COMPARISON OF HINDQUARTER METABOLITE UPTAKES IN BELGIAN BLUE DOUBLE-MUSCLED BULLS AT MAINTENANCE OR DURING FATTENING

Metabolism of muscle growth in the hindquarter was investigated by the arterio-venous difference (AVD) technique in Belgian Blue double-musc led type bulls at maintenance or at fattening. The bulls were fitted w ith an aortic ultrasonic blood flow probe and with catheters in the ao rta and vena cava. They were offered a diet allowing for maintenance ( MP) during a period of 15 d, at the end of which measurements were mad e over 3 d. Bulls were then given a fattening diet (FP) and the measur ements were repeated. Arterial blood flow was approximately 1 L/min gr eater when the bulls were standing than when lying. Blood flow was 2 L /min higher during FP than during MP. The AVD and uptake of glucose we re maximal at 1400 and 1600. Uptake of alpha-amino nitrogen decreased immediately after a meal. The increase in glucose from MP to FP fitted very well with the calculated energy needs for muscle growth. The AVD and uptake of alpha-amino nitrogen, total amino acids, and total none ssential amino acids were negative during MP and positive and signific antly higher during FP. There was also a significant increase in AVD a nd uptake of essential and branched-chain amino acids when the bulls w ere changed from MP to FP. When changing from maintenance to fattening , the incremental glucose and amino acid hindquarter uptake provided e nergy and supply for muscle protein accretion, respectively. The level of alanine transamination was also sharply reduced.