K+/Na+ antagonism at cytoplasmic sodium activation sites of the Na,K-ATPase is a tissue-specific mechanism of pump regulation

Analysis of the sodium activation kinetics of Na,K-ATPase as a function of varying K+ concentration indicates tissue-dependent differences in competitive inhibition by K+ at cytoplasmic Na+ activation sites. This holds true for the same pump isoform (alpha1) of various rat tissues, namely kidney, heart, axolemma, rat alpha1-transfected HeLa cells, red blood cells and small intestine. Indirect assays of the binding/occlusion of K+ to the enzyme supports a simple model whereby K+ competes with Na+ for binding to the cytoplasmic cation binding sites of E-1 driving the reaction in the reverse direction to form E-2(K). Our findings suggest that K+N+ antagonism at cytoplasmic sodium activation sites of the Na,K-ATPase is a tissue-specific mechanism of pump regulation.