4239The myogenic regulatory factors (MRFs) are a family of basic helix-loop-helix (bHLH) transcription factors essential to the specification and determination of the muscle cell Supplementary material available online at

SUMMARY Rainbow trout Oncorhynchus mykiss Walbaum were exercised at 0.8 and 1.6 body lengths s-1 for 18 h a day over a 30 day period. Exercise resulted in a 24-30% increase in the average cross-sectional area of fast muscle fibres relative to tank-rested controls. The concentrations of growth factors and transcription factors hypothesised to play a role in regulating exercise-induced muscle fibre hypertrophy were measured. Exercise training resulted in a minor increase in calcineurin localisation in the nucleus. However, nuclear factor of T-cells 2 (NFAT2) nuclear localisation did not follow a pattern that was consistent with NFAT2-mediated transcriptional activity and changes in calcineurin signaling. The active peptide of myostatin, a negative regulator of muscle growth in mammals, was downregulated in exercise groups relative to tank-rested controls, but only by 6-7%. It was concluded that myostatin and calcineurin signaling do not play a major role in regulating exercise-induced muscle hypertrophy in trout.

Endurance exercise training (2Á4-2Á6 body lengths s À1 , 16 h day À1 for 28 days) resulted in an increased density of myonuclei in fast muscle fibres relative to tank rested controls and induced slow muscle fibre hypertrophy. The results indicate that exercise is a powerful stimulus for the proliferation of myogenic cells and nuclear accretion.

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