Mechanism of triiodothyronine-induced creatinuria in the rat

Mechanism of triiodothyronine (T3)-induced creatinuria was studied in rats after a single injection of T3 (100 microgram/100 g per s). Urinary creatine excretion increased markedly during the 1st day after T3 injection. Peak of creatinuria was observed during the 2nd day after T3 injection. Excretions of nonprotein nitrogen and potassium were not different between control and T3-injected animals throughout the experimental period. Creatine reabsorption in the kidney was not influenced by T3 treatment. Radioactive creatines in both plasma and urine after [14C]creatine injection were higher in T3-injected animals than in controls, whereas the uptake of radioactive creatine by muscle was lower in the former. Creatine content in muscles decreased significantly 48 h after T3 injection. Moreover, T3 increased a fraction derived from muscle in urinary creatine, but it did not influence that derived from creatine synthesized de novo. These results suggest that both decreased creatine uptake by muscle and increased release of creatine from muscle play important roles in T3-induced creatinuria.