Expression of uncoupling protein-3 and mitochondrial activity in the transition from hypothyroid to hyperthyroid state in rat skeletal muscle

We sought a correlation between rat skeletal muscle triiodothyronine (T3)-m ediated regulation of uncoupling protein-3 (UCP3) expression and mitochondr ial activity, UCP3 mRNA expression increased strongly during the hypothyroi d-hyperthyroid transition. The rank order of mitochondrial State 3 and Stat e 4 respiration rates was hypothyroid < euthyroid < hyperthyroid. The State 4 increase may have been due to the increased UCP3 expression, as the prot on leak kinetic was stimulated in the hypothyroid-hyperthyroid transition a nd a good correlation exists between the State 4 and UCP3 mRNA level. As a significant proportion of an organism's resting oxygen consumption is dedic ated to opposing the proton leak, skeletal muscle mitochondrial UCP3 may me diate part of T3's effect on energy metabolism. (C) 1999 Federation of Euro pean Biochemical Societies.