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Phosphatase inhibitors potentiate adrenergic-stimulated cAMP and cGMP production in rat pinealocytes.

The role of Phosphoprotein Phosphatase in the regulation of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) accumulation in rat pinealocytes was investigated using the three Phosphatase inhibitors calyculin A, tautomycin, and okadaic acid. Calyculin A (0.1 microM) was found to enhance the isoproterenol- and norepinephrine-stimulated cAMP accumulation six- and threefold, respectively, whereas tautomycin and okadaic acid were less effective. The effect of calyculin A was rapid (within 5 min) and persisted in the presence of phosphodiesterase inhibition. However, in contrast to protein kinase C activation or intracellular calcium elevation, the Phosphatase inhibitors were less effective in potentiating the cAMP response stimulated by forskolin or cholera toxin, and their effects were not blocked by calphostin C or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide. The adrenergic-stimulated cGMP response was also less sensitive to the Phosphatase inhibition. Therefore, our results suggest that 1) the adrenergic-stimulated cAMP signal is subjected to the tonic inhibition by Phosphoprotein Phosphatase; 2) Phosphatase inhibitors enhance cAMP synthesis through their actions at the receptor level; and 3) the cAMP signal is more sensitive to the regulation by phosphorylation than cGMP in rat pinealocytes.