Effect of neurotensin on the canine gallbladder motility: in vivo and in vitro experiments

Neurotensin and cholecystokinin contract gallbladder circular muscle in chickens

The contractile effects of neurotensin (NT) and cholecystokinin octapeptide (CCK-8) on isolated circular smooth muscle strips of chicken gallbladder were investigated. The NT (0.25-300 nM) produced concentration-dependent contractions on smooth muscle with an EC50 of 8.5 nM (95% confidence limits = 5.3-13.6 nM). In comparison, CCK-8 produced concentration-dependent contractions with an EC50 of 13 nM (95% confidence limits of 9-20 nM). There were no statistical differences in contractile responses when comparing NT and CCK-8 at equimolar concentrations. The NT appears to act directly on smooth muscle tissue in the chicken; the contractile responses were not blocked by 10 µM atropine or tetrodotoxin. A portion of the activity is mediated by extracellular calcium as 100 nM nifedipine inhibited 30% of peptide-induced muscle tension. The NT receptor (NTR) type 1 antagonist SR 48692 (0.1 µM) did not significantly reduce NT potency. The contractile effects of CCK-8 remained unaltered in tissues pretreated with atropine, TTX, or nifedipine. The CCK-A antagonist lorglumide, at a concentration of 1 µM, reduced the contractile potency of CCK-8 by one-half. Avian receptors for NT and CCK may differ pharmacologically from their mammalian counterparts, but their contractile actions on the gallbladder resulting in increased biliary output by flow are further evidence of their role in the postprandial regulation of lipid digestion in chickens.

The peptide hormone xenin induces gallbladder contractions in conscious dogs

Xenin is a 25-amino acid peptide isolated from human gastric mucosa. The biological activities of xenin include modulating intestinal motility and affecting exocrine pancreatic secretion and gastric acid secretion. The physiological effect of xenin on the gastrointestinal tract, however, is incomplete. The objective of this study is to investigate the effects of xenin on the gastrointestinal tract motility of conscious dogs. Gastrointestinal tract and gallbladder contractions were monitored by chronically implanted force transducers. Synthetic xenin was injected intravenously during the interdigestive state with or without pretreatment with cholinergic blockers. The effects of xenin following cholecystectomy and truncal vagotomy were also investigated. Xenin induced gallbladder and jejunal contractions, although a dose-dependent response was shown only with gallbladder contractions. These effects were inhibited by pretreatment with cholinergic blockers, but were not enhanced by truncal vagotomy. The jejunal contractions were completely inhibited by cholecystectomy. The only direct effect of xenin in terms of gastrointestinal motility was to induce gallbladder contractions in conscious dogs. The neural pathway mediating xenin's action was cholinergic, but not the vagal. This novel finding indicates a new role of xenin.

Distribution of neurotensin-immunoreactive neurons in the digestive tract of the chicken

The origin of the neurotensin-containing nerve fibers in the digestive tract of the chicken has been investigated with the use of colchicine and immunohistochemistry. Neurotensin-immunoreactive nerve fibers were found in the smooth muscle layers from the esophagus to the duodenum. Their density of distribution was very high in the esophagus and crop (maximum mean value: 1315/mm2 of sectional area in the lamina muscularis mucosae of the crop) and decreased progressively to the duodenum. Neurotensin-immunoreactive neuronal cell bodies were observed after colchicine treatment in the submucosal plexuses of the esophagus and crop and in the myenteric plexuses of the esophagus, crop, proventriculus and gizzard, and they extended varicose fibers. The number of neurotensin-immunoreactive cell bodies was high in the myenteric plexus of the gizzard (28.3 +/- 2.7/ganglion) but low in the plexuses of the esophagus, crop and proventriculus. Seven days after cutting the glossopharyngeal nerve and vagus nerve unilaterally, the number and extent of neurotensin-immunostained structures in the smooth muscle layers from the esophagus to the gizzard did not show any significant difference between operated and unoperated sides. These results indicate that in the chicken the great majority of neurotensin-immunoreactive enteric fibers originate in the intramural plexuses of the upper digestive wall and are mainly distributed to smooth muscle cells.