Modulation of adenylate cyclase of human platelets by phorbol ester

The influence of the phorbol ester, 12‐ O ‐tetradecanoylphorbol 13‐acetate (TPA), a direct avtivator of the Ca 2+ ‐activated, phospholipid‐dependent protein kinase (protein kinase C), was studied on regulation of human platelet adenylate cyclase. Intact platelets were pretreated with the phorbol ester and, thereafter, membranes were prepared and the regulation of the hormone‐sensitive adenylate cyclase in these membranes was studied. The following data were obtained: The TPA treatment applied had apparently no effect on the activity of the catalytic moiety of the platelet adenylate cyclase nor on the stimulatory N s protein nor on stimulatory hormone receptors (prostaglandin E 1 ) and the mutual interactions of these components of the stimulatory hormone‐sensitive pathway. However, the TPA treatment of intact platelets largely impaired the GTP‐dependent, hormone‐sensitive inhibitory pathway to the adenylate cyclase, involving inhibitiory N i protein. The pretreatment led to a large reduction or loss of adenylate cyclase inhibition by GTP itself and by the inhibitory agonists, epinephrine and thrombin, inhibiting the untreated enzyme via separate receptors by an N i ‐mediated process. In contrast, platelet adenylate cyclase inhibition not involving the N i protein was not affected by the TPA treatment. The observed effects of TPA were very rapid in onset and were not shared by a derivative of TPA which did not activate protein kinase C. The data obtained suggest than protein kinase C activated by the phorbol ester interferes with the platelet adenylate cyclase system, leading to a specific alteration of the N i ‐protein‐mediated signal transduction to the adenylate cyclase.