Chronic heat exposure alters protein turnover of three different skeletal muscles in finishing broiler chickens fed 20 or 25% protein diets

Heat-exposed chickens exhibit a lower growth rate and a depressed protein r etention which may result from an alteration in protein metabolism. A high- protein diet seems to be beneficial under hot conditions because it tends t o improve growth. Effects of high ambient temperature (32 vs. 22 degrees C) and dietary crude protein (25 vs. 20%) on muscle protein turnover were inv estigated in finishing broiler chickens. At 5-6 wk of age, protein synthesi s was measured in vivo in the Pectoralis major, Sartorius and Gastrocnemius muscles (flooding dose of [H-3]-phenylalanine), Protein breakdown was dete rmined in the same muscles as the difference between protein synthesis and deposition. Chronic heat stress markedly reduced protein synthesis, irrespe ctive of muscle type (P < 0.05). This was mainly related to the lower capac ity for protein synthesis (muscle RNA/Protein) (P < 0.01). Chronic heat exp osure also decreased protein breakdown in the P. major and Sartorius; this effect was not observed in the Gastrocnemius. Protein synthesis was more af fected than breakdown, leading to reduced protein deposition, at least in t he P. major and Gastrocnemius muscles. Increasing dietary protein content h ad no significant impact on muscle protein turnover. Particularly at 32 deg rees C, the high-protein diet did not significantly modify either protein s ynthesis, ribosomal capacity or translational efficiency. However, it favor ed muscle protein deposition, which was probably related to reduced proteol ysis. In conclusion, we showed that chronic heat exposure decreased muscle protein deposition, mainly by reducing protein synthesis. Under these condi tions, the impaired protein synthesis was not restored by a 5% higher prote in intake.