Effects of Experimental Hypo- and Hyperthyroidism on the Rates of Muscle Protein Synthesis and Breakdown in Cockerels

Abstract

The effects of experimental hypo-(thyroidectomized) and hyperthyroidism (thyroxine-treated thyroidectomized) on the rates of synthesis and breakdown of myofibrillar proteins of skeletal muscle (estimated by Nτ-methylhistidine (Nτ-MH) excretion) and the plasma thyroid hormone concentration were investigated in White Leghorn cockerels. Thyroxine was orally given by mixing it in the diet at the level of 1.2 ppm. The growth was retarded by thyroidectomy which was recovered by thyroxine administration. In the thyroxine-administered group, growth rate was higher than that of the sham-operated control group. Both the rates of muscle protein synthesis and breakdown were decreased by thyroidectomy and tended to be increased by thyroxine administration. Plasma thyroxine concentration was 35% lower in the thyroidectomized group and was 340% higher in the thyroxine administered-group in comparison with the control group. However, there was no difference in the plasma triiodothyronine concentration between the control and the thyroxine-administered groups, even though the concentration was 60% lower in the thyroidectomized group than in the control group.