AJP - Cell Physiology, Vol 258, Issue 6 C988-C994, Copyright © 1990 by American Physiological Society
Kinetics of Ca2(+)-ATPase activation in platelet membranes of essential hypertensives and normotensives
J. Takaya, N. Lasker, R. Bamforth, M. Gutkin, L. H. Byrd and A. Aviv
Hypertension Research Center, University of Medicine and Dentistry of New Jersey, Newark 07103.
To explore the etiology of altered Ca metabolism in essential hypertension,
we studied parameters, i.e., maximal initial reaction velocity (Vmax) and
Michaelis constant (Km), 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 Vmax
of hypertensives was significantly lower than in normotensives without a
family history of essential hypertension (hypertensives, 14.99 +/- 1.71
nmol Pi.mg protein-1.min-1; normotensives, positive family history, 22.67
+/- 3.17 nmol Pi.mg protein-1.min-1; normotensives, negative family
history, 27.54 +/- 4.37 nmol Pi.mg protein-1.min-1; overall, P = 0.0078).
The Km 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
microM; normotensives, positive family history, 1.38 +/- 0.2 microM;
normotensives, negative family history, 2.79 +/- 0.58 microM; overall, P =
0.0251). Furthermore, the Km in whites was inversely related to plasma
renin activity (r = 0.50; P less than 0.005). We propose that a lower Vmax
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 Vmax. We also
suggest that a higher Km for Ca2(+)-ATPase in juxtaglomerular cells of
whites would result in blunting the release of renin.
