EFFECTS OF CARDIAC GLYCOSIDES ON RENAL ADENOSINE TRIPHOSPHATASE ACTIVITY AND Na⁺ REABSORPTION IN DOGS

¹ Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas

Ouabain was infused into the left renal artery. Na⁺ reabsorption was measured at different time intervals after ouabain. The kidneys were removed and adenosine triphosphatase (ATPase) activity was determined in homogenates of entire kidneys or in microsomal fractions from cortex and outer medulla. Na⁺ reabsorption was reduced only in the left kidney. The following qualitative correlations between the reduction of Na⁺ reabsorption and the reduction of ouabain-sensitive ATPaSe activity were observed in the left kidney: the enzyme activity was always reduced more in the left than in the right kidney; maximal effects occurred at similar doses of ouabain; and recovery of Na⁺ reabsorption and the enzyme activity was parallel and complete 72 hours after ouabain. At maximal effect Na⁺ reabsorption was 60 to 70% of the filtered load and ouabain-sensitive ATPase activity was almost totally reduced in the left kidney. In the right kidney Na⁺ reabsorption was not reduced when the ATPase activity was inhibited to the levels at which the left kidney had impaired Na⁺ reabsorption. The magnitude of ouabain effect on Na⁺ reabsorption was dependent on the saline load. Ouabain did not inhibit ATPase activity by interfering with the activation of Na⁺ + K⁺ ATPase by deoxycholate and ethylenediamine tetraacetic acid. In sucrose-Tris homogenates a larger fraction of ATPase activity was inhibited by ouabain in vivo than in vitro. Infusion of 0.9% NaCl i.v. at 10 ml/min for up to four hours did not alter ATPase activity. According to Ackermann-Potter plots, ouabain initially interacted reversibly with the enzyme in vitro, but with time the interaction acquired irreversible characteristics. The inhibition in vivo may be irreversible since the inhibition was the same at different dilutions of renal homogenates and perfusion of kidneys from animals treated with ouabain did not appreciably alter the inhibition. The 50% inhibitory concentration of ouabain was the same in enzyme preparations which had 12-fold difference in ouabainsensitive ATPase specific activity. The effects of digoxin two hours after administration into the left renal artery were similar to those of ouabain. It was concluded that the Na⁺ + K⁺ ATPase system may play a major role in the reabsorption of Na⁺, but that other mechanisms for Na⁺ reabsorption exist.