Bicarbonate secretion by the rabbit duodenum in vivo: effects of prostaglandins, vagal stimulation and some drugs.

Duodenal HCO-3 secretion in anaesthetized rabbits was measured by continuous titration of the recirculating luminal perfusate at pH 7.4. The segment under study started 3-4 cm distal to the pylorus and was devoid of pancreatic and biliary HCO-3 secretion. On histological examination the submucosa was seen to contain Brunner's glands, mainly of a mucous type. Duodenum in rabbit secreted HCO3- at a considerably higher basal rate (100-125 mu equiv h-1 cm-1 of intestine) than has previously been found in the rat, cat or dog. The cyclo-oxygenase inhibitor indomethacin (20 mg kg-1) reduced the secretion by 30%, while prostaglandin E2 (5-80 microM, luminal) caused a dose-dependent increase. Prostaglandins thus seem to be important in regulation of duodenal HCO3- secretion in the rabbit and may play a role in duodenal protection against acid. Carbachol (1 and 10 micrograms kg-1) and atropine (0.5 and 1 mg kg-1) had no effects whereas hexamethonium (10 mg kg-1) caused a persistent decrease (25%) in secretion. Effects of electrical stimulation of the vagal nerves or injection of the alpha 2-adrenergic agonist clonidine markedly depended on the agent used for anaesthesia. In urethane-anaesthetized animals, clonidine (0.75-75 micrograms kg-1) tended to increase the secretion whereas with nembutal, clonidine (5-150 micrograms kg-1) decreased it significantly. Electrical stimulation of the cervical vagal nerves decreased the HCO3- secretion in urethane-anaesthetized animals but had no significant effect during nembutal anaesthesia. The responses in the nembutal-anaesthetized rabbit are similar to those previously observed in the cat, rat or dog.