Myogenesis and muscle metabolism in juvenile Atlantic salmon (Salmo salar) made transgenic for growth hormone.

Atlantic salmon (Salmo salar) made transgenic for growth hormone (GH) and non-transgenic salmon were sampled at 4 and 7 months of age to estimate myogenic factors, satellite cell proliferation and metabolic enzyme activities. The growth rate of 4 month old transgenic salmon was higher than that of non-transgenic salmon. Myosatellite cell (MC) proliferation rates were higher in cells isolated from GH-transgenic salmon compared with cells from non-transgenic salmon of the same mass. Moreover, MCs extracted from non-transgenic salmon demonstrated a higher proliferation capacity when exposed in vitro to salmon GH. White muscle MyoD I mRNA content was higher in transgenic and non-transgenic salmon at 7 months compared with that at 4 months, indicating an effect of age on MyoD I mRNA expression. White muscle myogenin mRNA content varied with fish age and presence of the transgene, and was higher in transgenic fish at 7 months, suggesting a higher differentiation capacity. MyoD I, MyoD II and myogenin mRNA content was higher in red muscle of GH-transgenic fish at 7 months compared with non-transgenic salmon at 7 months. However, red muscle myogenic factor expression was not different between transgenic and non-transgenic fish of the same weight. Enzyme activities in white muscle and liver were highly affected by the presence of the transgene, although this effect was generally dependent on the age of the fish. Glycolytic and oxidative enzyme activities were increased in transgenic salmon liver, indicating a higher metabolic rate in transgenics. This study demonstrates that (1) the higher growth rate of transgenic salmon particularly at 4 months of age could be explained at least in part by higher numbers and proliferation rates of MCs, (2) GH can directly stimulate the proliferation of myosatellite cells extracted from salmon, indicating that GH is one possible factor involved in the higher myosatellite cell proliferation rates in transgenic salmon, (3) MyoD and myogenin mRNA expression are affected by fish age, and (4) metabolic enzyme activities are affected by the age of the fish at least in liver and white muscle, and any transgene effect is dependent upon the age of the fish.