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The aim of this study was to investigate the basis of reduced erythrocyte calcium-stimulated, magnesium-activated adenosine 5' triphosphatase (Ca2+,Mg2+-ATPase) activity in essential hypertension.Experiments were performed to establish whether the reduced erythrocyte Ca2+,Mg2+-ATPase activity in patients with essential hypertension, when compared with age- and sex-matched normotensive control subjects, was due to changes in enzyme properties or to an altered membrane environment.Erythrocyte membrane Ca2+,Mg2+-ATPase activity was determined by measuring ATP-dependent 45Ca2+uptake in inside-out vesicles and calcium-dependent γ-32P ATP hydrolysis in ghost membranes, prepared from the same sample of blood. Calcium-dependent γ-32P ATP hydrolysis activity was also measured in detergent extracts of erythrocyte membranes.In the absence and presence of calmodulin, both ATP-dependent Ca2+uptake and calcium-dependent ATP hydrolysis activities of erythrocyte membranes prepared from patients with essential hypertension were significantly reduced when compared with normotensive subjects. No difference in calmodulin affinity was observed between hypertensive and normotensive subjects, although the calcium dependence of calmodulin-independent Ca2+uptake activity in inside-out vesicles was altered. No significant difference in calcium-dependent ATP hydrolysis activity was observed between hypertensive and normotensive preparations after detergent solubilization of erythrocyte membrane proteins.These results suggest that the number of Ca2+,Mg2+-ATPase units is similar in erythrocytes of hypertensive and normotensive subjects and that the reduced activity in the intact erythrocyte membrane of hypertensive patients is due to an altered membrane environment.