Disposition of a Glucose Load into Hepatic Glycogen by Direct and Indirect Pathways in Juvenile Seabass and Seabream

Abstract

In carnivorous fish, conversion of a glucose load to hepatic glycogen is widely used to assess their metabolic flexibility towards carbohydrate utilization, but the activities of direct and indirect pathways in this setting are unclear. We assessed the conversion of an intraperitoneal glucose load (2 g.kg−1) enriched with [U-13C6]glucose to hepatic glycogen in juvenile seabass and seabream. 13C-NMR analysis of glycogen was used to determine the contribution of the load to glycogen synthesis via direct and indirect pathways at 48-hr post-injection. For seabass, [U-13C6]glucose was accompanied by deuterated water and 2H-NMR analysis of glycogen 2H-enrichment, allowing endogenous substrate contributions to be assessed as well. For fasted seabass and seabream, 47 ± 5% and 64 ± 10% of glycogen was synthesized from the load, respectively. Direct and indirect pathways contributed equally (25 ± 3% direct, 21 ± 1% indirect for seabass; 35 ± 7% direct, 29 ± 4% indirect for seabream). In fasted seabass, integration of 2H- and 13C-NMR analysis indicated that endogenous glycerol and anaplerotic substrates contributed an additional 7 ± 2% and 7 ± 1%, respectively. In fed seabass, glucose load contributions were residual and endogenous contributions were negligible. Concluding, direct and indirect pathways contributed equally and substantially to fasting hepatic glycogen repletion from a glucose load in juvenile seabream and seabass.

Impact of dietary starch on extrahepatic tissue lipid metabolism in farmed European (Dicentrarchus labrax) and Asian seabass (Lates calcarifer)

“…Alternatively, de novo synthesis of glycerol-3-phosphate must occur and as consequence, enrichment of the triglyceride glyceryl moiety is typically much higher than that of the FA, which also include FA from dietary (unlabeled) origin. Glucose can be a contributing precursor to glycerol synthesis (through dihydroxyacetone phosphate; DHAP) via an abbreviated pathway of glycolysis as confirmed by 13 C- (Rito et al, 2018) and 14 Cglucose (Walter et al, 2006) tracer studies. Higher levels of circulating (unlabeled) glucose may putatively explain why, glyceryl FSR was lower in the muscle of seabass fed with HS diet, as already observed in the liver if these fish (Viegas et al, 2016).…”

Limitations to Starch Utilization in Barramundi (Lates calcarifer) as Revealed by NMR-Based Metabolomics

“…MAG samples were reconstituted in 0.5 mL 90% acetonitrile/10% 2 H-depleted water with the addition of 50 µL of hexafluorobenzene. 2 H NMR spectra were obtained following the acquisition parameters described in Rito et al (2018).…”

“…Fish glucosyl units synthesized to glycogen during the 6days residence in 2 H 2 O become enriched in specific positions as a result of water hydrogen exchange and/or addition to metabolic intermediates catalyzed by specific enzymes of the glycogen synthesis pathways. Their positional 2 H-enrichments were determined using the MAG methyl signals (MAG-CH 3 ) as intramolecular standard (Nunes and Jones, 2009) and used to estimate the fractional metabolic provenance relative to tank water (Rito et al, 2018). The metabolic model for positional 2 Hlabeling is schemed in Figure 1 and a representative spectrum from which data is presented in Supplementary Figure S1.…”

“…Most carnivorous fish species such as European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) store excess starch in the form of glycogen (Enes et al, 2011), which is then readily available during fasting (Enes et al, 2009) and exercise (Felip et al, 2013). Hepatic conversion of glucose to glycogen is considered a key step in defining metabolic flexibility toward dietary CHO utilization (Rito et al, 2018). Glucose conversion to glycogen can occur by the so-called direct pathway (glucose -glucose-6-phosphate -glucose-1-phosphate -UDP-glucose -glycogen).…”

“…G-6-P, glucose-6-phosphate; Gly-3-P, glyceraldehyde-3-phosphate; DHAP, dihydroxyacetone phosphate; OA, oxaloacetate; PEP, phosphoenolpyruvate; AA, amino acids. stable isotope tracer methodologies, that are highly practical for juvenile fish raised in small recirculating systems in the research setting (Viegas et al, 2012(Viegas et al, , 2015(Viegas et al, , 2016Martins et al, 2013;Rito et al, 2018Rito et al, , 2019.…”