Roles of G(i) and G(q/11) in mediating desensitization of the luteinizing hormone/choriogonadotropin receptor in porcine ovarian follicular membranes

Although desensitization of most guanine nucleotide-binding (G) protein rec eptors is triggered by phosphorylation of the receptor, desensitization of the LH/CG receptor (-R) in porcine follicular ovarian membranes appears to be independent of LH/CG-R phosphorylation. We therefore evaluated whether d esensitization of the LH/CG-R reflected a direct inhibition of adenylyl cyc lase (AC) activity by either the alpha-subunit of G(i) or beta gamma-subuni ts derived from any of the membrane G proteins activated in response to LH/ CG-R activation or whether desensitization reflected a competition between G(s) and a G protein that activated phospholipase C for binding sites on th e LH/CG-R. The results showed that follicular membrane AC activity was not inhibited upon activation of the LH/CG-R despite evidence that the ACs in f ollicular membranes, when maximally activated by forskolin, could be inhibi ted when membrane G proteins were activated by guanyl-5'-yl imidodiphosphat e, and that pertussis toxin pretreatment of membranes raised forskolin-stim ulated AC activity, consistent with a tonic inhibition of follicular membra ne AC activity. Similarly, agonist-stimulated desensitization of LH/CG-R-st imulated AC activity was not inhibited by pertussis toxin. Therefore, desen sitization is not the result of inhibition of AC mediated by an inhibitory G(i) subunit. Follicular membrane AC was also not inhibited by G beta gamma subunits freed with activation of G(s), G(q/11), or G(13), based on the in abilities of exogenous G beta gamma to promote desensitization and of a pro tein that sequesters G beta gamma to inhibit desensitization. Desensitizati on was also not inhibited by a G(q/11) C-terminal peptide or antiserum dire cted toward the C-terminus of G(q/11), nor was it reversed with the additio n of G beta gamma to membranes exhibiting desensitized LH/CG-R, suggesting that desensitization is independent of coupling of the LH/CG-R to G(q/11). These results indicate that agonist-dependent desensitization of LH/CG-R-st imulated AC activity is mediated by a unique mechanism.