| Abstract: | Exposure of rat glioma C6 cells to either isoproterenol or 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in desensitization of isoproterenol-stimulated adenylate cyclase activity. After either treatment, the affinity of beta-receptors for isoproterenol was reduced. Thus, desensitization by TPA or isoproterenol appeared to involve an "uncoupling" of the beta-receptor from the stimulatory regulatory component (Ns) of adenylate cyclase. The activity of Ns, assayed by reconstitution of S49 cyc- adenylate cyclase activity, was found to be unchanged after desensitization. The activity of beta-receptors was measured by inactivating Ns and the catalytic component of adenylate cyclase in C6 membranes and fusing them with membranes lacking beta-receptors. Receptors from isoproterenol-treated C6 cells were less active in "coupling" to the foreign adenylate cyclase than receptors from untreated cells, whereas receptors from TPA-treated cells were fully active. This unexpected latter result was explored further. Lysates from C6 cells were centrifuged on linear sucrose density gradients and the gradient fractions assayed for beta-receptor binding activity. Most of the receptors were recovered in a "heavy" plasma membrane peak but some receptors also appeared in a "light" membrane peak. After treatment of the cells with isoproterenol or TPA, the proportion of receptors in the light peak increased. Prior treatment of the cells with concanavalin A prevented the increase in light receptors caused by isoproterenol or TPA. In addition, the concanavalin A treatment prevented the desensitization of adenylate cyclase caused by TPA but not that caused by isoproterenol. Finally, desensitization of adenylate cyclase was reversed by polyethylene glycol-induced fusion of membranes from cells treated with TPA but not isoproterenol. We conclude that beta-agonists and phorbol esters desensitize adenylate cyclase by distinct mechanisms. Agonists cause a reduction in the functional activity of the beta-receptors followed by a segregation of the receptors into a light membrane fraction devoid of Ns. Phorbol esters do not alter the activity of the receptors but do cause their segregation. |
