Despite being a carnivorous fish species, cobia (Rachycentron canadum) can utilize high levels of dietary carbohydrate (up to 360 g kg-1). By contrast, rainbow trout (also carnivorous) cannot, due to the absence of molecular induction of glycolytic enzyme and inhibition of gluconeogenic enzyme gene expressions such as pyruvate kinase (PK) and phosphoenolpyruvate carboxykinase (PEPCK). We hypothesized that this phenomenon is species-specific and will not be observed in cobia. Our results show that, at the molecular level, the mRNA abundance of the important glycolytic (PK) and gluconeogenic (PEPCK) enzymes in cobia liver are regulated by dietary carbohydrate-to-lipid (CHO:L) ratios and nutritional status (fed, unfed, and refed). Significantly upregulated hepatic PK and depressed PEPCK gene expressions were observed when the fish were fed with an increasing CHO/L-ratio diet or were refed. However, in contrast to gene expression, there was no significant effect of dietary CHO/L ratios on PK enzyme activity. The decrease in PEPCK activity was significantly found between low CHO/L ratio and high CHO/L ratio diets, whereas the moderate CHO/L ratio group showed intermediate values. But PEPCK activity appeared to be independent of nutritional status. These results suggest that nutritional regulation is obvious, at least at the molecular level, in the key hepatic enzymes (PK and PEPCK) of the glucose metabolism pathway, in response to different dietary CHO/L ratios and to the transition from being starved to fed. Determining whether other key enzymes involved in hepatic glucose metabolism contribute to glucose tolerance in cobia is necessary for further investigation of this phenomenon at the enzymatic and molecular levels.
Keywords: Carbohydrate-to-lipid ratios; Nutritional regulation; Phosphoenolpyruvate carboxykinase; Pyruvate kinase; Rachycentron canadum; Starvation and refeeding.