All human Na+-K+-ATPase alpha-subunit isoforms have a similar affinity forcardiac glycosides

Three zeta -subunit isoforms of the sodium pump, which is the receptor for cardiac glycosides, are expressed in human heart. The aim of this study was to determine whether these isoforms have distinct affinities for the cardi ac glycoside ouabain. Equilibrium ouabain binding to membranes from a panel of different human tissues and cell lines derived from human tissues was c ompared by an F statistic to determine whether a single population of bindi ng sites or two populations of sites with different affinities would better fit the data. For all tissues, the single-site model fit the data as well as the two-site model. The mean equilibrium dissociation constant (K-d) for all samples calculated using the single-site model was 18 +/-6 nM (mean +/ - SD). No difference in K-d was found between nonfailing and failing human heart. samples, although the maximum number of binding sites in failing hea rt was only similar to 50% of the number of sites in nonfailing heart. Meas urement of association rate constants and dissociation rate constants confi rmed that the binding affinities of the different human alpha -isoforms are similar to each other, although calculated K-d values were lower than thos e determined by equilibrium binding. These results indicate both that the a ffinity of all human alpha -subunit isoforms for ouabain is similar and tha t the increased sensitivity of failing human heart to cardiac glycosides is probably due to a reduction in the number of pumps in the heart rather tha n to a selective inhibition of a subset of pumps with different affinities for the drugs.