Analysis of the role of receptor number in defining the intrinsic activity and potency of partial agonists in neuroblastoma x glioma hybrid NG108-15 cells transfected to express differing levels of the human beta 2-adrenoceptor.

Many agonist ligands are known experimentally to display a range of efficacies and potencies in different tissues and preparations. To analyze the role of the levels of receptor expression and availability in the intrinsic activities and potencies of agonists, the function of a number of beta-adrenoceptor ligands was examined in clones of neuroblastoma x glioma hybrid NG108-15 cells transfected to express differing levels of the human beta 2-adrenoceptor, as well as after treatment of these cell lines with the irreversible beta-adrenoceptor antagonist bromoacetyl alprenolol menthane (BAAM). Clone beta N22 expressed approximately 10-fold higher levels of the receptor than did clone beta N17. In measurements of agonist stimulation of adenylyl cyclase activity in membranes of these cells or agonist stimulation of the formation of the complex of Gs alpha and adenylyl cyclase, which acts as the high affinity binding site for [3H]forskolin in whole cells, a series of beta-adrenoceptor agonists, including dichloroisoprenaline, ephedrine, dobutamine, and salbutamol, displayed higher intrinsic activity and showed concentration-response curves that were substantially to the left (lower EC50 values) in clone beta N22, compared with clone beta N17. Treatment of clone beta N22 cells with varying concentrations of BAAM reduced the intrinsic activity of these ligands and shifted the concentration-response curves for these agents to the right. In clone beta N22 cells and membranes, reduction in the observed intrinsic activity for ephedrine required elimination of a smaller fraction of the beta 2-adrenoceptor reserve than for salbutamol and reduction in the effect of the full agonists isoprenaline and epinephrine was noted only with high fractional elimination of the receptor pool. The effect of isoprenaline was substantially reduced, however, by BAAM treatment of clone beta N17 cells, where the beta 2-adrenoceptor number approached extremely low levels. Analysis of the data using the formalisms of Whaley et al. [Mol. 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.