SUMMARY

heterogeneity. Mol. Pharmacol. 18: 362-369 (1980). $-Adrenergic agonists appear to induce a conformational change in 65 % of the $-adrener-gic receptors in wild-type S49 lymphoma cell membranes as shown by the increased sensitivity of a subpopulation of receptor to inactivation by the aJ.kylating agent N-ethylmaleimide (NEM). This inactivation is conveniently monitored through the induc-tion with NEM of a biphasic (-)-isoproterenol displacement curve of [‘251]IHYP binding the receptors. In marked contrast, NEM alone or NEM plus a $-adrenergic antagonist hs no effect on receptor binding. The final percentage (65%) of total receptor sites inactivated with agonist plus NEM is not dependent on the concentration of NEM used or on the time of exposure. Agonist plus NEM also causes inactivation of about 75 % of the receptors in the 549 lymphoma /id clone, which has about 25 % of the /-athenergic receptor density of wild-type cells. However, agonist plus NEM treatment does not affect receptor binding in unc and cyc 549 cell membranes, in which the receptors are functionally uncoupled from the adenylate cyclase complex. Further, agonists plus NEM also cause inactivation of 45 to 60 % of the /1-adrenergic receptors in turkey erythrocyt