Calcium ATPase in erythrocytes of spontaneously hypertensive rats of the Milan strain.

Calcium (Ca)-dependent ATPase activity was determined in erythrocyte membrane ghosts from normal and spontaneously hypertensive rats of the Milan strain in order to detect changes in enzyme activity which had previously been shown in the spontaneously hypertensive rat strain. Activity was similar in control and hypertensive rats in the absence of calmodulin. In contrast, activity in the presence of saturating amounts of calmodulin was significantly lower in the hypertensive rats. At the Vmax (free Ca2+ concentration 10 mumol/l) the decrease was about 30% (70.1 +/- 8.94 versus 49.1 +/- 4.75 nmol of ATP split/mg of ghost proteins per min; P less than 0.05). The affinity of the ATP-dependent Ca2+ pump for Ca2+ (Km about 1 mumol/l) was not altered in the hypertensive rats. It is possible that deficient Ca ATPase activity sustains an increase of intracellular free Ca in cells of hypertensive rats concomitant with the intracellular sodium (Na) decrease typical of this strain.