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.
All Science Journal Classification (ASJC) codes
- Cell Biology
- Michaelis constant
- cellular calcium regulation
- maximal initial reaction velocity