Regulation by Thyroid Hormones of Glucose Transport in Cultured Rat Myotubes

Primary cultures of skeletal muscle obtained from neonatal rats possess a saturable process for active glucose uptake, the myotubes having a relatively high affinity for the substrate with a Km of 1 mM. The expression of the glucose transport system was most apparent after fusion of single myoblasts to multinucleated myotubes [3-4 days in vitro (DIV)], at which time glucose uptake increased sharply to reach plateau values at about 6-8 DIV. Treatment of the cells at age 6 DIV with triiodothyronine or thyroxine caused a marked increase in glucose uptake beginning 4 h after treatment and reaching a maximum at 24 h. Thyroid hormone-induced increase in glucose uptake was not reduced by either tetrodotoxin or verapamil, thus indicating that the effect was not secondary to the ability of the hormone to increase contractile activity. The effect of thyroid hormones was eliminated completely by inhibition of protein synthesis. The results indicate that thyroid hormones play an important role in regulation of glucose transport in skeletal muscle.