Gastric emptying in rats: role of afferent neurons and cholecystokinin.

Peptone, acid, and hyperosmolal saline delay gastric emptying in conscious gastric fistula rats. We have now studied the emptying of these solutions in animals pretreated with capsaicin to lesion small diameter primary afferents and in rats with both a gastric and duodenal cannula. In capsaicin-treated rats, hyperosmolal saline did not significantly inhibit gastric emptying, whereas the inhibitory action of acid and peptone was reversed but not abolished. In control rats, the action of peptone was inhibited by the selective cholecystokinin antagonist L364,718, but in capsaicin-treated rats, L364,718 enhanced the action of peptone in delaying gastric emptying. In rats with a duodenal cannula approximately 5 cm from the pylorus, intragastric peptone or hyperosmolal solutions only delayed emptying when the duodenal cannula was closed; in contrast, intragastric acid inhibited gastric emptying when the duodenal cannula was open or closed. The results suggest 1) that all three test meals delay emptying by mechanisms depending at least in part on afferent neurons; 2) peptone delays emptying by at least two mechanisms: one is mediated by cholecystokinin A-type receptors and afferent neurons, and the other requires neither these receptors nor small diameter afferents; and 3) acid, but not peptone or hyperosmolal saline, regulates emptying by an action localized to the stomach or proximal duodenum. The results suggest that there are several different reflex pathways by which liquid test meals act to delay gastric emptying.