The Na+/H+ exchanger role in the intracellular pH regulation of human platelets.

The intracellular pH of human platelets was measured with a fluorescent intracellular probe. When platelets were in basal conditions (pHo 7.4, [Na+]o 140 mM) the pHi was 6.98 +/- 0.04. Five minutes after the addition of EIPA 60 microM, an inhibitor of Na+/H+ exchange, the pHi fell in 0.075 +/- 0.022 pH units (P less than 0.05). Preincubation in a sodium free, acid medium (pHo 6.3) induced a cell acidification to pH 6.61 +/- 0.03 (P less than 0.01). Preacidified platelets showed a recovery in sodium-containing solution that is a function of [Na+]o. The initial rate of recovery depends on [Na+]o in a Michaelis-Menten fashion, showing a Km of 35.6 mM and a Vmax of 0.213 pH units/min. These results show that the pHi is maintained in human platelets by a Na+/H+ exchange that is active even in basal conditions. The properties of the Na+/H+ exchanger in human platelets and in the less accessible smooth vascular cells are similar; generalized pathological alterations, like hypertension, could be reflected by both tissues.