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. Insulin 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 PCO2 exceeded blood values (U - B PCO2) by 41 plus or minus 2 (mean plus or minus SE) mmHg (1 mmHg equals 133.22 Pa) in the controls and 25 plus or minus mmHg in the hypothyroid rats (p less than 0.001). Although maximal urinary bicarbonate concentrations achieved during bicarbonate loading were lower in the hypothyroid animals. U - B PCO2 was consistently less in controls when compared at similar urine bicarbonate concentrations. Thus, the difference in U - B PCO2 was not explained by lower urinary bicarbonate concentrations in the hypothyroid animals. Maximal U - B PCO2 during neutral phosphate loading was significantly less in the hypothyroid rats (38 plus or minus 2 mmHg) than in the controls (68 plus or minus 5 mmHg), p less than 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.