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)