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Serum proteins as carriers of amino acids to and from the hindlimbs of fed and fasted calves.

Venoarterial differences in the amino acid composition of serum proteins were measured across the hindlimbs of growing Holstein steer calves (147 kg) surgically fitted with abdominal aorta and caudal vena cava cannulae. Steers were maintained in near steady-state metabolism by feeding at hourly intervals in a continuously lighted environment. The steady-state blood samples were obtained 10 d post-surgery; then the steers were fasted and resampled. Arterial and venous serum proteins were chromatographically separated into four fractions and the amino acid composition of each was determined. Under steady-state conditions, there appeared to be a tendency for serum protein amino acids to appear on the venous side of the tissue in either greater or lesser concentrations than on the arterial side, depending upon the protein fraction. However, after the steers had fasted for 72 h, virtually all amino acids from all protein fractions were present in greater concentrations in venous serum than in arterial serum. Furthermore, the quantitative differences were several-fold larger than during steady-state sampling. It appears that serum proteins may contribute to the transport of amino acids to and from peripheral tissues. Data further suggest that all serum proteins do not appear to function in the same manner, and physiological state influences this process.