Spironolactone- and Canrenone-Induced Changes in Hepatic (Na+, K+)ATPase Activity, Surface Membrane Cholesterol and Phospholipid, and Fluorescence Polarization in the Rat†

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Abstract
We studied changes in hepatic membrane (Na+,K+)ATPase activity and membrane lipids induced by canrenoate, the water-soluble congener of canrenone, the active metabolite of spironolactone. (Na+,K+)ATPase activity was decreased after canrenoate in a dose- and time-dependent manner. Decreased activity was demonstrated at the lowest dose (91% of control after 5 μmoles per 100 gm body weight per day × 3 days); the maximum dose (30 μmoles per 100 gm body weight per day × 3 days) resulted in activity 38% of untreated control values. A 20 μmoles per 100 gm body weight per day dose decreased enzyme activity to 89 and 55% of control after 24 and 72 hr, respectively. The nonionic detergent Triton WR- 1339 partially reversed drug-induced inhibition, suggesting that the enzyme changes may be related to altered membrane lipids. Membrane cholesterol increased 17% after 3 days of 30 μmoles canrenoate per 100 gm body weight per day; phospholipids decreased by 12%. The cholesterol to phospholipid molar ratio increased from 0.419 to 0.555. Membrane fluidity, as measured by the fluorescent probe 1,6- diphenyl- 1,3,5-hexatriene decreased after treatment with 20 μmoles canrenoate per 100 gm body weight per day for 3 days. These results describe in vivo and in vitro inhibition of hepatic (Na+,K+)ATPase activity. Increased membrane cholesterol with decreased phospholipid alters membrane fluidity and may be partially responsible for the change in (Na+,K+)ATPase activity.