FREE AMINO ACIDS IN BLOOD OF RUMINANTS-PHYSIOLOGICAL AND NUTRITIONAL REGULATION 1,2 Werner G. Bergen 3 Michigan State University, East Lansing 48824 Summary Tissue free amino acid pools account for only a small fraction of the total amino acid content in the body. Blood or plasma free amino acid pools represent only a very small proportion of tissue free amino pools. Plasma free amino acid profiles have been used as a tool to study nutritional and metabolic aspects of protein status. Static measurements of plasma amino acid levels have limited value as sensitive indicators of protein status because such measurements do not reflect the magnitude of amino acid fluxes in and out free amino acid pools. Use of plasma amino acid profiles, under well defined experimental conditions, in assessing limiting amino acids and essential amino acid requirements in ruminants is dis- cussed. (Key Words: Plasma Free Amino Acids, Physio- logical Regulation, Nutritional Regulation, Ruminants.) useful in assessing overall nitrogen metabolism in many diverse circumstances. Plasma alpha amino nitrogen and free animo acid (AA) levels have been determined in an effort to assess protein status more clearly. Although plasma free AA profiles are technically easy to obtain, physiological implications of these profiles are often difficult to ascertain due to the constant flux and turnover of free AA in plasma. Munro (1964) has called free AA the "currency" of protein metabolism. AA enter the body by absorption from the intestine after protein digestion, are utilized for tissue protein synthesis and represent the medium of exchange of nitrogenous compounds between tissues. This review has two purposes: first, to present a general framework of AA metabolism and how it is reflected by AA levels in blood plasma and second, to review specific nutritional studies in ruminants using plasma (P) AA levels as response criteria. I ntroduction Determination of metabolite or nutrient levels in readily available body fluids (blood and urine) is a long established practice in studying metabolic changes in intact animals. More recently, with the advent of multi-faceted automatic analytical equipment, metabolic profiling in blood has become a tool of re- searchers as well as clinicians. Blood and urinary nitrogenous metabolite levels have been used extensively to assess the nutritional/protein status of animals. Urinary nitrogen excretion data, although cumbersome to obtain, are 1 Michigan Agr. Exp. Sta. Journal Arcticle No. 9048. 2 Invited paper, presented at the 70th Annu. Meet. of the ASAS, Michigan State Univ., East Lansing, July 11, 1978. 3 Ruminant Nutrition Laboratory, Dept. of Animal Husbandry. Free Amino Acid Pools in Animal Tissues: General Considerations The sum total of all factors influencing total body AA flux, that is protein synthesis, protein degradation, tissue uptake and efflux (including the role of hormones or dietary factors on these processes) and influx from the small intestine and AA catabolism are reflected by concentra- tions of AA in the plasma. In nonruminants, variations in dietary AA levels, whether from various protein sources of differing AA composi- tions, from excess intakes of single AA or from AA imbalances or antagonisms, are reflected, at least during the absorptive phase, by AA profiles in the peripheral circulation (Harper et al., 1970). Portal blood resembles the AA makeup of the exogenous influx of AA even more distinctly (Denton and Elvehjem, 1954). The reflection of dietary AA profiles in blood plasma is seen more readily for essential (or indispensible-EAA) AA than for nonessential 1577 JOURNAL OF ANIMAL SCIENCE, Vol. 49, No. 6, 1979