Skeletal muscle growth dynamics and expression of related genes in white and red muscles of rainbow trout fed diets with graded levels of a mixture of plant protein sources as substitutes for fishmeal

Juvenile rainbow trout were fed to visual satiation for 12 weeks with four diets containing fishmeal as the primary protein source (FM diet) or FM substituted with graded levels of a mix of plant proteins to replace 50% (PP50 diet), 75% (PP75) or 100% (PP100) of fishmeal with addition of crystalline amino acids to meet the requirements in indispensable amino acids. Growth performance, feed efficiency and protein efficiency were not affected by 50% fishmeal substitution but all decreased with higher substitution levels (FM = PP50 > PP75 > PP100). Ammonia excretion was inversely changed with FM substitution level (FM = PP50 < PP75 < PP100). Trout fed the FM diet had the highest level of nitrogen gain and those fed diet PP100 the lowest. Distribution of white muscle fibre diameter and expression of selected muscle genes were not affected by 50% fishmeal substitution. At higher substitution levels (75% and 100%) there was a significant decrease in the median diameter of white muscle fibres. Total substitution of fishmeal by plant protein sources (PP100) significantly increased (× 2.4) cathepsin D expression in white muscle but not in red muscle. Expression of MyoD, myogenin, PCNA, myosin heavy chains and calpain 2 was not modified by dietary treatments in either muscle type. This study thus demonstrates that the decrease in growth of trout fed a high level of plant protein sources is linked to changes in the dynamics of white muscle growth and correlated with changes in expression of a gene involved in lysosomal proteolysis in the white muscle.