Gastroduodenal HCO3 - secretion is a key process that aids in preventing acid-peptic injury. Endogenous prostaglandins (PGs) play a particularly important role in the local control of this secretion. The secretion of HCO3 - in both the stomach and duodenum was increased in response to PGE2 as well as mucosal acidification, the latter occurring with concomitant enhancement of mucosal PG generation. These HCO3 - responses in the duodenum were markedly reduced by prior administration of the EP4 antagonist in rats, and profoundly decreased in the animals lacking EP3 receptors but not EP1 receptors. In contrast, gastric HCO3 - responses induced by PGE2 and mucosal acidification were prevented by the EP1 antagonist and disappeared in EP1, but not EP3-knockout mice. Consistent with these findings, duodenal HCO3 - secretion was stimulated by both EP3 and EP4 agonists but not EP1 or EP2 agonists, while gastric HCO3 - secretion was increased by the EP1 agonist but not EP2, EP3 or EP4 agonists. In addition, the HCO3 - stimulatory action of sulprostone (EP1/EP3 agonist) in the stomach was inhibited by the Ca2+ antagonist verapamil but not affected by IBMX, the inhibitor of phosphodiesterase, while that in the duodenum was inhibited by verapamil and enhanced by IBMX. Forskolin, the stimulator of adenlate cyclase, increased HCO3 - secretion in the duodenum but not the stomach. Thus, the HCO3 - stimulatory action of PGE2 in the duodenum is mediated by both EP3 and EP4 receptors being coupled intracellularly with both Ca2+ and cAMP, while that in the stomach is mediated by EP1 receptors, coupled with Ca2+.