TISSUE LOCALIZATION OF ACTIVE Na,K-ATPase ISOENZYMES BY DETERMINATION OF THEIR PROFILE OF INHIBITION WITH OUABAIN, DIGOXIN, DIGITOXIGENIN AND LND 796. A NEW AMINOSTEROID CARDIOTONIC

Recently, Na,K-ATPase isoforms with differential affinities for digitalis have been identified that may contribute to different toxicity profiles. Our objectives were to localize them and to define. tissue receptor patterns by examining the effect of different glycosides on the Na,K-ATPase activity. The digitalis derivatives used exhibit variation in lipophilicity and rate of enzyme inhibition. Membrane fractions enriched in Na,K-ATPase were prepared from canine heart, brain, aorta and peripheral nerves. The inhibition of enzyme activities indicates a pattern of differential sensitivities with IC50 values starting from 3 nM in heart and 30 nM in brain. Therefore, high and low affinity active forms of the Na,K-ATPase enzyme coexist in these tissues. The data also suggest the existence of two Na,K-ATPase isoforms in aorta and peripheral nerves as identified by the action of digitoxigenin and LND 796 where the predominant expression is that of a high affinity form. The comparison of the patterns of digitalis sensitivities in these different tissues, suggests a more complex molecular interaction than that which can be explained by the presence of only two forms.