The differential effect of Li⁺ on microsomal ATPase in cortex, medulla and papilla of the rat kidney

1. 1. The effect of Li⁺ on microsomal ATPase was studied in the cortex, medulla and papilla of the rat kidney. 2. 2. At Li⁺ concentrations, in vitro, of 5-50 mM no significant effect on Mg²⁺-ATPase activity in any part of the kidney was observed. At concentrations of 100-150 mM a slight inhibition of the activity of this enzyme was found. 3. 3. At in vitro Li⁺ concentrations of 5-50 mM (Na⁺ + K⁺)-dependent ATPase was found only at 10 mM LiCL. At a concentrations of 150 mM Li⁺ inhibited (Na⁺ + K⁺-dependent activity in the medulla and cortex. 4. 4. In vivo administration of LiCl at a dose of 2 mequiv/kg daily for 14 days caused a significant increase in (Na⁺ + K⁺)-dependent ATPase in the medulla and papilla but not in the cortex of the kidney. 5. 5. The significance of these findings with respect to the clinical effect of Li⁺ is discussed. 6. 6. The experiments reported demonstrate a selective effect of Li⁺ on (Na⁺ + K⁺-dependent ATPase in the kidney medulla as compared to this enzyme in the kidney cortex.