Gastric mucus is secreted to form a gel layer on the gastric mucosa, which acts as a lubricant for ingested food and protects against injury from digestive juice, alcohol, hypertonic or hypotonic foods, spices, drugs, etc. However, even in the absence of food in the stomach, the gastric mucosa should be coated with mucus against unexpected invasions. Therefore it is thought that gastric mucus secretion is controlled by many factors in different ways. Prostaglandin E 2 (PGE 2 ) and secretin, which are known inhibitors of gastric acid secretion, stimulate mucus secretion.1,2) Generally, gastric mucus is secreted via two or three pathways 3): receptor-mediated exocytosis; apical expulsion; and possibly exfoliation. In receptormediated mucus secretion, many physiologically active substances including neurotransmitters, autacoids, hormones, and their related substances are known to be secretagogues.
3)We have reported that carbachol, cholecystokinin octapeptide (CCK-8), secretin, and PGE 2 strongly stimulated mucus secretion from cultured rat gastric epithelial cells and gastrin I stimulated it weakly, but histamine did not.1) Although secretagogues of mucus secretion resemble those of gastric acid and pepsinogen secretion, the functions of secreted substances are different; gastric acid and pepsinogen are used for digestion, but mucus protects the gastric mucosa. If the regulatory systems of gastric acid, pepsinogen, and mucus secretion were completely the same, digestion in the stomach would be unsuccessful. Since common secretagogues stimulate secretion by specific cells, the different regulatory mechanisms of secretion must work in each type of cell. However, the signal transduction mechanisms of gastric mucus secretion in the rat are obscure. Candidates for the second mediators have been reported to be cAMP, 4) phosphatidylinositol (PI) turnover (Ca 2ϩ and protein kinase C), 5) guanosine 3Ј,5Ј-cyclic monophosphate, 6) and nitric oxide. 6) To clarify the signal transduction mechanisms of receptor-mediated exocytosis, we focused on cAMP and PI turnover. Although it is known that cAMP accumulation and PI turnover are related to mucus secretion, no direct results have been obtained in the rat. In this paper, we investigate the mucus secretion through cAMP accumulation and PI turnover.