Accumulating evidence suggests an increased activity of the Na(+)-H+ exchanger in essential hypertension. The present investigation aimed at developing a test for routine measurements. Platelet-rich plasma was added directly to a cuvette placed into an aggregometer containing 140 mmol/l sodium propionate medium (pH 6.7, 37 degrees C). The accumulation of intracellular sodium due to activation of Na(+)-H+ exchange results in an osmotic cell swelling, which is detectable as a decrease in optical density (OD). This reaction reflects activation of the Na(+)-H+ exchanger since we observed (1) a dose-dependent inhibition by amiloride (inhibition constant, Ki = 10 mumol/l) and ethylisopropylamiloride (Ki = 0.07 mumol/l) and (2) a dependence on extracellular sodium of the OD changes. Electron microscopy of sodium propionate-treated platelets revealed a general swelling and a distinct decrease in electron density of the cytosol without other significant alterations. Quantification of Na(+)-H+ exchange activities was accomplished by calculating rate constants of the recorded changes in OD. Application of this assay to 20 essential hypertensives and 32 normotensives demonstrated an increased activity of the Na(+)-H+ exchanger in essential hypertensives (rate constants 29.8 x 10(-3) per s versus 21.7 x 10(-3) per s).