ANALYSIS OF THE ROLE OF RECEPTOR NUMBER IN DEFINING THE INTRINSIC ACTIVITY AND POTENCY OF PARTIAL AGONISTS IN NEUROBLASTOMA X GLIOMA HYBRIDNG108-15 CELLS TRANSFECTED TO EXPRESS DIFFERING LEVELS OF THE HUMAN BETA(2)-ADRENOCEPTOR

Many agonist ligands are known experimentally to display a range of ef ficacies and potencies in different tissues and preparations. To analy ze the role of the levels of receptor expression and availability in t he intrinsic activities and potencies of agonists, the function of a n umber of beta-adrenoceptor ligands was examined in clones of neuroblas toma x glioma hybrid NG108-15 cells transfected to express differing l evels of the human beta(2)-adrenoceptor, as well as after treatment of these cell lines with the irreversible beta-adrenoceptor antagonist b romoacetyl alprenolol menthane (BAAM). Clone beta N22 expressed approx imately 10-fold higher levels of the receptor than did clone beta N17. In measurements of agonist stimulation of adenylyl cyclase activity i n membranes of these cells or agonist stimulation of the formation of the complex of G(s alpha) and adenylyl cyclase, which acts as the high affinity binding site for [H-3]forskolin in whole cells, a series of beta-adrenoceptor agonists, including dichloroisoprenaline, ephedrine, dobutamine, and salbutamol, displayed higher intrinsic activity and s howed concentration-response curves that were substantially to the lef t (lower EC(50) values) in clone beta N22, compared with clone beta N1 7. Treatment of clone beta N22 cells with varying concentrations of BA AM reduced the intrinsic activity of these ligands and shifted the con centration-response curves for these agents to the right. In clone bet a N22 cells and membranes, reduction in the observed intrinsic activit y for ephedrine required elimination of a smaller fraction of the beta (2)-adrenoceptor reserve than for salbutamol and reduction in the effe ct of the full agonists isoprenaline and epinephrine was noted only wi th high fractional elimination of the receptor pool. The effect of iso prenaline was substantially reduced, however, by BAAM treatment of clo ne beta N17 cells, where the beta(2)-adrenoceptor number approached ex tremely low levels. Analysis of the data using the formalisms of Whale y et al. [Mel. Pharmacol. 45:481-489 (1994)] showed that prediction of alterations in agonist potency with receptor number for full agonists can be adequately extended to partial agonists.