Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2011
Publication Date: 7/3/2011
Citation: Cleveland, B.M., Burr, G.S. 2011. Proteolytic response to feeding level in rainbow trout (Oncorhynchus mykiss). Aquaculture. DOI: 10.1016/j.aquaculture.2011.06.043.
Interpretive Summary: Growth rate and nutrient retention, specifically protein retention, are important performance variables that directly affect the profitability of an aquaculture operation. However, growth and nutrient retention are often mutually exclusive, with optimal feed efficiency and protein retention occurring at moderate levels of nutrient restriction. Therefore, understanding the proteolytic mechanisms that contribute to the reduction in protein retention during satiable feeding will help develop new strategies that enable the colocalization of maximal growth rates with optimal feed efficiency. This research suggests that increases in protein degradation in white muscle reduce protein retention, which negatively impacts fish performance. Therefore, reducing the activity of metabolic pathways that control protein degradation may be important for improving feed efficiency. This may be achievable by increasing the lipid content of the diets, which would make more energy available for nutrient deposition. Alternatively, strategies that selectively intervene with protein degradation pathways may have positive impacts on protein retention in aquaculture species.
Technical Abstract: In fish there is a quadratic relationship between feed intake and feed efficiency, with optimal nutrient retention occurring at moderate levels of feed restriction. Determining how mechanisms controlling nutrient utilization are affected by feed intake, with a particular emphasis on those that impact protein turnover, will enable a better understanding of the metabolic processes that contribute to variations in nutrient retention. In the present study, rainbow trout were fed for 10 wks at different levels of satiation and indices of growth, nutrient retention, and protein degradation were determined. Feed efficiency, specific growth rate, and protein, lipid, and energy retention, as well as plasma concentrations of insulin-like growth factor-I (IGF-I) displayed a positive quadratic response to feeding level (P<0.01). The myofibrillar protein degradation product, 3-methylhistidine (3-MH), displayed a negative quadratic response (P<0.02) whereas free amino acids increased linearly with feed intake (P<0.001). These data suggest that an increase in protein degradation occurs during high levels of feed intake that is partially driven by changes in plasma IGF-I, which displayed a slight decrease in plasma concentration approaching satiation and may contribute to the associated reductions in protein retention and feed efficiency. Proteolytic genes that also displayed quadratic responses included the ubiquitin ligase, Fbx32 (P<0.01), and cathepsin D (P<0.01). Therefore, interrupting the up-regulation of the ubiquitin-proteasome and lysosomal systems during high levels of feed intake may impede increases in protein degradation, reduce rates of protein turnover and subsequently improve protein retention and feed efficiency.