Glucose, alpha-amino nitrogen and amino acid exchange across the hindlimb in young bulls maintained at two growth rates

The effect of growth rate and protein supplementation on muscle metabolism of eight bulls from the Belgian Blue breed, double-muscled type, was investigated by the arterio-venous difference technique. A low growth (LG) group was maintained at a low growth rate over 36 d, and a rapid growth (RG) group for 28 d before receiving a fattening diet allowing for a rapid growth. At the end of the RG period the RG bulls received a supplement of protected soybean meal. Animals were fitted with an aortic ultrasonic blood flow probe and with catheters in the aorta and the vena cava. The blood flow in the hindlimbs of bulls varied greatly by time of the day but was higher in the RG group. The RG group had a higher arterio-venous difference (AVD) and uptake of alpha-amino nitrogen while AVD in essential amino acids was four times higher and uptake eight times higher. Significant higher AVD or uptake was observed in individual amino acids such as leucine, isoleucine and lysine. The supplementation with protected soybean meal had significant negative effect on the uptake of several amino acids. It was concluded that caution should be exercised when measuring punctually blood flow in muscle tissue, for example by dilution techniques. At high growth rate, the requirements for amino acids are larger than for glucose. Excess protein provides no additional benefit