Direct characterization of the Na+/H+ exchanger in human platelets

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Abstract
The kinetic properties of the Na+/H+ exchanger in human platelets were investigated by direct measurements of pHi 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 (pHo 7.2 at 25°C) whereas its pHi‐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.