Tissue specificity of inhibitory action of excess thyroid hormone on creatine transport in the rat.

Tissue specificities in inhibitory action of excess triiodothyronine (T3) on creatine uptake from the plasma and the effect of denervation of the muscle on the inhibitory action were studied in rats with the use of radioactive creatine. The uptake of radioactive creatine of all muscles studied significantly diminished after T3 (100 microgram/100 g, s.c.) injection, while that of brain was not affected by T3 treatment. Tissue specificity of the T3 action on creatine uptake was consistent with that of previously known T3 action on oxygen consumption. The responses to inhibitory action of T3 on creatine uptake were not different in different types of the skeletal muscles. Although the uptake of radioactive creatine of denervated muscles was significantly lower than that of controls, inhibitory action of T3 on creatine uptake was similarly observed in denervated muscles as well as in normal ones. The results indicate that T3 has a direct effect on muscle cell per se and the inhibitory action of T3 on creatine uptake by the muscle is closely related to T3 action on the energy-requiring process in cell membrane.