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Metabbolism of perfused ruminant muscle

SUMMARY Substrate and nitrogen metabolism of ruminant muscle was studied in vitro using a perfused sheep hemidiaphragm preparation. Glucose, acetate, propionate and butyrate were taken up by the perfused muscle at a constant rate over 2 h. Rate of acetoacetate uptake declined over the perfusion period. Net production of 3-hydroxybutyrate over the perfusion period was observed. Release of alanine and glycine by the perfused muscle could not be accounted for by protein breakdown, implicating muscle as a site of synthesis of these amino acids. The perfused muscle metabolized aspartate and arginine to a considerable extent, but the branched-chain amino acids only to a small extent. Only a small proportion, approximately 5%, of the alanine produced by the perfused muscle was derived from glucose. In the presence of amino acids, at twice the concentration found in plasma, and insulin, some amino acids showed a net uptake by the diaphragm but when cysteine was omitted from the perfusate, all amino acids with the exception of arginine showed a net output. Evidence was obtained that protein synthesis was reduced in the absence of cysteine. The output of Nτ methylhistidine by the preparation was much higher than could be accounted for by estimates of protein breakdown using isotope dilution techniques. These data are, however, consistent with the view that Nτ methylhistidine is not a good index of muscle protein breakdown in sheep.