Evidence for Enhanced Distal Tubule Sodium Reabsorption in Chronic Salt-Depleted Dogs

AB STRA CT In order to assess the renal tubular site(s) at which sodium reabsorption is enhanced in chronic sodium-depletion, seven normal dogs, six salt-depleted dogs, and three normal dogs receiving aldo-sterone were studied during a steady-state water diuresis under Pentothal anesthesia and during progressive hy-potonic saline diuresis. For both maintenance of the water diuresis and progressive hypotonic saline diuresis 0.45 % NaCl was used. During the steady state water diuresis delivery of sodium to the diluting segment of the nephron as approximated by solute-free water clear-ance + sodium clearance/glomerular filtration rate (CH2O + CN./GFR) was significantly lower in salt-de-pleted dogs compared to normal dogs with or without aldosterone. During progressive hypotonic saline infu-sion fractional free water excretion (CH20/GFR) was similar in all three groups as CHn>O + CNa/GFR increased up to 12-14 ml /min. 100 ml GFR. Thereafter, CH20/ GFR continued to rise in virtually a straight line in salt-depleted dogs but leveled off in normal dogs with or without aldosterone. These data demonstrate that en-hanced sodium reabsorption in the diluting segment of the nephron is an important determinant of the renal sodium retention in chronic extracellular volume con-traction in dogs in addition to confirming the presence of increased proximal tubule sodium reabsorption in these animals. Part of this material appeared in abstract form in Clin