Platelet membrane Ca(2+)-ATPase: blunted calmodulin-stimulation in essential hypertension.

To determine whether defective calcium-efflux pump activity may be responsible for calcium overload in platelets from patients with essential hypertension, the properties of a calmodulin-stimulated Ca(2+)-ATPase in membranes from 25 normotensive and 27 hypertensive subjects were compared. Calcium-ATPase did not differ in affinity for calmodulin or Ca2+ between groups. Stimulation of Ca(2+)-ATPase activity at saturating calmodulin concentrations is diminished in calmodulin-deficient membranes from established essential hypertensive patients (64%) compared with normotensive subjects (125%) (P < 0.01). The capacity for Ca(2+)-ATPase activity (basal and calmodulin-activated) is markedly greater (1.5 to 1.8-fold) in both native and calmodulin-deficient platelet membranes from hypertensive compared with normotensive subjects. The data suggest that calmodulin-stimulation of Ca(2+)-ATPase in platelets from patients with established essential hypertension does not effectively maintain Ca2+ homeostasis, and that the increased Ca(2+)-ATPase capacity may reflect an adaptation to increased intracellular calcium concentration.