Sodium Content of the Renal Medulla During Osmotic Diuresis †

The hypertonicity of interstitial fluids of renal medulla and papilla, present during antidiuresis, is reduced during water diuresis. This is the result of a decrease in the absolute amount of sodium in the medulla and papilla as well as dilution by an increase in the tissue content of water."2 It is not clear whether the decrease in tissue content of sodium which accompanies water diuresis is a specific result of the removal of endogenous antidiuretic hormone (ADH) or whether it is a passive consequence of the increase in flow of urine and blood through the medulla, which might be associated with any diuresis. Massive osmotic diuresis has been shown to wash out the gradient of sodium content per unit of dry tissue which normally exists between cortex and papilla.' Lesser degrees of osmotic diuresis induced by urea and sulfate4 were found to diminish the concentration of sodium in the medulla, but it is not clear from these experiments whether the total content of sodium was decreased. The present experiments were designed to compare water diuresis (absence of ADH) with mannitol diuresis of the same magnitude (presence of ADH) from the standpoint of their effects upon renal medullary content of sodium, at comparable rates of urine flow. Both procedures were found to lower sodium in medullary tissue to the same extent. These results indirectly support the view that the most important factor lowering medullary sodium during water diuresis is an increase in medullary blood flow, rather than the removal of a specific influence of ADH upon sodium transport by medullary tubules.