Abstract
In cholera toxin-treated gastric chief cells, incubation with a cholinergic agonist (carbamylcholine), a regulatory peptide (cholecystokinin), or a calcium ionophore (A23187) causes a dose- and time-dependent potentiation of cAMP levels. Because this augmented response is calcium/calmodulin-dependent, we hypothesized that it was mediated by calcineurin (protein phosphatase 2B). To test this hypothesis, we examined the actions of calcineurin inhibitors on secretagogue-induced potentiation of cAMP levels in guinea pig chief cells. Preincubation of cells with 0.1 microM FK-506 completely prevented carbachol-induced augmentation of cAMP levels and pepsinogen secretion from cholera toxin-treated cells. Cyclosporin-A, another calcineurin inhibitor, also prevented the augmented cAMP response. FK-506 and cyclosporin inhibited augmentation of cAMP levels following treatment with cholecystokinin(26-33) and A23187, but not the smaller increase in cAMP following treatment with a phorbol ester that activates protein kinase C. Hence, the actions of calcineurin inhibitors were limited to secretagogues that increase cellular calcium. Rapamycin, an agent that competes with FK-506 for the immunophilin, 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 cells. However, a molar excess of rapamycin reversed the inhibitory actions of FK-506. These experiments provide further evidence that the actions of FK-506 on cholera toxin-treated gastric chief cells are caused by its inhibitory actions on calcineurin. FK-506 also inhibited potentiation of cAMP levels when carbachol was added to cells that were preincubated with forskolin, an agent that directly activates adenylyl cyclase. We conclude that, in gastric chief cells, calcineurin mediates cross-talk between the calcium/calmodulin and adenylyl cyclase signaling pathways.
Highlights
In cholera toxin-treated gastric chief cells, incubation with a cholinergic agonist, a regulatory peptide, or a calcium ionophore agents that increase intracellular calcium concentration ([Ca2ϩ]i)1 results in potentiation of pepsinogen secretion
In cells incubated with 0.1 M cholera toxin, there was a steady increase in cAMP to approximately 40 pmol/106 cells by
We show that, in gastric chief cells, calcineurin mediates an interaction between the calcium and adenylyl cyclase signaling systems that results in augmentation of cAMP levels and potentiation of pepsinogen secretion
Summary
In cholera toxin-treated gastric chief cells, incubation with a cholinergic agonist (carbamylcholine), a regulatory peptide (cholecystokinin), or a calcium ionophore agents that increase intracellular calcium concentration ([Ca2ϩ]i)1 results in potentiation of pepsinogen secretion. The actions of calcineurin inhibitors were limcontrast to observations in other tissues [2, 3], in cholera toxintreated chief cells, potentiation of enzyme secretion by agents that increase [Ca2ϩ]i is mediated by an augmentation in cellular levels of cAMP [1]. The additive rapamycin did not prevent carbachol-induced augmentation of cAMP levels in cholera toxin-treated chief cells.
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