Okadaic acid inhibits amylase exocytosis from parotid acini stimulated by cyclic AMP

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Abstract
To evaluate the role of protein phosphorylation in amylase exocytosis, we studied the effects of okadaic acid, a potent inhibitor of protein phosphatase types I and 2A, on amylase release and protein phosphorylation in rat parotid acini. Although okadaic acid by itself weakly stimulated amylase release, it did not potentiate amylase release stimulated by half‐maximum doses of isoproterenol or cAMP, and markedly inhibited their maximum effects. Okadaic acid dose‐dependently increased cAMP‐independent phosphorylation of some proteins and enhanced cAMP‐dependent phosphorylation of 21‐ and 26‐kDa proteins. These results indicate that increase in protein phosphorylation does not necessarily enhance the exocytosis of amylase from parotid acini.