Platelet Na+-H+ exchanger activity in normotensive and hypertensive subjects

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Abstract
Objective Primary hypertension has been reported to be associated with an enhancement of Na−+-H+ exchange. However, details of the kinetic properties of the Na+-H+ exchanger in hypertensives and its dependence upon age and gender in normotensives are unknown. Participants We determined the activity of the platelet Na+-H+ exchanger in 20 normotensives and 26 untreated primary hypertensives. Interventions In eight hypertensive individuals antihypertensive treatment was interrupted for 1 week. Treatment for 6 weeks with a daily single dose of 10 mg enalapril decreased mean arterial pressure to 105.7 ± 11.6 mmHg. Methods Platelets were loaded with the intracellular pH (pHj) indicator 2′-7′-bis-carbo-xyethyl-5(6)-carboxyfluorescein (BCECF) and acidified by propionic acid. Initial velocities of pH recovery were determined and used for calculation of maximum velocity (Vmax), baseline pHj and the pHj value for half maximal activation (pH05) of the Na+-H+ exchanger in each individual. Results In normotensives, Vmax averaged 0.05 ± 0.01 dpH/min independently of age, gender and actual diastolic blood pressure. In hypertensives, two different subgroups were defined bearing either low or high Na+-H+ exchange activity. Values of pHj and pH05 were identical in all subgroups irrespective of Vmax. The twofold enhancement of Na+-H+ exchange in the second group was preserved in thrombin-stimulated platelets. Vmax values remained unaffected by enalapril treatment. Conclusions Enhanced Na+-H+ exchange activity in hypertensives is primarily characterized by an increase in Vmax. This enhancement is refractory to antihypertensive treatment and therefore appears to be a relatively fixed parameter.

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