Activatory effect of two cardioglycosides on Cavia cobaya kidney Na+/K+-ATPase activity.

Ouabain and K-strophanthoside promote an enhancement of Na+/K+-ATPase activity in a range of cardioglycoside concentrations from 100 nM to 100 pM, with a maximum (+30%) between 10 and 4 nM. Binding experiments with [3H]ouabain show upward-curved Scatchard plots and evidence two intrinsic affinity constants for the ligand: (a) High-affinity constant: 350 nM (microsomes) and 15 nM (purified enzyme). (b) Low-affinity constant: 2100 nM (microsomes) and 890 nM (purified enzyme). The reaction velocity trend indicates that at ouabain concentrations higher than 20 nM but lower than the minimal inhibiting level, the enhanced reaction velocity is tending towards the control values.