Pattern of protein phosphorylation in rat hepatocytes stimulated by glucagon or by the Ca2+-linked hormones

We have adapted the high-resolution electrophoretic technique of O'Farrell to analyze phosphorylated proteins from rat hepatocytes. Total proteins were extracted from rat hepatocytes which had been incubated in the presence of [32P]phosphate and with two types of stimuli: glucagon on the one hand and the Ca2+-linked hormones on the other hand. About 200 phosphorylated polypeptides have been separated. Glucagon modifies the incorporation of [32P]phosphate in at least 17 polypeptides and dibutyryladenosine 3',5'-monophosphate mimics this hormonal effect, implying a common mechanism of action. Phenylephrine (in the presence of the beta-antagonist propranolol), vasopressin and angiotensin all modify the incorporation of [32P]phosphate in about 13 polypeptides; since the Ca2+ ionophore A23 187 reproduces the effect of these agents it may be concluded that Ca2+ mediates their effect. Not all the substrates affected by the two types of hormones are identical. Both types of stimuli increase the phosphorylation of a same set of seven proteins and decrease the phosphorylation of a same set of three proteins but seven proteins have their phosphorylation uniquely enhanced by glucagon whereas three other specific proteins get more phosphorylated by the Ca2+ -linked hormones. The clear differences between the patterns of protein phosphorylation observed in the presence of glucagon and dibutyryladenosine 3',5'-monophosphate on the one hand and by the Ca2+-linked hormones on the other hand strongly suggest different mechanisms of action for these two types of stimuli.