The effect of exercise on muscle growth and muscle-specific gene expression in the common carp (Cyprinus carpio L.) and rainbow trout (Oncorhynchus mykiss Walbaum)

Abstract

Forced exercise training has long been known to be a powerful stimulus for muscle growth in several families of teleosts. The effect of exercise on muscle growth in the common carp (Cyprinus carpio L.) was studied in five experiments, at a range of swimming velocities between 1.7 and 3.0 body lengths per second (bls⁻¹) Muscle fibre cross-sectional area was measured in fast, slow and where identified, intermediate muscle fibres. Exercise was shown to affect the fast and slow musculature differently in a manner analogous to the teleost starvation response. In response to moderate endurance exercise training fast muscle fibres were recruited to the myotome, but this process was balanced by atrophy of larger existing fast fibres. In contrast, slow to moderate exercise training induced slow muscle fibre hypertrophy. In response to a hyperplastic stimulus, the number of myonuclei per myofibre increased, implying a single population of myogenic progenitor cells provides the cells for post-embryonic muscle growth. A comparative study was conducted on rainbow trout (Oncorhynchus mykiss Walbaum), a species associated with a high inherent swimming capacity. Exercise training at 0.8 and 1.6 bls⁻¹ induced a 24 - 30% increase in mean fast muscle cross-sectional area through hypertrophy. To identify putative molecular signaling pathways underlying teleost muscle growth, the nuclear localisation and/or overall expression of myogenic regulatory factors (MRFs), calcineurin and associated substrate transcription factors and myostatin was compared between exercised and tank rested controls. The overall expression of MRFs was invariant in all experimental treatments. In common carp, increased nuclear localisation of the primary MRFs, especially MyoD, was positively correlated with larger mean fibre cross-sectional area. Increased nuclear localisation of the calcineurin protein was associated with hypertrophic growth of fast and slow muscle fibres in the common carp. Exercise-induced fast fibre hypertrophy was strongly correlated with increased calcineurin nuclear localisation in rainbow trout. However, a second experiment on common carp revealed no association between extensive fibre hypertrophy and nuclear localisation of the calcineurin protein. Furthermore, NFAT2 nuclear localisation did not follow a pattern that suggested NFAT2-mediated transcriptional activity was concurrent with muscle growth in the common carp or rainbow trout. In rainbow trout, myostatin active peptide was downregulated slightly (6 - 7%) in groups exhibiting exercise-induced hypertrophy of fast muscle fibres, consistent with its proposed role as a negative regulator of muscle growth. It was concluded that the molecular pathways examined might contribute to the regulation of muscle growth in teleosts but only as part of a more complex regulatory network.