K+/Na+ antagonism at cytoplasmic sites of Na+-K+-ATPase: a tissue-specific mechanism of sodium pump regulation

Tissue-distinct interactions of theNa⁺-K⁺-ATPasewith Na⁺ andK⁺, independent ofisoform-specific properties, were reported previously (A. G. Therien,N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol.Chem. 271: 7104-7112, 1996). In this paper, wedescribe a detailed analysis of tissue-specific kinetics particularlyrelevant to regulation of pump activity by intracellularK⁺, namelyK⁺ inhibition at cytoplasmicNa⁺ sites. Our results show thatthe order of susceptibilities of ₁ pumps of various rat tissuestoK⁺/Na⁺antagonism, represented by the ratio of the apparent affinity forNa⁺ binding at cytoplasmicactivation sites in the absence ofK⁺ to the affinity constant forK⁺ as a competitive inhibitor ofNa⁺ binding at cytoplasmic sites,is red blood cell < axolemma  rat₁-transfected HeLa cells < small intestine < kidney < heart. In addition, we havecarried out an extensive analysis of the kinetics ofK⁺ binding and occlusion to thecytoplasmic cation binding site and find that, for most tissues, thereis a relationship between the rate ofK⁺ binding/occlusion and theapparent affinity for K⁺ as acompetitive inhibitor of Na⁺activation, the order for both parameters being heart  kidney > small intestine  rat₁-transfected HeLa cells. Thenotion that modulations in cytoplasmicK⁺/Na⁺antagonism are a potential mode of pump regulation is underscored byevidence of its reversibility. Thus the relatively highK⁺/Na⁺antagonism characteristic of kidney pumps was reduced when rat kidneymicrosomal membranes were fused into the dog red blood cell.