Effect of intake on whole body plasma amino acid kinetics in sheep

Reproduction Nutrition Development, Jan 2003

Isabelle Savary-Auzeloux, Simone O. Hoskin, Gerald E. Lobley

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Effect of intake on whole body plasma amino acid kinetics in sheep

Reprod. Nutr. Dev. Effect of intake on whole body plasma amino acid kinetics in sheep Isabelle SAVARY-AUZELOUX 0 2 Simone O. HOSKIN 1 2 Gerald E. LOBLEY 2 0 Present address: INRATheix , Clermont-Ferrand, 63122 St-Genès-Champanelle , France 1 Present address: Institute of Food, Nutrition and Human Health, Massey University , Private Bag 11222, Palmerston North , New Zealand 2 Rowett Research Institute , Bucksburn, Aberdeen, AB21 9SB , UK - While both the quantity and quality of food ingested are potent regulators of whole body protein metabolism in ruminants, little data are available on responses across a wide range of intakes. The current study examined the responses in whole body protein flux (PrF) to such intake changes and compared these with the responses across the hind-quarters (in a companion study). Six growing sheep (6-8 months, 30-35 kg) received each of four intakes of dried grass pellets (0.5, 1.0, 1.5 and 2.5 times maintenance energy; M) for a minimum of 7 days. At each intake, a mixture of U-13C amino-acids (AA) was infused intravenously for 10 h. Arterial plasma and blood were obtained over the last 4 h of infusion and the concentrations and the enrichments of thirteen 13C labelled AA were determined. The absolute values for plasma Irreversible Loss Rate (ILR) but also converted PrF varied between the AA. PrF values were lower for histidine, methionine, aspartate, glycine and proline (range 68 to 174 g·d-1 at 1.5 M) than for isoleucine, leucine, valine and glutamate (range 275 to 400 g·d-1 at 1.5 M). These discrepancies may be explained by (1) the differential AA removal by the splanchnic tissues, (2) the de novo synthesis of the non-essential AA, (3) the transfer of AA from the erythrocytes or plasma to the tissues. The first two assumptions require further investigation whereas recent work has shown a minor role for AA transfers between erythrocytes and tissues. For most AA, ILR and PrF responded linearly to intake but curvilinear responses were observed for phenylalanine, lysine, leucine, isoleucine and tyrosine. These differences were not due to hind-quarter metabolism and may involve the digestive tract and liver. 1. INTRODUCTION In ruminants, and other mammals, whole body and tissue amino acid (AA) fluxes are sensitive to many physiological and nutritional stimuli. While intake has been shown to be a potent regulator of such fluxes (e.g. Pell et al. [1]; Young et al. [2]; Harris et al. [3]) there is still controversy as to how different tissues might respond and the effect this has on whole body kinetics. This may be because of the technique and tracer AA used (e.g. Tessari et al. [4, 5]) or because tissues may respond differently to changes in the nutrient supply, or both. Whole body protein metabolism in larger animals, including humans, is often estimated based on continuous infusion of a single labelled AA (usually an essential AA), during steady-state conditions [6, 7]. The advantages include limited surgical invasion, use of longitudinal studies and lower tracer costs compared with the large dose procedure [ 8, 9 ]. This technique is, however, subject to some limitations [ 10 ], especially when a single AA is monitored (such as leucine or phenylalanine [ 11–13 ]), which may, or may not, be distributed similarly between the different metabolic routes (protein synthesis, oxidation or synthesis of metabolites: [ 5, 9, 14 ]). To date, there have been relatively few comparisons between kinetic data obtained with a number of different AA (e.g. Lobley et al. [10]; MacRae et al. [ 15 ]) and even less that have investigated the responses across a wide range of intakes. From various data, based on the kinetics of a limited number of amino acids (especially phenylalanine and leucine), it has been proposed [ 7, 16 ] that ruminant peripheral tissue protein metabolism responds in a curvilinear manner as intake is increased between fasting and supra-maintenance levels. This was then postulated to relate to muscle tissue responsiveness to the anabolic polypeptide hormones, insulin and insulin-like-growth factor-1, with differential responses in protein synthesis and degradation when intake is altered. Unfortunately, the individual experiments available from the literature for such an analysis only provide a limited number of intakes [ 16 ]. Furthermore, whether the same pattern of response occurs for other tissues (and thus whole body protein metabolism) has not been examined. Metabolic responses and utilisation of the AA by other tissues (notably the liver and the digestive tract) may differ with intake [ 10, 17, 18 ]. As a consequence, the influence of the tissues that may respond curvilinearly to intake (e.g. muscle) can be masked when whole body flux is measured, if more metabolically active tissues, such as the splanchnic tissues, do not respond similarly. The current study quantified the effect of four intakes, between 0.5 to 2.5 times maintenance, on the whole body (...truncated)


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Isabelle Savary-Auzeloux, Simone O. Hoskin, Gerald E. Lobley. Effect of intake on whole body plasma amino acid kinetics in sheep, Reproduction Nutrition Development, 2003, pp. 117-129, Volume 43, Issue 1, DOI: doi:10.1051/rnd:2003010