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 o 7.2 at 25°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 + o ( 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.