypertrophied circular musculature were enlarged and distorted and did not contain diaphorase activity, whereas the activity in the myenteric plexus and the longitudinal musculature was preserved.

CONCLUSIONS

We suggest that a lack of nitric oxide synthase in pyloric tissue is responsible for pylorospasm in infantile hypertrophic pyloric stenosis.

Ascending neural pathways in the rat ileum in vitro — Effect of capsaicin and involvement of nitric oxide

The aim of the present study was to develop and characterize an in vitro model of the rat ileum in which activation of the orally projecting neural excitatory pathway of the myenteric reflex is produced by electrical field stimulation anally to the recording site. The motility of a 10-cm segment of rat ileum was recorded using a perfused manometric assembly with side holes 2 and 4 cm orally to the stimulation site. Electrical field stimulation caused a contractile response in the oral but not in the aboral direction of the stimulation site. The contractile response, which was maximal using low stimulus frequencies (3 or 5 pulses per second (pps)) and decreased with higher frequencies (10 or 20 pps), was blocked by atropine (10(-6) M) at all frequencies tested after acute and after prolonged (greater than 30 min) treatment. The maximal contractile response at 3 pps was abolished by hexamethonium (10(-4) M), tetrodotoxin (5 x 10(-7) M) and by complete transection of the muscular wall between the stimulation and the recording site. Acute administration of capsaicin (8 x 10(-7) M) to the bath reduced the lag between the start of the electrical stimulation and the onset of the contractile response. Higher concentrations of capsaicin (10(-5) M) reduced the contractile response, but this was partly due to an unspecific effect of capsaicin. Blockade of nitric oxide (NO) synthesis by L-NG-nitro-arginine-methyl ester (L-NAME) (3 x 10(-4) M) augmented the contractile response to anal stimulation by 222.4% and reduced the lag period by 54.5%, whereas the stereoisomer D-NAME had no significant effect. The potentiating effects of L-NAME were reversed in the presence of L-arginine (3 x 10(-3) M) but not in the presence of the stereoisomer D-arginine (3 x 10(-3) M). This model can be used to study ascending neural pathways in the rat small intestine. The ascending excitatory response is abolished by atropine and hexamethonium and is modulated by capsicin-sensitive fibers. The ascending pathway is under tonic inhibition of metabolites of the L-arginine-NO pathway.

Histochemistry of NADPH-diaphorase, a marker for neuronal nitric oxide synthase, in the peripheral autonomic nervous system of the mouse

In order to identify possible sites of synthesis of nitric oxide in the peripheral nervous system, several mouse organs were investigated for the presence of NADPH-diaphorase activity. Diaphorase-positive neurons and fibers were localized in the tongue, submandibular salivary glands, gastrointestinal and biliary duct systems, lower urinary tract and pelvic ganglia. By thionin counterstaining it was found that a distinct subpopulation of neurons was labeled. The present study indicates that nitric oxide synthase may be present in intrinsic neurons of various organs, suggesting a widespread function of nitric oxide in the peripheral autonomic nervous system.

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.

Spontaneous release of nitric oxide inhibits electrical, Ca2+ and mechanical transients in canine gastric smooth muscle

1. In canine antrum, rhythmic electrical activity consists of a rapid upstroke phase followed by a plateau depolarization. In response to slow waves, cytosolic Ca2+ ([Ca2+]cyt) and tension increased. 2. Addition of sodium nitroprusside (SNP, 0.5 microM) decreased the amplitude of the plateau phase of slow waves without significant effects on the upstroke depolarization. SNP also inhibited changes in [Ca2+]cyt and tension associated with the plateau potential. SNP induced a negative chronotropic effect at concentrations above 0.1 microM. 3. Similar to the effects of SNP, illumination of muscles during slow waves with ultraviolet (UV) light caused premature repolarization. UV illumination is known to release NO in some tissues. 4. L-NG-monomethyl-arginine (L-NMMA, 300 microM), Methylene Blue (MB, 5 microM) and oxyhaemoglobin (oxy-Hb, 5 microM) increased the force of contractions. In contrast, L-arginine (L-Arg, 300 microM) decreased contractile force and antagonized the effects of L-NMMA. 5. During the upstroke phase, SNP caused a small reduction in [Ca2+]cyt and a large reduction in force, suggesting that SNP caused a decrease in Ca2+ sensitivity. 6. In muscles permeabilized by alpha-toxin, cyclic GMP (100 microM) and UV illumination inhibited Ca(2+)-induced contraction (at pCa 5.5). 7. These data suggest that NO or NO-related compounds are spontaneously released in gastric muscles. These agents have two effects on excitation-contraction coupling: (i) inhibition (directly and/or indirectly) of the voltage-dependent Ca2+ channels that participate in the plateau phase of slow waves, and (ii) reduction in the Ca2+ sensitivity of the contractile element.

Endogenous nitric oxide modulates morphine-induced constipation

Administration of morphine in mice causes inhibition of the gastrointestinal transit of a charcoal meal. Morphine-induced constipation in mice seems to depend predominantly on action(s) on the central nervous system since N-methyl morphine, a quaternary derivative, inhibits intestinal transit only when administered intracerebroventricularly (i.c.v.). L- but not D-arginine, given intraperitoneally, reversed the constipation induced by both morphine and its quaternary analogue. L-arginine was ineffective when given i.c.v. and did not reverse atropine-induced constipation. These results suggest that L-arginine preferentially modulates opioid-induced constipation through a stereospecific and peripheral action(s). It is possible that the effect of L-arginine is achieved by increasing the amount of nitric oxide released by non-adrenergic, non-cholinergic nerves in the gut. Thus, L-arginine may represent a useful agent for the treatment of undesirable constipation associated with the use of narcotic analgesics.

Nerve-induced tachykinin-mediated vasodilation in skeletal muscle is dependent on nitric oxide formation

Nerve-induced vasodilatation was studied by intravital microscopy of the rabbit tenuissimus muscle, pretreated with pancuronium, phentolamine, and guanethidine. Nerve stimulation of the tenuissimus nerve induced a vasodilatation which was frequency and pulse duration-dependent and insensitive to atropine and propanolol but abolished by tetrodotoxin. The nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM), but not its enantiomer, D-NAME, markedly inhibited the vasodilation induced by nerve stimulation or by exogenous substance P or neurokinin A. Vasodilatation due to calcitonin gene-related peptide, prostaglandin E2 or nitroprusside was unaffected. The substance P antagonist, spantide (30 microM), significantly attenuated nerve-induced vasodilatation, in parallel with L-NAME. Our results indicate that nerve-induced vasodilatation in skeletal muscle can be attributed to the release of substance P and/or other tachykinins and that nitric oxide subsequently mediates the response to endogenous tachykinins released from nerves.

Modulation of neuroeffector transmission in the guinea pig pulmonary artery by endogenous nitric oxide

The influence of endogenous nitric oxide (NO) on neuroeffector transmission in segments of guinea pig pulmonary artery was analyzed by application of NG-monomethyl-L-arginine (L-NMMA). L-NMMA enhanced contractile responses to nerve stimulation and this enhancement was counteracted by L-arginine. The enhancement remained after removal of the endothelium. L-NMMA enhanced contractions to exogenous noradrenaline. After blockade of adrenergic transmission by phentolamine, L-NMMA enhanced contractions induced by nonadrenergic-noncholinergic (NANC) neurotransmission. Stimulation-induced release of [3H]noradrenaline was unchanged by L-NMMA. The results suggest that endogenous NO exerts a postjunctional inhibition on adrenergic neurotransmission in the guinea pig pulmonary artery. A concomitant pre- and/or postjunctional inhibition of NANC transmission is implicated. The neuromodulation by NO does not require an intact endothelium.