Urinary Concentrating Ability in Rats Treated with Thyroxine

Abstract

The maximal antidiuretic response of thyroxine treated rats fed normal and low sodium diets was studied by analyzing urine volume, sodium, potassium, urea and total solute concentrations in urine, and plasma. Sodium, potassium, ammonia and urea were also measured in kidney cortex, medulla and papilla of rats fed a low sodium diet. The volume of urine collected, Cosm and T^CH₂O were significantly higher in treated than in control rats; whereas no significant difference could be demonstrated in the other parameters studied, except in V × U/P urea, which was significantly higher in thyroxine treated rats, suggesting an increase in GFR. These results support the hypothesis that the increased urine flow observed in thyroxine treated rats is the result of an osmotic diuresis. The conservation of the urinary concentrating ability in spite of the osmotic diuresis may be due either to the fact that the flow attained is not sufficiently high as to limit the urinary concentrating process, or because the concentrating mechanism of these rats is more efficient than that of the controls, a possibility which could be explained by the greater urea excretion of these animals or by an increase of the amount of sodium transported by the ascending limb of the loops of Henle.