~ ~~~Whole-body protein synthesis, estimated by the irreversible loss rate procedure, and hind-leg protein metabolism determined by arterio-venous techniques were monitored in response to three nutritional conditions (approximately 0.6, 1 2 and 1.8 x energy maintenance (M)) in ten wether lambs (33 kg average live weight). In all lambs and treatments measurements were based on radiolabelled phenylalanine, but the terminal procedures (five a t 0.6 x M and five at 1.8 x M) also included infusion of[ I-'3Clleucine ; this permitted comparison of amino acids catabolized (leucine) and non-metabolized (phenylalanine) by the hind-limb tissues. Whole-body protein synthesis increased with intake and the relationship with energy expenditure was slightly lower than that reported previously for pigs and cattle. The efficiency of protein retention: protein synthesis did not exceed 0.25 between the two intake extremes. Effects of intake on amino acid oxidation were similar to those observed for cattle. Hind-limb protein synthesis also increased significantly (P < 0001) in response to intake. Estimates of protein gain, from net uptake values, indicated that the tissues made a greater proportional contribution to total protein retention above M and to protein loss below M, emphasizing the role played by muscle tissue in providing mobile protein stores. The rates of protein synthesis calculated depended on the selection of precursor (blood) metabolite, but rates based on leucine always exceeded those based on phenylalanine when precursor from the same pool was selected. The incremental efficiency of protein retained: protein synthesis was apparently unity between 0.6 and 1.2 x M but 0.3 from 1.2 to 1.8 x M. Blood flow through the iliac artery was also proportional to intake. Leucine and 0x0-acid catabolism to carbon dioxide increased with intake such that the metabolic fate of the amino acid was distributed in the proportion 2: I between protein gain and oxidation. The rates of oxidation were only 1-3% the reported capacity of the rate-limiting dehydrogenase enzyme in muscle, but sufficient enzyme activity resides in the hindlimb adipose tissue to account for such catabolism. 1987) and, thus, small changes in the balance of the two processes can produce marked effects on net anabolism and production efficiency. One effective modulator of protein turnover is intake and for a variety of commercial species, including pig (Reeds et ul. 1980),

In ten lambs (average live weight 33 kg), five offered 300 g/d (approximately 0 6 x maintenance; L) and five 900 g/d (1.8 x maintenance; H), tissue protein synthesis was measured by three procedures simultaneously. The techniques involved continuous infusion of leucine over 7-8 h followed by a terminal large dose of 115Nlphenylalanine during the last 30 or 60 min. Rates of protein synthesis were then calculated based on the free amino acid or 0x0-acid isotopic activity in either arterial, iliac venous blood or tissue homogenate for the continuous-infusion studies, or on plasma or tissue homogenate for the large-dose procedure. For muscle ( > 99 YO), and to a lesser extent skin amino or 0x0-acid were significantly less, more so at the lower intake. In contrast, for skin, a tissue dominated by export protein synthesis, values from the large-dose procedure (L 6.37 %/d, H 10.98 %Id) were similar to those derived with arterial or venous metabolites as precursor (L 5.23 and 693%/d, H 9.98 and 11.71 %/d for leucine), but much less than those based on homogenate data. Based on the large-dose technique, protein synthesis increased with intake in muscle (P < 0.001), skin (P = 0.009) and liver (26.7 v. 30.5 %/d; P = 0.029). The contributions of muscle and skin to total protein synthesis were approximately equal. The incremental efficiency of conversion for muscle of synthesized protein into deposition appeared to be similar to values reported for rodents.Tissue protein synthesis: Protein intake: LambThe dynamic nature of protein metabolism has been much investigated over the past two decades, with particular emphasis on understanding responses in tissues to a variety of nutritional and physiological stimuli. In both laboratory and commercial species most attention has focused on measurement of protein synthesis, for which a range of tracerbased techniques are available (see Lobley, 1988). Data for the larger species are more limited than for rodents, due to both accessibility and cost. Furthermore, much of the information on protein metabolism in individual tissues of farm animals is based on the continuous infusion of tracer amino acid developed by Waterlow and his colleagues (e.g. Garlick et al. 1973). The problem with this technique is that the various free amino acid pools of the body (e.g. vascular, interstitial, intracellular) become labelled to different

1. Five Greyface wethers (4245 kg) fed on various fixed amounts of dried grass pellets (either approximately 1.3 times maintenance or 2 times maintenance) by means of belt-type continuous feeders were housed in opencircuit respiration chambers for periods of 45 d. Between days 15 and 35 they received daily oral doses of 1.5 mg of the p-adrenergic agonist clenbuterol (adsorbed on to the feed). Continuous energy and nitrogen balance measurements each of 5 d duration were conducted throughout the chamber confinement. 2.On six occasions (twice during the 15 d pre-clenbuterol period, on days +4, + 11 and + 18 of clenbuterol administration and once during the post-treatment period) animals were infused with [l-14C]leucine to determine the rates of leucine oxidation and the amounts of leucine available for protein synthesis. 4. During clenbuterol treatment leucine oxidation was reduced ( P < 001). However, values for the amounts of leucine available for protein synthesis were equivocal, with an increase (P < 0.001) on day 11 of treatment, but no change on days 4 and 18.5. Withdrawal of the clenbuterol resulted in rapid alterations of N and energy metabolism towards those expected of control animals of that weight.The assertion that over-consumption of animal fat is detrimental to human health (National Advisory Committee on Nutrition Edwation, 1983 ; Department of Health and Social Security, 1984) has intensified the interest in methods which induce a leaner carcass conformation in growing animals. Some manipulation can be achieved by nutritional means (Turgeon et al. 1986) but economic considerations limit the extent of such applications. Within the genetic constraints of any particular breed, alternative ways of decreasing the ratio of fat gain:protein gain in growing and fattening animals usually involve the administration of exogenous compounds, e.g. ionophores, anabolic steroids, growth hormone, etc.The phasing-out, within the European Economic Community at least, of the use of anabolic steroids has given added impetus to the search for alternative manipulators of the growth process. Recent interest has focussed on the range of /3-adrenergic agonists, e.g. clenbuterol and cimaterol, which in growth trials involving poultry, pigs, sheep and cattle have been shown markedly to increase the areas of specific muscles, while concomitantly reducing body fat Dalrymple et al. 1984a, b ;Beerman et al. 1986).At present the majority of metabolic studies on the action of the P-agonists have been confined to rats where, for muscle protein metabolism at least, the evidence as to their mode of action is contradictory, with reports of both increases in protein synthesis (Emery et al. 1984) and decreases in protein degradation (Reeds et al. 1986). The present study represents the first stage in an examination of the action of clenbuterol on protein and energy metabolism in sheep. The protocol involved determination of the kinetics of energy https://www.cambridge.org/core/terms. https://doi

The effects of episodic infusion of testosterone into the vascular system on energy expenditure, nitrogen retention and whole body protein synthesis (determined from [1-14C]leucine kinetics) were studied in castrated male lambs under conditions of controlled food intake. Comparisons were made between a 10-day control period and a 10-day treatment period for each lamb. Infusion of testosterone produced a significant increase in heat production, but the magnitude (198 kJ/day, +2.5%) was less than the differences in energy expenditure expected between entire and castrated male ruminants. The retention of nitrogen improved by 1.24 g/day (+22%) in response to the administration of androgen, and this was accompanied by a decrease in amino acid oxidation. Total protein synthesis also declined, and the anabolic nature of testosterone supply must, therefore, be effected through a reduction in the breakdown of protein, the mechanism being similar to that proposed for certain anabolic steroids and the beta-agonist, clenbuterol. Contrary to other reports, the presence of testosterone had no effect on the plasma concentration of GH.