Effects of steroid treatment on growth, nutrient partitioning, and expression of genes related to growth and nutrient metabolism in adult triploid rainbow trout (Oncorhynchus mykiss).

The contribution of sex steroids to nutrient partitioning and energy balance during gonad development was studied in rainbow trout. Specifically, 19-mo old triploid (3N) female rainbow trout were fed treatment diets supplemented with estradiol-17β (E2), testosterone (T), or dihydrotestosterone at 30-mg steroid/kg diet for a 1-mo period. Growth performance, nutrient partitioning, and expression of genes central to growth and nutrient metabolism were compared with 3N and age-matched diploid (2N) female fish consuming a control diet not supplemented with steroids. Only 2 N fish exhibited active gonad development, with gonad weights increasing from 3.7% to 5.5% of body weight throughout the study, whereas gonad weights in 3N fish remained at 0.03%. Triploid fish consuming dihydrotestosterone exhibited faster specific growth rates than 3N-controls ( P < 0.05). Consumption of E2 in 3N fish reduced fillet growth and caused lower fillet yield compared with all other treatment groups ( P < 0.05). In contrast, viscera fat gain was not affected by steroid consumption ( P > 0.05). Gene transcripts associated with physiological pathways were identified in maturing 2N and E2-treated 3N fish that differed in abundance from 3N-control fish ( P < 0.05). In liver these mechanisms included the growth hormone/insulin-like growth factor (IGF) axis ( igf1 , igf2 ), IGF binding proteins ( igfbp1b1 , igfbp2b1 , igfbp5b1 , igfbp6b1 ), and genes associated with lipid binding and transport ( fabp3 , fabp4 , lpl , cd36 ), fatty acid oxidation ( cpt1a ), and the pparg transcription factor. In muscle, these mechanisms included reductions in myogenic gene expression ( fst , myog ) and the proteolysis-related gene, cathepsin-L, suggesting an E2-induced reduction in the capacity for muscle growth. These findings suggest that increased E2 signaling in the sexually maturing female rainbow trout alters physiological pathways in liver, particularly those related to IGF signaling and lipid metabolism, to partition nutrients away from muscle growth toward support of maturation-related processes. In contrast, the mobilization of viscera lipid stores appear to be mediated less by E2 and more by energy demands associated with gonad development. These findings improve the understanding of how steroids regulate nutrient metabolism to meet the high energy demands associated with gonad development during sexual maturation. [ABSTRACT FROM AUTHOR]