GROWTH-HORMONE STIMULATES HEPATIC LIPID MOBILIZATION IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Rainbow trout were used to characterize the direct influence of growth hormone on hepatic lipid mobilization in vitro. Liver was removed from fish fasted 72 h, sliced into 1-mm3 pieces and incubated in Hanks' medium containing ovine or salmon growth hormone (0.28 ng . ml-1-28 mug . ml-1). Lipid mobilization, resulting from triacylglycerol hydrolysis, was evaluated by fatty acid and glycerol release into culture medium. Control rates of fatty acid release and glycerol release were 0.95 +/- 0.16 (mean +/- SE) and 0.88 +/- 0.28 mumol . l-1 . mg fresh weight, respectively. Both ovine growth hormone (28 ng . ml-1) and salmon growth hormone (28 ng . ml-1) stimulated fatty acid release into culture medium, increasing rates by 112% and 97%, respectively, during the course of a 24-h incubation. Glycerol release was similarly augmented by growth hormone treatment. Growth hormone-stimulated metabolite release became evident within 12 h and persisted for up to 72 h. The presence of amino acids in the culture medium potentiated the lipolytic action of growth hormone. Ovine growth hormone (28 ng . ml-1) in the presence of amino acids stimulated a 53% increase in fatty acid, and a 108% increase in glycerol, release over rates observed in the absence of amino acids. Salmon growth hormone (28 ng . ml-1) in the presence of amino acids stimulated a 53% increase in fatty acid, and a 44% increase in glycerol, release over rates observed in the absence of amino acids. Ovine growth hormone (28 ng . ml-1) also stimulated gluconeogenesis, as indicated by a 75% increase in phosphoenolpyruvate carboxykinase activity in liver pieces incubated in the presence of amino acids. These results indicate that growth hormone directly stimulates lipid breakdown in the liver of trout and that amino acids potentiate growth hormone-stimulated lipolysis.