Tonic suppression of gastric acid secretion by endogenous peptides in neonatal rats.

Stimulation of gastric acid secretion by secretagogues was measured in developing rats by in vivo and in vitro techniques. Basal acid outputs in vivo were very low in 8- and 14-day-old rats compared with those in 20- and 30-day-old rats. In 20-day-old rats, all secretagogues increased acid output in vivo, whereas only carbachol, pentagastrin, and sulfated cholecystokinin octapeptide (CCK-8S) were active in 14-day-old rats. In contrast, basal acid output in vitro and stimulation by secretagogues did not differ significantly with age. CCK-8S-stimulated acid output in vitro in 14-day-old rats was blocked by L-365,260, L-364,718, tetrodotoxin, and atropine, but not by hexamethonium, whereas gastrin-stimulated acid output was blocked only by L-365,260. Furthermore, acid output in vivo was elevated three- to fourfold by subcutaneous naloxone-methiodide or L-364,718, but not by L-365,260, in 14-day-old rats; none of these antagonists produced an effect in 20-day-old rats. These studies show that low basal gastric acid output in neonatal rats is caused by tonic inhibitory regulation by endogenous regulatory peptides.