Impaired renal bicarbonate reabsorption in the hypothyroid rat.

We previously demonstrated a decrease of sodium reabsorption in the proximal superficial nephron of the hypothyroid rat. The present clearance studies were designed to examine the renal handling of bicarbonate in hypothyroid rats. Hypothyroidism was induced by the intraperitoneal injection of Na131I and the animals were studied 10 or more wk thereafter under anesthesia and compared to their age-matched litter-mates. As is characteristic of hypothyroid animals, insulin clearance was lower and fractional urine flow and sodium excretion rates were higher than in the control rats. At the peak of the bicarbonate diuresis and at similar blood pH, PCO2, and bicarbonate levels, fractional renal bicarbonate excretion was significantly higher in the hypothyroid animals: 22 +/- 2 vs. 15 +/- 1%, P less than 0.02. This perturbation of bicarbonate reabsorption of the hypothyroid rats was independent of the influence of extracellular fluid volume expansion and appears closely related to the impaired sodium reabsorptive capacity characteristic of experimental hypothyroidism.