Mg^2+- or Ca^2+-Activated ATPase Activities of Plasma Membranes Isolated from Vascular Smooth Muscle

Abstract

Studies of ATP hydrolysis by various subcellular fractions isolated from rat mesenteric arteries and veins indicate that an apparent ATPase activity, which can be activated by Mg^2+ or Ca^2+, is primarily associated with the plasma membranes. Although both Mg^2+- activated and Ca^2+-activated ATPase activities under the optimal condition are substantially lower in venous than in arterial plasma membrane fraction, their dependence on the concentration of Mg^2+ and Ca^2+ are quite similar in arterial as well as venous plasma membrane fractions. No synergistic effect on ATP hydrolysis was observed in the presence of both Mg^2+ and Ca^2+. In addition, Mg^2+-activated and Ca^2+-activated ATPase activities show similar pH dependence, inhibition by deoxycholate, stability toward heat inactivation and substrate specificity. Furthermore, Mg^2+-activated and Ca^2+-activated ATPase activities were similarly reduced in vascular smooth muscles of spontaneously hypertensive rats. These results suggest that the activation of ATP hydrolysis by Mg^2+ or Ca^2+ may represent a single enzyme moiety in the plasma membrane of vascular smooth muscle. The possible involvement of such ATPase in the Ca^2+ transport function of vascular smooth muscle is discussed.