Responses of myogenesis and metabolism to temperature and growth factors in fish

Abstract

Fish are becoming more important in the human diet as they represent a high protein source and a source of essential free fatty acids. Fish production from aquaculture has been improved by the selection of faster growing individuals, but fish continue to remain an expensive nutrient source. A better understanding of fish growth mechanisms and regulation will help to further improve aquaculture techniques. Moreover, studying those factors that stimulate cells implicated in fish muscle growth could provide useful knowledge and possible applications in mammalian muscle illness. In contrast to mammals and birds, fish growth is generally indeterminate and most fish grow throughout their life by the formation of new muscle cells (hyperplasia) and/or by increasing muscle cell volume (hypertrophy). Proliferation and differentiation of muscle cells are regulated by transcription factors called myogenic regulatory factors (MRF). MyoD and myogenin are MRF involved in cell proliferation and differentiation in vertebrates. Many factors affect fish growth including external factors such as temperature and food availability, and internal factors, principally hormones. The effects of temperature on fish growth and metabolism were assessed in Atlantic cod (Gadus morhua) and rainbow trout (Oncorhynchus mykiss ), and the effects of hormones were investigated using Atlantic salmon (Salmo salar) made transgenic for growth hormone (GH) and rainbow trout fed beta-agonists. In order to understand the mechanisms and regulation of myogenesis, morphometric parameters were recorded, metabolic enzymes were assayed in liver and skeletal muscle and myogenesis was measured in white and red muscles in each experiment. Moreover, several growth factors and hormones were tested in vitro on myosatellite cell proliferation and differentiation. The results demonstrate that metabolism and myogenesis are not affected by the same factors, and an increase in growth is not absolutely accompanied by an effect on tissue metabolism. High and constant water temperatures increased juvenile cod growth though there were no significant alterations in liver and white muscle metabolism. In contrast, factors such as photoperiod and internal cycles appear to be more important in the control of cod tissue metabolism. However, myogenesis was significantly decreased and metabolism increased in rainbow trout exposed to cold compared with high temperatures. Growth hormone, insulin-like-growth factor I (IGF-1) and fibroblast growth factor (FGF) were found to increase in vitro myosatellite cell proliferation in salmonids. My results also show that proliferation and differentiation of salmon muscle does not happen simultaneously but may alternate in red and white muscles. These results taken together contribute to a better understanding offish muscle growth and metabolism and will be used in future studies to optimize fish growth in aquaculture.