Dissociation of Na-K-ATPase specific activity and net reabsorption of sodium.

Prevoius studies have suggested that the increase in specific activity of Na-K-ATPase in renal tissue during treatment with glucocorticoids occurred as a result of aconcurrent rise in net tubular reabsorption of sodium. Since recent data have indicated a specific effect of glucocortiocoids on epithelial cells, experiments were performed to determine whether enzyme activity and net sodium reabsorption could be dissociated. Evidence is provided demonstrating that base-line specific activity of Na-K-ATPase in rat renal cortex and outer medulla does not correlate directly with net sodium reabsorption since enzyme activity did not change after a chronic reduction in glomerularfiltration rate and the rate of sodium reabsorption. Further studies showed a markedrise in Na-K-ATPase after 4 days of treatment with methylprednisolone despite a fall in sodium absorption. These results suggest a direct effect of glucocorticoids onrenal Na-K-ATPase and illistrate the difficulty in assigning a transport role tothis enzyme from the correlation of specific activity with rates of net electrolyte transport.