PGE generates intracellular cAMP and accelerates mucin secretion by cultured dog gallbladder epithelial cells

Mucin is the main secretory product of gallbladder epithelial cells. Increased gallbladder mucus secretion has been implicated in gallstone formation in humans. The mechanisms underlying control of mucin synthesis and secretion by the gallbladder are not known. This study aimed to elucidate the efficacy of a panel of secretagogues to stimulate mucin secretion and to determine the intracellular second messengers involved. Studies were carried out on normal well-differentiated epithelial cells from dog gallbladder grown in monolayer culture. Intracellular adenosine 3',5'-cyclic monophosphate (cAMP) as measured by radioimmunoassay increased in response to prostaglandin (PG) E2, PGE1, vasoactive intestinal peptide, epinephrine, and isoproterenol. The greatest effect, a 37-fold increase in cAMP level, was noted with PGE2 at 1.0 microM concentration. In contrast, three breakdown products of phosphatidylinositol (inositol triphosphate, inositol bisphosphate and inositol monophosphate) were not detected with any of the secretagogues tested. Assay of mucin secretion using tritiated N-acetyl-D-glucosamine, a mucin precursor, showed that the same secretagogues noted to increase intracellular cAMP led to an increase in mucin secretion. No correlation was noted, however, between the magnitude of the intracellular cAMP rise and the amount of mucin secreted. A membrane-permeable form of cAMP, dibutyryl cAMP, mimicked PGE2-induced mucin secretion. The results unequivocally show that secretagogue-stimulated mucin secretion in these normal gallbladder epithelial cells can proceed via a cAMP signal transduction pathway.