DESENSITIZATION OF INOSITOL 1,4,5-TRISPHOSPHATE CA2-INDUCED CL- CURRENTS BY PROLONGED ACTIVATION OF G-PROTEINS IN XENOPUS OOCYTES()

Expression of G protein a subunits of the G(q) family with various G p rotein-coupled receptors induces activation of an inositol 1,4,5-trisp hosphate (IP3)/Ca2+-mediated Cl- conductance in Xenopus oocytes, Our p resent data show that two members of this family, the human G alpha(16 ) subunit and the murine homologue G alpha(15) can induce both activat ion and inhibition of these agonist-induced currents, Although extreme ly low amounts (10-50 pg) of injected G alpha(16) subunit cRNA cause m odest (similar to 2-fold) enhancement of ligand-induced Cl- currents i n oocytes co-injected with thyrotropin-releasing hormone (TRH) recepto r cRNA 48 h postinjection, larger G alpha(16) and G alpha(15) cRNA inj ections cause > 10-fold inhibition of TRH or 5HT2c receptor responses, The inhibition is analyzed in this study, The inhibited currents are recovered if various G beta gamma subunit combinations are also expres sed with the G alpha subunits. The constitutively active mutant, G alp ha(16)Q212L, also causes a strong attenuation of the ligand-induced Cl - currents, but this inhibition is not recovered by co-expression of G beta gamma subunits, These results indicate that the free G alpha sub unit is responsible for the inhibitory signal, Although expression of TRH receptor alone produces maximum responses approximately 48 h after injection, co-expression of TRH receptor with G alpha(16) results in enhanced responses 6-12 h postinjection, followed by complete attenuat ion at 36 h, Furthermore, injection of G alpha(16) cRNA alone at compa rable levels gives rise to spontaneous Cl- currents within 6-12 h post injection, suggesting that the early spontaneous activation underlies the later suppression, Expression of other G protein a subunits of the G(q) family, at cRNA levels considerably higher than effective for G alpha(16) produces both analogous spontaneous Cl- currents and, later, inhibition of ligand-induced Cl- currents, Experiments with direct in jection of IP3 and of Ca2+ suggest that this inhibition is consistent with the down-regulation of IP3 receptors, These data indicate that bo th enhancement and inhibition of signaling through G protein-coupled r eceptors can be mediated by the expression level and/or activity of an individual G protein.