Effects of dietary carbohydrate sources on growth, digestive enzyme activity, gene expression of hepatic GLUTs and key enzymes involved in glycolysis-gluconeogenesis of giant grouper Epinephelus lanceolatus larvae

This study aimed to evaluate effects of dietary carbohydrate sources on growth, digestive enzyme activity, gene expression of hepatic glucose transporters (GLUTs) and key enzymes involved in glycolysis-gluconeogenesis in giant grouper Epinephelus lanceolatus larvae through a 21-day growth trial. Four isonitrogenous (50%) and isolipidic (15.4%) experimental diets, including a cellulose control diet and three diets with glucose, maltose or maize starch, were formulated. Dietary CHO level was 18%. The 29 DPH (days post hatching) grouper larvae (initial average weight: 0.078 ± 0.0018 g) were cultured in small floating cages (L 120 cm × W 70 cm × H 50 cm) at a density of 80 fish per cage. Experimental cages were labeled and located in three connective 6-m3 indoor concrete tanks (L 3 m × W 3 m × H 2 m) with 4 cages occurring in each tank. Results showed that at 50 DPH, fish fed with maize starch diet had significantly higher weight gain (WG) and thermal growth coefficient (TGC) than fish fed with glucose, maltose or cellulose. Fish fed with glucose had the lowest WG among all experimental treatments. Fish fed with maize starch or cellulose had significantly higher mRNA expression levels of hepatic fructose 1,6 bisphosphatase (FBP) gene compared to fish fed with glucose or maltose at 50 DPH. The highest mRNA expression level of hepatic glucose-6-phosphate dehydrogenase (G6PDH) gene was observed in fish fed with glucose at 50 DPH. Fish fed with maize starch had higher mRNA expression levels of hepatic pyruvate kinase (PK) and hexokinase (HK) genes than fish fed with cellulose, glucose and maltose at 50 DPH. The mRNA expression level of hepatic glucose transporter 4 (GLUT4) gene was significantly lower in fish fed with cellulose compared to that in fish fed with digestible CHOs at 50 DPH. There were no significant differences in mRNA expression levels of hepatic glucose transporter 1 (GLUT1) gene among all experimental fish at 50 DPH. In terms of digestive enzymes, activities of pancreatic trypsin and intestinal lipase increased during the development of fish larvae, but pancreatic α-amylase specific activity showed an opposite trend of variation as those of trypsin and lipase. During the development of experimental fish larvae from 29 DPH to 50 DPH, pancreatic trypsin specific activity in fish fed with glucose was significantly lower than that in fish fed with cellulose or maltose but did not differ from that in fish fed with maize starch. Intestinal lipase specific activity in fish fed with maize starch or maltose were significantly higher than those in fish fed with cellulose but did not differ from those in fish fed with glucose. Generally, giant grouper larvae fed maize starch had better growth than those fed cellulose, glucose or maltose; the mRNA expressions of hepatic HK, PK, FBP and G6PDH genes involved in glycolysis-gluconeogenesis were significantly affected by dietary CHO sources.