662 0026-895X/96/030662-08$3.OO/O Copyright © by The American Society for Pharmacology and Experimental Therapeutics All rights of reproduction in any form reserved. MOLECULAR PHARMACOLOGY, 50:662-669 (1996). Agonist-Induced Modulation of Inverse Agonist Efficacy at the 132-Adrenergic Receptor PETER CHIDIAC,1 SANDRINE NOUET, and MICHEL BOUVIER D#{233}partement de Biochimie and Groupe de Recherche sur le Systeme Netveux Autonome, Universit#{235}de Montr#{233}al,H3C 3J7 Canada Received February 23, 1 996; Accepted May 1 6, 1996 SUMMARY Sustained stimulation of several G protein-coupled receptors is known to lead to a reduction in the signaling efficacy. This phenomenon, named agonist-induced desensitization, has been best studied for the p2-adrenergic receptor (AR) and is characterized by a decreased efficacy of 3-adrenergic agonists to stimulate the adenylyl cyclase activity. Recently, several [3-adrenergic ligands were found to inhibit the spontaneous agonist-independent activity of the f32AR. These compounds, termed inverse agonists, have different inhibitory efficacies, ranging from almost neutral antagonists to full inverse agonists. The current study was undertaken to determine whether, as is the case for agonists, desensitization can affect the efficacies of inverse agonists. Agonist-promoted desensitization of the human f32AR expressed in Sf9 cells potentiated the inhibitory actions of the inverse agonists, with the extent of the potenti- ation being inversely proportional to their intrinsic activity. For example, desensitization increased the inhibitory action of the weak inverse agonist labetalol by 29%, whereas inhibition of the spontaneous activity by the strong inverse agonist timolol was not enhanced by the desensitizing stimuli. Interestingly, dichloroisoproterenol acted stochastically as either a weak par- tial agonist or a weak inverse agonist in control conditions but always behaved as an inverse agonist after desensitization. These data demonstrate that like for agonists, the efficacies of inverse agonists can be modulated by a desensitizing treat- ment. Also, the data show that the initial state of the receptor can determine whether a ligand behaves as a partial agonist or an inverse agonist. Recent studies on GPCRS have shown that antagonists can regulate activity in a manner seemingly opposite that of the corresponding agonists ( 1-5). The measurement of this phe- nomenon, usually referred to as inverse agonism, requires that the receptor exhibits a detectable level of agonist-inde- pendent, spontaneous activity. Although such spontaneous activity can be detected in systems expressing relatively low concentrations of receptors (2-4), it is more easily detectable in overexpression systems, such as the baculovirus/Sfi cells (2, 5). Similar to differences among activating ligands, which can be classified as either full or partial agonists, maximal inhib- itory activity has been observed to differ among inverse ago- nists at GPCRS (1, 2, 5, 6). The intrinsic activity ofan agonist refers to its propensity to increase the activity of a receptor (7); inverse agonists analogously are said to have negative intrinsic activity (1 ). The maximal effect of a partial agonist observed experimentally is derived from both the intrinsic This work was supported by grants from the Heart and Stroke Foundation of Canada, the Medical Research Council of Canada, and the International Program for Animal Alternatives of the Procter and Gamble Company. P. C. held a fellowship from the Heart and Stroke Foundation ofCanada, S. N. holds a fellowship from Ia Fondation pour la Recherche Mbdicale, and MB. is a Medical Research Council Scientist. 1 Current affiliation: Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX 75235-9041. activity of the ligand and the sensitivity of the preparation being studied; partial agonists thus may fully activate a receptor response in a highly sensitive system while having relatively little effect in an insensitive one (7). The same principles presumably apply to full and partial inverse ago- nism, but the aforementioned dependence on spontaneous receptor activity implies a further level of complexity. The net effect of an inverse agonist in a particular system thus may reflect at least three factors: the sensitivity of the sys- tem to inverse agonism, the level of spontaneous receptor activity, and the negative intrinsic activity of the ligand. Recent reports in the literature seem to support the gen- eral idea that receptors isomerize rapidly between an active state (R*) and an inactive state (R) and that agonists produce their effects by preferentially binding to and thereby increas- ing the proportion of the active state, whereas inverse ago- nists analogously favor the inactive state (8, 9). In terms of such models, the intrinsic activity of a ligand reflects the difference in its affinities for the two states. Thus, compared with full agonists and full inverse agonists, partial agonists and partial inverse agonists, respectively, are thought to be less selective for the active and inactive states of the receptor. Sustained treatment of the f32AR-expressing cells with ag- onist is known to alter signaling efficacy of agonists. This phenomenon, referred to as agonist-promoted desensitiza- ABBREVIATIONS: AR, adrenergic receptor; DCI, dichloroisoproterenol; GPCRG_prot n-cou#{231} ed receptor. at ASPET Journals on August 30, 2016 molpharm.aspetjournals.org Downloaded from