Vol. 274, Issue 3, G480-G486, March 1998
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 |
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
N
-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 |
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 
- and 
-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 |
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- and - 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-(410) (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-(413)
(118%), NT-(813) (167%), and NT-(913) (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
N-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 |
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.
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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).
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.
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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 N-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.
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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.
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| |
DISCUSSION |
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 - (by phentolamine or phenoxybenzamine)
or -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 - and -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 - and -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 - and
-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 - but not by -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.
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Abstract
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