Effect of two new cholecystokinin antagonists on gallbladder emptying in opossums.

In this investigation we evaluated the effect of two new cholecystokinin (CCK) antagonists, CR 1409 and L364,718, on gallbladder emptying in the opossum. Gallbladder emptying was elicited by both exogenous and endogenous CCK. The three test challenges were 1) intravenous infusion of CCK octapeptide (OP) (10 ng.kg-1.min-1), 2) feeding, and 3) intraduodenal infusion of Isocal (0.4 ml/min), a fat-containing nutrient. During control conditions each test challenge elicited approximately 60% gallbladder emptying within 30 min and 70% emptying by 60 min. At given doses both CR 1409 and L364,718 substantially antagonized or abolished the gallbladder emptying elicited by each of the test challenges. The antagonism for postprandial gallbladder emptying was diminished between 30 and 50 min compared with that for CCK-OP infusion and intraduodenal infusion of Isocal. Unexpectedly, the gallbladder emptying induced by infusion of motilin (5 micrograms.kg-1.h-1) was antagonized by either CR 1409 or L364,718. In anesthetized animals, gallbladder contraction was induced by a variety of agonists, such as bethanechol, histamine phosphate, 5-hydroxytryptamine, and phenylephrine. In this later model CR 1409 and L364,718 functioned solely as selective antagonists. We conclude that for the opossum gallbladder 1) the CCK antagonists CR 1409 and L364,718 antagonize or abolish gallbladder emptying induced by exogenous or endogenous CCK; 2) the pattern of CCK antagonism after feeding suggests that the early phase of postprandial gallbladder emptying is mediated by a mechanism other than endogenous CCK, whereas late postprandial emptying is mediated by release of endogenous CCK; and 3) CR 1409 and L364,718 are not totally specific antagonists for gallbladder CCK receptors alone but also antagonize gallbladder contraction induced by motilin.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Cholecystokinin-induced contraction of opossum sphincter of Oddi. Mechanism of action.

In this study, we evaluated the mechanism of action whereby cholecystokinin increases spike-burst rate of the opossum sphincter of Oddi (SO). Each spike burst corresponds to a peristaltic SO contraction. Two types of animal preparations were evaluated: (1) awake chronic animal preparations and (2) anesthetized animals. A total of 19 chronic animals were prepared by implantation of electrodes on the SO, gastric antrum, duodenum, and jejunum. SO spike-burst rate was stimulated by intravenous infusion of CCK-OP (10 ng/kg/min), feeding, or intraduodenal infusion of fat-containing nutrient. Each stimulus was begun 20 min after cessation of phase III duodenal MMC activity and caused an increase in SO spike-burst rate from about 2 to 6/min that lasted for less than or equal to 1 hr. Such increases were antagonized substantially by hexamethonium, atropine, or methysergide. The CCK antagonist, L364718, antagonized the excitatory SO response to CCK-OP infusion or intraduodenal infusion of fat-containing nutrient (Isocal) but did not antagonize the response to feeding; CR1409 had no antagonistic effect on SO response to any of the three types of stimuli. In the acute studies in anesthetized animals, an intravenous bolus dose of CCK-OP (800 ng/kg) caused a substantial increase in SO spike-burst rate that was antagonized by CR1409 but not by atropine, hexamethonium, methysergide, L364718, or TTX.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Fasting and postprandial hepatic bile flow in unanesthetized opossums.

In conscious opossums, we evaluated the relationship between hepatic bile flow and the intestinal motor function during fasting as well as after feeding. In six opossums, bipolar electrodes were implanted from the gastric antrum to the terminal ileum. After cholecystectomy, the common duct was ligated, and a catheter was tied into the proximal common duct for collecting hepatic bile. During subsequent studies, hepatic bile flow was measured, and bile was returned to the duodenum through an externalized duodenal catheter. Cyclic increases in bile flow during fasting did not show a close correlate with the duodenal migratory motor complex (MMC) cycle. Rather, bile flow showed peak values [0.11 +/- 0.02 (SE) ml/min] when phase III MMC activity reached the midileum. Hepatic bile flow correlated closely with the amount of bile acid secreted by the liver. When the bile acid pool was depleted by diverting bile from the intestine, hepatic secretion of bile fell to uniformly low values of approximately 0.04 ml/min that did not show cyclic variation. Hepatic bile flow after feeding increased to a maximal value of 0.12 +/- 0.01 ml/min at 90 min. We conclude that increases in hepatic bile flow during fasting and after meals are determined mainly by variations in intestinal motor activity that alter small bowel transit and thereby affect the enterohepatic circulation of bile acids.

Diaphragmatic contribution to gastroesophageal competence and reflux in dogs.

Events associated with gastroesophageal reflux have been determined by concurrent diaphragmatic and esophageal body electromyography, video radiography, and manometry in four conscious dogs. Three characteristic phenomena occurred in parallel immediately before and during gastroesophageal reflux: 1) transient lower esophageal sphincter relaxation, 2) profound (99.5%) and selective inhibition of crural diaphragmatic activity, and 3) a previously unrecognized dorsal movement of the gastroesophageal junction (mean 1.3 cm) demonstrated by implanted radiological markers. The patterns associated with spontaneous acid and gas reflux were indistinguishable from those induced by gastric distension. Costolumbar diaphragmatic activity was stable up until the instant of sphincter opening, when there was a single costolumbar contraction of short duration and high amplitude. Esophageal shortening did not occur before reflux. Reflux that occurred after atropine-induced inhibition of lower esophageal sphincter tone to < 2 mmHg was intermittent and coincided with selective crural inhibition. These studies demonstrated that selective crural inhibition is a prerequisite for gastroesophageal reflux and suggest that the crural diaphragm is an important factor for the maintenance of gastroesophageal competence.