Nociceptive inhibition of migrating myoelectric complex by nitric oxide and monoaminergic pathways in the rat

Per M. Hellström1, Mikael Thollander1, and Elvar Theodorsson2

1 Departments of Medicine, Karolinska Hospital, SE-171 76 Stockholm; and 2 Clinical Chemistry, University Hospital, SE-581 85 Linköping, Sweden

    ABSTRACT
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Abstract
Introduction
Methods
Results
Discussion
References

This study investigated the role of nitric oxide (NO) and adrenergic and dopaminergic mechanisms in reflex inhibition of the migrating myoelectric complex (MMC) after intraperitoneal administration of acid in rats. Acid instilled immediately after an activity front inhibited the migrating complex and prolonged the cycle length from 13.0 ± 0.7 to 98.5 ± 17.2 min (P < 0.001). Administration of Nomega -nitro-L-arginine, reserpine, or guanetidine before acid decreased the prolonged cycle length to 18.1 ± 2.8 (P < 0.001), 19.0 ± 2.0 (P < 0.001), and 27.5 ± 9.3 min (P < 0.001), respectively. Similarly, haloperidol given before acid shortened the prolonged cycle length to 46.7 ± 5.2 min (P < 0.05). There was no effect of phentolamine in combination with propranolol or hexamethonium given alone. After intraperitoneal instillation of acid there was an increase in the plasma levels of somatostatin and a decrease of calcitonin gene-related peptide, but there was no change of neuropeptide Y, vasoactive intestinal peptide, substance P, neurokinin A, or neurotensin. The results indicate that NO and adrenergic, dopaminergic, and somatostatinergic mechanisms cooperate in inhibiting the MMC after nociceptive stimulation of the peritoneum.

acid; calcitonin gene-related peptide; somatostatin

    INTRODUCTION
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Abstract
Introduction
Methods
Results
Discussion
References

IT IS WELL RECOGNIZED that nociceptive stimulation of the peritoneum inhibits gastrointestinal motility. In 1922 Arai (2) demonstrated that intraperitoneal injection of iodine or bacteria decreased propulsion of barium through the gastrointestinal tract. In later studies of small bowel motility in rats, small intestinal transit of contents was inhibited by stimulation of peritoneal nociceptors by intraperitoneal injection of chemical irritants, such as iodine (25). With the use of intraperitoneal acid the migrating myoelectric complex (MMC) was also inhibited for 1-2 h (14), resulting in paralytic ileus. The iodine-induced paralysis was slightly ameliorated by capsaicin treatment but was not affected by alpha - and beta -adrenoceptor blockade (25), suggesting that mediators other than adrenergic transmitters may be involved in this inhibitory response.

Splanchnic nerve resection prevents ileus induced by peritoneal irritation, suggesting involvement of sympathetic nervous pathways including a spinal reflex (2). Bueno and co-workers (5) reported that inhibition of the small bowel myoelectric activity in rats was reduced by demedullation of the spinal cord and abolished by splanchnicectomy, whereas vagotomy had no effect. These findings suggest that intestinal paralysis induced by nociceptive peritoneal stimulation is effected through sympathetic pathways that relay in the prevertebral ganglia. Furthermore, stimulation of intra-abdominal nociceptors in cats causes marked inhibition of gastric motility, mediated through nonadrenergic noncholinergic (NANC) vagal fibers (1, 18). Because the afferents for these two reflexes are essentially the same, the activation of inhibitory NANC reflex mechanisms also in the small intestine may lead to development of paralytic ileus.

Because nitric oxide (NO) is thought to be involved in NANC transmitter functions (35) and NO has been implicated in the inhibition of small bowel motility in rats (6, 23) and dogs (32), we investigated the possible inhibitory role of NO in paralytic ileus in conjunction with adrenergic, dopaminergic, and possibly also serotonergic mechanisms. Because regulatory peptides are also important mediators in the control of small intestinal motility, we measured the plasma concentrations of neuropeptide Y (NPY), somatostatin (Som), vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), substance P (SP), neurokinin A (NKA), and neurotensin (NT) after intraperitoneal acid administration, to clarify possible associations between these neuropeptides and changes in small bowel motility.

    METHODS
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Abstract
Introduction
Methods
Results
Discussion
References

Electromyographic recordings of motility. Eighty-two male Sprague-Dawley rats (B&K, Sollentuna, Sweden) weighing 300-350 g were used. The animals were anesthetized with pentobarbital (50 mg/kg; Apoteksbolaget, Umeå, Sweden). Then, three bipolar, insulated, stainless steel electrodes (SS-5T, Clark Electromedical Instruments, Reading, UK) were implanted into the muscular wall in the small intestine 5, 15, and 25 cm distal to the pylorus. Each animal was implanted with an intraperitoneal catheter for acid administration and a venous catheter for drug administration in the jugular vein. The electrodes and catheters were tunneled subcutaneously and exited at the back of the neck of each rat. A 7-day recovery period was provided after surgery.

Before each experiment the animals were fasted for 24 h with free access to water. For the experiments rats were placed in Bollman cages and electrodes were connected to an electroencephalogram preamplifier 7P5B operating a Grass Polygraph 7B (Grass Instruments, Quincy, MA). The time constant was set at 0.015 s, and the low and high cut-off frequencies were set at 10 and 35 Hz, respectively, for recordings of MMC. For recordings of slow waves a time constant of 0.1 s was used.

All experiments were initiated by recording basal myoelectric activity for 1 h with four propagated activity fronts at all three registration sites. After the fifth activity front passed the duodenal electrode, acid was administered intraperitoneally and the subsequent myoelectric pattern was monitored for 3 h. Computerized calculation was employed for detailed analysis of the characteristics of activity fronts.

The activity front, or phase III, of the MMC was identified as a period of clearly distinguishable intense spiking activity, with an amplitude at least twice that of the preceding baseline, propagating aborally through the portion of the intestine being monitored and followed by a period of quiescence, phase I of the MMC. Phase II of the MMC was characterized by irregular spiking preceding the activity front. Prolonged periods of >30 min with spike potentials, but no discernible cyclic activity, were considered as periods of irregular spiking activity. Acid-induced disruption of MMC cyclicity was assessed by measuring the period between onset of phase III immediately before administration of acid and the reappearance of propagated phase III.

RIA of regulatory peptides. Immunochemical measurements of plasma concentrations of NPY, Som, VIP, CGRP, SP, NKA, and NT were taken after intraperitoneal acid administration. For this part of the study 16 animals were used to obtain plasma concentrations of NPY-like immunoreactivity (LI), Som-LI, VIP-LI, CGRP-LI, SP-LI, NKA-LI, and NT-LI. Through direct heart puncture blood samples were taken 15 min after acid administration in all study groups. The samples were centrifuged at 3,000 rpm for 10 min, and plasma was collected. Plasma samples were then stored frozen at -80°C until extraction procedures and radioimmunoassay (RIA).

The regulatory peptides were adsorbed onto and eluted from Sep-Pak C18 cartridges (Waters, Millipore, Milford, MA). Eluent A consisted of 0.1% trifluoroacetic acid, 0.06 M NaCl, and 99.9% water. Eluent B contained 0.1% trifluoroacetic acid, 19.9% water, 0.06 M NaCl, and 80% methanol. The Sep-Pak was primed using 5 ml eluent A containing 1 mg/ml Polypep (Sigma Chemical, St. Louis, MO), followed by 10 ml eluent B, followed by 10 ml eluent A. The sample to which 0.1% trifluoroacetic acid had been added was then applied, and the column was washed with 2 ml eluent A, followed by 5 ml of a 4:1 mixture of eluent A and eluent B. The samples were eluted with 4 ml of eluent B and evaporated to dryness at 45°C under nitrogen before RIA.

NPY-LI was analyzed using antiserum N1, which cross-reacts 0.1% with avian pancreatic polypeptide but not with other peptides (37). The detection limit of the assay was 11 pmol/l. Intra- and interassay coefficients of variation were 7 and 12%, respectively.

Som-LI was analyzed as described by Grill and collaborators (19). The detection limit was 2 pmol/l, and the intra- and interassay coefficients of variation were 7 and 11%, respectively.

VIP-LI was analyzed using antiserum VIP-2 raised against conjugated porcine VIP. This antiserum did not cross-react with gastrin, pancreatic polypeptide, glucagon, NPY, or NT. Intra- and interassay coefficients of variation were 9 and 13%, respectively (40).

CGRP-LI was analyzed using antiserum CGRPR8 raised in a rabbit against conjugated rat CGRP. High-performance liquid chromatography-purified 125I-histidyl rat CGRP was used as radioligand and rat CGRP as standard. The detection limit of the assay was for rat CGRP and is 9 pmol/l, and the cross-reactivity of the assay to SP, NKA, neurokinin B (NKB), neuropeptide K (NPK), gastrin, NT, bombesin, NPY, and calcitonin was <0.01%. Cross-reactivity toward human CGRP-alpha and -beta was 100 and 120%, respectively.

SP-LI was analyzed using antiserum SP2 (3), which reacts with SP and SP sulfoxide but not with other tachykinins. The detection limit was 10 pmol/l. Intra- and interassay coefficients of variation were 7 and 11%, respectively.

NKA-LI was analyzed using antiserum K12, which reacts with NKA (100%), NKA-(3---10) (48%), NKA-(4---10) (45%), NKB (26%), NPK (61%), and eledoisin (30%) but not with SP (38). The detection limit of the assay was 12 pmol/l. Intra- and interassay coefficients of variation were 7 and 12%, respectively.

NT-LI was analyzed using antiserum H, which reacts with NT, NT-(4---13) (118%), NT-(8---13) (167%), and NT-(9---13) (15%) but not with NH2-terminal fragments of NT. The detection limit of the assay was 8 pmol/l. Intra- and interassay coefficients of variation were 8 and 13%, respectively (39).

Design of studies of paralytic ileus. In the first experimental series, we studied the effect of 0.1 M hydrochloric acid on MMC. After a basal recording period, hydrochloric acid was administered as a 0.5-ml bolus via the intraperitoneal catheter (n = 7). The cycling pattern of MMC before and after administration of hydrochloric acid was compared in the same animal.

In the second series of experiments involving six separate groups, we studied the effect of different drugs that inhibit NO and adrenergic, dopaminergic, serotonergic, as well as preganglionic cholinergic pathways on acid-induced paralytic ileus. Because NO is synthesized from L-arginine by NO synthase (NOS), the NOS inhibitor Nomega -nitro-L-arginine (L-NNA) was used to assess the role of NO in acid-induced ileus. Guanethidine, phentolamine, and propranolol were used to block adrenergic transmission; haloperidol was used to block dopaminergic transmission, and reserpine was used to deplete adrenergic, dopaminergic, and serotonergic stores. In addition, hexamethonium was used as an inhibitor of preganglionic cholinergic pathways. Intraperitoneal acid was administered after each drug, and the myoelectric pattern was continuously monitored until MMC reappeared.

In the first group hydrochloric acid alone at a dose of 0.5 ml of 0.1 mol/l hydrochloric acid was administered intraperitoneally. The MMC was then monitored for the next 3 h (n = 7).

The second group was pretreated with L-NNA intravenously at a dose of 1 mg/kg and then followed 10 min after the onset of the next activity front by an intraperitoneal injection of 0.5 ml of 0.1 mol/l hydrochloric acid (n = 8). In preliminary studies we examined the specificity of L-NNA for nitrergic mechanisms by studying the effect of L-NNA alone at a dose of 1 mg/kg intravenously as well as after administration of 300 mg/kg L-arginine intravenously (n = 4). In all cases L-NNA alone induced a rapid stimulation of myoelectric activity at all electrode sites. This response was inhibited by prior administration of L-arginine (data not shown).

In the third group reserpine was administered intravenously as a bolus of 10 mg/kg. After 24 h intraperitoneal hydrochloric acid was administered (n = 8).

The fourth group was pretreated with guanethidine intravenously at a dose of 3 mg/kg. Four hours later the animals received hydrochloric acid intraperitoneally (n = 7).

In the fifth group 3 mg/kg phentolamine were combined with propranolol at 1 mg/kg, administered intravenously, and followed 10 min after the onset of the subsequent phase III by intraperitoneal hydrochloric acid (n = 9).

In the sixth group 4 mg/kg haloperidol were administered intravenously and followed 10 min after the next phase III by intraperitoneal hydrochloric acid (n = 9).

In the seventh group 10 mg/kg hexamethonium were administered intravenously, followed by intraperitoneal hydrochloric acid in a similar fashion 10 min after the onset of the next phase III of MMC (n = 8).

In the third series of experiments plasma concentrations of regulatory peptides were measured in control animals (n = 8) and in animals that received intraperitoneal hydrochloric acid, resulting in intestinal paralysis (n = 8). Plasma samples were collected 15 min after administration of acid in the latter group or at a similar time point in the control group.

Drugs and other chemicals. Hydrochloric acid, 0.1 mol/l (pH 1.2, 290 mosmol/l), was obtained from Chemicon (Sollentuna, Sweden). L-Arginine, L-NNA, and hexamethonium were purchased from Sigma Chemical. Injectable formulations of reserpine (Serpasil), guanethidine (Ismelin), and phentolamine (Regitin) were kindly supplied by Ciba (Basel, Switzerland). Propranolol (Inderal) was obtained from Zeneca (Cheshire, UK) and haloperidol (Haldol) from Janssen Pharmaceutica (Beerse, Belgium). All compounds were dissolved in saline, with the exception of L-NNA, which was dissolved in alkaline saline at pH 8 before use. All drugs were administered intravenously in volumes of 0.1-0.2 ml.

Statistics. Values are expressed as means ± SE in n animals. Statistical significance was evaluated using the Student's t-test for paired data or analysis of variance followed by the Bonferroni multiple comparisons test where appropriate.

Ethical considerations. The study was approved by the Regional Ethics Committee for the Humane Use of Research Animals in Northern Stockholm, Sweden. Surgical procedures and experiments were performed in accordance with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health).

    RESULTS
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Abstract
Introduction
Methods
Results
Discussion
References

MMC under control conditions. Under control conditions, a regular motility pattern with recurring MMCs was recorded in all animals (Fig. 1). The MMC cycle length under basal conditions in the different study groups is shown in Table 1.


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Fig. 1.   Prolonged inhibition of migrating myoelectric complex (MMC) after intraperitoneal injection of 0.5 ml of 0.1 mol/l hydrochloric acid (arrow). D, electrode site in duodenum 5 cm from pylorus; J1 and J2, electrode sites in jejunum 15 and 25 cm from pylorus. Paper speed, 1 cm/min in tracing.

Effects of drug pretreatment on MMC. L-NNA shortened the MMC cycle length (P < 0.001). Reserpine, guanethidine, phentolamine, and propranolol in combination, as well as haloperidol or hexamethonium given alone had no effect on the MMC cycle length (Table 1).

                              
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Table 1.   MMC cycle length of small intestine under basal conditions and after administration of drugs

Effects of intraperitoneal acid on MMC. Hydrochloric acid instilled intraperitoneally promptly abolished the MMC at all registration levels for a duration of 98.5 ± 17.2 min (P < 0.001, Fig. 1), but there was persistence of slow waves with a frequency of 36.3 ± 1.2 cycles/min (Figs. 2 and 3). After inhibition and reappearance of the MMC, the cycle length was gradually resumed. The cycle length was 37.1 ± 6.3 min between the onset of the first and second activity fronts and 16.2 ± 2.3 min between the second and third activity fronts. Thereafter, the MMC cycle length was completely normalized to 13.2 ± 0.7 min.


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Fig. 2.   During inhibition of MMC induced by intraperitoneal injection of hydrochloric acid (arrow) slow-wave rhythm persisted until activity fronts of MMC reappeared. D, electrode site in duodenum 5 cm from pylorus; J1 and J2, electrode sites in jejunum 15 and 25 cm from pylorus. Paper speed increased from 1 to 10 cm/min in middle of tracing.


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Fig. 3.   Recording showing persistent slow wave rhythm after intraperitoneal injection of hydrochloric acid (arrow). D, electrode site 5 cm from pylorus; J1 and J2, respective electrode sites 15 and 25 cm from pylorus. Paper speed, 10 cm/min in tracing.

Pretreatment with L-NNA prevented the acid-induced reflex inhibition of intestinal motility. The onset of the next activity front was observed already after 18.0 ± 3.1 min (P < 0.001, Fig. 4). The next two MMCs were recorded after 11.8 ± 2.0 and 14.1 ± 2.1 min, respectively.


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Fig. 4.   Effect of Nomega -nitro-L-arginine (L-NNA, 1 mg/kg), reserpine (10 mg/kg), guanethidine (3 mg/kg), phentolamine (3 mg/kg), and propranolol (1 mg/kg) in combination, as well as haloperidol (1 mg/kg) or hexamethonium (10 mg/kg) compared with control on inhibitory motor response to hydrochloric acid administered intraperitoneally. Values are means ± SE. ### P < 0.001 in comparisons between controls and acid alone. * P < 0.05 and *** P < 0.001 in comparisons between group receiving acid alone and after pretreatment with different pharmacological blockers.

Similarly, reserpine prevented the acid-induced reflex inhibition of motility; an activity front was recorded 19.0 ± 5.7 min after acid challenge (P < 0.001, Fig. 4), followed by another activity front after 17.0 ± 1.0 min.

Guanethidine also prevented the acid-induced inhibition of motility; an activity front emerged 27.5 ± 9.3 min after acid challenge (P < 0.001, Fig. 4). Thereafter the MMC cycle length was measured to be 11.9 ± 2.3 min.

Haloperidol reduced the acid-induced inhibition to a lesser extent (P < 0.05), whereas phentolamine and propranolol in combination and hexamethonium did not affect the acid-induced reflex inhibition of motility (Fig. 4).

Effects of intraperitoneal acid on neuropeptides. Intraperitoneal acid increased the circulating levels of Som increase (P < 0.05), whereas the concentrations of CGRP markedly decreased (P < 0.05) and the concentrations of NPY did not change (Fig. 5). There were no detectable levels of VIP, SP, NKA, and NT in peripheral blood before or after challenge with acid.


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Fig. 5.   Effect of intraperitoneally administered hydrochloric acid on plasma concentrations of immunoreactive somatostatin (Som), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP). Values are mean ± SE. * P < 0.05.

    DISCUSSION
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Methods
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In this study, nociceptive stimulation of the peritoneum with hydrochloric acid resulted in prompt inhibition of the MMC. Because the basic electrical slow-wave rhythm was preserved, this effect is not likely to be due to a high acidity with an ensuing nonspecific cell damage. Rather, it appeared to be a specific inhibitory action on motility-regulating systems suggested to involve nitrergic, as well as adrenergic, dopaminergic, and somatostatinergic mechanisms.

The observed acid-induced intestinal paralysis appeared to be a reflex inhibition of MMC, rather than a peritonitis with an inflammatory reaction that induced a disturbance of the MMC. In favor of a reflex mechanism the paralysis occurred immediately after acid administration, and the slow-wave rhythm persisted throughout the period of intestinal paralysis. In contrast, however, peritonitis secondary to bowel perforation has been shown to be associated with a perturbed intestinal motility that first appears 24 h after induction of peritonitis and persists 48-72 h (4). Furthermore, our findings point in favor of a neuronally mediated inhibition of MMC, rather than a humoral catecholamine-induced effect. Adrenoceptor blockade failed to reduce intestinal paralysis, whereas reserpine and guanethidine, which both act at a neuronal site, effectively reduced the inhibitory motility response to intraperitoneal acid. As an explanation for these apparent contradictory results between reserpine and guanethidine, and the adrenergic antagonists, it has been shown that under physiological conditions blockade of alpha - (by phentolamine or phenoxybenzamine) or beta -adrenoceptors (by propranolol) does not influence the occurrence of the MMC (13), indicating that the adrenergic nervous system is not involved in the control of MMC. In addition, under pathophysiological conditions alpha - and beta -adrenoceptor blockade has been shown to be ineffective in inhibiting iodine-induced paralysis, even if a nerve toxin such as capsaicin produces intestinal disinhibition (25), suggesting that mediators in addition to norepinephrine may be involved in inhibitory responses of the gut. In line with this we have previously found that high doses of alpha - and beta -adrenergic blockers in combination do not produce as profound an inhibition of motility as seen with guanethidine. In these studies it was speculated that NPY might be involved in the inhibitory motility response to sympathetic nerve stimulation of the colon in cats (21, 24). It was also demonstrated that NPY is released from sympathetic nerves in the splanchnic area by a guanethidine-sensitive mechanism in cats (27). These findings indicate that other mediators apart from norepinephrine acting on alpha - and beta -adrenoceptors may be involved in inhibitory responses of the gut. Because NPY also has been shown to inhibit the MMC and propulsion of contents through the small intestine (22), it is a most likely candidate for additional sympathetic inhibitory mechanisms in the gut. Furthermore, in this project we tried to measure NPY to verify an increase of the peptide in plasma after challenge with acid. However, the high basal levels of NPY found prevented us from detecting any significant increases in the levels of circulating NPY. Hence, a local inhibitory effect of NPY or some other related transmitter with a similar inhibitory function, such as Som or VIP, cannot be excluded.

Supportive findings for a neuronal mechanism for the acid-induced inhibition of motility come from Smith and co-workers (34), who reported a transient increase in plasma epinephrine simultaneously with a sustained increase of norepinephrine after laparotomy in the dog. In their study, ileus persisted for a long time after plasma concentrations of epinephrine returned to basal values. Furthermore, studies in the rat have demonstrated that adrenalectomy does not reduce the duration of postoperative ileus (8).

In our study we found that reserpine prevented the acid-induced inhibition of motility. This effect seems to be confined to a reserpine-induced depletion of norepinephrine and dopamine stores, whereas the depletory effect of reserpine on serotonin stores is of limited importance because serotonin is generally considered a motility-stimulating transmitter.

Two different neuronal pathways have been implicated in the intestinal sympathetic inhibitory reflex (16). One pathway has been described to consist of afferent neurons from the gut wall that reach the spinal cord. These neurons connect via short interneurons with efferent preganglionic splanchnic neurons that synapse in the prevertebral ganglia with postganglionic sympathetic neurons innervating the myenteric plexus of the gut. Another pathway consists of short afferents from the gut, which are conveyed to the prevertebral ganglia, where they directly connect with efferent postganglionic sympathetic neurons that finally innervate the myenteric plexus. It has been suggested that the source and intensity of peritoneal irritation determine whether the intestinal inhibitory reflex is restricted to the spinal pathways or whether it also involves sympathetic interconnections via the prevertebral ganglia (16). In our hands, hexamethonium, a ganglionic nicotinic receptor antagonist that inhibits the fast excitatory postsynaptic potential induced by acetylcholine, failed to block the acid-induced intestinal paralysis, suggesting that the activated reflex arc is primarily of the short type with afferent fibers that connect with efferent fibers within the prevertebral ganglia. However, another possible ganglionic transmitter that mediates fast ganglionic transmission is 5-hydroxytryptamine (5-HT, serotonin) by activating 5-HT3 receptors (29). Furthermore, a number of neuropeptides such as SP, VIP, and cholecystokinin have accounted for the mediation of a noncholinergic slow excitatory postsynaptic potential (11, 30). Therefore, the possibility that both long and short reflex arcs are involved in the intestinal inhibitory response to intraperitoneal acid cannot be entirely excluded.

As indicated by our findings that guanethidine and reserpine blocked the inhibition of MMC after intraperitoneal acid, an increased sympathetic activity prevails in this type of paralytic ileus. Our results are in agreement with previous pharmacological data (34). In addition, chemical destruction of sympathetic nerves by pretreatment with 6-hydroxydopamine prevents inhibition of gastric emptying and intestinal transit after abdominal surgery in the rat (10). Increased synthesis and release of norepinephrine from the intestinal wall in the rat have been reported (9, 10). In rats, impaired gastrointestinal motility was restored by alpha - but not by beta -adrenoceptor blockade (31). Furthermore, blockade of adrenoceptors prevented inhibition of gastric activity fronts in the dog but had no effect on gastric emptying or small intestinal myoelectric activity and transit of contents (34). Thus it seems that the adrenergic pathway is not the only mechanism responsible for the reflex inhibition evoked by peritoneal irritation.

An important mechanism for the inhibition of motility is dopamine acting at neural D2 receptors. Previous studies have shown that stimulation of D2 receptors decreases acetylcholine release from cholinergic motoneurons innervating the gastrointestinal tract (26). In our study, haloperidol was used as an antagonist on inhibitory D2 neural receptors. Presumably, haloperidol removed dopamine-mediated inhibition and facilitated acetylcholine release, resulting in increased acetylcholine levels (36), which should counteract acid-induced intestinal paralysis.

During intestinal paralysis we observed an increase in plasma concentrations of Som-LI and a decrease in CGRP-LI. In the rat, cell bodies reactive to Som are located mainly in the myenteric plexus (33) and are considered to participate in abolishing peristalsis. Nerve cell bodies reactive to CGRP are found within the myenteric plexus as well, but also in nerve fibers around ganglia, in the mucosa, and around arterioles as peripheral endings of sensory neurons (17). Speculative reasoning would infer that the observed increase in Som may contribute to the inhibition of motility, as Som inhibits the firing rate of myenteric neurons (15) and decreases acetylcholine release (20). The decrease in CGRP is interesting because this peptide has been demonstrated to disrupt MMC and stimulate irregular spiking in the rat small intestine (28). Even if speculative, the observed changes in plasma concentrations of these peptides from the gastrointestinal tract and nervous system may have an association with inhibition of motility as seen after intraperitoneal acid.

L-NNA diminished the period of acid-induced inhibition of the gut. In agreement with this a high density of NOS-positive neurons has been demonstrated mainly confined to the myenteric plexus in the mammalian gastrointestinal tract (12). From immunohistochemical studies of autonomic ganglia, NO appears to be a mediator both in parasympathetic postganglionic neurons as well as in preganglionic sympathetic neurons (7). NO can inhibit gastrointestinal motility either through actions in the autonomic nervous system or within the myenteric plexus to exert a local inhibitory action on the smooth muscle itself. The latter mechanism would be more consistent with the profound inhibition of motility as seen in our acid-induced paralysis. Because L-NNA by itself stimulated the MMC with a shortening of the cycle length, we cannot exclude the possibility that the effect of L-NNA in conjunction with intraperitoneal acid is also related to an alteration of the regulation of the MMC rather than a block of the nociceptive inhibitory response of the gut.

In conclusion, our results indicate that intraperitoneal administration of hydrochloric acid activates an intestinal inhibitory reflex mechanism. Of the different mediators involved, NO in addition to adrenergic and dopaminergic, and possibly also peptidergic mechanisms cooperate in the acid-induced inhibition of the MMC after nociceptive stimulation of the peritoneum.

    ACKNOWLEDGEMENTS

The study was supported by the Swedish Medical Research Council (Grant 7916), the Magnus Bergvall Foundation, the Åke Wiberg Fund, and the Prof. Nanna Svartz Fund.

    FOOTNOTES

Address for reprint requests: P. M. Hellström, Gastroenterology Section, Dept. of Medicine, Karolinska Hospital, SE-171 76 Stockholm, Sweden.

Received 26 March 1997; accepted in final form 24 November 1997.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

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AJP Gastroint Liver Physiol 274(3):G480-G486
0193-1857/98 $5.00 Copyright © 1998 the American Physiological Society




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