Cyclic AMP has distinct effects from Ca²⁺ in evoking priming and fusion/exocytosis in parotid amylase secretion

Abstract.

Rat parotid acinar cells were perfused in small quartz columns to examine the role of cAMP and Ca²⁺ in the priming and fusion/exocytosis of amylase secretion. Carbachol (CCh) evoked a biphasic response of amylase secretion with an initial rapidly occurring large peak and a subsequent sustained plateau. Isoproterenol produced slowly increasing amylase secretion that reached the plateau greater 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 was similar to those induced by CCh but not by isoproterenol, suggesting that the potentiation is caused by isoproterenol-induced modification of the CCh effect. Concentration-dependent responses of CCh-induced amylase secretion with and without isoproterenol showed that isoproterenol greatly enhances both the sensitivity and maximum effect of CCh. Similar potentiation was observed when the Ca²⁺ effect was directly examined in cells permeabilized to Ca²⁺ with ionomycin instead of CCh. In a Ca²⁺-free medium, CCh evoked an initial peak but did not produce a sustained plateau. Isoproterenol did not enhance the effect of CCh on [Ca²⁺]_i. 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 Ca²⁺, has two distinct effects on amylase secretion: triggering of fusion/exocytosis and the priming of secretory granules. Isoproterenol, via cyclic AMP, also has two distinct effects: direct stimulation of priming and enhancement of the sensitivity to the Ca²⁺ 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.