Itsuo Takahashi

Comparison of Gallbladder Contractions Induced by Motilin and Cholecystokinin in Dogs

Contractile activity in the gallbladder was measured by means of chronically implanted force transducers in conscious dogs. Intravenous infusion o f motilin (0.3–0.9 mg/kg·h) induced transient contractions during the interdigestive but not during the digestive period. The contractions of the gallbladder occurred simultaneously with the initiation of phase 2 contractions in the duodenum. The duration (5.9 ± 0.15 min) and the contractile force (11.5 ± 0.57 g) were not different from those with three doses o f motilin. The motilin-induced contractions, however, started with dose-related periods of latency and terminated before the cessation of motilin infusion. In contrast, cholecystokinin-octapeptide (0.03–0.09 μg/kg·h) induced dose-dependent tonic contractions (7.8 ± 0.41 g to 17.7 ± 0.77 g) not only during the interdigestive period but also during the digestive period. These contractions also started with dose-related periods of latency and lasted the duration of the infusion. Atropine sulfate (0.025 mg/kg) inhibited both motilin and cholecystokinin—octapeptide-induced contractions of the gallbladder. The present study reveals that motilin has a stimulus effect on the gallbladder only during the interdigestive period and that motilin-induced contractions were different from those induced by cholecystokinin—octapeptide, a common stimulator of the gallbladder.

Mechanism of Cholecystokinin-Induced Contraction of the Opossum Gallbladder

The present study evaluated the mechanism of cholecystokinin-induced gallbladder contraction in awake opossums. Each of 19 chronic animal preparations had an indwelling gallbladder cannula for monitoring changes in gallbladder volume and a jugular catheter for administration of cholecystokinin octapeptide and drugs. An intraduodenal catheter allowed intraduodenal infusion of Isocal (Mead Johnson Laboratories, Evansville, Ind.). Bipolar electrodes in the stomach, duodenum, and jejunum enabled monitoring of the duodenal migratory myoelectric complex cycle. One-hour infusions of cholecystokinin octapeptide (10 ng/kg/min), intraduodenal Isocal (0.4 ml/min), or feeding were started 20 min after cessation of phase 3 duodenal migratory myoelectric complex activity. Bolus intravenous doses of potential pharmacological antagonists were given 10 min before and 20 min after the onset of cholecystokinin octapeptide infusion, Isocal infusion, or feeding. Each test challenge induced about 60% gallbladder emptying at 30 min and 70% at 60 min. Cholecystokinin octapeptide-induced gallbladder emptying was converted to filling by hexamethonium and nearly abolished by atropine. Similar results were obtained for gallbladder emptying induced by feeding or duodenal infusion of Isocal. Phentolamine caused a modest decrease of cholecystokinin octapeptide-induced gallbladder contraction at 30 min, whereas postprandial or Isocal-induced gallbladder contraction was unaffected. Cholecystokinin octapeptide-induced gallbladder contraction was not affected by pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methiodide, prazosin, pyrilamine, cimetidine, methysergide, naloxone, or propranolol. In acute studies of anesthetized animals, gallbladder contraction induced by a D100 intravenous bolus of cholecystokinin octapeptide (800 ng/kg) was not antagonized by hexamethonium, atropine, or tetrodotoxin. It is concluded (a) that cholecystokinin-induced physiologic contraction of the opossum gallbladder occurs through neural mechanisms rather than by a direct action of cholecystokinin on gallbladder smooth muscle, and (b) that studies using pharmacologic bolus doses of cholecystokinin octapeptide and tetrodotoxin indicate that cholecystokinin receptors exist on the gallbladder smooth muscle which do not seem to have any physiological role in gallbladder emptying.

Motilin: a mechanism incorporating the opossum lower esophageal sphincter into the migrating motor complex.

The lower esophageal sphincter (LES) exhibits cyclical phasic contractile activity synchronous with phases II and III of the gastric migrating motor complex. Motilin has been implicated in this process, although the exact mechanism is unknown. The effect of motilin on LES pressure and on gastrointestinal myoelectric activity was examined in 8 unanesthetized opossums. Intraluminal pressure was recorded by a manometric assembly incorporating a sleeve device. Myoelectric activity was recorded from the stomach, duodenum, and jejunum via implanted electrodes. The opossum LES exhibited cyclical periods of phasic contractions synchronous with phases II and III of the gastric migrating motor complex cycle. Variations in the occurrence and magnitude of the phasic LES pressure waves paralleled the spontaneous cyclic fluctuations in the level of circulating plasma motilin. Pulse doses of exogenous motilin (25-400 ng/kg) elicited a contractile LES response that mimicked the spontaneous migrating motor complex-related phasic LES contractions. This effect was dose related with the maximal response occurring at a motilin dose of 100 ng/kg. The LES response to motilin was abolished by hexamethonium and significantly antagonized by atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide, but was not affected by pirenzepine, phentolamine, or naloxone. The study findings support the hypothesis that cyclic increases in circulating endogenous motilin incorporate phasic LES as well as gastric contractile activity into the gastrointestinal migrating motor complex cycle. Motilin acts on the LES by the preganglionic stimulation of cholinergic nerves.

Interdigestive Gallbladder Bile Concentration in Relation to Periodic Contraction of Gallbladder in the Dog

Interdigestive changes in the concentration of bilirubin and sodium in gallbladder bile collected through a chronically indwelling tube in the gallbladder were measured in conjunction with periodic contractions of the gallbladder and immunoreactive motilin concentration in plasma in conscious dogs. It was found that the bilirubin concentration in the gallbladder bile increased as stepped arcs that show periodic decreases with each cycle of the interdigestive migrating contraction in the stomach. The cycle for bilirubin concentration was then repeated, with higher maximum bilirubin values occurring during successive interdigestive migrating contractions cycles. In contrast, the sodium concentration did not progressively increase but fluctuated widely in association with changes in the bilirubin concentration. Peaks in the concentration of bilirubin and sodium generally coincided and occurred in the phase 3 period of the interdigestive migrating contractions in the stomach and the duodenum. However, the initial rise in the sodium concentration after the fall took place earlier than that in the bilirubin concentration in almost one-half of all experiments. Peaks of plasma immunoreactive motilin concentration coincided with the maximum concentration of gallbladder bile in each of the concentrating cycles, but there was no correlation between the two when compared over the entire period of the experiments. These findings suggest that the interdigestive periodic contractions in the gallbladder may play a role in the progressive concentration of bilirubin in gall bladder bile; that is, gallbladder relaxation after termination of the immunoreactive motilin concentration may be associated with aspiration by the gallbladder of dilute hepatic bile. This may in turn stimulate further gallbladder reabsorption. These studies do not allow firm conclusions to be drawn regarding the role of motilin in gallbladder absorption.

Effect of vagotomy on biliary-tract motor activity in the opossum.

In this study, we evaluated the effect of truncal vagotomy in the opossum on changes in gallbladder and sphincter-of-Oddi (SO) contractile activity that occur normally during fasting and after a meal. In six animals, bipolar electrodes were implanted on the SO gastric antrum, duodenum, and jejunum. A catheter secured in the gallbladder fundus was used to monitor gallbladder volume. After control studies, truncal vagotomy and pyloroplasty were done in each animal. Before vagotomy, the SO exhibited cyclic changes in spike-burst rate, from 2 to 5/min, that were synchronized with corresponding phases of the duodenal MMC cycle. The gallbladder exhibited partial emptying during the second half of the duodenal MMC cycle with refilling during the first half of the next cycle. By two weeks after vagotomy, the MMC-related changes in SO and gallbladder contractile activity were normal. In contrast, vagotomy retarded the increase in SO spike-burst rate and delayed as well as diminished the pronounced gallbladder emptying that normally occurs after a meal. The same effects were observed for changes in SO and gallbladder activity induced by intraduodenal infusion of Isocal. After vagotomy, the SO and gallbladder exhibited exaggerated responses to intravenous infusion of CCK-OP or motilin. We conclude that truncal vagotomy in the opossum does not affect the fasting pattern of SO and gallbladder contractile activity, but it significantly alters the normal postprandial pattern of enhanced SO spike-burst rate and pronounced gallbladder emptying. The precise mechanism(s) whereby vagotomy produces these changes remains to be determined.

Computerized analysis of spike-burst activity of the upper gastrointestinal tract

Although the electrical recording of spike-burst activity is a well-established technique, the visual-manual analysis of the electrical tracings is laborious and time consuming. Current automated methods for analyzing upper gastrointestinal spike-burst activity involve the use of computer systems capable of storing large quantities of data. We describe herein an automated system for counting myoelectrical spike bursts that provides online detection of spike bursts using a laboratory computer. Because only the parameters that describe each burst are stored, only a minimal amount of computer memory is required. Subsequent analysis of stored information yields tabular or graphical output of data. Results of the computer analysis are comparable with those obtained by visual-manual assessment.

Variability of Pressure and Myoelectrical Activity Recorded from the Lower Esophageal Sphincter of Fasted Conscious Opossums

The opossum has served as an important model for studies of lower esophageal sphincter (LES) function. Previous investigations, however, have been confined to studies in anesthetized animals. Our aim in this study was to record LES pressure and electrical activity concurrently from conscious opossums. In four animals, we performed a cervical esophagostomy and implanted bipolar electrodes on the abdominal esophagus, LES, gastric antrum and duodenum. Beginning several weeks after surgery, 15 to 20 recording sessions, each lasting 4–6 hr, were obtained in each animal. Intraluminal pressure from the gastric antrum, LES and esophageal body was recorded by a manometric assembly that incorporated a sleeve device, while myoelectric activity from comparable sites was recorded via the implanted electrodes. In fasted conscious animals, interdigestive migratory myoelectric complexes (MMC’s) were readily recorded from the stomach and duodenum. These MMC’s had a cycle length of 86±2.9 (SE) min. The LES exhibited cyclic changes in its intraluminal pressure and myoelectrical activity that occurred in synchrony with the gastric MMC cycle. Basal LES pressure (24.1±2.1 mmHg) was lowest during phase I of the gastric MMC cycle and reached a maximal value (29.5±1.4 mmHg) during phase III of the gastric MMC. Phasic LES contractions began to appear during phase II of the gastric MMC, became pronounced during phase III and disappeared during phase I.