Relationship between mineralocorticoids and renal Na+-K+-ATPase: sodium reabsorption

To evaluate the mechanism responsible for the effect of deoxycorticosterone acetate (DOCA) on renal Na+-K+-ATPase, we compared the relative contribution of this hormone and of increased absolute sodium reabsorption (TNa) to the restoration of the enzyme in kidneys of adrenalectomized rats. In study A, adrenalectomized animals maintained on a salt-free diet received 5 mg/kg per day DOCA i.m., while sham-operated and untreated adrenalectomized rats receiving the same diet served as controls. Absolute TNa and Na+-K+-ATPase specific activity in the cortex and outer medulla of DOCA-treated rats were similar to those measured in untreated adrenalectomized animals, but were significantly lower than in sham-operated controls. In study B, the adrenalectomized rats did not receive DOCA but were fed a high salt diet and received isotonic saline, 50 ml/kg per day s.c. Absolute TNa and cortical and medullary Na+-K+-ATPase specific activity were significantly higher in the salt-loaded group than in both adrenalectomized and sham-operated rats deprived of salt. These results suggest that absolute sodium reabsorption is a major determinant of renal Na+-K+-ATPase activity, and that the effect of DOCA on this enzyme is secondary to its stimulation of absolute tubular sodium transport.