Expression of functional Na,K-ATPase isozymes in normal human cardiac biopsies

In human heart failure, disturbances in Ca2+ homeostasis are well known but the fate of the Na,K-ATPase isoforms (alpha1 beta1, alpha2 beta1 and alpha3 beta1), the receptors for cardiac glycosides, still remains under study. Microsomes have been purified from nonfailing human hearts. As judged by the sensitivities of Na,K-ATPase activity to ouabain (IC50 values: 7.0 +/- 2.5 and 81 +/- 11 nM), H-3-ouabain-binding measurements at equilibrium with and without 10 mM K+ and by a biphasic ouabain dissociation process, at least two functionally active Na,K-ATPase isozymes coexist in normal human hearts. These are demonstrated as a very high- and a high affinity ouabain-binding site. The K-D values are 3.6 +/- 1.6 nM and 17 +/- 6 nM, respectively. The two dissociation rate constants are 42 x 10(-4) min(-1) and 360 x 10(-4) min(-1). Addition of 10 mM K+ ions shifted the respective K-D values for ouabain from 3.6 +/- 1.6 to 20 +/- 5 nM and from 17 +/- 6 nM to 125 +/- 25 nM, respectively. The isozymes involved are identified by comparing these three pharmacological parameters to those of each alpha/beta -isozyme separately expressed in Xenopus oocytes (9). In human heart, the very high affinity site for ouabain is the alpha1 beta1 dimer and the high affinity site is alpha2 beta1.