Energy-Dependent Uptake of 3,5,3′-Triiodo-LThyronine in Rat Skeletal Muscle

Abstract

The uptake of [125I]T3 in rat skeletal muscle was investigated by incubating intact soleus muscles with a tracer amount of [125I]T3. At 37.degree. C [125I]T3 uptake increased asymptotically; at 60 min the muscle contained 10% of the total [125I]T3 or 0.238 .+-. 0.021% per mg wet tissue. At 0.degree. C the [125I]T3 uptake was 1/5 of that at 37.degree. C. The specific [125I]T3 uptake, determined by subtracting the uptake in the presence of 10 .mu.M unlabeled T3 from the total [125I]T3 uptake, attained a plateu after 60 min. Washout experiments done by first incubating the muscle for 60 min at 37.degree. C or 0.degree. C with [125I]T3 and then at 0.degree. C for 3 h with unlabeled T3, showed that 21 .+-. 2% or 58 .+-. 4% of the radioactivity, respectively, was released, indicating an intracellular location of the hormone after incubation at 37.degree. C. Addition of increasing concentrations of L-T3, D-T3 and L-T4 caused a progressive inhibition of the [125I]T3 uptake; the 50% inhibitory concentrations being 400 nM, 7 .mu.M, and more than 15 .mu.M, respectively. Preincubation of soleus muscles with metabolic inhibitors almost completely inhibited [125I]T3 specific uptake, with oligomycin and antimycin causing 98 .+-. 4% and 81 .+-. 3% reduction, respectively. Monodansyl cadaverine and bacitracin, inhibitors of receptor-mediated endocytosis, reduced the specific [125I]T3 uptake in a dose-dependent manner up to 67 .+-. 3% and 62 .+-. 2%, respectively. These results indicate the presence of a saturable, stereospecific, and energy-dependent process responsible, at least in part, for T3 uptake in rat skeletal muscle. This specific T3 uptake may be a receptor-mediated endocytosis process.