It is widely agreed that the gastric mucosa secretes bicarbonate in addition to acid. Gastric secretion of bicarbonate is correctly characterized by which of the following statements?

b. Gastric bicarbonate secretion is stimulated by acetylcholine

d. Prostaglandin E2 is a potent stimulant of gastric bicarbonate secretion

The gastric cells responsible for bicarbonate secretion are believed to be surface mucous cells facing the gastric lumen between crypts. Although the total amount of gastric bicarbonate secreted is only a small fraction of total acid secretion, pH close to neutrality is maintained near the mucosal surface while bulk luminal pH is highly acidic. Cholinergic agonists, vagal stimulation, and sham feeding all increase gastric bicarbonate secretion. Prostaglandin E2 and its synthetic derivatives are potent stimulants of bicarbonate secretion as well. Conversely, indomethacin and other drugs that inhibit prostaglandin formation decrease mucosal bicarbonate secretion.