Role of nitric oxide in the peristalsis in the isolated guinea-pig ileum

The role of enteric inhibitory neurons in intestinal motility

The enteric nervous system controls much of the mixing and propulsion of nutrients along the digestive tract. Enteric neural circuits involve intrinsic sensory neurons, interneurons and motor neurons. While the role of the excitatory motor neurons is well established, the role of the enteric inhibitory motor neurons (IMNs) is less clear. The discovery of inhibitory transmission in the intestine in the 1960's in the laboratory of Geoff Burnstock triggered the search for the unknown neurotransmitter. It has since emerged that most neurons including the IMNs contain and may utilise more than one transmitter substances; for IMNs these include ATP, the neuropeptide VIP/PACAP and nitric oxide. This review distinguishes the enteric neural pathways underlying the 'standing reflexes' from the pathways operating physiologically during propulsive and non-propulsive movements. Morphological evidence in small laboratory animals indicates that the IMNs are located in the myenteric plexus and project aborally to the circular muscle, where they act by relaxing the muscle. There is ongoing 'tonic' activity of these IMNs to keep the intestinal muscle relaxed. Accommodatory responses to content further activate enteric pathways that involve the IMNs as the final neural element. IMNs are activated by mechanical and chemical stimulation induced by luminal contents, which activate intrinsic sensory enteric neurons and the polarised interneuronal ascending excitatory and descending inhibitory reflex pathways. The latter relaxes the muscle ahead of the advancing bolus, thus facilitating propulsion.

Effect of intravenous infusion of glyceryl trinitrate on gastric and small intestinal motor function in healthy humans

BACKGROUND

Glyceryl trinitrate is a donor of nitric oxide that relaxes smooth muscle cells of the gastrointestinal tract. Little is known about the effect of glyceryl trinitrate on gastric emptying and no data exist on the possible effect of glyceryl trinitrate on small intestinal transit.

AIM

To examine the effect of intravenous infusion of glyceryl trinitrate on gastric and small intestinal motor function after a meal in healthy humans.

METHODS

Nine healthy volunteers participated in a placebo-controlled, double-blind, crossover study. Each volunteer was examined during intravenous infusion of glyceryl trinitrate 1 microg/kg x min or saline. A gamma camera technique was used to measure gastric emptying and small intestinal transit after a 1600-kJ mixed liquid and solid meal. Furthermore, duodenal motility was assessed by manometry.

RESULTS

Glyceryl trinitrate did not change gastric mean emptying time, gastric half emptying time, gastric retention at 15 min or small intestinal mean transit time. Glyceryl trinitrate did not influence the frequency of duodenal contractions, the amplitude of duodenal contractions or the duodenal motility index.

CONCLUSIONS

Intravenous infusion of glyceryl trinitrate 1 microg/kg x min does not induce major changes in gastric or small intestinal motor function after a 1600-kJ meal in healthy volunteers.

Measurement of the propelled liquid by isolated hamster ileum as a parameter to evaluate peristalsis

We present a method to measure the volume of liquid propelled by peristaltic movements of isolated hamster ileum as a novel means to assess peristaltic activity. The oral and aboral ends of the dissected ileum were attached to cannulas fixed horizontally. The application of intraluminal pressure by raising the level of liquid in the bottle connected to the oral end evoked peristalsis and intermittent propulsion of the intraluminal liquid. The inhibition of intrinsic neurons by tetrodotoxin stopped propulsion; this indicated that the liquid propulsion was correlated with neuron-regulated peristalsis. The volume of liquid propelled by one complete peristaltic movement was significantly greater than that by incomplete peristalsis, whereas recordings of pressure changes were indistinguishable. Inhibitors of nitric oxide synthase decreased the volume of liquid propelled by peristaltic movements, suggesting a role of nitrergic neurons in peristalsis. Our data show that the method described above might be suitable for analyzing peristalsis.

Upregulation of muscarinic receptors by long-term nitric oxide inhibition in the rat ileum

1. The aim of the present study was to examine the effects of long-term nitric oxide (NO) blockade on contractions of the rat ileum induced by muscarinic agonists. 2. Male Wistar rats received the NO synthesis inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 20 mg/rat per day) in drinking water for 7, 15, 30 and 60 days. Concentration-responses curves to methacholine and carbachol were obtained and pEC50 values were calculated. Saturation binding assays were performed in membranes prepared from rat ileum after 60 days of l-NAME treatment and the dissociation constant (KD) and maximal number of binding sites (Bmax) were determined by Scatchard analysis. 3. The NO synthase activity of the ileum was markedly reduced in all l-NAME-treated groups. At 60 days after l-NAME treatment, a significant increase in the potency of methacholine (fourfold) and carbachol (threefold) was observed. In binding studies, we found a significant increase in Bmax for [3H]-quinuclidinyl benzilate of approximately 57% in the l-NAME treated group without any significant change in KD values. The contractile response to methacholine was not modified by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (3 micro mol/L). No morphological alterations in the rat ileum were observed in l-NAME-treated rats. 4. Our findings suggest that treatment with l-NAME for 60 days induces a marked increase in the potency of methacholine and carbachol, as well as an increase in receptor number in the rat ileum.

Propagating contractions of the circular muscle evoked by slow stretch in flat sheets of guinea-pig ileum

Flat sheet preparations of guinea-pig ileum were stretched circumferentially and the propagation of circular muscle contractions along the preparation was investigated. Slow stretch, at 100 microm s-1, of a 50-mm long flat sheet of intestine, evoked circular muscle contraction orally, which propagated, without decrement, for up to 30 mm. This occurred despite circular muscle shortening being prevented, and in the absence of propulsion of contents. Thus, propagation in this flat sheet preparation could not explained on the basis of neuro-mechanical interactions, as previously proposed. Irrespective of the length of preparations, contraction amplitude decreased significantly in the most aboral 10-15 mm of intestine. This was not due to descending inhibitory pathways, but was associated with interruption of ascending excitatory pathways near the aboral end. Slow waves were not detected in circular muscle cells in any preparation (n=8). Smooth muscle action potentials evoked in circular muscle cells, in the presence of tetrodotoxin (TTX, 0.6 micromol L-1), did not propagate for more than 1 mm in the longitudinal axis. Propagation of circular muscle activity, evoked by slow stretch of flat sheet preparations, reveals the presence of a mechanism other than myogenic spread or the neuro-mechanical interactions previously proposed to account for propagation; the nature of this mechanism remains to be determined.

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

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

Neuronal nitric oxide synthase (NOS) regulates the expression of inducible NOS in rat small intestine via modulation of nuclear factor kappa B

We previously reported that neuronal nitric oxide synthase (nNOS) is the predominant NOS in the intestine. Inducible NOS (iNOS), an enzyme involved in the inflammatory response, is regulated by cytokines via the transcriptional factor NF-kappaB. We examined a new mechanism of intestinal iNOS regulation with respect to the role of nNOS and its effect on NF-kappaB. Young Sprague-Dawley rats were treated for 4 days with 1) saline, 2) 7-nitroindazole (7-NI, specific nNOS inhibitor), 3) 7-NI + pyrrolidine dithiocarbamate (PDTC, NF-kappaB inhibitor), or 4) PDTC. Intestinal iNOS mRNA, NF-kappaB activity, and the tissue content of the regulatory IkappaBalpha were examined. We found that 7-NI-treated animals had higher intestinal NF-kappaB (p50-p65) activity, lower IkappaBalpha content, and increased intestinal iNOS mRNA, iNOS protein, and iNOS activity compared with controls. All of these changes were abolished when PDTC was given together with 7-NI. PDTC alone had no effect. 7-NI induces a delayed increase in intestinal myeloperoxidase activity (after elevation in NF-kappaB and iNOS), which could be abrogated by PDTC. We conclude that in normal rat small intestine, nNOS suppresses the gene expression of iNOS through NF-kappaB down-regulation and that nNOS suppression leads to IkappaBalpha degradation, NF-kappaB activation, and iNOS expression.

Nitric oxide-containing neurons in the bovine gut, with special reference to their relationship with VIP and galanin

The presence and distribution of nicotinamide dinucleotide phosphate diaphorase (NADPH-d)-containing neurons have been studied by means of NADPH-d histochemistry in different regions of the adult cow gut, from the esophagus to the rectum. NADPH-d and nitric oxide synthase (NOS) were constantly recognized to be colocalized in the same neuron. The colocalization of vasoactive intestinal polypeptide (VIP) and galanin in such nitrergic neurons was also studied by means of combined histochemical and immunofluorescence techniques. NADPH-d-positive neurons were present along the myenteric plexus of the entire gut, and in the submucous plexus from the abomasum to the rectum. Notably, they formed two types of nerve networks in the submucous connective tissue of the jejunum-ileum. NADPH-d-positive innervation of the muscle layers occurred throughout the tract, and sometimes a clear correspondence was noted between the number of reactive fibres and the thickness of the muscle. Nitrergic fibres also occurred in the mucosa and often were in relation to glands and blood vessels. The nitrergic neurons varied in size, shape, and intensity of staining, and often their terminals were seen to surround unstained perikarya. Various types of neurons were recognized on the basis of their chemical content; one of them contained galanin, VIP and NOS simultaneously. The present results suggest that the nitrergic neurons of the bovine gastrointestinal tract play roles presumably for controlling the motility of the gut and the conduction of interneuronal impulses.

Morphine contracts the guinea pig ileal circular muscle by interfering with a nitric oxide mediated tonic inhibition

The effect of morphine was examined on the circular muscle of guinea pig ileal segments in vitro, with special regard to its interaction with enteric nitric oxide (NO) releasing neurons. In the presence of atropine (10(-6) M), morphine (10(-6) M) caused tonic contraction (approximately 7% of the maximal spasm) which was reversed by naloxone (10(-6) M). Tetrodotoxin (TTX; 10(-6) M) also caused contraction (14% of maximum); morphine completely lost its effect in the presence of TTX. Likewise, the NO synthase inhibitor N(G)-nitro-L-arginine (L-NOARG, 10(-4) M) elicited a tonic circular muscle contraction (12% of maximum) and completely prevented the excitatory action of TTX or morphine. The NO donor sodium nitroprusside (10(-7) to 10(-4) M) caused relaxation. In longitudinally oriented preparations in the presence of atropine (10(-6) M), no change in tone was observed upon administration of morphine (10(-6) M), TTX (10(-6) M), or L-NOARG (10(-4) M). In the circular muscle in the absence of atropine, cholecystokinin octapeptide (CCK-8; 10(-9) M) evoked a tonic-phasic contractile response which spontaneously faded away within 3 min. L-NOARG (10(-4) M) failed to affect intensity or duration of the response to CCK-8. It is concluded that NO-releasing myenteric neurons exert a tonic inhibitory influence upon the circular, but not longitudinal muscle of the guinea pig ileum. Morphine and TTX probably contract the circular muscle by reducing the amount of NO released. A release of NO seems to play no role in the contractile effect of CCK-8 or in its spontaneous termination.

Initiation of peristalsis by circumferential stretch of flat sheets of guinea-pig ileum

1. Segments of isolated guinea-pig intestine, 12 mm long, were distended slowly by intraluminal fluid infusion or by mechanical stretch as either a tube or flat sheet. In all cases, at a constant threshold length, a sudden, large amplitude contraction of the circular muscle occurred orally, corresponding to the initiation of peristalsis. 2. Circumferential stretch of flat sheet preparations evoked graded contractions of the longitudinal muscle (the 'preparatory phase'), which were maintained during circular muscle contraction. This suggests that the lengthening reported during the emptying phase of peristalsis is due to mechanical interactions. 3. The threshold for peristalsis was lower with more rapid stretches and was also lower in long preparations (25 mm) compared with short preparations (5-10 mm), indicating that ascending excitatory pathways play a significant role in triggering peristalsis. 4. Stretching a preparation beyond the threshold for peristalsis evoked contractions of increasing amplitude; thus peristalsis is graded above its threshold. However, during suprathreshold stretch maintained at a constant length, contractions of the circular muscle quickly declined in amplitude and frequency. 5. Circular muscle cells had a resting membrane potential approximately 6 mV more negative than the threshold for action potentials. During slow circumferential stretch, subthreshold graded excitatory motor input to the circular muscle occurred, prior to the initiation of peristalsis. However, peristalsis was initiated by a discrete large excitatory junction potential (12 +/- 2 mV) which evoked bursts of smooth muscle action potentials and which probably arose from synchronized firing of ascending excitatory neuronal pathways.

Roles of nitric oxide synthases in platelet-activating factor-induced intestinal necrosis in rats

OBJECTIVE

To examine the role of constitutive and inducible nitric oxide synthases (cNOS and iNOS) in platelet-activating factor (PAF)-induced shock and intestinal injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

Hospital research laboratory.

SUBJECTS

Young adult male Sprague-Dawley rats were anesthetized and studied.

INTERVENTIONS

Rats were injected with PAF, either alone or after the following pretreatments: a) selective iNOS inhibitors aminoguanidine or S-methylisothiourea; b) 3-morpholinosydnonimine, a NO donor; c) S-methylisothiourea + 3-morpholinosydnonimine; and d) antineutrophil antibody (to deplete neutrophils).

MEASUREMENTS AND MAIN RESULTS

Blood pressure, hematocrit, white blood cell counts, intestinal injury, and intestinal cNOS and iNOS activities were assessed. We found that: a) cNOS is the predominant NOS in the intestine and its activity is inversely correlated to the level of tissue injury; b) there is a time-dependent increase in cNOS activity in sham-operated animals, which was abolished by PAF; c) Western blotting and immunohistochemistry showed iNOS present in the normal intestine, localizing mainly in crypt cells; d) iNOS inhibitors attenuated PAF-induced injury in animals with high cNOS activity, but had no protective effect in animals with low cNOS activity; e) 3-morpholinosydnonimine, alone or together with S-methylisothiourea, alleviated PAF-induced injury; and f) neutrophil depletion blocked the suppressive effect of PAF on cNOS and prevented injury.

CONCLUSIONS

We conclude that cNOS and iNOS play different roles in PAF-induced intestinal injury. Caution should be exerted concerning potential therapeutic uses of iNOS inhibitors.

Histamine H3 receptors do not modulate reflex-evoked peristaltic motility in the isolated guinea-pig ileum

We investigated the role played by histamine H3 receptors in the control of intestinal peristalsis, using two different in vitro preparations of guinea-pig ileum. (a) Ileal segments were perfused from the oral end, inducing peristaltic movements (emptying waves), due to the activation of intramural reflexes. Such peristaltic motility was measured as changes in the perfusion pressure during the emptying phase and the threshold pressure for triggering the emptying wave was determined. (b) Ileal segments were mounted horizontally and circular muscle contraction evoked by the ascending peristaltic reflex was triggered by caudal distension of the intestinal wall. In perfused ileal segments, specific agonists acting at histamine H3 receptors, ((R)-alpha-methylhistamine and immepip, 1 nmol-10 micromol/l), did not cause any change in the threshold pressure for triggering the peristaltic wave, or in the rise of the perfusion pressure during the emptying phase. Similarly, circular muscle contractions evoked by caudal distension of the wall were not affected by these histamine H3 receptor agonists up to 10 micromol/l. In the same conditions, a complete inhibition of peristaltic movements was elicited by agonists acting at alpha2-adrenoceptors or adenosine A1 receptors (compound UK 14,304 and N6-cyclopentyladenosine, respectively), their effects being prevented by the respective receptor antagonists, idazoxan and 8-cyclopentyl-1,3-dimethyl-xanthine. These data demonstrate that, contrary to alpha2-adrenoceptors and adenosine A1 receptors, histamine H3 receptors are not primarily involved in the modulation of intramural reflexes that modulate the peristaltic motility of the isolated guinea-pig ileum.

Nitric oxide modulates cholinergic neurotransmission in cat duodenum

1. Longitudinal muscle strips isolated from cat proximal duodenum were characterized by spontaneous phasic contractions. 2. Electrical field stimulation (EFS) (0.5 ms, 1-20 Hz, supramaximal voltage intensity for 40 sec) produced frequency-dependent contractions, and maximal amplitude was achieved at 10 Hz. The EFS-induced contractions were abolished either by atropine (10(-6) M) or by tetrodotoxin (3 x 10(-7) M). 3. The nitric oxide (NO) synthase blocker N infinity-nitro-L-arginine (L-NNA, 10(-4) M) or the inhibitor of the soluble guanylyl cyclase methylene blue (MB, 3 x 10(-5) M) increased the amplitude of the electrically evoked contractions. 4. L-Arginine (10(-3) M) or sodium nitroprusside (SNP, 10(-4) M) significantly decreased the amplitude of the EFS-induced, L-NNA- or MB-potentiated contractions as the effect of SNP was much more pronounced. 5. Neither L-NNA nor MB affected the contraction evoked by exogenous acetylcholine. 6. The L-NNA or MB-induced interruption of the L-arginine-NO pathway potentiated the electrically evoked cholinergic contractions, suggesting the inhibitory role of NO in the cholinergic neurotransmission realized probably at the pre-synaptic level in cat duodenum.

Neurogenic "off" contractions are mediated by NK2-receptors in the circular muscle of guinea pig ileum

In guinea pig ileal circular muscle, electrical stimulations by a train of 10-100 pulses (0.05-msec duration, 10 Hz) produced tetrodotoxin-sensitive "off" contractions that were initiated upon the termination of stimulations. Atropine at 10(-6) M did not inhibit the "off" contractions. FK224 at 10(-5) M, a dual antagonist for NK1- and NK2-receptors, but not 10(-7) M CP-96,345, an antagonist for NK1-receptors, almost abolished the "off" contractions in the presence of atropine. These results suggest that the neurogenic "off" contraction was mediated mainly by NK2-receptors in the circular muscle of guinea pig ileum.