Interactions of exogenous thyroxine and dietary protein levels on growth and muscle protein metabolism in broiler chickens.

Effects of exogenous thyroxine (T4) and dietary protein on growth, feed efficiency, breast muscle weights (pectoral profundus and superficial muscles), abdominal fat content, and muscle protein synthesis (Ks) and breakdown (Kd)rates based on Nτ-methylhistidine excretion were investigated in male broiler chickens (15-27 days of age). The inclusion of T4 (1.2ppm) in the low-protein diet (10%) increased rate of muscle protein breakdown and plasma concentration of T4, and decreased body weight gain, feed efficiency and breast muscle weight.On the other hand, abdominal fat content and plasma T3 concentrations were not influenced although there was a trend in T3 concentration. The inclusion of T4 in the adequate-protein diet (20%) increased rates of muscle protein synthesis and breakdown, feed efficiency, breast muscle weight and plasma concentration of T4, but abdominal fat content and plasma T3 concentrations were not changed. The inclusion of T4 in the high-protein diet (30%) reduced both rates of muscle protein synthesis and breakdown, abdominal fat content and plasma T3 concentrations.Body weight gain, feed efficiency and muscle weight were, however, not altered, whereas plasma concentration of T4 was increased by T4 inclusion. The present results clearly show that the effects of dietary thyroxine on feed efficiency, breast muscle weight, rates of muscle protein synthesis and breakdown and abdominal fat deposition are modified by dietary protein. The present results also indicate that the retarding effect of T4 on muscle growth in the low-protein group is due to the enhancement of the rate of muscle protein breakdown, and the T4 induced muscle growth in the adequate-protein group is due to the increased rate of muscle protein synthesis; when, however, a high-protein diet is given, muscle growth is not improved by T4 since both rates of muscle protein synthesis and breakdown are decreased.