Properties of Heart Sarcolemmal Na⁺-K⁺ ATPase

Abstract

Heart sarcolemmal fraction containing Na⁺-K⁺ stimulated ATPase [EC 3. 6. 1. 3], Mg²⁺ ATPase, Ca²⁺ ATPase and adenylate cyclase was isolated by hypotonic shock and 0.4M LiBr treatments. The time required for the isolation was about 5hr. The specific activities of Na⁺-K⁺ ATPase (lOμmoles P_i/mg per hr) and adenylate cyclase (280pmoles cyclic AMP/mg per min) in the dog heart membrane fraction were 7-8 fold of those in the heart homogenate. The sarcolemmal fraction consisted of sacs of varying shapes and sizes and was devoid of most of the mitochondrial, microsomal and myofibril contaminants. The properties of Na⁺-K⁺ ATPase in the membrane fraction were compared with those of the Nal treated membrane fraction. The enzyme activity in the membrane fraction was lower than that of the Nal treated preparation. The K_m values for the Na⁺-K⁺ ATPase of the membrane fraction and Nal treated membrane fraction were 0.77 and 0.70mM (MgAPT) respectively. The pH optimum for Na⁺-K⁺ ATPase activity in these preparations was about 7.5. Different monovalent cations such as Rb⁺, NH₄⁺, Cs⁺, Li⁴, and choline⁴ were poor substitutes for K⁺ in activating the Na⁺-K⁺ ATPase in these preparations. The mean values for the concentrations of K⁺ varied from 1.2-1.5mM and those for Na varied from 12-16mM for half maximal activation of Na⁺-K⁺ ATPase in these preparations. The mean values for the concentrations of ouabain varied from 3.1-3.2μM and those for calcium varied from 1.0-1.2mM for 50% inhibition of Na⁺-K⁺ ATPase in these preparations. Various other cardioactive agents failed to influence Na⁺-K⁺ ATPase activity of the heart sarcolemma.