CALCINEURIN MEDIATES CALCIUM-INDUCED POTENTIATION OF ADENYLYL-CYCLASEACTIVITY IN DISPERSED CHIEF CELLS FROM GUINEA-PIG STOMACH - FURTHER EVIDENCE FOR CROSS-TALK BETWEEN SIGNAL-TRANSDUCTION PATHWAYS THAT REGULATE PEPSINOGEN SECRETION

In cholera toxin treated gastric chief cells, incubation with a cholin ergic agonist (carbamylcholine), a regulatory peptide (cholecystokinin ), or a calcium ionophore (A23187) causes a dose- and time-dependent p otentiation of cAMP levels. Because this augmented response is calcium /calmodulin dependent, we hypothesized that it was mediated by calcine urin (protein phosphatase 2B). To test this hypothesis, we examined th e actions of calcineurin inhibitors on secretagogue-induced potentiati on of cAMP levels in guinea pig chief cells. Preincubation of cells wi th 0.1 CCM FK-506 completely prevented carbachol-induced augmentation of cAMP levels and pepsinogen secretion from cholera toxin-treated cel ls. Cyclosporin-A, another calcineurin inhibitor, also prevented the a ugmented cAMP response, FK-506 and cyclosporin inhibited augmentation of cAMP levels following treatment with cholecystokinin(26-33) and A23 187, but not the smaller increase in cAMP following treatment with a p horbol ester that activates protein kinase C. Hence, the actions of ca lcineurin inhibitors were limited to secretagogues that increase cellu lar calcium, Rapamycin, an agent that competes with FK-506 for the imm unophilin, FK binding protein 12, does not inhibit calcineurin. In the present study, preincubation with rapamycin did not prevent carbachol -induced augmentation of cAMP levels in cholera toxin-treated chief ce lls, However, a molar excess of rapamycin reversed the inhibitory acti ons of FK-506. These experiments provide further evidence that the act ions of FK-506 on cholera toxin-treated gastric chief cells are caused by its inhibitory actions on calcineurin. FK-506 also inhibited poten tiation of cAMP levels when carbachol was added to cells that were pre incubated with forskolin, an agent that directly activates adenylyl cy clase. We conclude that, in gastric chief cells, calcineurin mediates crosstalk between the calcium/calmodulin and adenylyl cyclase signalin g pathways.