Exercise training in skeletal muscle of brook trout (Salvelinus fontinalis).

1. The differentiation of myotomal muscles in the brook trout (Salvelinus fontinalis Mitchill) has been investigated using p-phenylene diamine stained semi-thin sections and cytochemical and quantitative determinations of enzyme activities. 2. Evidence is presented that the range of fibre size in white muscle represents stages in growth rather than distinct fibre types. 3. Electromyography shows that both red and white muscles are recruited for sustained swimming. The threshold swimming speed for recruitment of white fibres is around 1.8 body length/s (L/s). 4. White muscle citrate synthetase and cytochorome oxidase activities are 25-35% that of red muscle. Hexokinase, phosphorylase and phosphofructokinase activities are 2, 4 and 2 times higher in white than red muscles. It is considered that the aerobic capacity of white muscle is sufficient to support sustained swimming, and that blood glucose could be an important fuel source. 5. Endurance exercise training has been investigated in fish swimming, continuously, for 21 days at 3 L/s. This training regime restricts spontaneous high-speed swimming activity and resulted in a general decline of white muscle glycolytic enzyme activities. Red fibres underwent hypertrophy relative to non-exercised controls (530 +/- 64 microns2 non-exercised, 901 +/- 63 microns2 trained). Aerobic enzyme activities in red muscle and the fraction of fibre volume occupied by mitochondria (30.2 +/- 0.8%) did not change in response to the training programme, but glycolytic enzyme activities were elevated. 3-OH Acyl CoA dehydrogenase activities increased in both red and white muscles indicating an enhanced capacity for fatty acid catabolism with training. 6. Plasma and muscle lactate levels were not statistically different between tank-rested and trained fish swimming a 3 L/s. 7. Adaptations of fish muscle to endurance training are discussed and compared with results for other vertebrates.