Phorbol esters inhibit alpha1 adrenergic stimulation of glycogenolysis in isolated rat hepatocytes

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Abstract

Tumor promoting phorbol esters can stimulate Ca++-phospholipid-dependent protein kinase. It has been suggested that this enzyme may mediate the effects of calcium-dependent hormones. In this paper the effects of phorbol 12-myristate 13-acetate (TPA) on isolated rat hepatocyte metabolism were studied. Phorbol esters completely blocked alpha1-adrenergic stimulation of glycogenolysis. This effect is quite specific for alpha1-adrenergic actions, as the stimulations of glycogenolysis by vasopressin, angiotensin II, ionophore A-23187 and glucagon were unaffected by TPA. The potencies of the different phorbol esters used in this study suggests that the inhibitory effects of these agents may be due to activation of protein kinase C. The effect of phorbol esters on alpha1-adrenergic actions seems to occur at an early step of the alpha1-adrenergic action. TPA (10−11–10−6M) was unable to stimulate glycogenolysis. Urea synthesis, which is stimulated by vasopressin and alpha1-adrenergic agents, was not stimulated by phorbol ester, neither alone nor in combination with the Ca++ ionophore A-23187.

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