AGE-RELATED-CHANGES OF PROTEIN-TURNOVER IN SPECIFIC TISSUES OF THE CHICK

The effect of age on muscle and liver protein turnover was analyzed in growing broilers. Protein turnover was compared in the Pectoralis maj or (PM), the Anterior Latissimus dorsi (ALD), and the Sartorius (SART) muscles, as well as in the liver in broilers at three ages (2, 3, and 4 wk). Protein fractional synthesis rates (FSR, in percentage per day ) were measured by a flooding dose of L-[4-H-3] Phe. Protein fractiona l breakdown rates (FBR, in percentage per day) were estimated as the d ifference between FSR and the gain rates of tissue protein. When expre ssed in absolute rates (grams per day), tissue protein deposition incr eased during chick growth (approximately 1.7-fold, whatever the tissue ). This phenomenon was accompanied by increased protein synthesis (ASR ) and proteolysis (ABR) in the PM and in the ALD muscles as well as in the liver. However, ASR and ABR did not seem to be significantly modi fied in SART. The FSR in skeletal muscles significantly decreased with age. This developmental decline was associated with decreases in RNA content and RNA:protein ratio (Cs, capacity for protein synthesis). In contrast to muscle, we found no significant variation in liver FSR be tween 2 and 4 wk of age. In addition, liver Cs and translational effic iencies were not decreased. Finally, in muscle as in liver, FBR did no t show any clear age-related pattern.