THYROID HORMONE UPTAKE AND T4DERIVED T3FORMATION IN DIFFERENT SKELETAL MUSCLE TYPES OF NORMAL AND HYPERTHYROID RATS

In this study hind-limb perfusion was used to investigate conversion of T4to T3in skeletal muscle tissue. For this purpose the rats were depleted of thyroid hormones by thyroid ablation with 0.75 mCi 131I and were perfused 2 weeks later, when the skeletal muscle tissue consumed oxygen at a normal rate due to one subcutaneous dose of 10 μg T3/100 g b. w. 3 days before the perfusion experiments were started. T4*1) of high specific activity (> 2000 μCi/μg) was added to the perfusate. In the muscle (mixed type) a mean T4→ T3conversion of 2 % (range 0.5–3.9) was found after 120 min of perfusion. T3generation from T4in skeletal muscle did not correspond with T3muscle uptake. This observation makes a significant overestimation of T3by selective uptake of a small contamination of T3* in the T4* preparation highly improbable. In red muscle the T4and T3uptake was about 50 % higher than in white muscle. The observed Tetraccand T3cwere significantly higher in red than in white muscle. The uptake of thyroid hormones by both muscle types was not changed in hyperthyroid rats. The Tetrac and T3formation from T4, however, was increased in red muscles of hyperthyroid rats. The results show that thyroid hormone metabolism can vary markedly depending upon the type of muscle studied and they present a basis for a better understanding of clinical and biochemical evidence for a different susceptibility of red and white muscle fibers to thyroid hormones.