Ca²⁺, Mg²⁺-ATPase of Microsomal Membranes from Bovine Aortic Smooth Muscle: Effects of Sr²⁺ and Cd²⁺ on Ca²⁺ Uptake and Formation of the Phosphorylated Intermediate of the Ca²⁺, Mg²⁺-ATPase

Abstract

The effects of various divalent cations on the Ca²⁺ uptake by microsomes from bovine aortic smooth muscle were studied. High concentrations (1mM) of Co²⁺, Zn²⁺, Mn²⁺, Fe²⁺, and Ni²⁺ inhibited neither the Ca²⁺ uptake by the microsomes nor the formation of the phosphorylated intermediate (E_??_P) of the Ca²⁺, Mg²⁺-ATPase of the microsomes. The cadmium ion, however, inhibited both the Ca²⁺ uptake and the E_??_P formation by the microsomes. Dixon plot analysis indicated that Cd²⁺ inhibited (K₁=135μM) the Ca²⁺ dependent E_??_P formation in a non-competitive manner. The inhibitory effect of Cd²⁺ was lessened by cysteine or dithiothreitol. The strontium ion inhibited the Ca²⁺ uptake competitively, while the E_??_P formation increased on the addition of Sr²⁺ at low Ca²⁺ concentrations. At a low Ca²⁺ concentration (1μM), Sr²⁺ was taken up by the aortic microsomes in the presence of 1mM ATP. It is thus suggested that Sr²⁺ replaces Ca²⁺ at the Ca²⁺ binding site on the ATPase.