Urea and sodium: Effect on microsomal ATPase in different parts of the kidney

Microsomal ATPase activity was studied in three regions of the rat kidney: cortex, medulla and papilla. (Na+K)-ATPase activity was highest in the medulla but a substantial activity, comparable to that in the cortex, was also present in the papilla. In the presence of high sodium (200-320 mM) or urea (100-900 mM) progressive inhibition of Mg-ATPase activity was observed in all three regions of the kidney. Urea (900 mM) or Na (320 mM) caused activation of (Na+K)-dependent ATPase in the medulla and inhibition of this enzymatic activity in the papilla of the kidney. Total microsomal ATPase activity in the medulla was unchanged in the presence of urea or sodium but was reduced in the papilla. Urea inhibited non-selectively Mg-p-nitrophenylphosphatase and K-activated p-nitrophenylphosphatase in all three parts of the kidney. These findings may point to a molecular basis for the function of urea and of sodium in the concentrating mechanism of the kidney.