Effect of nitric oxide on circular muscle of the canine small intestine

Functional changes in nonadrenergic, noncholinergic inhibitory neurons in ileal circular smooth muscle after small bowel transplantation in rats

This experiment was designed to determine mechanisms of change in nonadrenergic, noncholinergic (NANC) inhibitory neurons in the ileum after small bowel transplantation (SBT) in the rat and whether nitric oxide (NO) serves as an important NANC inhibitory neurotransmitter in the rat ileum. Eight groups of rats (N > or =8 rats/group) were studied: neurally intact unoperated controls; rats one week after anesthesia and sham celiotomy; and separate groups one and eight weeks after either 40 min of cold ischemia of the jejunoileum, combined jejunal and ileal intestinal transection/reanastomosis, or orthotopic SBT of the entire jejunoileum. Contractile activity was evaluated in full-thickness ileal circular muscle strips under isometric conditions. Spontaneous activity did not differ among groups. In all groups, exogenous NO, NG-monomethyl-L-arginine (L-NMMA, an NO synthase inhibitor), and methylene blue (soluble guanylate cyclase inhibitor) had no effect on spontaneous activity, while 8-bromocyclic guanosine monophosphate (8Br-cGMP) inhibited contractile activity in all groups. Low frequency (2-10 Hz) electrical field stimulation (EFS) inhibited contractile activity only in control and SBT groups; L-NMMA and methylene blue did not alter the response to EFS in any group. These results suggest that each aspect of the SBT procedure, ischemia/reperfusion injury, disruption of enteric neural continuity by intestinal transection, and extrinsic denervation, alter function of enteric ileal inhibitory neurons separately early (one week) after operation. NO, a known inhibitory neurotransmitter in other gut regions, does not affect ileal circular muscle in neurally intact tissue nor mediate functional changes in inhibitory nerve function nor smooth muscle contractility after SBT.

Surgical manipulation of the gut elicits an intestinal muscularis inflammatory response resulting in postsurgical ileus

OBJECTIVE

To investigate the pathophysiologic mechanisms that lead to ileus after abdominal surgery.

SUMMARY BACKGROUND DATA

The common supposition is that more invasive operations are associated with a more extensive ileus. The cellular mechanisms of postsurgical ileus remain elusive, and few studies have addressed the mechanisms.

METHODS

Rats were subjected to incremental degrees of surgical manipulation: laparotomy, eventration, "running," and compression of the bowel. On postsurgical days 1 and 7, muscularis infiltrates were characterized immunohistochemically. Circular muscle activity was assessed using mechanical and intracellular recording techniques in vitro.

RESULTS

Surgical manipulation caused an increase in resident phagocytes that stained for the activation marker lymphocyte function-associated antigen (LFA-1). Incremental degrees of manipulation also caused a progressive increase in neutrophil infiltration and a decrease in bethanechol-stimulated contractions. Compression also caused an increase in other leukocytes: macrophages, monocytes, dendritic cells, T cells, natural killer cells, and mast cells.

CONCLUSION

The data support the hypothesis that the degree of gut paralysis to cholinergic stimulation is directly proportional to the degree of trauma, the activation of resident gut muscularis phagocytes, and the extent of cellular infiltration. Therefore, postsurgical ileus may be a result of an inflammatory response to minimal trauma in which the resident macrophages, activated by physical forces, set an inflammatory response into motion, leading to muscle dysfunction.

Protein kinase G expression in the small intestine and functional importance for smooth muscle relaxation

In functional experiments, the nitric oxide (NO) donor N-morpholino-N-nitroso-aminoacetonitrile or the cGMP analog 8-(4-chlorophenylthio)-cGMP caused a concentration-dependent, tetrodotoxin-resistant relaxation of precontracted strips from rat small intestine. The inhibitory effect of both substances was completely blocked at lower concentrations and was significantly attenuated at higher concentrations by the selective cGMP-dependent protein kinase (cGK) antagonist KT-5823 (1 microM). cGK-I was identified by immunohistochemistry in circular and longitudinal muscle, lamina muscularis mucosae, and smooth muscle cells of the villi and in fibroblast-like cells of the small intestine. Additionally, there was staining of a subpopulation of myenteric and submucous plexus neurons. Double staining for neuronal NO synthase (nNOS) and cGK-I demonstrated a colocalization of these two enzymes. Western blot analysis of smooth muscle preparations and isolated nerve terminals demonstrated that these structures predominantly contain the cGK-Ibeta isoenzyme, whereas the cGK-Ialpha expression is about threefold less. The isoform cGK-II was entirely confined to mucosal epithelial cells. These results show that cGK-I is expressed in different muscular structures of the small intestine and participates in the NO-induced relaxation of gastrointestinal smooth muscle. The presence of cGK-I in NOS-positive enteric neurons further suggests a possible neuronal action site.

Contractile activity of circular smooth muscle in rats one year after small bowel transplantation: differing adaptive response of the jejunum and ileum to denervation

The aim of the present study was to determine the long-term effects of isogeneic small bowel transplantation (SBT) on jejunal and ileal circular smooth muscle contractile activity in the rat. Transmural strips of circular muscle were prepared from proximal jejunum and distal ileum of 1-year-old control rats and rats 1 year after SBT (SBT-1Y) to measure isometric force. Spontaneous contractile activity and the dose-responses to bethanechol and norepinephrine were studied. Electrical field stimulation (EFS) at varying frequencies (1 to 20 Hz) was evaluated under adrenergic and cholinergic blockade to investigate inhibitory nerves. Spontaneous activity both in the jejunum and ileum in SBT-1Y rats was not different compared to control rats. Sensitivity to bethanechol did not differ between control and SBT-1Y rats in the jejunum or ileum. Sensitivity to norepinephrine, however, was significantly increased after SBT in the ileum but not in the jejunum. During EFS, inhibition was seen at low frequencies, and contractions were induced at high frequencies in all groups. The degree of inhibition did not differ between control and SBT-1Y rats in the jejunum; however, it tended to be increased in the ileum after SBT. The long-term adaptive response of smooth muscle to the extrinsic denervation accompanying SBT differs between the jejunum and the ileum.

Involvement of intracellular Ca2+ stores in inhibitory effects of NO donor SIN-1 and cGMP

We investigated the role of K+ channels and intracellular Ca2+ stores in the relaxations induced by the NO donor 3-morpholinosydnonimine (SIN-1) and 8-bromo-cGMP (8-BrcGMP), 8-(4-chlorophenylthio)-cGMP (pCPT-cGMP), and alpha, beta-methylene-ATP in isolated segments of rat ileum. The inhibitory responses to SIN-1 and the cGMP analogs were not influenced by the K+ blockers apamin, charybdotoxin, iberiotoxin, or glibenclamide, whereas relaxations induced by alpha,beta-methylene-ATP were abolished by apamin and tetraethylammonium. The NO-donor SIN-1 and the cGMP analogs were able to inhibit contractions induced by activation of L-type Ca2+ channels (BAY-K-8644), by carbachol (CCh), and by cyclopiazonic acid (CPA), a blocker of sarcoplasmic Ca2+-ATPase. However, the inhibition of the combined CPA and CCh response was reduced and the dose-response curve of SIN-1 shifted to the right. Intracellular Ca2+ stores were emptied by incubation in Ca2+-free buffer and repetitive stimulation with CCh or BAY-K-8644. After restoration of extracellular Ca2+, the inhibitory effect of SIN-1 and pCPT-cGMP was only attenuated, whereas in the additional presence of CPA, the inhibitory effect of SIN-1 was blocked and the effect of 8-BrcGMP reduced. Thus depleting intracellular Ca2+ stores attenuated the effect of SIN-1 and 8-BrcGMP, suggesting an involvement of functional Ca2+ stores.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCa channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCa current and the activity of KCa channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration. NO (0.5% or 1%) in the bath increased NPo of KCa channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPo of KCa channels. In the on-cell patch recording configuration, methylene blue (1 microM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of KCa channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 microM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activating KCa channels through a cGMP pathway.

Colocalization of inhibitory mediators, NO, VIP and galanin, in canine enteric nerves

The colocalization of three putative inhibitory mediators of enteric nerves, vasoactive intestinal peptide (VIP), galanin (GAL) and nitric oxide synthase (nNOS), was examined in the myenteric plexus of canine antrum, intestine and colon. Many ileal and colonic neurons contained nNOS-immunoreactive (nNOS-IR) activity with some also containing VIP-IR; only a few neurons also contained GAL-IR. Ileal and colonic VIP-IR nerves often appeared to be interneurons innervating nNOS nerves. Many antral neurons contained VIP-IR with nearly all also containing GAL-IR. A few also contained nNOS-IR. The predominance of nNOS-IR neurons relative to VIP-IR and GAL-IR neurons in the ileal and colonic, but not the antral, myenteric plexus is consistent with NO being the primary inhibitory mediator in the intestine but not in the antrum.

Effects of ethyl alcohol on canine jejunal circular smooth muscle

Ethyl alcohol has many symptomatic effects on the gastrointestinal tract. Our aim was to determine the effects of ethyl alcohol on circular smooth muscle contractility of the canine small bowel. Mechanical and intracellular electrical recordings were made in vitro from the circular muscle of full-thickness strips of muscularis externa from canine jejunum. Ethyl alcohol (20-120 mM) dose-dependently decreased spontaneous contractile amplitude, hyperpolarized the resting membrane potential, and decreased the amplitude of the slow wave. Ethyl alcohol also decreased the amplitude of the inhibitory junction potential (during electrical field stimulation-EFS) but did not alter the maximum absolute cell-polarized potential reached during EFS. An increase in extracellular calcium (15 mM) partially restored spontaneous contractile amplitude, resting membrane potential, and slow-wave amplitude. Ethyl alcohol decreased the amplitude of contractions evoked by acetylcholine, CCK, and substance P. These data suggest that ethyl alcohol has direct effects on jejunal smooth muscle contractility. These effects can be partially reversed by increasing the availability of extracellular calcium.

Altered mRNA expression of the neuronal nitric oxide synthase gene in Hirschsprung's disease

Nitric oxide has been described as an inhibitory neurotransmitter to mediate smooth muscle relaxation in the mammalian gastrointestinal tract. The enzyme neuronal nitric oxide synthase (NOS) catalyzes the formation of NO. The authors examined the expression of neuronal NOS gene at the mRNA level in intestinal specimens from seven patients who had Hirschsprung's disease (HD) using reverse transcription polymerase chain reaction (RT-PCR) technique. With 35 cycles of PCR reaction, substantial signals of neuronal NOS mRNA were observed in the ganglionic bowel in all seven patients, whereas in the aganglionic bowel, neuronal NOS signals were weak in three patients, and undetectable in four patients. By increasing the PCR cycle to 40 cycles, barely detectable signals were observed in two of the latter four patients. Semiquantitative analysis in the three patients who showed weak signals with 35 cycles of PCR indicated that neuronal NOS mRNA expression in aganglionic bowel was decreased at least 1/50 to 1/100 of the level expressed in ganglionic bowel. Because absence or low level of expression of neuronal NOS mRNA may lead to impaired production of NO, our observations suggest that motility dysfunction in HD may be as a result of markedly decreased or no expression of the neuronal NOS gene at the mRNA level.

Non-adrenergic non-cholinergic (NANC) transmission to smooth muscle: 35 years on

In 1963, two substances were thought to mediate all transmission between neurons, as well as between nerve and muscle in the peripheral nervous system, namely acetylcholine and noradrenaline. This paradigm primarily was due to the research of Dale, Loewi and von Euler in the first half of the century [Dale, 1937 (Transmission of nervous effects by acetylcholine, Harvey Lect. 32, pp. 229-245)]. However, in 1963, a series of experiments were carried out using recently introduced electrophysiological techniques, which showed unequivocally for the first time that the classical paradigm was not correct. Both inhibitory and excitatory junctions between nerves and smooth muscle cells were shown to exist in which transmission was mediated by non-adrenergic, non-cholinergic (NANC) transmitters. In the succeeding 35 years, identification of these NANC transmitters has been a major task of neuropharmacology, with nitric oxide, neuropeptides, and purines being isolated. This review presents an historical account of the developments this century of the classical paradigm, of how it was displaced, and of the progress made in identifying the neuromuscular transmitters of the autonomic nervous system.

Pharmacological properties of non-adrenergic, non-cholinergic inhibitory transmission in chicken gizzard

1. Non-adrenergic, non-cholinergic (NANC) inhibitory transmission in chicken gizzard was studied by use of intracellular microelectrode techniques. Changes in membrane potential in response to NANC nerve stimulation were recorded in the gizzard smooth muscle pretreated with atropine (1 microM) and guanethidine (1 microM). 2. Field stimulation of the intramural nerves (FS) evoked inhibitory junction potentials (i.j.ps) which were abolished by tetrodotoxin (1 microM), but not inhibited at all by K+ channel blockers including apamin (0.5 microM), tetraethylammonium (TEA, 10 mM), charybdotoxin (0.2 microM) and glibenclamide (10 microM). 3. NG-nitro-L-arginine (3 mM), an inhibitor of nitric oxide (NO) synthase, inhibited i.j.ps. The effect was reversed by L-arginine (3 mM), but not by D-arginine (3 mM). 4. 8-Bromo cyclic GMP (100 microM), a membrane permeable analogue of cyclic GMP, produced a membrane hyperpolarization which was blocked by TEA (10 mM) or glibenclamide (10 microM). 5. NO at concentrations of up to 400 microM affected neither i.j.ps nor resting membrane potential. On the other hand, NO (80 microM) caused the membrane to hyperpolarize in the smooth muscle of guinea-pig ileum. 6. These results suggest that in the chicken gizzard, NANC i.j.ps may not arise from opening of conventional types of K+ channel and that NO seems unlikely to be involved in the generation of i.j.ps. A possible mechanism by which the inhibitory effect of NG-nitro-L-arginine on i.j.ps was brought about will be discussed.

Pathophysiology of adynamic ileus

We hypothesized that the inhibitory neurotransmitters nitric oxide (NO) and vasoactive intestinal peptide (VIP) may play a role in the disrupted gastrointestinal motility of endotoxemia. Strain gauge transducers on the stomach and small intestine of dogs determined interdigestive gastrointestinal motility. Tissue levels of NO synthase and VIP and serum levels of nitrite/nitrate (NO(2)-/NO(3)-) and VIP were measured. Following completion of the baseline studies, dogs were given a single dose of E. coli lipopolysaccharide, 200 microg/kg intravenously, and the studies were repeated for the next three days. Following endotoxin bolus, the migrating motor complex (MMC) was delayed for two days while serum VIP was increased on postendotoxin day 1 and serum NO(2)-/NO(3)- was increased on postendotoxin day 2. There were no changes in gut smooth muscle levels of NO synthase or VIP. We conclude that a single, sublethal dose of endotoxin results in prolongation of the MMC with distinct but independent increases in serum levels of VIP and NO(2)-/NO(3)-.

Basal release of nitric oxide induces an oscillatory motor pattern in canine colon

1. The consequences of intrinsic, basal nitric oxide release on electrical and contractile activity of canine proximal colon were examined. Membrane potential and contraction were simultaneously recorded from the circular muscle in the presence of drugs to block adrenergic and cholinergic responses. 2. Electrical slow waves were recorded from muscle cells near the submucosal surface of the circular layer. Spontaneous contractions were initiated by each slow wave. Contractile amplitude increased 1.9-fold when nerves were blocked with tetrodotoxin (TTX, 1 microM). 3. Muscle cells near the myenteric surface displayed myenteric potential oscillations (MPOs) averaging 16 cycles per minute (c.p.m.) in frequency and 10 mV in amplitude. Twenty-five per cent of muscles displayed an additional slow, neurogenic oscillation (mean frequency, 1 c.p.m.; amplitude, 14 mV) superimposed upon the MPO rhythm. 4. The nitric oxide (NO) synthase inhibitor N omega -nitro-L-arginine (L-NA, 100 microM; n = 16) abolished neurogenic oscillations, depolarized cells, and increased MPO upstroke velocity, amplitude and frequency. The actions of L-NA were mimicked by N omega-nitro-L-arginine methylester (L-NAME, 100 microM) and oxyhaemoglobin (3%). 5. Spontaneous contractions were increased 2.3-fold by L-NA, and TTX had no effect on contractions after addition of L-NA. 6. The NO-donor sodium nitroprusside (SNP, 1 microM) reversed the electrical and mechanical effects of L-NA and initiated slow oscillations similar to the neurogenic oscillations. Slow oscillations were also evoked with S-nitroso-N-acetylpenicillamine (SNAP, 1 microM). The effects of NO donors were blocked by oxyhaemoglobin. 7. Slow electrical oscillations could not be elicited by SNP after removal of a thin strip of circular muscle along the myenteric edge. 8. These data suggest that the spontaneous electrical and contractile activity of the proximal colon is tonically suppressed by basal release of NO. Basal NO causes an oscillatory pattern of electrical and mechanical activity. This activity does not require patterned firing of nerves; rather a continuous, low level release of NO would be capable of producing the neurogenic oscillatory behaviour. The slow oscillatory activity depends upon the presence of the myenteric region of the circular muscle layer, which contains cell bodies of enteric neurons and interstitial cells of Cajal.

Regulation of citrulline recycling in nitric oxide-dependent neurotransmission in the murine proximal colon

1. We investigated the contribution of nitric oxide (NO) to inhibitory neuromuscular transmission in murine proximal colon and the possibility that citrulline is recycled to arginine to maintain the supply of substrate for NO synthesis. 2. Intracellular microelectrode recordings were made from circular smooth muscle cells in the presence of nifedipine and atropine (both 1 microM). Electrical field stimulation (EFS, 0.3-20 Hz) produced inhibitory junction potentials (i.j.ps) composed of an initial transient hyperpolarization (fast component) followed by a slow recovery to resting potential (slow component). 3. L-Nitro-arginine-methyl ester (L-NAME, 100 microM) selectively abolished the slow component of i.j.ps. The effects of L-NAME were reversed by L-arginine (0.2-2 mM) but not by D-arginine (2 mM). Sodium nitroprusside (an NO donor, 1 microM) reversibly hyperpolarized muscle cells. This suggests that NO mediates the slow component of i.j.ps. 4. L-Citrulline (0.2 mM) also reversed the effects of L-NAME, and this action was maintained during sustained exposures to L-citrulline (0.2 mM). This may reflect intraneuronal recycling of L-citrulline to L-arginine. 5. Higher concentrations of L-citrulline (e.g. 2 mM) had time-dependent effects. Brief exposure (15 min) reversed the effects of L-NAME, but during longer exposures (30 min) the effects of L-NAME gradually returned. In the continued presence of L-citrulline, L-arginine (2 mM) readily restored nitrergic transmission, suggesting that during long exposures to high concentrations of L-citrulline, the ability to generate arginine from citrulline was reduced. 6. Aspartate (2 mM) had no effect on i.j.ps, the effects of L-NAME, or the actions of L-citrulline in the presence of L-NAME, L-Citrulline (0.2-2 mM) alone had no effect on i.j.ps under control conditions. 7. S-methyl-L-thiocitrulline (10 microM), a novel NOS inhibitor, blocked the slow component of i.j.ps. The effects of this inhibitor were reversed by L-arginine (2 mM), but not by L-citrulline (2 mM). 8. These results suggest that i.j.ps in the murine colon result from release of multiple inhibitory neurotransmitters. NO mediates a slow component of enteric inhibitory neurotransmission. Recycling of L-citrulline to L-arginine may sustain substrate concentrations in support of NO synthesis and this pathway may be inhibited when concentrations of L-citrulline are elevated.

Effect of extrinsic denervation in a canine model of jejunoileal autotransplantation on mechanical and electrical activity of jejunal circular smooth muscle

Little is known about the acute and chronic effects of the intestinal transplantation on smooth muscle contractile physiology. Our aim was to determine the effects of the denervation necessitated by jejunoileal autotransplantation on membrane potential and contractile activity. Six dogs underwent a model of jejunoileal autotransplantation that specifically avoids ischemia/reperfusion injury (by maintaining blood flow to the gut during the "transplantation" procedure). Strips of jejunal circular muscle were studied sequentially before and 2 and 8 weeks after denervation by recording mechanical and intracellular electrical activities in vitro. The amplitude of spontaneous contractions (X +/- SD) was increased (P < 0.05) at 2 compared to 0 weeks (126 +/- 19 vs 77 +/- 32 g/g; P < 0.05) but markedly decreased at 8 weeks (7 +/- 2 g/g). Contraction frequency, resting membrane potential, and amplitude of slow waves were unchanged across these time points. Bethanechol (10(-7)-10(-4) M) and substance P (10(-8)-10(-6) M) dose-dependently increased contractile activity at all time points, but the absolute change in amplitude was decreased at 8 weeks. The amplitude of inhibitory junction potentials (IJPs) and duration of inhibition of contractile activity in the presence of cholinergic and adrenergic blockade increased at 2 and 8 weeks; off-contraction amplitude was decreased at 8 weeks (P < 0.05). These effects may occur via changes in neurotransmitter release, changes in regulation of membrane receptors, or alteration of characteristics of the membrane threshold potential.

Altered messenger RNA expression of the neuronal nitric oxide synthase gene in infantile hypertrophic pyloric stenosis

Nitric oxide (NO) has been described as a mediator of smooth muscle relaxation in the mammalian gastrointestinal tract. The enzyme neuronal nitric oxide synthase (NOS) catalyzes the formation of NO. We examined the expression of the neuronal NOS gene at the messenger RNA (mRNA) level in pyloric smooth-muscle biopsy specimens from six patients with infantile hypertrophic pyloric stenosis (IHPS) using a reverse transcription-polymerase chain reaction (RT-PCR) technique. For controls, smooth-muscle layer specimens of pylorus (n=3), ileum (n=2), and colon (n=2) were used. With 31 cycles of PCR reaction, control specimens revealed detectable signals for neuronal NOS mRNA. In contrast, signals of IHPS specimens were undetectable in five cases and very weak in one. By increasing the PCR to 37 cycles, detectable signals for neuronal NOS mRNA were observed in all IHPS specimens, but they were significantly weaker than those of controls. Since a low level of neuronal NOS mRNA may lead to impaired production of NO, our observations indicate that the excessively contracted, hypertrophied pyloric muscle in IHPS is a result of reduced expression of the neuronal NOS gene at the mRNA level.

Contribution of acetylcholine, vasoactive intestinal polypeptide and nitric oxide to CNS-evoked vagal gastric relaxation in the rat

Several in vitro models of gastric relaxation have elucidated a role of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) in non-adrenergic, non-cholinergic (NANC) vagally mediated gastric relaxation. However, these models do not necessarily mimic the events leading to gastric relaxation in the whole animal. We have recently described a vagally mediated gastric relaxation evoked by micro-injection of substance P (SP) into the nucleus raphe obscurus (NRO). The present study was performed to elucidate whether this CNS-stimulated in vivo gastric relaxation involved acetylcholine, NO and VIP. Atropine (1 mg kg-1 i.v.), reduces both the rapid nadir and sustained gastric relaxation evoked by SP in the NRO, and the residual responses are abolished by NG-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 10 mg kg-1 i.v.), an NO synthase inhibitor. Blockade of NO synthase alone is not sufficient to abolish the effect of SP into the NRO on intragastric pressure. A VIP antagonist, [p-chloro-D-Phe6, Leu17]VIP (32 micrograms i.v.) alone, or with the addition of L-NAME, does not affect the nadir of the gastric relaxation in response to SP microinjected into the NRO; however, both antagonists reduce the CNS-evoked sustained intragastric pressure relaxation. We conclude that, in CNS-evoked gastric relaxation, inhibition of cholinergic pathways is potentially important for both the rapid nadir and sustained gastric relaxation, and both NO and VIP contribute to sustained gastric relaxation.

VIP- and PACAP-mediated nonadrenergic, noncholinergic inhibition in longitudinal muscle of rat distal colon: involvement of activation of charybdotoxin- and apamin-sensitive K+ channels

1. The mediators of nonadrenergic, noncholinergic (NANC) inhibitory responses in longitudinal muscle of rat distal colon were studied. 2. An antagonist of pituitary adenylate cyclase activating peptide (PACAP) receptors, PACAP6-38, concentration-dependently inhibited the rapid relaxation of the longitudinal muscle induced by electrical field stimulation (EFS), resulting in a maximal inhibition of 47% at 3 microM. 3. PACAP6-38 inhibited the relaxation by 75% in the presence of the vasoactive intestinal peptide (VIP) receptor antagonist, VIP10-28 at 3 microM, which inhibited the relaxation by 44%. 4. An antagonist of large conductance Ca(2+)-activated K+ channels, charybdotoxin, concentration-dependently inhibited the rapid relaxation of the longitudinal muscle, resulting in a maximal inhibition of 58% at 100 nM. 5. An antagonist of small conductance Ca(2+)-activated K+ channels, apamin, concentration-dependently inhibited the relaxation (58% at 1 microM). 6. Treatment with both K+ channel antagonists resulted in 84% inhibition of the EFS-induced relaxation, which is comparable to the extent of inhibition induced by PACAP6-38 plus VIP10-28. 7. The inhibitory effect of VIP10-28 and of apamin, but not of charybdotoxin was additive: the same applied to PACAP6-38 and charybdotoxin, but not apamin. 8. Exogenously added VIP (100 nM 1 microM) induced a slow gradual relaxation of the longitudinal muscle. Charybdotoxin, but not apamin significantly inhibited the VIP-induced relaxation VIP10-28, but not PACAP6-38 selectively inhibited the VIP-induced relaxation. 9. Exogenously added PACAP (10-100 nM) also induced slow relaxation. Apamin and to a lesser extent, charybdotoxin, inhibited the PACAP-induced relaxation. PACAP6-38, but not VIP10-28 selectively inhibited the PACAP-induced relaxation. 10. Apamin at 100 nM inhibited inhibitory junction potentials (i.j.ps) induced by a single pulse of EFS Apamin also inhibited a rapid phase, but not a delayed phase of i.j.ps induced by two pulses at 10 Hz. VIP10-28 did not inhibit i.j.ps induced by a single pulse, but significantly inhibited the delayed phase at two pulses. A combination of apamin and VIP10-28 abolished the i.j.ps induced by two pulses. 11. Both VIP and PACAP induced slow hyperpolarization of the cell membrane of the longitudinal muscle. Apamin inhibited the PACAP-, but not VIP-induced hyperpolarization. 12. From these findings it is suggested that VIP and PACAP are involved in NANC inhibitory responses of longitudinal muscle of the rat distal colon via activation of charybdotoxin- and apamin-sensitive K+ channels, respectively.

The influence of polyethylene glycol conjugation on bovine hemoglobin's intrinsic effect on the gastrointestinal system of the rat

The purpose of this study was to determine the impact of polyethylene glycol (PEG) conjugation on bovine hemoglobin's effect on gastrointestinal (GI) blood flow and motility in the Sprague Dawley rat. This study was divided into two parts: part one assessed blood flow, while the other evaluated bolus transit time through the GI. To examine blood flow, thirty-two rats were divided into four experimental groups (PEG-hemoglobin, bovine hemoglobin, Ringer's Lactate and autologous blood sham). Blood flow within the superior mesenteric artery was monitored during graduated isovolemic hemodilution. In the second part of the study, GI motility was estimated by bolus transit time. Thirty-six rats were assigned to four groups (PEG-hemoglobin, bovine hemoglobin, Ringer's Lactate and no treatment sham) and following an overnight fast, the rats were given a bolus injection (25 mL/kg) of test article. Three hours following injection, they received an oral 0.3 mL gavage of a charcoal/arabic gum mixture and were later sacrificed and their GI tract evaluated. Results indicated that the infusion of bovine hemoglobin reduced both baseline blood flow through the mesenteric artery and gastrointestinal transit time. In contrast, PEG-hemoglobin maintained baseline blood flow through the mesenteric artery and had no effect on GI transit time or morphology. Therefore, PEG conjugation of bovine hemoglobin significantly attenuated its intrinsic effect on the GI system of the rat.

Effects of nitric oxide (NO) and NO donors on the membrane conductance of circular smooth muscle cells of the guinea-pig proximal colon

1. The membrane conductance changes underlying the membrane hyperpolarizations induced by nitric oxide (NO), S-nitroso-L-cysteine (NC) and sodium nitroprusside (SNP) were investigated in the circular smooth muscle cells of the guinea-pig proximal colon, by use of standard intracellular microelectrode recording techniques. 2. NO (1%), NC (2.5-25 microM) and SNP (1-1000 microM) induced membrane hyperpolarization in a concentration-dependent manner, the hyperpolarizations to NO and NC developing more rapidly than those to SNP. The slower-developing responses to SNP were mimicked by the membrane permeable analogue of guanosine 3':5' cyclic-monophosphate (cyclic GMP), 8-bromo-cyclic GMP (500 microM), and by isoprenaline (10 microM). 3. The hyperpolarizations to NC and SNP were reduced in a low Ca2+ (0.25 mM) saline and upon the addition of haemoglobin (20 microM), but were not effected by NG-nitro-L-arginine (L-NOARG) (100 microM) or omega-conotoxin GVIA (100 nM). the hyperpolarizations to SNP were also significantly reduced by methylene blue (50 microM). 4. Apamin (250 nM) depolarized the membrane potential approximately 10 mV and reduced the initial transient component of the hyperpolarization to NO (1%) and NC (25 microM), but had no effects on the hyperpolarizations to SNP and cyclic GMP. Tetraethylammonium (TEA) (5-15 mM), had little effect on the membrane responses to NO(1%), NC(2.5-25 microM), SNP(100(-1000) microM) or cyclic GMP(500 microM). However, TEA (5-15 mM) reduced the membrane hyperpolarizations to SNP (10 microM) and isoprenaline (10 microM) in a concentration-dependent manner. The hyperpolarization to isoprenaline (10 microM) remaining in the presence of 15 mM TEA was blocked by ouabain (10 microM). 5. The amplitude of electronic potentials (1 s duration) elicited during NO donor hyperpolarizations were little changed or only slightly reduced (5-25%). However, the amplitude of the electrotonic potentials elicited during maintained electrically-induced hyperpolarizations of similar amplitude were significantly increased (30-150%), suggesting that the non-linear membrane properties of the proximal colon partially mask an increase in membrane conductance elicited during the NO donor hyperpolarizations. 6. Membrane hyperpolarization in the presence of an NO donor, 8-bromo-cyclic GMP, isoprenaline, or upon application of a maintained hyperpolarizing electrical current, often evoked oscillations of the membrane potential. These oscillations were prevented by Cs+ (1 mM). 7. These results indicate that NO and NC hyperpolarize the circular muscle of the proximal colon by activating at least two TEA-resistant membrane K+ conductances, one of which is sensitive to apamin blockade. The K+ conductance increases activated by SNP or 8-bromo-cyclic GMP were little effected by apamin, perhaps suggesting a common mechanism. In contrast, the hyperpolarization to isoprenaline appears to involve the activation of TEA-sensitive Ca2(+)-activated K+ ('BK') channels, as well as a Na:K ATPase. Finally, the 'background' membrane conductance of the circular muscle cells of the proximal colon decreased upon membrane hyperpolarization to reveal oscillations of the membrane potential which may well represent 'pacemaker' or 'slow wave' activity.

Apamin-sensitive and -insensitive components of inhibitory junction potentials in rat caecum: role of nitric oxide

1. The non-adrenergic non-cholinergic (NANC) inhibitory response to electrical field stimulation (EFS) in circular muscle from rat caecum was investigated using the single sucrose-gap technique. EFS with single pulses evoked hyperpolarization oral inhibitory function potential (IJP) of the membrane associated with muscular relaxation or with transient inhibition of spontaneous contractile activity. 2. The amplitude and the duration of the IJPs were enhanced by using train stimulation at increasing frequency. 3. Apamin (10(-7) M) reduced the amplitude of IJPs at all frequencies tested. 4. N omega-nitro-L-arginine methyl ester (L-NAME) (10(-4) M, 5 x 10(-4) M), but not D-NAME, caused a concentration dependent decrease in the amplitude of IJPs at all frequencies tested. L-Arginine (10(-3) M) prevented these effects. 5. L-NAME (5 x 10(-4) M) caused the disappearance of the apamin-resistant IJP-component, evoked by single pulse or by low frequency trains. 6. Sodium nitroprusside (SNP) (10(-4) M), a nitric oxide (NO) donor, induced hyperpolarization of membrane potential and muscular relaxation. SNP-induced effects were not affected by pretreatment of the muscle strips with effective concentrations of tetrodotoxin, apamin, and L-NAME. 7. P2-purinergic antagonists, reactive blue 2 (up to 5 x 10(-4) M) and suramin (up to 3 x 10(-4) M), failed to affect the evoked IJPs. 8. These results show that, in rat caecum, the NANC response to electrical stimulation is composed of two distinguishable components: an apamin-resistant and an apamin-sensitive component. NO or a related compound is mainly involved in the mediation of the apamin-resistant component, while ATP is not the mediator responsible for the apamin-sensitive component.

Nitric oxide synthesis inhibition: the effect on rabbit pyloric muscle

The relaxation mechanism of the pyloric smooth muscle is largely dependent on a nonadrenergic noncholinergic (NANC) inhibitory innervation mediated in part by nitric oxide (NO). The aim of the present study was to investigate the effect of NO antagonists on the contractility of the pyloric smooth muscle. In the clinical trial, 10 anesthetized experimental rabbits were infused intraarterially with the NO synthesis inhibitor N-nitro-L-arginine (L-NNA), at a concentration of 10(-4) mol/L; 10 controls received normal saline intraarterially. Pyloric contractility was assessed by balloon manometry. L-NNA infusion produced a dose-dependent increase in the frequency of the pyloric contraction. The maximal increase in frequency occurred during the slow L-NNA infusion rate of 146 ng/min (baseline-adjusted frequencies of experimental v control: 1.267 +/- 0.389 v 0.632 +/- 0.375; P = .001). The increased frequency level was sustained over the subsequent fast infusion rate of 292 ng/min (experimental v control: 1.362 +/- 0.604 v 0.704 +/- 0.579; P = .022). Both the duration and the amplitude of the pyloric contractions were not affected by the L-NNA infusion. These findings suggest that blockage of the L-arginine-NO pathway may have resulted in inhibition of the NANC-induced gastric muscle and relaxation of the pyloric sphincter. The authors speculate that the decreased NO production may be responsible for the sustained contraction of the pyloric smooth muscle with secondary hypertrophy, characteristic of hypertrophic pyloric stenosis.

Nitric oxide activates multiple potassium channels in canine colonic smooth muscle

1. Nitric oxide (NO), an inhibitory neurotransmitter released from peripheral neurones, hyperpolarizes smooth muscle cells and inhibits contraction. The mechanism of this hyperpolarization is unknown. 2. We have identified three classes of K+ channels activated by NO and NO donors in colonic smooth muscle cells. NO and NO donors increased the open probability of 80 pS channels (KNO1), very small channels (< 4 pS, KNO2), and 270 pS Ca(2+)-activated K+ channels (BK channels) in cell-attached patches. 3. Dibutyryl cGMP and 8-bromo cGMP also increased the open probability of KNO1 and KNO2 in cell-attached patches. 4. In excised patches of membrane, direct application of NO or the NO donor, S-nitroso-N-acetyl penicillamine (SNAP), increased the open probability of KNO1 and KNO2, but cGMP or dibutyryl cGMP had no effect. SNAP had no effect on the open probability of BK channels in excised patches. 5. The reducing agent dithiothreitol and the alkylating agent N-ethylmaleimide blocked NO-induced channel openings. 6. In summary, the hyperpolarization response to NO in smooth muscles may be mediated by multiple K+ channels. At least two of these classes of channels may be activated by dual pathways involving direct activation by NO and cGMP-mediated mechanisms.

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.

Clinical biology of nitric oxide

Nitric oxide is a pluripotential molecule that acts as both an autocrine and paracrine mediator of homoeostasis, and derangement of its metabolism can be linked with many pathophysiological events. This review provides a broad overview of the basic and clinical scientific aspects of nitric oxide.

Diffuse esophageal spasm: a malfunction that involves nitric oxide?

OBJECTIVE

As recently suggested, nitric oxide (NO) may play an important role in the regulation of esophageal motility, being partly responsible for the latency period and latency gradient between the onset of a swallow and contractions of esophageal circular smooth muscles. Diffuse esophageal spasm appears to be a classical example in which the mechanisms normally responsible for the physiologic timing of the contractions occurring in the esophageal body after swallowing are disturbed.

METHODS

Five patients (one male and four female; age, 18-48 years) with symptomatic esophageal spasm were give glyceryl trinitrate (GTN) intravenously in gradually increasing doses or L-arginine on two separate occasions and underwent manometric measurements of esophageal motility after wet swallows, using a multilumen perfused catheter system (Synetics Medical, Stockholm, Sweden). The amplitude, duration, and propagation of the contractions and the latency period were analyzed, using specially designed software. Additionally, during the GTN infusion period arterial blood pressure was measured every 5 min,

RESULTS

GTN infusion given at a dose of 100 to 200 micrograms/kg-h intravenously caused the occurrence of and a dose-dependent elongation of the latency period after swallowing. The mean amplitude of the contractions did not show any significant alterations, whereas the mean duration of the contractions decreased significantly, from 11.2 +/- 4.8 sec to 5.4 +/- 0.8 sec. These effects were accompanied by significant alleviation of symptoms during swallowing. Interestingly, no adverse side effects such as headache or flush were observed at any dose of GTN. The blood pressure did not show any changes during the studies in any of the five patients. Administration of L-arginine (300 mg/kg-h intravenously) did not cause any significant alterations of motility pattern or alleviation of dysphagia.

CONCLUSIONS

1) NO may play an important role in the control of human esophageal motility, being involved in the mechanisms responsible for the timing of propulsive contractions in the body after swallowing; 2) GTN may to be of benefit in the treatment of diffuse esophageal spasm in symptomatic patients; and 3) patients with diffuse esophageal spasm may have a malfunction in endogenous NO synthesis and/or degradation.

Nitric oxide: an overview

Nitric oxide (NO), a paracrine-acting gas enzymatically synthesized from L-arginine, is a unique biologic mediator that has been implicated in a myriad of physiologic and pathophysiologic states. It is an important regulator of vascular tone and may be the mediator of the hemodynamic changes involved in sepsis and cirrhosis. In addition, there is increasing evidence that NO is involved in coagulation, immune function, inhibitory innervation of the gastrointestinal tract, protection of gastrointestinal mucosa, and the hepatotoxicity of cirrhosis. It has already been speculated that NO may represent a point of control or intervention in a number of disease states. The purpose of this paper is to provide the surgeon with a broad overview of the scientific and clinical aspects of this important molecule.

Characteristic features of inhibitory junction potentials evoked by single stimuli in the guinea-pig isolated taenia caeci

1. Changes in membrane potential of the guinea-pig isolated taenia caeci evoked by single stimuli have been investigated using intracellular recording techniques. Nifedipine (10 microM) was used to arrest spontaneous muscle action potentials. Single stimuli elicited complex junction potentials which consisted of both excitatory and inhibitory components. 2. The excitatory component of the compound junction potential was unaffected by hexamethonium (100 microM) but abolished by atropine (1 microM) and omega-conotoxin GVIA (10-100 nM). 3. In the presence of atropine, single stimuli elicited fast inhibitory junction potentials (IJPs). IJPs were sometimes biphasic during repolarization with a noticeable 'slow tail'. Apamin (30-100 nM) potently inhibited the fast IJP and revealed an underlying slow IJP. 4. The fast IJP was also abolished by omega-conotoxin GVIA (100 nM). However, the slow IJP was insensitive to omega-conotoxin GVIA but was abolished by cadmium (30 microM). 5. Guanethidine (3 microM) and N omega-nitro-L-arginine (10-100 microM) had no detectable effects on either of the IJPs. The dye Reactive Blue 2 reduced the amplitude of the fast IJP but this reduction was associated with a membrane hyperpolarization. 6. The existence of two distinct IJPs in the guinea-pig taenia caeci has been demonstrated. The ability of omega-conotoxin GVIA to selectively abolish the fast IJP leaving the slow IJP intact suggests that separate nerves are involved in mediating these responses.

Modulation of nitric oxide-dependent relaxation of pig tracheal smooth muscle by inhibitors of guanylyl cyclase and calcium activated potassium channels

Nitric oxide is released from intrinsic nonadrenergic, noncholinergic (NANC) nerves of pig tracheal smooth muscle (TSM) in response to electrical field stimulation (EFS). In this study, we investigated the role of guanylyl cyclase in the NANC relaxation by using guanylyl cyclase inhibitors, LY83583 and methylene blue (MB). The role of large conductance calcium-activated potassium (KCa) channels in mediating NANC relaxation was studied by using inhibitors of this channel, charybdotoxin and iberiotoxin. In carbachol-contracted TSM strips, LY83583 (10-20 microM) and MB (10-100 microM) resulted in inhibition of EFS-induced relaxations at all frequencies studied. Relaxations induced by exogenous 8-Bromo-cyclic 3',5'-guanosine monophosphate (8-Br-cGMP) were unaffected by LY83583. The concentration-relaxation curves to isoproterenol, which acts by elevating adenosine-3',5'-cyclic monophosphate (cAMP), and the nitric oxide donors sodium nitroprusside (SNP) or S-nitroso-n-acetylpenicillamine (SNAP) were unaffected by LY83583. Both charybdotoxin (240 nM) and iberiotoxin (180 nM) attenuated relaxations induced by EFS and SNAP. The role of guanylyl cyclase activation in the relaxation to EFS of pig TSM is suggested by the sensitivity of the responses to MB. The selective inhibitory effects of LY83583 on relaxation to neurally released, but not to the nitric oxide donors, suggests that it acts by inhibiting nitric oxide release. The lack of any effect of LY83583 on isoproterenol- or guanosine, 3'5'-cyclic monophosphate (cGMP)-mediated relaxation suggests a mechanism that does not involve elevation of cAMP but lies proximal to the generation of cGMP. The susceptibility of the relaxations to EFS and SNAP to charybdotoxin and iberiotoxin suggests a mechanism that involves the selective activation of KCa channels in airway smooth muscle cells.

Neurogenic control of myoelectric complexes in the mouse isolated colon

BACKGROUND/AIMS

Little is known about the mechanisms controlling colonic migrating electrical activity. This study investigates the neural processes involved in the generation of migrating myoelectric complexes in the isolated mouse colon.

METHODS

Intracellular electrophysiological recordings were obtained from the circular muscle layer of the mouse colon in vitro in the presence of 2 mumol/L nifedipine.

RESULTS

Complexes occurred approximately every 3 minutes and consisted of 1 mumol/L hyoscine-sensitive rapid oscillations (approximately 2 Hz) superimposed on a slow depolarization (approximately 17 mV); the latter was often preceded by a precomplex hyperpolarization (approximately 7 mV) that was reduced by 250 nmol/L apamin. Five hundred micromolars of hexamethonium or 2 mumol/L of tetrodotoxin abolished the complexes and depolarized the muscle by 8.7 +/- 1.3 mV (n = 9) or 12.1 +/- 1.4 mV (n = 5), respectively. Carbachol (50 nmol/L to 5 mumol/L) produced dose-dependent depolarizations but without rapid oscillations. The nitric oxide synthase inhibitor NG-nitro-L-arginine (100 mumol/L) depolarized the tissue by 17.2 +/- 1.6 mV (n = 8) but had no effect on the rapid oscillations. In the presence of 2 mumol/L tetrodotoxin, 5 mumol/L sodium nitroprusside produced a sustained hyperpolarization (15.5 +/- 2.0 mV; n = 5) but did not restore complexes.

CONCLUSIONS

In the isolated mouse colon, the membrane potential between complexes is maintained by the release of inhibitory neurotransmitters (including nitric oxide), and the formation of complexes involves disinhibition and the simultaneous activation of cholinergic motor nerves.

Nitric oxide is involved in non-adrenergic, non-cholinergic inhibitory neurotransmission in rat duodenum

1. In rat duodenum, electrical field stimulation (EFS) induced a relaxation due to activation of non-adrenergic, non-cholinergic (NANC) inhibitory intramural neurones. 2. Nitric oxide synthase (NOS) inhibitors, N omega-nitro-L-arginine (L-NNA) and N omega-nitro-L-arginine methyl ester (L-NAME), caused a dose-dependent reduction in amplitude of the NANC relaxation. Responses to low frequencies of stimulation were more sensitive to NOS inhibitors than those to high frequencies. 3. Effects induced by NOS inhibitors were stereospecific since D-NNA and D-NAME did not affect NANC relaxation. L-arginine, but not D-arginine, partially prevented the effects induced by NOS inhibitors on NANC relaxation. 4. The nitrovasodilator drug, sodium nitroprusside, caused muscle relaxation which was not affected by preincubation with either tetrodotoxin (TTX), L-NNA or L-NAME. 5. alpha-Chymotrypsin reduced relaxations elicited by stimulation of NANC nerves, especially when high frequencies of stimulation were used. The residual NANC relaxation was further reduced by NOS inhibitors. In the same way, alpha-chymotrypsin was able to further reduce the relaxation observed after NOS inhibitors. 6. These results suggest that nitric oxide (NO) and a peptide are involved in NANC relaxation of rat duodenal smooth muscle. NO and peptidergic pathways act in parallel to produce muscle relaxation and they are preferentially activated by stimuli at low and high frequencies, respectively.

Distribution pattern, neurochemical features and projections of nitrergic neurons in the pig small intestine

The presence and topographical distribution of nitrergic neurons in the enteric nervous system (ENS) of the pig small intestine have been investigated by means of nitric oxide synthase (NOS) immunocytochemistry and nicotinamide dinucleotide phosphate diaphorase (NADPHd) histochemistry. Both techniques yielded similar results, thus confirming that within the pig ENS the neuronal isoform of NOS corresponds to NADPHd. Intrinsic nitrergic neurons were not confined to the myenteric plexus; considerable numbers were also present in the outer submucous plexus. In the inner submucous plexus, NOS immunoreactivity or NADPHd staining was restricted to a few nerve fibres and nerve cell bodies. The nitrergic neurons displayed a wide variety in size and shape, but could all be characterized as being multidendritic uniaxonal. Nerve lesion experiments showed that the majority of the myenteric nitrergic neurons project in an anal direction. Evidence is at hand to show that a substantial proportion of these neurons contribute to the dense nitrergic innervation of the tertiary plexus and the circular smooth muscle layer. Some of the nitrergic neurons of the outer submucous plexus were equally found to send their axons towards the circular muscle layer. In some of the nitrergic enteric neurons, VIP, neuropeptide Y, galanin or protein 10 occurred colocalized, but not calbindin or serotonin. The present findings provide morphological evidence for the presence of NOS in a proportion of the enteric neurons in the small intestine of a large omnivorous mammal, i.e. the pig. The topographical features of the staining patterns of NOS and NADPHd are in accord with the results of neuropharmacological studies and argue for the existence of distinct nitrergic subpopulations acting either as interneurons or as motor neurons.

Participation of nitric oxide in the nicotine-induced relaxation of the cat lower esophageal sphincter

The participation of nitric oxide in the relaxation of the cat lower esophageal sphincter muscle strip in response to electrical field stimulation or administration of nicotine was studied. The nicotine-induced relaxation was mediated via a neuronal pathway, since it was inhibited by administration of hexamethonium or tetrodotoxin. Inhibition of nitric oxide biosynthesis by N-nitro-L-arginine decreased the relaxation induced by nicotine (50 microM) or field stimulation. With the maximal concentration of N-nitro-L-arginine (1 mM) electrical field stimulation-induced relaxation was abolished, while nicotine-induced relaxation decreased by 70%. L-Arginine (1 mM) partly restored this relaxation. Desensitization of P2x receptors by alpha, beta methylene-adenosine 5-triphosphate (alpha, beta-m-ATP) did not change the relaxation induced by either electrical field stimulation or administration of nicotine. It is therefore suggested that the field stimulation-induced relaxation is mediated by the release of nitric oxide, but in the nicotine-produced relaxation is only partly due to nitric oxide, other factor(s) might be also be involved.

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.

Evidence for coexistence of ATP and nitric oxide in non-adrenergic, non-cholinergic (NANC) inhibitory neurones in the rat ileum, colon and anococcygeus muscle

The possible coexistence of the two non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitters, adenosine 5'-triphosphate and nitric oxide in the myenteric plexus was investigated using whole-mount preparations of rat ileum, proximal colon and anococcygeus muscle. The presence of adenosine 5'-triphosphate in neurones was examined using the quinacrine fluorescence technique. After localizing and taking photographs of quinacrine-fluorescent neurones and nerve fibres, the same tissues were then fixed and processed for NADPH-diaphorase activity, a marker for nitric oxide-containing neurones. We have demonstrated for the first time that almost all quinacrine-fluorescent myenteric neurones in the proximal colon are also NADPH-diaphorase reactive, while only a subpopulation of quinacrine-fluorescent neurones in ileum and anococcygeus muscle were also NADPH-diaphorase reactive.

Nerve-mediated nitric oxide production by opossum lower esophageal sphincter

Antagonists of nitric oxide synthesis inhibit nerve-induced hyperpolarization and relaxation of muscle from the opossum lower esophageal sphincter. These studies test the hypothesis that nitric oxide is released during stimulation of intrinsic esophageal nerves. The intrinsic nerves were stimulated with an electrical field (10-sec trains of 1-msec, 30-V pulses delivered at 10 Hz). Nitric oxide production was measured with a DASIBI model 2108 Chemiluminescence NO Analyzer. NG-Nitro-L-arginine, an inhibitor of NO synthase, antagonized nerve-induced relaxation the lower esophageal sphincter. Nerve stimulation increased NO production from 0.50 +/- 0.04 nmol/min/100 mg tissue to 0.87 +/- 0.07 nmol/min/100 mg tissue (P < 0.01). NG-nitro-L-arginine inhibited both basal (0.030 +/- 0.09 nmol/min/100 mg tissue, P < 0.05 vs baseline) and stimulated (0.38 +/- 0.10 nmol/min/100 mg tissue, P < 0.01 vs stimulated). These studies support the hypothesis that nerve stimulation releases nitric oxide from the lower esophageal sphincter.

Quantitative study of the NADPH-diaphorase-positive myenteric neurons of the rat ileum

The subpopulation of myenteric neurons able to synthesize nitric oxide was studied quantitatively in the adult rat, using the NADPH-diaphorase histochemical method on whole-mount preparations of distended distal ileum. The spatial density of NADPH-diaphorase-positive myenteric neurons was 2388 +/- 193/cm2 (S.D.; five rats), comprising about 27% of the nerve cell bodies per ganglion. Most neurons were intensely stained and displayed predominantly a Dogiel type I morphology; about 8% of the labelled nerve cells were ovoid neurons, exhibiting a pale cytoplasmic reaction product. The mean somatic size of all NADPH-diaphorase-positive myenteric neurons was 446 +/- 40 microns2, with a mean nuclear size of 96 +/- 18 microns2 (mean values +/- S.D.; five rats). Such values fell exactly within the range of neuronal sizes of the total myenteric population, marked by means of NADH-diaphorase histochemistry. Therefore, the morphometric analysis did not identify any peculiar cell size feature, characterizing this specific nerve cell subpopulation. Thus, the present study provides quantitative data on the size, density and proportion of those myenteric neurons that may synthesize nitric oxide in the distal ileum of the rat.

Nitric oxide-mediated inhibitory response of rat proximal colon: independence from changes in membrane potential

1. We studied the relation of nitric oxide-mediated relaxation of smooth muscle to changes in membrane potential of cells in the proximal colon of rats. 2. The resting membrane potential and electrical field stimulation (EFS)-induced junction potentials were recorded from the circular and longitudinal muscle cells. 3. Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region (descending relaxation). Relaxation of the longitudinal muscle was also induced by EFS. 4. Inhibitory junction potentials (i.j.ps) were recorded from all circular muscle cells tested, but rarely from the longitudinal muscle cells. 5. The i.j.ps were recorded only in the presence of atropine but relaxations of both muscles were induced even in the absence of atropine. 6. Apamin (100 nM) completely abolished the i.j.ps recorded in both circular and longitudinal muscle cells, but had no significant effect on the relaxations of either. 7. In contrast to apamin, Ng nitro-L-arginine (10 microM) inhibited the relaxations of both muscles, but did not affect the i.j.ps. 8. Exogenously added nitric oxide (0.1-10 microM) induced relaxations of both muscles concentration-dependently, but did not affect the membrane potentials at these concentrations. 9. These data strongly suggest that nitric oxide-mediated relaxation of rat proximal colon is not associated with the i.j.ps of the cell membrane.

Nitric oxide producing neurons in the monkey and human digestive system

Nitric oxide has been proposed as an inhibitory transmitter molecule that plays a role in muscle relaxation and vasodilation in the gastrointestinal tract. The present study analyzes the distribution of nitric-oxide-producing neurons in the monkey and human digestive system by means of nicotinamide-adenine-dinucleotide-phosphate-diaphorase histochemistry. This histochemical method is reliable and convenient for the visualization of neuronal nitric-oxide synthase, the enzyme responsible for nitric-oxide generation. In the gastrointestinal tract, nitric-oxide-synthase-related diaphorase activity was present in nerve fibers running throughout the muscular layer (circular > longitudinal) and in numerous ganglion cells and processes in the myenteric plexus of monkeys and humans. Labelled ganglion cells and fibers also were observed in the submucous plexus, although they were much less numerous than those seen in the myenteric plexus. In the submucosa, a few positive fibers were seen around blood vessels. In the mucosa, stained fibers were sparse at the base of the villi and crypts, whereas they were quite abundant in the muscularis mucosae, especially in the small intestine and colon. In the gallbladder (human), labelling was found in ganglion cells and processes of the innermost and outermost ganglionated plexuses. Stained fibers also were distributed to the muscular layer and, less abundantly, to the mucosa and vasculature. Labelled fibers were more abundant in the sphincter of Oddi (human) than in the gallbladder. In the monkey and human pancreas, nicotinamide-adenine-dinucleotide-diaphorase staining was seen mainly in ganglion cells and fibers of intrapancreatic ganglia, and in processes running among acini, around ducts and in the stroma. A moderate density of stained fibers also was distributed to the vasculature, whereas the islets showed few positive processes. Finally, double label experiments performed in the pancreas showed that the vast majority of neurons producing nitric oxide are immunoreactive for vasoactive intestinal peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of NADPH-diaphorase-positive neurons in the enteric nervous system of the human colon

Recent investigations have demonstrated the colonization of NOS-immunoreactivity and NADPH-diaphorase staining in central and peripheral neurons. The current study investigates the presence and distribution of NADPH-diaphorase-stained nerve cells and fibres in whole-mount preparations of the enteric nervous system of the human colon. Numerous NADPH-diaphorase-stained nerve cell bodies were found in the plexus myentericus and in the plexus submucosus externus. In contrast, we found but very few NADPH-diaphorase-stained nerve cells in the plexus submucosus internus. The majority of the NADPH-diaphorase-stained nerve cells had morphological characteristics similar to those of the Dogiel type I neuron, i.e. possessing broad flat dendrites and one major axonal projection. NADPH-diaphorase-stained nerve fibres can be observed in all three ganglionic networks and in the aganglionic plexus of the circular muscle layer.

Ultrastructural localization of nitric oxide synthase in canine small intestine and colon

The ultrastructural distribution and subcellular localization of nitric oxide synthase (NOS) immunoreactivity and its possible colocalization with vasoactive intestinal polypeptide (VIP) and substance P in the muscularis externa in canine ileum and colon were studied by using polyclonal antisera raised against VIP, substance P, and cerebellar NOS. Immunogold staining, with or without silver enhancement, was carried out directly on ultrathin sections using single and two-faced double immunogold methods. NOS immunoreactivity was observed in nerve profiles in myenteric plexus and circular muscle layer. Immunoreactivity was occasionally detected in smooth muscle cells and interstitial cells of Cajal. The double immunostaining revealed NOS and VIP in the same nerve varicosities but never in the same organelles. NOS was localized in electron-dense material of undetermined nature, whereas VIP was associated with large granular vesicles. Substance P and NOS were never found in the same nerves. These results indicate that NOS is present in the enteric nerves containing VIP but in different organelles and that nitric oxide release probably does not occur by an exocytotic mechanism.

In vivo study of the role of muscarinic receptors in the parasympathetic control of rabbit colonic motility

The aim of the present study was to elucidate the role of the non-M1 muscarinic receptors, in the extrinsic and intrinsic nerve control of in vivo colonic motility. Experiments were performed on the proximal colon of anaesthetized rabbits. In this species, the parasympathetic innervation of the proximal colon originates from the vagus nerves. The action of methoctramine and 4-diphenyl-acetoxy-N-methylpiperidine methobromide (4-DAMP) was studied on excitatory junction potentials (EJPs), and on inhibitory junction potentials (IJPs) elicited in smooth muscle cells by stimulating parasympathetic efferents. The effects of the same drugs on spontaneous spiking activity were also investigated. The EJPs either decreased or disappeared after intra-arterial (i.a.) administration of 4-DAMP (45 pg to 450 ng). In the presence of 4-DAMP, further intravenous (i.v.) administration of pirenzepine (0.1 mg.kg-1) had facilitatory effects on the inhibitory pathway, i.e., after abolition of the EJPs, vagal stimulation elicited IJPs. With the highest dose of 4-DAMP, vagal stimulation immediately elicited IJPs the amplitude of which still increased after pirenzepine. In the presence of 4-DAMP, the spontaneous spike discharge was not noticeably altered. Methoctramine (0.37 to 75 micrograms, i.a. or 50 micrograms to 0.2 mg.kg-1, i.v.) increased the amplitude of the EJPs, whereas it decreased that of the IJPs. In addition, at the same doses, it either initiated or increased spike discharges that were not altered by pirenzepine up to 0.2 mg.kg-1, i.v. The so-called rebound excitation occurring after IJPs was not affected by methoctramine. No change in the EJP or IJP amplitude was observed with gallamine at sufficiently high doses to paralyse striated muscles (up to 3 mg.kg-1.h-1). It is concluded that the parasympathetic excitatory pathway to smooth muscle is blocked by 4-DAMP, whereas it is facilitated by methoctramine. 4-DAMP has no effect on the inhibitory pathway which is strongly depressed by methoctramine; however, the fact that these two drugs have opposite effects indicates that 4-DAMP and methoctramine may act on different muscarinic receptor subtypes. In addition, the facilitatory effects of pirenzepine on IJPs observed in animals pre-treated with 4-DAMP, indicates that the latter drug may act on non-M1 and non-M2 (presumably M3) muscarinic receptors. Methoctramine acts on non-M1 and non-M3 (presumably M2) receptors. The spike discharge induced by methoctramine is presumably due to an increased release of acetylcholine, and possibly also of a non-cholinergic transmitter which has excitatory effects on smooth muscle, the identification of which requires further investigations.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ dependency of the release of nitric oxide from non-adrenergic non-cholinergic nerves

1. The role of Ca2+ in nitrergic neurotransmission was studied in the canine ileocolonic junction. 2. The specific N-type voltage-sensitive Ca2+ channel blocker omega-conotoxin GVIA (CTX, 10-100 nM) significantly reduced the electrically-evoked (2-16 Hz, 1-2 ms pulse width) non-adrenergic non-cholinergic (NANC) relaxations, preferentially affecting those to low frequency stimulation, in circular muscle strips of the ileocolonic junction. In contrast, the nerve-mediated NANC-relaxations in response to acetylcholine (30 microM), gamma-aminobutyric acid (100 microM) and adenosine 5'-triphosphate (100 microM), as well as the relaxations to nitric oxide (NO) (3-10 microM) and nitroglycerin (1 microM), remained unaffected. 3. A NO-related substance (NO-R), released from the ileocolonic junction in response to NANC nerve stimulation (4 and 16 Hz, 2 ms pulse width), was assayed with a superfusion bioassay cascade. CTX (50 nM) reduced the release of NO-R induced by electrical impulses (4 Hz: from 18 +/- 4% to 6 +/- 4%; 16 Hz: from 33 +/- 2% to 14 +/- 4%, n = 5), but not that in response to the nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 0.3 mM). In Ca(2+)-free medium, the release of NO-R evoked by electrical impulses or DMPP was inhibited. The L-type Ca2+ channel blockers verapamil (1-3 microM) and nifedipine (1 microM) had no effect. 4. From these results we conclude that the release of NO-R in response to NANC nerve stimulation is Ca(2+)-dependent. The electrically-evoked release of NO-R results from Ca2+ entry through CTX-sensitive N-type voltage-sensitive Ca2+ channels, whereas that induced by nicotinic receptor activation involves CTX-insensitive Ca2+ channels, different from the L- or N-type.

New transmitters and new targets in the autonomic nervous system

Several recent findings have made research into the autonomic nervous system even more exciting, such as the revelation that nitric oxide is a major neurotransmitter, the delineation of the physiological roles for purines and vasoactive intestinal peptide, and the discovery that the interstitial cells of Cajal are major target cells for enteric innervation. Nitric oxide is probably the major neurotransmitter evoking inhibitory junction potentials in smooth muscle. ATP is a mediator of non-adrenergic non-cholinergic enteric innervation, as well as being a fast neurotransmitter in peripheral and autonomic neuro-neuronal synapses. The interactions between enteric nerves and both immune cells and interstitial cells of Cajal (as pacemaker cells of gut smooth muscle) are forcing a rethink of many aspects of gut physiology.

Nitric oxide mediates nonadrenergic, noncholinergic neural relaxation in the Australian possum

BACKGROUND

Nitric oxide has been shown to play an important role in neurally mediated relaxations of gastrointestinal smooth muscle. The aim of this study was to determine whether NO may be the inhibitory transmitter to circular smooth muscle from the sphincter of Oddi of the Australian brush-tailed possum (Trichosurus vulpecula).

METHODS

The effects of drugs on relaxations evoked by electrical-field stimulation of circular muscle strips precontracted with either erythromycin or carbachol were studied. Preparations were also processed histochemically to determine the presence of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reactivity in the tissue.

RESULTS

NG-nitro-L-arginine methyl ester reduced the amplitude of relaxations; this effect could be partially reversed by millimolar concentrations of L-arginine but not by D-arginine. Oxyhemoglobin also reduced the amplitude of the relaxations, and sodium nitroprusside mimicked the relaxations in precontracted strips. Histochemical processing revealed the presence of nerve cell bodies and nerve fibers associated with the circular muscle layer, which are reactive for NADPH-diaphorase and are thus likely to contain NO synthase.

CONCLUSIONS

These results are all consistent with NO released from nerve cells mediating a significant part of the nonadrenergic, noncholinergic relaxation of the circular muscle layer of the sphincter of Oddi.

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.

Peptide YY stimulates circular muscle contractions of the isolated perfused canine ileum by inhibiting nitric oxide release and enhancing acetylcholine release

Peptide YY (PYY) and neuropeptide Y (NPY), infused into the quiescent isolated perfused canine ileum, dose-dependently increased phasic activity of the circular muscle and decreased tonic output of immunoreactive vasoactive intestinal peptide (VIP) in the venous effluent. The contractions subsided before the prolonged inhibition of VIP output. Motor excitation by PYY, an abundant neuropeptide in this tissue, was reduced by blockade of muscarinic or nicotinic receptors, or inhibition of nitric oxide (NO) synthase, despite continued inhibition of VIP output. A combination of atropine and hexamethonium eliminated the PYY-induced decrement in VIP output and left motor excitation unchanged. Blockade of NO synthase eliminated motility increases under these conditions. Intracellular microelectrode recordings of myenteric plexus-free circular muscle strips found no effect of NPY on the resting membrane potential, or on the field stimulation-induced inhibitory junction potential. Inhibition of VIP release plays no essential role in changing motility. These results suggest that PYY/NPY induce motility by stimulating release of acetylcholine and inhibiting NO release at a locus proximal to but not on nerve terminals.