G-PROTEIN REGULATION OF THE NA+ H+ ANTIPORTER IN XENOPUS-LAEVIS OOCYTES - INVOLVEMENT OF PROTEIN-KINASE-A AND PROTEIN-KINASE-C/

We have characterized the regulation of the endogenous Na+/H+ exchange r in Xenopus laevis oocytes by G proteins and protein kinases by measu ring the ethylisopropylamiloride-sensitive Li+ uptake. Injection of oo cytes with the stable GTP analog GTP gamma S stimulated Li+ uptake up to almost 4-fold, an effect blocked by coinjection with the GDP analog guanyl-5'-yl thiophosphate. Injection into oocytes of beta gamma subu nits of the heterotrimeric G protein transducin enhanced Li+ uptake by about 3-fold. This stimulation was blocked by transducin alpha subuni ts, which by themselves did not influence Li+ uptake. Using various ac tivators and inhibitors of protein kinases, it is demonstrated that th e X. laevis oocyte Na+/H+ antiporter can be stimulated by activation o f both protein kinase A and C. Stimulation of Na+/H+ exchanger activit y by GTP gamma S but not that induced by transducin beta gamma subunit s was blocked by the protein kinase A inhibitor H-89. On the other han d, transducin beta gamma subunit-stimulated activity was prevented by the protein kinase C inhibitor, calphostin C. The non-selective protei n kinase inhibitor H-7 blocked both GTP gamma S- and transducin beta g amma subunit-stimulated Na+/H+ exchanger activity. The results suggest that the Na+/H+ exchanger of X. laevis oocytes can be activated by G proteins and that this activation is not direct but mediated by protei n kinase A- and/or protein kinase C-dependent pathways.