Localization of parathyroid-hormone-independent sodium bicarbonate inhibition of tubular phosphate reabsorption.

We investigated phosphate transport along superficial nephrons in two groups of acutely parathyroidectomized (APTX) rats. Animals of group 1 were infused with 0.5 M sodium chloride; and those of group 2, with 0.5 M sodium bicarbonate at the same rates. Compared to the sodium chloride infusion, the sodium bicarbonate infusion was associated with a significant increase in urinary excretion of phosphate: the fractional phosphate excretion was 2.3 +/- (SD) 1.3% in the sodium chloride group and 14.4 +/- 3.2% in the sodium bicarbonate group, P less than 0.01, whereas the fractional sodium excretion was identical, 7.4 +/- 0.60% and 7.5 +/- 0.50%. Micropuncture studies performed at the late accessible proximal and early accessible distal sites of the same superficial nephrons indicate that the reabsorptive capacity for phosphate (absolute reabsorption/absolute delivered phosphate per nephron segment) is decreased during sodium bicarbonate infusion in the convoluted proximal tubule as well as in the loop (segment located between late proximal and early distal accessible convolutions) and the terminal nephron. Such an effect is independent of both parathyroid hormone secretion and extracellular fluid volume expansion.