Expressional regulation of key hepatic enzymes of intermediary metabolism in European seabass (Dicentrarchus labrax) during food deprivation and refeeding

Effects of a single-phase fasting period and subsequent re-feeding on compensatory growth, digestive enzyme activities, and antioxidant capacity of Sobaity (Sparidentex hasta) and Yellowfin seabream (Acanthopagrus latus)

An eight-week research was carried out to examine the influence of fasting (FA) and refeeding (RF) episodes on the compensatory growth responses (CGR) in sobaity (Sparidentex hasta, 10 g) and yellowfin seabreams (Acanthopagrus latus, 4.3 g) juveniles. Fish were fed with a commercial feed (contained 500 g kg−1 crude protein and 150 g kg−1 crude lipid) as following regimes: control (C, fish were fed three times every day), T1 (two weeks of feeding, one week of FA, and five weeks of RF), T2 (one week of feeding, two weeks of FA and five weeks of RF) and T3 (three weeks of FA and five weeks of RF). Two hundred and forty S. hasta juveniles were stocked into twelve 300-L tanks (20 fish tank−1), and 360 A. latus juveniles were allocated into other 12 tanks (30 fish tank−1). Each treatment was carried out in triplicates for each species, and each tank held only one of the species. The experiment was carried out for both species simultaneously. The weight and length of fish from the four groups were measured individually after the third week (after FA episode) and after eight weeks (after RF episode). After finishing the RF episode (eighth week), six fish of each tank were sacrificed with an overdose of 2-phenoxyethanol (1000 mg L−1), and the liver and the whole gut of the sacrificed fish were sampled, dissected, and then kept in a freezer (−80 °C) until further analyses. Survival rate was decreased in S. hasta juveniles with increasing the FA period mainly due to their cannibalistic behavior, which was triggered by starvation, but it was not affected in A. latus. The fasted groups in both species were significantly lost their weight after FA episodes. After five weeks of RF, S. hasta showed full compensatory growth response; meanwhile A. latus had a partial compensatory response (P<0.05). Hepatosomatic index value decreased after the FA period in both species, but it was restored to the normal level after RF phase. The activities of liver catalase, superoxide dismutase, glutathione-S-transferase, and glutathione peroxidase were increased in T2 group in S. hasta, but liver antioxidant enzymes were not affected in A. latus. In both species, the amount of the lipid peroxidation was significantly increased in the liver of fish groups subjected to T2 or T3 compared to T1 and control groups (P < 0.05). Liver alanine aminotransferase (ALT) and alkaline phosphatase (ALP) in S. hasta fasted for two weeks were higher than the other groups. The activities of trypsin, chymotrypsin, α-amylase, and lipase in S. hasta fasted for a week (T1) were higher than control. In addition, the activities of trypsin and chymotrypsin in A. latus fasted for two or three weeks were lower than C and T1 groups. The activity of ALP was increased with increasing FA period in both species. The findings of the present study showed that single-phase FA episodes reduce survival and induce oxidative stress in S. hasta juveniles; meanwhile A. latus juveniles did not show complete compensatory growth after RF episode.

RNA-Seq Transcriptome Analysis of the Liver and Brain of the Black Carp (Mylopharyngodon piceus) During Fasting

The black carp (Mylopharyngodon piceus) is an important carnivorous freshwater-cultured species. To understand the molecular basis underlying the response of black carp to fasting, we used RNA-Seq to analyze the liver and brain transcriptome of fasting fish. Annotation to the NCBI database identified 66,609 unigenes, of which 22,841 were classified into the Gene Ontology database and 15,925 were identified in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Comparative analysis of the expression profile between fasting and normal feeding fish revealed 13,737 differentially expressed genes (P < 0.05), of which 12,480 were found in liver tissue and 1257 were found in brain tissue. The KEGG pathway analysis showed significant differences in expression of genes involved in metabolic and immune pathways, such as the insulin signaling pathway, PI3K-Akt signaling pathway, cAMP signaling pathway, FoxO signaling pathway, AMPK signaling pathway, endocytosis, and apoptosis. Quantitative real-time PCR analysis confirmed that expression of the genes encoding the factors involved in those pathways differed between fasting and feeding fish. These results provide valuable information about the molecular response mechanism of black carp under fasting conditions.

Contribution of dietary starch to hepatic and systemic carbohydrate fluxes in European seabass (Dicentrarchus labrax L.)

In the present study, the effects of partial substitution of dietary protein by digestible starch on endogenous glucose production were evaluated in European seabass (Dicentrarchus labrax). The fractional contribution of dietary carbohydrates v. gluconeogenesis to blood glucose appearance and hepatic glycogen synthesis was quantified in two groups of seabass fed with a diet containing 30% digestible starch (DS) or without a carbohydrate supplement as the control (CTRL). Measurements were performed by transferring the fish to a tank containing water enriched with 5% (2)H2O over the last six feeding days, and quantifying the incorporation of (2)H into blood glucose and hepatic glycogen by (2)H NMR. For CTRL fish, gluconeogenesis accounted for the majority of circulating glucose while for the DS fish, this contribution was significantly lower (CTRL 85 (SEM 4) % v. DS 54 (SEM 2) %; P < 0.001). Hepatic glycogen synthesis via gluconeogenesis (indirect pathway) was also significantly reduced in the DS fish, in both relative (CTRL 100 (SEM 1) % v. DS 72 (SEM 1) %; P < 0.001) and absolute terms (CTRL 28 (SEM 1) v. DS 17 (sem 1) μmol/kg per h; P < 0.001). A major fraction of the dietary carbohydrates that contributed to blood glucose appearance (33 (sem 1) % of the total 47 (SEM 2) %) had undergone exchange with hepatic glucose 6-phosphate. This indicated the simultaneous activity of hepatic glucokinase and glucose 6-phosphatase. In conclusion, supplementation of digestible starch resulted in a significant reduction of gluconeogenic contributions to systemic glucose appearance and hepatic glycogen synthesis.