Effect of triiodothyronine on renal growth and renal sodium reabsorption in hypothyroid rats.

Experiments were carried out to compare temporal changes in the paraaminohippuric acid clearance (CPAH), renal sodium reabsorption (RNa+), ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) content in hypothyroid rats after a single injection of triiodothyronine (T3) (50 micrograms/100 g body wt). CPAH and RNa+ showed no changes at 24 and 48 h. At 72 h, however, significant increases of 41% and 42% (per g kidney wet wt) were observed in CPAH and RNa+, respectively. The cortex in T3-treated hypothyroid rats showed a significant increase in the protein/DNA and RNA/DNA ratios at 24 h and progressive increases to a level of 24%, and 37%, respectively, at 48 h. No changes in DNA content were observed at either time-points. The results show that the increases in RNA/DNA and protein/DNA ratios upon T3 treatment preceded the increases in CPAH and RNa+, suggesting a direct effect of T3 on renal cortical growth, rather than a secondary response to a primary increase in renal functions.