Impact of Dietary Carbohydrate/Protein Ratio on Hepatic Metabolism in Land-Locked Atlantic Salmon (Salmo salar L.)

Abstract

A common-garden experiment was carried out to compare two genetically distinct strains of Atlantic salmon (Salmo salar) fed diets with either high (CHO) or low (NoCHO) digestible carbohydrate (starch). Twenty salmon from either a commercial farmed strain (F) or a land-locked population (G) were placed in two tanks (10 fish of each population in each tank) and fed either CHO or NoCHO feeds. At the end of the experiment fish were fasted for 8 h, euthanized and blood and liver collected. Both diet and population had an effect on circulating glucose levels with G showing hypoglycaemia and dietary starch increasing this parameter. In contrast, G showed increased plasma triacylglycerol levels regardless of dietary treatment suggesting faster conversion of glucose to triacylglycerol. This different ability to metabolize dietary starch among strains was also reflected at a molecular (gene) level as most of the metabolic pathways evaluated were mainly affected by the factor population rather than by diet. The data are promising and suggest different regulatory capacities toward starch utilization between land-locked salmon and the farmed stock. Further analyses are necessary in order to fully characterize the capacity of land-locked salmon to utilize dietary carbohydrate.

Keywords: dietary carbohydrates; glucose metabolism; land-locked; salmon populations; transcriptomics.

FIGURE 1

(A) Plasma glucose and (B) triglyceride levels in land-locked (G) and farmed (F) Atlantic salmon after being fed with a diet high (dark bars) or low (light bars) in carbohydrate for 32 days. ^∗p < 0.05; ^∗∗p < 0.05; and ^∗∗∗p < 0.001.

FIGURE 2

Venn diagram representing mRNA transcripts in liver differentially expressed in response to diet, population, and those showing interaction between diet and population.

FIGURE 3

(A) Pathway clustering analysis performed on the 5327 genes exclusively affected by population. (B) Enriched functions and pathways of the metabolism-related genes exclusively related to population. The network of pathways was created with ClueGo and CluePedia Cytoscape apps. The pathways were functionally grouped and interconnected based on the kappa score. The size of the nodes indicates the number of genes associated. Pathways are colored based on their significance after Bonferroni correction, where the most significant terms are shown in dark red and the least significant terms are illustrated in white.

FIGURE 4

GO analysis of differentially expressed metabolism-related genes indicating the percentage of genes related (X axis) as well as the number of genes affected. Gray bar, cellular oxidation detoxification; yellow bar, oxygen transport; red bar, cofactor metabolic process; pink bar; citrate cycle; green bar, cellular respiration; and blue bar, organic acid metabolism.

FIGURE 5

(A) Venn diagram representing mRNA transcripts differentially expressed in liver of farmed (F) or land-locked (G) Atlantic salmon fed either high (CHO) or low (NoCHO) carbohydrate feeds. (B) Distribution by category of common differentially expressed genes (2170) in liver of Atlantic salmon fed high or low CHO feeds.

FIGURE 6

Gene expression of a glucose transporter and selected glycolytic and gluconeogenic genes in the liver of land-locked (G) or farmed (F) Atlantic salmon fed a diet high (dark bars) or low (light bars) in carbohydrate for 32 days. glut2b, Glucose transporter type 2; gckb, glucokinase paralog b; pkl, liver pyruvate kinase; g6pca, glucose 6-phosphatase paralog a; g6pcb1, glucose 6-phosphatase paralog b1; fbp1a, fructose 1,6-bisphosphatase 1 paralog a; fbp1b1, fructose 1,6-bisphosphatase 1 paralog b1; fbp1b2, fructose 1,6-bisphosphatase 1 paralog b2; pck2, mitochondrial phosphoenol pyruvate kinase. ^∗p < 0.05; ^∗∗p < 0.05; and ^∗∗∗p < 0.001.

FIGURE 7

Gene expression of selected mitochondrial metabolism, β-oxidation, lipogenesis and biosynthesis of pentose phosphate genes in the liver of land-locked (G) or farmed (F) Atlantic salmon fed a diet high (dark bars) or low (light bars) in carbohydrate for 32 days. qcr2, ubiquitinol cytochrome c reductase core protein 2; atp5a, ATP synthase form 5; cox4, cytochrome oxidase 4; sdhb, succinate dehydrogenase complex iron sulfur subunit B; cs, citrate synthase; cpt1a, carnitine palmitoyl transferase isoform 1a; cpt1b, carnitine palmitoyl transferase isoform 1a; hoad, hydroxyacyl-CoA dehydrogenase; acly, ATP citrate lyase; fas, fatty acid synthase; g6pd, glucose 6-phosphate dehydrogenase. ^∗p < 0.05; ^∗∗p < 0.05; and ^∗∗∗p < 0.001.