Activation of mitogen-activated protein kinase in freshly isolated rat hepatocytes by both a calcium- and a protein kinase C-dependent pathway

In the present study, we investigated the role of calcium and protein kinase C (PKC) in the activation of mitogen activated protein kinase (MAPK) in isolated rat hepatocytes. We found that the glycogenolytic hormone norepinephrine (NE), acting through the alpha(1)-adrenergic receptor and the G protein Gq, was able to induce a dose- and time-dependent activation of MAPK in hepatocytes. Vasopressin, which acts through a different receptor but also through stimulation of the dq-dependent pathway, also caused a twofold activation of MAPK. Activation of MAPK by both agonists required the presence of free extracellular calcium and was blocked by the specific PKC inhibitor. Ro 31-8220. MAPK activation was also induced by phorbol myristate acetate (PMA), confirming that a PKC-dependent pathway exists for MAPK activation in liver. Furthermore, calcium-mobilizing agents such as thapsigargin and ionomycin were able to induce an activation of MAPK by a PKC-independent pathway that was totally abolished by preincubation of cells with EGTA. A second pathway for MAPK activation that relies solely on calcium may therefore exist, Ro 31-8220 did not affect phosphorylase activation by ME, vasopressin, thapsigargin, and ionomycin, indicating that PKC inhibition did not interfere with the signaling pathway leading to inositol-1,4,5-triphosphate (IP3)-induced calcium mobilization or with changes in calcium fluxes. The role of MAPK activation by NE and vasopressin in the regulation of hepatic carbohydrate metabolism is discussed. Copyright (C) 1997 by W.B. Saunders Company.