Effect of thyroid hormones on the in vivo protein synthesis in rats

The studies were made in order to estimate the rate of tissue protein synthesis in euthyroid, hyperthyroid and hypothyroid rats (110 g body weight). Hyperthyroidism was simulated by daily oral application of 4,44 micrograms thyroxine (T4) and 0,89 micrograms triiodothyronine (T3)/100 g body weight. Hypothyroidism was induced by oral application of 17,8 mg of methylthiouracil (MTU) per day and 100 g body weight. In both cases the treatment lasted for 7 days. The concentration of both hormones in blood serum was twice that of the control group in the T3 + T4 treated group. The MTU application produced a decrease in T3 and T4 concentration to levels below the lower detection limits of the methods used. The fractional rate of tissue protein synthesis was estimated using the constant intravenous infusion technique with L-14C-leucine and L-14C-lysine simultaneously. The range of the fractional rate of protein synthesis was found to be higher in hyperthyroid than in control animals in all organs under study (liver, small intestine, large intestine, gastrocnemius muscle and pancreas). Protein synthesis was stimulated to the greatest extent in muscle (4,5--5,1% . d-1 in controls vs, 5,5--6,7% . d-1 in hyperthyroid animals). The influence of hypothyroidism on the rate of protein synthesis was uncertain.