Effects of prolonged fasting on substrate metabolism in female Japanese quails

Food stands as a pivotal environmental element that exerts a profound influence on the survival of animals. Faced with food shortages, animals need to develop morphological, physiological, and behavioral strategies to improve their survival adaptability. Animals undergoing fasting tend to mobilize the reserved substances in the body to meet the energy needs for metabolism. In the present investigation, we assessed the influence of prolonged fasting on various physiological parameters related to fat catabolism, carbohydrate metabolism, and protein catabolism in female Japanese quails (Coturnix japonica). The treatment of Japanese quails was divided into four stages: the pre-fasting stage, Phase I (fasting for 1 day), Phase II (fasting for 4 days), and Phase III (fasting for 6-11 days). Compared with the pre-fasting stage, the following indicators changed significantly during prolonged fasting. (1) Fat catabolism: In the liver, the level of lipid droplets and free fatty acids (FFA), the activity of triacylglycerol lipase (TGL), the activity and mRNA level of hydroxymethylglutaryl-CoA synthetase (HMGCS), and the level of β-hydroxybutyric acid (BHBA) in the serum increased significantly, while the activity and mRNA level of carnitine acyltransferase I (CPTI) and carnitine acyltransferase II (CPT-II) decreased significantly. (2) Carbohydrate metabolism: The activity and mRNA levels of the pyruvate carboxylase (PC) and phosphoenolpyruvate carboxykinase (PEPCK) genes, and that of the glucose-6-phosphatase (G6Pase) in the liver increased significantly, while the mRNA level of the hexokinase domain containing 1 (HKDC1) and pyruvate kinase (PK) genes in the liver decreased significantly. (3) Protein catabolism: free amino acid levels in the liver and pectoral muscle increased significantly, whereas the mRNA levels of the asparagine synthetase (ASNS) and glutamate dehydrogenase (GLUD) genes in the liver decreased significantly, while the mRNA level of the nuclear factor-kappa B (NF-kB1 and NF-kB2) in the pectoral muscle increased significantly. Additionally, glucocorticoid levels significantly rose in Phase III compared with Phases I and II. Therefore, for prolonged fasting female Japanese quails, the mobilization of fat, fat decomposition, and generation of ketone bodies increased significantly, and gluconeogenesis in the liver also increased significantly, while glycolysis decreased significantly; protein decomposition, particularly in pectoral muscle, increased significantly. These results indicate that enhanced fat catabolism, protein catabolism, and gluconeogenesis, along with reduced glycolysis, could play an important role in the tolerance of female Japanese quails to prolonged fasting. These mechanisms might be significant for the birds to establish a temporary balance to maintain homeostasis under conditions of restricted exogenous energy supply.