Estimation of Plasma Histidine Flux in Cows

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Abstract
A tracer method is described for estimating the plasma amino acid flux in animals during steady state, and its application is illustrated using lactating cows that were injected intravenously with 14C- or 3H-labeled histidine. The method involves an extension of the occupancy principle in which the fraction of tracer recovered in a metabolic sink, casein or hemoglobin, provides the basis for calculating the amino acid flux. The plasma histidine flux was 25 g/day based on recovery of istope in milk casein and 31 g/day based on recovery of isotope in hemoglobin.