Evidence for the involvement of protein phosphorylation in cyclic AMP-mediated amylase exocytosis from parotid acinar cells.

We evaluated the role of protein phosphorylation in cAMP-mediated amylase exocytosis from parotid acinar cells by using H89, a new protein kinase A (PKA) inhibitor, which is more lipophilic and 25 times more potent than H8. In our previous studies, H8 markedly inhibited protein phosphorylation without decreasing amylase release [Takuma, T. (1988) Biochem. J. 256, 867-871]. These findings were completely reproduced even in the small acini that were prepared by trypsin treatment before collagenase digestion. In the present study, however, H89 strongly inhibited both amylase release and protein phosphorylation in a dose-dependent manner. The inhibitory effect was specific for PKA at least up to 33 microM, since 33 microM H89 did not block amylase release stimulated by PMA. H85, a closely related compound of H89 without inhibitory effect on PKA, did not prevent amylase release or protein phosphorylation at least up to 33 microM. These results suggest that protein phosphorylation by PKA is involved in cAMP-mediated amylase exocytosis. The inhibition of protein phosphorylation by H8 might be insufficient or inadequate for blocking of amylase release.