G_i affects the agonist‐binding properties of β‐adrenoceptors in the presence of G_s

Pertussis‐toxin‐catalyzed ADP‐ribosylation of G_i in S49 membranes, but not in S49AC⁻ membranes, which lack G_s, induces a threefold reduction of isoproterenol affinity to the β‐adrenoceptors. A similar treatment of turkey erythrocyte membranes, which are devoid of functional G_i, has no effect on β‐agonist affinity to their β‐adrenoceptors. Non‐hydrolyzable analogs such as GTP[S] induce a larger decrease in β‐adrenoceptor affinity in S49 cells towards the agonist isoproterenol as compared to pertussis‐toxin‐catalyzed ADP‐ribosylation of G_i. These results suggest that G_i affects β‐adrenoceptor affinity to its agonist and that this interaction requires the presence of G_s. It seems, therefore, that G_i physically interacts with G_s to exert its effects on the receptor and probably on adenylate cyclase as well. Our ability to detect (a) the effect of pertussis‐toxin‐catalyzed ADP‐ribosylation in S49 cells on β‐agonist affinity and (b) the quantitative difference between the effect of pertussis toxin (approx. threefold) and GTP[S] (fivefold to sevenfold) depends on the use of a simple but rigorous method to study in detail the affinity of β‐agonists to their receptors. This method seems to be superior to the analysis of displacement curves as a means to examine receptor‐ligand interactions.