Sodium-dependent cardiac glycoside binding: experimental evidence and hypothesis

1 The influence of increasing Na+ concentrations on the binding of digitoxin, digoxin and ouabain was examined in a Na+-K+-ATPase preparation of guinea-pig hearts. 2 Two distinct processes seem to be involved in this interaction: one binding process was activated at low Na+ concentrations. The maximum binding capacities were different and the K0.5 values were nearly identical for the cardiac glycosides studied. 3 In contrast, the second binding process was activated at appreciably higher Na+ concentrations, the maximum binding capacities were almost identical and the K0.5 values were different for the cardiac glycosides studied. 4 On the basis of these results attempts are made to explain the well known differences in the myocardial accumulation of cardiac glycosides.