Kinetic study of the Ca2+ pump in erythrocytes from essential hypertensive patients

Ca2+ pump kinetics were investigated in erythrocytes from 22 essential hypertensive patients and 20 normotensive controls (under initial-rate and steady-state conditions, using Sr2+ as a Ca2+ analogue). The mean value of the apparent dissociation constant for total internal Ca2+ (KCa) was slightly but significantly increased in the hypertensive population (73 +/- 7 versus 55 +/- 3 mumol/l cells, mean +/- s.e.m., P = 0.042 Mann-Whitney U-test). The statistical analysis showed that this was due to six essential hypertensives who exhibited a dissociation constant for Ca2+ that was higher than the upper 95% normal confidence limit (KCa = 116 +/- 7 mumol/l cells), and abnormally high maximal pump rates (7.7 +/- 0.6 versus 5.0 +/- 0.2 mmol/l cells per h in normotensives, P less than 0.001). In addition, the apparent dissociation constant for Ca2+ was inversely correlated with plasma renin activity, although the correlation was only borderline (P = 0.076). In the remaining 16 hypertensive patients, all kinetic parameters of the Ca2+ pump were within the normal range. Finally, a simultaneous study of Na+ transport kinetics suggested that erythrocyte Ca2+ and Na+ transport abnormalities were independent phenomena. Our results do not support the concept that primary hypertension (as a whole entity) is associated with a ubiquitous defect in the plasma membrane Ca2+ pump. However, in some essential hypertensive patients (about 25%) the erythrocyte Ca2+ pump exhibited an apparent decreased affinity for internal Ca2+. A similar defect in vascular smooth muscle may induce a delayed Ca2+ extrusion after the opening of Ca2+ channels, a disturbance likely to be translated into increased vascular reactivity.