Negative regulation of the platelet Na+/H+ exchanger by trimeric G-proteins

Human platelets contain a Na+/H+ exchanger (NHE) that regulates the cytosol ic pH. The role of trimeric G-proteins in NHE control was investigated in p lasma membrane vesicles by measuring exchange of intravesicular protons for extravesicular Na+. Exchange was saturable, independent of membrane potent ial and inhibited by ethylisopropyl amiloride (K-i 0.05 mu mol.L-1), demons trating the involvement of NHE-1. The G-protein activators ALF(4)(-) and GM P-P(NH)P reduced exchange by increasing the K-m for Na+ from 11.3 +/- 2.1 m M to 21.6 +/- 1.4 mM (ALF(4)(-)) and 19.8 +/- 1.1 mM (GMP-P(NH)P), leaving V-max and the Hill coefficient unchanged. This effect was abolished by inhi bitors of G(i)-proteins (N-ethylmaleimide, holoenzyme and A-protomer of per tussis toxin) and by an anti-G alpha Ig and GDP(beta )S. Activation of G(i) -proteins by mastoparan and its synthetic analogue Mas7 also strongly reduc ed NHE activity. These data show that in platelets NHE-1 is under negative control of the G(i)-family of trimeric G-proteins.