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 Rb-86 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 1.8x10(-9)M to 1.4x10(-11)M for the higher affinity binding site and from 1.8X10(-6)M to 5.5X10(-9)M for the lower affinity site. Digitoxigenin is the most active showing the higher active site at 1.4X10(-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. (C) 2000 Elsevier Science Inc. All rights reserved.