AGONIST-SPECIFIC ALTERATIONS IN RECEPTOR-PHOSPHOLIPASE COUPLING FOLLOWING INACTIVATION OF G(I2)ALPHA GENE

Different forms of phospholipase A(2), together with pertussis toxin-s ensitive G-proteins, [Ca2+](i) (intracellular Ca2+ concentration), pro tein kinase C, calmodulin, protein tyrosine kinases, mitogen-activated protein kinases and Ca2+/calmodulin-dependent protein kinase appear t o play a role in agonist-mediated release of arachidonic acid. Here we report that fibroblasts from 14-day-old mouse embryos with inactivate d G(i2)alpha (alpha-subunit of the heterotrimeric G-protein G(i2)) gen e display a marked decrease in the ability of lysophosphatidic acid, t hrombin and Ca2+ ionophores to release arachidonic acid compared with their normal counterparts. The requirement for G(i2)alpha in the relea se of arachidonic acid following increased [Ca2+](i) may be explained by the incomplete translocation of cytosolic phospholipase A(2) observ ed in G(i2)alpha-deficient cells. Paradoxically, inactivation of the G (i2)alpha gene resulted in up-regulation of bradykinin receptors and t heir coupling to increased arachidonic acid release, phospholipase C a ctivity and [Ca2+](i). A concomitant increase in basal phospholipase C activity was also observed in the G(i2)alpha-deficient cells. These o bservations establish a pleiotropic and essential role for G(i2)alpha in receptor-phospholipase coupling that contrasts with its less obliga tory participation in agonist-mediated inhibition of adenylate cyclase .