Renal acidification in the hypothyroid rat. Evaluation by urinary CO2 tension

We have previously demonstrated an abnormal renal response to chronic acid loading in the hypothyroid rat. The present study was designed to characterize further this defect by examining the renal response to bicarbonate loading. Hypothyroidism was induced by the intraperitoneal injection of Na131I and animals were studied 10 or more weeks thereafter under anesthesia and compared with their age-matched littermates. Inulin clearance of hypothyroid animals was lower and fractional urine flow and sodium excretion rates were higher than in the control rats. At the time of maximal urinary bicarbonate levels, the urine [Formula: see text] exceeded blood values [Formula: see text] by 41 ± 2 (mean ± SE) mmHg (1 mmHg = 133.322 Pa) in the controls and 25 ± 3 mmHg in the hypothyroid rats (p < 0.001). Although maximal urinary bicarbonate concentrations achieved during bicarbonate loading were lower in the hypothyroid animals, [Formula: see text] was consistently less than in controls when compared at similar urine bicarbonate concentrations. Thus, the difference in [Formula: see text] was not explained by lower urinary bicarbonate concentrations in the hypothyroid animals. Maximal [Formula: see text] during neutral phosphate loading was significantly less in the hypothyroid rats (38 ± 2 mmHg) than in the controls (68 ± 5 mmHg), p < 0.001, arguing against a gradient defect of distal hydrogen ion handling. These data indicate that hypothyroid rats exhibit an impairment of distal hydrogen ion secretion.