Duodenal mucosal bicarbonate secretion in humans: a brief review.

The human proximal duodenum serves as the crucible for the neutralization of gastric acid. Methods have been developed and validated that permit isolation of 4-cm segments of either the proximal or distal duodenum. These isolated segments were free of contamination from gastric, pancreatic, and biliary secretions. At rest the healthy human proximal duodenum produced approximately 175 mumol cm-h: or, assuming that the duodenal bulb is approximately 4 cm in length, 700 mumol h. The distal duodenum (the third part) produced significantly less bicarbonate, approximately 25 mumol/cm-h. HCl produced a prompt and sustained increase in bicarbonate output from both duodenal segments. Bicarbonate output was less in the distal duodenum, indicating a proximal-to-distal gradient. Synthetic prostaglandin E1 caused a dose-related increase in output. Substitution of the NaCl perfusate with Na2SO4 produced a brief decrease, suggesting a chloride bicarbonate exchange mechanism. Vasoactive intestinal polypeptide significantly increased proximal duodenal bicarbonate output. Bicarbonate production by the duodenal mucosa is probably an important defensive factor in maintaining mucosal integrity.