Effects of Some Opiates and Vasoactive Intestinal Peptide (VIP) on Duodenal Surface Epithelial Bicarbonate Secretion in the Rat

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Abstract
Bicarbonate secretion by 12 mm segments of duodenum just distal to the Brunner's glands area and devoid of pancreatic bicarbonate was titrated in situ in anaesthetised rats. The secretion increased significantly after intravenous injection of small amounts (20 ng/kg) of the endogenous opioid peptides β-endorphin and methionine enkephalin and maximal (∼twofold) stimulation occurred after 200–500 ng/kg. Morphine (50 μg/kg) caused a similar stimulation and the μ-opiate antagonist naloxone prevented stimulation by morphine. The synthetic analogue [D-Ala2, D-Leu5]- enkephalin (500 ng/kg) which is an agonist at 6-opiate receptors, did not affect the secretion, further suggesting that stimulation is mediated by μ-receptors. VIP (5–100 μg/kg) increased the secretion dose-dependently to levels considerably higher than those observed with opiates, and pretreatment with atropine or indomethacin did not affect the response to VIP. The results suggest a role of endogenous opioid peptides and VIP in the humoral and/or nervous control of duodenal surface epithelial bicarbonate secretion and mucosal protection.

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