The duodenal mucosal bicarbonate secretion.

The duodenal lumen is exposed to aggressive factors with a high potential to cause damage to the mucosa. Bicarbonate secretion by the duodenal mucosa is accepted as the primary important defense mechanism against the hydrochloric acid intermittently expelled from the stomach. The present work concerns both the influence of the central nervous system and the effects of the hormone melatonin on duodenal bicarbonate secretion in anesthetized rats in vivo as well as effects of melatonin on intracellular calcium signaling by duodenal enterocyte in vitro examined in tissues of both human and rat origin. The main findings were as follows: Melatonin is a potent stimulant of duodenal mucosal bicarbonate secretion and also seems to be involved in the acid-induced stimulation of the secretion. Stimulation elicited in the central nervous system by the alpha1-adrenoceptor agonist phenylephrine induced release of melatonin from the intestinal mucosa and a four-fold increase in alkaline secretion. The melatonin antagonist luzindole abolished the duodenal secretory response to administered melatonin and to central nervous phenylephrine but did not influence the release of intestinal melatonin. Central nervous stimulation was also abolished by synchronous ligation of the vagal trunks and the sympathetic chains at the sub-laryngeal level. Melatonin induced release of calcium from intracellular stores and also influx of extracellular calcium in isolated duodenal enterocytes. Enterocytes in clusters functioned as a syncytium. Overnight fasting rapidly and profoundly down-regulated the responses to the duodenal secretagogue orexin-A and the muscarinic agonist bethanechol but not those to melatonin or vasoactive intestinal polypeptide.