Regulation of muscle protein metabolism in ruminants
A series of in vivo experiments was conducted in sheep, to evaluate the effects of limiting amino acids, growth hormone and the β-adrenergic agonists, cimaterol and clenbuterol, on protein metabolism. As a complement to substrate studies, investigations into bicarbonate kinetics were performed. A multi-tracer amino acid model was developed, based on the large-dose procedure to measure protein synthesis. The response in nitrogen retention to a parenteral infusion of methionine, lysine and histidine was evaluated. This experiment was an attempt to define an experimental model to evaluate the responsiveness of peripheral tissue to supplies of limiting amino acids under conditions which minimise alterations in systemic substrate and hormonal conditions. The animals were probably not dietary-nitrogen limited, as initially calculated, and a poor responsive model was created. In a series of three experiments, the extent of ^13CO_2 sequestration in both the whole body and across the hind-leg preparation was studied in order to establish correction factors for planned substrate oxidation studies. Mean whole body sequestration was 30% with a fractional sequestration across the hind-limb of 3%. The results from these experiments suggested that the fraction of the bicarbonate sequestered did not appear to vary with either the level of intake or the duration of the infusion. The involvement of rumen microorganisms in the carbon dioxide sequestration was also excluded. The effects of two levels of protein nutrition on the response to exogenous bovine growth hormone was evaluated. Growth hormone infusion significantly increased nitrogen retention at both levels of intake, over an 8d period, suggesting that, under the established experimental conditions, the level of protein supply in the diet was not a factor limiting the hormone-induced increases in protein accretion. Rates of muscle protein synthesis (k_s %d) were measured by flooding with L-[1-^13C]phenylalanine, L-[1-^13C]leucine and L-[1-^13C]valine. The fractional synthesis rates obtained with the three amino acids were not significantly different. The results of these study were of particular relevance since it was important to establish a model which allowed serial measurements in the same animal.