Kinetics of Ca²⁺-ATPase activation in platelet membranes of essential hypertensives and normotensives

To explore the etiology of altered Ca metabolism in essential hypertension, we studied parameters, i.e., maximal initial reaction velocity (V(max)) and Michaelis constant (K(m)), of Ca activation kinetics of Ca²⁺-ATPase in membrane fractions (isolated by a sucrose gradient) from platelets of blacks and whites, 27 of whom were essential hypertensives, 17 of whom were normotensives with a family history of essential hypertension, and 10 of whom were normotensives without a family history of the disease. The V(max) of hypertensives was significantly lower than in normotensives without a family history of essential hypertension (hypertensives, 14.99 ± 1.71 nmol P(i) · mg protein^-1 · min^-1; normotensives, positive family history, 22.67 ± 3.17 nmol P(i) · mg protein^-1 · min^-1; normotensives, negative family history, 27.54 ± 4.37 nmol P(i) · mg protein^-1 · min^-1; overall, P = 0.0078). The K(m) was lower in both hypertensives and normotensives with a positive family history of essential hypertension as compared with normotensives with a negative family history of the disease (hypertensives, 1.70 ± 0.23 μM; normotensives, positive family history, 1.38 ± 0.2 μM; normotensives, negative family history, 2.79 ± 0.58 μM; overall, P = 0.0251). Furthermore, the K(m) in whites was inversely related to plasma renin activity (r = 0.50; P < 0.005). We propose that a lower V(max) for Ca²⁺-ATPase may play a role in the higher level of free Ca in platelets of essential hypertensives and that a higher affinity of the enzyme to Ca may reflect a process compensating for the lower V(max). We also suggest that a higher K(m) for Ca²⁺-ATPase in juxtaglomerular cells of whites would result in blunting the release of renin.