Ca2+-dependent and -independent secretagogue action on gastric mucus secretion in rabbit mucosal explants

The secretion of high-molecular weight glycoprotein was studied in rabbit antral and fundic explants in response to acetylcholine (ACh), the calcium ionophore A23187, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), forskolin, histamine, pentagastrin, and prostaglandin E2 (PGE2). Glycoprotein secretion was measured in pulse-chase experiments by incorporation of [14C]N-acetyl-D-glucosamine into explants and quantification of the secreted high-molecular weight-labeled glycoprotein. All tested agents except histamine and pentagastrin stimulated glycoprotein secretion in a concentration-dependent manner. The response to A23187 was abolished by removal of extracellular Ca2+ and greatly reduced by the calmodulin antagonist N-(6-amino-hexyl)-l-naphthalenesulfonamide (W7). The response to ACh was abolished by equimolar concentrations of atropine and by depletion of intracellular Ca2+ stores, an effect which was reversible on addition of Ca2+, and reduced by W7. The response to forskolin and PGE2 was not significantly affected by either extracellular or intracellular Ca2+ depletion. The combination of ACh with forskolin and A23187 with TPA had a synergistic effect on secretion. The absolute dependence of cholinergic stimulation on the presence of intracellular Ca2+ and the Ca2+-independent stimulatory effect of forskolin and PGE2 suggest that gastric mucus secretion can be elicited by at least two distinct pathways.