Adipocytes from rat epididymal rat pads contain a complex of receptors that either stimulate (Rs) or inhibit (Ri) adenylate cyclase and, within the plasma membrane, the actions of these receptors are regulated by their respective GTP-binding proteins, Ns and Ni. Previously, we demonstrated that ligands for Rs and Ri receptors regulate cellular response, such as the activity of the glucose transporter, in a manner independent of changes in cellular cAMP concentrations. Treatment of fat cells with cholera toxin to modify Ns, and treatment with pertussis toxin to modify Ni, reveals that in regulating their adenylate cyclase-independent actions on the glucose transporter, the Rs and Ri receptors act via the same GTP-binding proteins used to regulate adenylate cyclase activity. In order to continue our examination of the mechanism whereby insulin inhibits lipolysis in the face of elevated cellular cAMP-dependent protein kinase activity, we have purified the hormone sensitive lipase from fat cells. By drwing on experience gained from purification of the adrenal cholesterol esterase, thought to be identical to the fat cell lipase, a rapid purification scheme was developed, based both on selective management of the detergent environment and on developing procedures for early removal of cytoskeletal proteins in cell extracts. Previously, we showed that adenylate cyclase activity in purified adipocyte membranes is stimulated by the calcium/phospholipid-dependent enzyme protein kinase C. Since the effect of the kinase on adenylate cyclase occurs under conditions highly unfavorable for phosphorylation, we conclude that the kinase associates with and thus activates cyclase.
