Tubular handling of bicarbonate in dogs with experimental renal failure

In this study, micropuncture experiments were performed to examine the segmental reabsorption of bicarbonate and chloride in the normal dog kidney (stage 1) and in the remnant kidney (stage 3) of dogs with experimental renal failure. The protocol consisted of three phases: (1) 3% extracellular fluid volume expansion, (2) infusion of Ringer's solution containing 90 mM of sodium bicarbonate, and (3) infusion of 150 mM of sodium bicarbonate in Ringer's solution. In the animals with remnant stage 3 kidneys, the ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was increased compared to dogs with stage 1 kidneys at both the proximal and distal sampling sites. These data suggest that bicarbonate reabsorption was elevated in both the distal and the proximal tubules in experimental chronic renal failure. In contrast to the reabsorption of bicarbonate, chloride reabsorption was depressed in stage 3 kidneys at the late proximal puncture site. This resulted in greater delivery to the distal nephron. The distal segments reabsorbed a constant fraction of delivered chloride, resulting in an increase in fractional chloride excretion in chronic renal failure compared to that in normal animals.