Effect of phorbol esters on mitochondrial actions of glucagon

Glucagon generates different second messenger signals in liver. It increases cAMP levels and elevates cytosolic Ca/sup 2 +/ levels by degradation of polyphosphoinositides. The phorbol ester 12-0-tetradecanoyl phorbol 13-acetate (TPA) inhibits glucagon-induced calcium mobilization, but not cAMP formation. TPA can thus be used to assess the role of Ca/sup 2 +/ and cAMP in the activation of mitochondrial processes. In isolated hepatocytes, glucagon increased the steady state NAD(P)H level, probably by activating mitochondrial Ca/sup 2 +/ dependent dehydrogenases. TPA inhibited the glucagon-induced NAD(P) reduction without affecting phosphorylase activation. The effects of glucagon and TPA on mitochondrial respiratory activity and calcium retention were tested after isolation of the mitochondria from perfused livers. Electron transport rates were increased by 15-25% and calcium retention time was increased four-fold after glucagon treatment. When livers were pretreated with TPA, glucagon had no effect on electron transport activity, but calcium retention was increased by the same factor. The results suggest that glucagon-induced calcium mobilization is required for the stimulation of the respiratory activity but not for the increased capacity to retain a calcium overload in the mitochondria.