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

Tissue-distinct interactions of the Na+-K+-ATPase with Na+ and K+, independent of isoform-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, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K+, namely K+ inhibition at cytoplasmic Na+ sites. Our results show that the order of susceptibilities of α1 pumps of various rat tissues to K+/Na+antagonism, represented by the ratio of the apparent affinity for Na+ binding at cytoplasmic activation sites in the absence of K+ to the affinity constant for K+ as a competitive inhibitor of Na+ binding at cytoplasmic sites, is red blood cell < axolemma ≈ rat α1-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K+ binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K+ binding/occlusion and the apparent affinity for K+ as a competitive inhibitor of Na+activation, the order for both parameters being heart ≥ kidney > small intestine ≈ rat α1-transfected HeLa cells. The notion that modulations in cytoplasmic K+/Na+antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K+/Na+antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.