Distal tubular function in superficial rat tubules during volume expansion

Free-flow micropuncture experiments were carried out on superficial late proximal and distal tubules during hydropenic conditions and during extracellular volume expansion. Fluid collected from tubules was analyzed for inulin and sodium. During volume expansion, renal perfusion pressure to one kidney was reduced so that the increase in distal sodium and fluid delivery that normally occurs after saline loading was prevented. Although urinary sodium excretion remained significantly elevated in such kidneys, the rate of sodium reabsorption along the distal tubules was not different from that occurring under nondiuretic conditions. It is concluded that those factors that reduce net sodium transport in the proximal tubule during extracellular volume expansion do not act on superficial distal tubules. Additional factors beyond the distal tubule contribute to increased sodium excretion and can be shown to be activated even when delivery of fluid and sodium out of superficial distal tubules is normal.