Dietary protein source significantly alters growth performance, plasma variables and hepatic gene expression in rainbow trout (Oncorhynchus mykiss) fed amino acid balanced diets

Abstract

The objective of the study was to evaluate the effect of dietary protein source on fish growth, nutrient utilization, plasma variables and hepatic gene expression in juvenile rainbow trout (Oncorhynchus mykiss). Fishmeal (FM) and soy protein isolate (SPI) were used as the main sources of protein in six isonitrogenous, isolipidic and isocaloric diets. The amino acid profiles of the diets were completely balanced to minimize differences between experimental treatments and formulated to contain increasing levels of branched-chain amino acids (BCAA) based upon dietary requirements for trout (NRC, 1993). Dietary protein source more consistently changed the measured variables while BCAA supplementation had an unexpected effect over whole body lipid content. Growth performance and protein retention efficiency were significantly reduced in fish fed SPI diets independently of BCAA supplementation. Total concentration of amino acids as well as circulating indispensable amino acids (IAA) were significantly elevated in the plasma of fish receiving SPI diets compared to fish fed FM diets. The change in IAA was large enough to increase (p<0.05) plasma IAA/DAA (DAA: dispensable amino acids) proportion even when the diets were formulated to have a ratio close to 1. Levels of circulating BCAA and alanine were also elevated in the fish fed SPI diets, possibly indicating a change in protein turnover. The use of SPI caused a reduction (p<0.05) in the hepatic expression levels of alanine amino transferase (alt1) and glutamine synthetase 2 (gls02), while an increase was observed for aspartate aminotransferase (got2), and asparagine synthetase (asns) compared with FM diets. Expression of the gene tor (target of rapamycin) declined over time for all treatments, while expression of a gene known to repress tor function, redd-1, was consistently higher in the liver of fish fed SPI diets. Glucose 6 phosphate dehydrogenase (g6pd) also showed a significantly higher expression in the liver of fish fed SPI diets but only at higher levels of BCAA supplementation.In summary dietary protein source has a significant effect over growth performance, body composition and hepatic gene expression in rainbow trout. We also identified for the first time in fish changes in the expression of redd-1, which may represent another regulatory point in the TOR cascade.