Effects of chronic sodium depletion on tubular sodium and water reabsorption in the dog

In order to assess further the effects of chronic sodium depletion on tubular sodium and water reabsorption, renal clearance studies were performed on dogs on each of the following regimens: a control diet; after sodium depletion (SD); and a control diet with desoxycortone (DOCA, 'Percorten') administered prior to each study. Alterations in renal function were evaluated under the following conditions: hydropenia and superimposed hypertonic mannitol loading; water hydration; superimposed hypotonic mannitol laoding; and hypotonic saline loading. Glomerular filtration rate (GFR) was unchanged, whereas C(Na)/GFR was significantly reduced after SD. After water hydration, an index of the rate of delivery of sodium to the distal tubule [C(H₂O)/GFR + C(Na)/GFR] was reduced after SD. Moreover, during hypotonic mannitol loading, the distal sodium load rose at a significantly lower rate after SD as distal fluid delivery (V/GFR) increased. Similarly, during hypotonic saline loading, the distal sodium load rose at a significantly lower rate after SD as the cumulative administered sodium load increased to 150 meq. During hypotonic mannitol or saline loading, an index of distal sodium transport [C(H₂O)/GFR], per level of distal sodium load, was unchanged after SD. These data suggest that chronic SD in dog enhances fractional sodium reabsorption primarily within the proximal tubule without evidence of a major alteration in distal tubular sodium reabsorption.