Hepatic glycogen synthesis in farmed European seabass (Dicentrarchus labrax L.) is dominated by indirect pathway fluxes

Hepatic glycogen synthesis fluxes from direct and indirect pathways were quantified in seabass by postmortem 2H NMR analysis of plasma water (PW) and glycogen glucosyl 2H enrichments from 2H-enriched seawater. Eighteen fish (28.0 ± 1.7 cm and 218.0 ± 43.0 g) were divided into three groups of 6 and studied over 24 days with transfer to 5% 2H-seawater after day 21. Over this period, one group was fed daily with fishmeal, a second group was fasted, and a third group was fasted for 21 days followed by 3 days refeeding. Glycogen turnover and sources were determined from the ratio of glucosyl position 5 enrichment to that of plasma water (H5/PW). Glycogen levels of fed fish were significantly higher than fasted (665.4 ± 345.2 μmol.g− 1 liver versus 77.2 ± 59.5 μmol.g− 1 liver, P < 0.05) while refed fish had comparable levels to fed (584.6 ± 140.4 μmol.g− 1 liver). Glycogen enrichment of fed fish was undetectable indicating negligible turnover over 3 days. For fasted fish, H5/PW was ~ 50% indicating that half of the glycogen had turned over via indirect pathway flux. For refed fish, H5/PW was ~ 100% indicating that the indirect pathway accounted for all net glycogen synthesis. Direct pathway conversion of dietary carbohydrate to glycogen was not detected in any of the groups.