Selective inhibition of human erythrocyte Na+/K+ ATPase by cardiac glycosides and by a mammalian digitalis like factor

Na+/K+ATPase is a transport membrane protein which contains the functional receptor for digitalis compounds. In this work we compare the inhibition curves of Na+/K+ATPase measured by the inhibition of 86Rb uptake in human red blood cells by cardiac glycosides and by an endogenous digitalis like factor (EDLF) extracted from human newborn cord blood. The curves of Na+/K+TPase inhibition show a monophasic shape for ouabain, strophantidin, digitoxin, proscillaridin and EDLF whereas a biphasic shape for ouabagenin, digoxin, digoxigenin and digitoxigenin. All the drugs are potent inhibitors of erythrocyte Na+/K+ATPase with an IC50 ranging from 1.8 x 10(-9) M to 1.4 x 10(-11) M for the higher affinity binding site and from 1.8 x 10(-6) M to 5.5 x 10(-9) M for the lower affinity site. Digitoxigenin is the most active showing the higher active site at 1.4 x 10(-11) M. Ouabain and digoxin have higher affinity compared with their corresponding genins, while digitoxigenin shows a binding site with higher affinity than the respective cardiac glycosides. The increased affinity of the drugs to Na+/K+ATPase may be related to a lipophilic region in correspondence of the carbons 10, 9, 11, 12, 13 of the steroid nucleus, situated in the opposite side with respect of the C-OH-14. The comparison of the inhibition curves and the HPLC profile of newborn EDLF and of the investigated cardenolides suggest that EDLF may be a compound identical or very similar to ouabain.