Role of cyclic AMP and Ca^<2+> in priming and fusion/exocytosis of parotid amylase secretion

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Rat parotid acinar cells were perfused in the column to examine the role of cAMP and Ca^<2+> in the priming and fusion/exocytosis of amylase secretion. Carbachol (Cch) evoked a biphasic response of amylase secretion with an initial rapid, large peak and a subsequent sustained plateau. Isoproterenol produced slowly increasing amylase secretion and reached the plateau that was larger than that induced by Cch. Combined stimulation with isoproterenol and Cch greatly potentiated amylase secretion. The rise and decay of amylase secretion induced by the combined stimulation were similar to those induced by Cch alone but not by isoproterenol suggesting that the potentiation is caused by isoproterenol-induced modification of the Cch effect. Concentration-responses of Cch-induced amylase secretion with and without isoproterenol showed that isoproterenol largely enhances the effect of Cch in both the sensitivity and maximum effect. Similar potentiation was observed when the Ca^<2+> effect was dir … More ectly examined in cells permeabilized to Ca^<2+> with ionomycin instead of Cch. In a Ca^<2+> -free medium, Cch evoked only an initial peak, but did not produce a sustained plateau, Isoproterenol did not enhance the effect of Cch on [CA^<2+>] 2,4-Dintrophenol and carbonyl cyanide m-chlorophenyl hydrazone did not decrease the Cch-induced initial peak of amylase secretion, but markedly decreased the sustained responses induced by isoproterenol and Cch. These results suggest that Cch, via Ca^<2+>, has two distinct effects on amylase secretion ; triggering of fusion/exocytosis and priming of secretory granule. Isoproterenol, via cyclic AMP, also has two distinct effects ; direct stimulation of priming and enhancement of the sensitivity to the Ca^<2+> effects. Thus, isoproterenol stimulates amylase secretion by increasing the primed pools of secretory granules, to the Ca^<2+> effects. Thus, isoproterenol stimulates amylase secretion by increasing the primed pools of secretory granules, whereas Cch increases the flux of secretory granules into/from the primed pools, which is greatly enhanced by isoproterenol. Less