THE INTERACTION BETWEEN NUTRITIONAL-STATUS AND GROWTH-HORMONE IN YOUNG CATTLE - DIFFERENTIAL RESPONSIVENESS OF FAT AND PROTEIN-METABOLISM

The effect of dietary intake level on in vivo plasma leucine and plasm a palmitate flux rates and on the response to a bolus injection of bov ine growth hormone (GH) was investigated in six young steers. Animals were fed on a pelleted diet of dried grass-barley (0.7:0.3, w/w) in qu antities sufficient to supply 0.8, 1.2, 1.6, 2.0, 2.4 or 2.65 x mainte nance energy requirement, offered in hourly portions. Continuous intra venous infusions of [1-C-13]leucine or [1-C-13]palmitate were used to determine the flux of amino acid and fatty acid through the plasma poo l before, immediately (1-3 h) after and 22-24 h after a subcutaneous i njection of bovine GH (0.55 mg/kg body weight). Hourly blood samples w ere taken for 27 h to monitor the temporal responses of circulating ho rmones and metabolites following GH administration. The animal on the lowest plane of nutrition had elevated plasma GH and reduced insulin-l ike growth factor-1 concentrations compared with those fed on higher i ntake levels. Plasma leucine flux and leucine concentration increased with intake while palmitate flux and plasma non-esterified fatty acid (NEFA) concentrations were inversely related to intake. Leucine flux r ate decreased in the animals fed on the two highest intake levels in r esponse to GH 22-24 h after administration, but plasma leucine concent rations were reduced in all animals at this time. Only the animal fed on the lowest intake level showed an immediate response to GH (within 3 h of administration) with increased palmitate flux and plasma NEFA c oncentrations but a lipolytic response was apparent in other animals 2 2-24 h post-administration although the magnitude of the response was markedly reduced at high intakes. We conclude that lipid and protein m etabolism are differentially responsive to GH and nutritional status.