We have been investigating the mechanism of densensitization of hormone-stimulated adenylate cyclase in mammalian cells. Exposure of a clonal line of murine Leydig tumor cells to human chorionic gonadotropin (hCG) resulted in a rapid attenuation of hCG-stimulated adenylate cyclase activity. This desensitiation was both time and dose dependent and occurred without any loss of cell surface hCG-receptors or any change in their affinity for hCG. Desensitization did not occur when the cells were exposed to hCG at OoC but did occur when protein synthesis was blocked by cycloheximide. Desensitization of hCG-stimulated adenylate cyclase also occured in cells exposed to phorbol esters which activate protein kinase C. There was increased incorporation of 32p into the hCG-receptor of cells desensitized by either hCG or phorbol esters. Thus, receptor phosphorylation may be the basis for desensitization. Both agonists and phorbol esters caused desensitization of the adrenergic-stimulated adenylate cyclase in rat glioma C6 cells but by distinct mechanisms. Either treatment caused a redistribution of beta-receptors from the plasma membrane to a lighter density membrane fraction which was devoid of adenylate cyclase activity. Prior treatment of the cells with concanaval in A prevented this shift in receptors. The lectin pretreatment also prevented the phorbol ester-, but not the agonist-mediated desensitization. Finally, beta-receptor functional activity was reduced in cells desensitized by it physical separation from adenylate cyclase. In contrast, phorbol ester-mediated desensitization involves only the physical separation of the receptor from adenylate cyclase. Receptor phosphorylation may be involved in both types of desensitization but at different sites on the receptor.
