DIRECT CHARACTERIZATION OF THE NA+/H+ EXCHANGER IN HUMAN PLATELETS

The kinetic properties of the Na+/H+ exchanger in human platelets were investigated by direct measurements of pH(i) as detected with the fluorescent dye, BCECF. In acid-loaded cells, the antiporter displayed a hyperbolic dependence regarding external Na+ with an apparent K(m) of 38+/-4 mM (pH-omicron 7.2 at 25-degrees-C) whereas its pH(i)-dependent activation between 7.3 to 6.4 did not obey a Michaelian model. External acidification from 7.7 to 6.5 decreased significantly the initial rate of Na+-dependent H+ efflux. The amiloride derivative, ethylisopropylamiloride blocked this exchanger and exerted a non-competitive inhibition with respect to Na+-omicron (K(i) = 17 nM). The cation selectivity of the external site of the antiporter was Na+ > Li+ > K+ and choline. These results indicate that the BCECF technique allows to evaluate the main features of the Na+/H+ exchanger in human platelets, which possesses kinetic properties similar to those reported in other cell types.