Effect of cardiac glycosides and sodium ethacrynate on transepithelial sodium transport in in vivo micropuncture experiments and on isolated plasma membrane Na-K ATPase in vitro of the rat

In parallel micropuncture experiments employing peritubular capillary perfusion techniques in vivo and biochemical techniques on the simultaneously isolated Na−K ATPase from kidney cortexin vitro, the effects of various cardiac glycosides and sodium ethacrynate on transepithelial sodium transport in proximal tubules of the rat were examined. Cardiac glycosides with an inhibitory potency for this ATPase varying over approximately a 100 fold range were used. The in vitro sensitivity of this rat ATPase to Scilliroside, a glycoside from red squill, the most potent of them and used here for the first time in micropuncture experiments was comparable to that shown to Ouabain by a similar ATPase from kidneys of more glycoside sensitive animals. However, even at 250–300 × the I50 concentration, maximal glycoside inhibition of Na transportin vivo was only 40–50%. ETA at only 7 x its I50 concentration inhibited Na transport by 73%, consistent with an effect on general metabolism rather than on the Na−K ATPase. These results are discussed as compatible with the Na−K ATPase/Na pump complex being a reasonably coherent single unit using ATP as a source of energy