Mechanisms of Gastric Mucus Secretion from Cultured Rat Gastric Epithelial Cells Induced by Carbachol, Cholecystokinin Octapeptide, Secretin, and Prostaglandin E₂

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The effects of carbachol, cholecystokinin octapeptide (CCK-8), secretin, prostaglandin E₂ (PGE₂), and second mediator-like substances (A23187, phorbol 12-myristate 13-acetate, and dibutyryl cAMP) on mucus secretion from cultured gastric epithelial cells were investigated. Gastric mucus was measured by an enzyme-linked lectin assay with soybean agglutinin and wheat germ agglutinin. Intracellular cAMP and Ca²⁺ were measured with a cAMP assay kit and an image analysis system using fura-2-loaded cells, respectively. Secreted mucus induced by any combination of receptor agonists was almost equal to the summation of each stimulated mucus secretion. On the other hand, combined stimulation with second mediator-like substances secreted mucus synergistically. These results suggest the existence of interactions among receptors for mucus secretion. Based on these results, the secretagogue induced intracellular cAMP and free calcium ([Ca²⁺]_i) levels were measured in cultured gastric epithelial cells incubated with secretagogues. Secretin and PGE₂ induced cAMP accumulation, and carbachol and CCK-8 induced a [Ca²⁺]_i increase. To confirm these results, the effects of protein kinase A and C inhibitors and intracellular calcium chelator on mucus secretion were investigated. An intracellular calcium chelator inhibited the mucus secretion induced not only by carbachol and CCK-8 but also by secretin and PGE₂. These results suggest that the [Ca²⁺]_i plays an important role in mucus secretion through cAMP accumulation.