Salinity Effects on Strategies of Glycogen Utilization in Livers of Euryhaline Milkfish (Chanos chanos) under Hypothermal Stress

Abstract

The fluctuation of temperature affects many physiological responses in ectothermic organisms, including feed intake, growth, reproduction, and behavior. Changes in environmental temperatures affect the acquisition of energy, whereas hepatic glycogen plays a central role in energy supply for the homeostasis of the entire body. Glycogen phosphorylase (GP), which catalyzes the rate-limiting step in glycogenolysis, is also an indicator of environmental stress. Here, we examined the effects of salinity on glycogen metabolism in milkfish livers under cold stress. A reduction of feed intake was observed in both freshwater (FW) and seawater (SW) milkfish under cold adaptation. At normal temperature (28°C), compared to the FW milkfish, the SW milkfish exhibited greater mRNA abundance of the liver isoform of GP (Ccpygl), higher GP activity, and less glycogen content in the livers. Upon hypothermal (18°C) stress, hepatic Ccpygl mRNA expression of FW milkfish surged at 3 h, declined at 6 and 12 h, increased again at 24 h, and increased significantly after 96 h. Increases in GP protein, GP activity, and the phosphorylation state and the breakdown of glycogen were also found in FW milkfish livers after 12 h of exposure at 18°C. Conversely, the Ccpygl transcript levels in SW milkfish were downregulated after 1 h of exposure at 18°C, whereas the protein abundance of GP, GP activity, and glycogen content were not significantly altered. Taken together, under 18°C cold stress, FW milkfish exhibited an acute response with the breakdown of hepatic glycogen for maintaining energy homeostasis of the entire body, whereas no change was observed in the hepatic glycogen content and GP activity of SW milkfish because of their greater tolerance to cold conditions.

Changes in hypothermal stress-induced hepatic mitochondrial metabolic patterns between fresh water- and seawater-acclimated milkfish, Chanos chanos

“…Under hypothermal stress (18 °C), glycogen breakdown increased in FW milkfish livers to maintain blood glucose levels. In contrast, this response was not observed in SW milkfish 24 . Lactate dehydrogenase (LDH) is an indicator of hepatic gluconeogenesis identified by the two-dimensional gel electrophoresis of SW milkfish livers 25 .…”

“…Our previous studies have also demonstrated that at low temperatures, FW milkfish livers exhibited higher oxidative stress than SW milkfish livers 31 , leading to more anabolic demands in FW individuals to support their antioxidative mechanisms. In addition, the temperature was the major factor for feed consumption according to the same depression pattern in FW and SW milkfish under hypothermal stress 24 . However, milkfish acclimated to FW affect low-temperature tolerance ability and leading to hepatic glycogen breakdown to maintain whole body or liver fuel source 23,24 .…”

“…Increases in oxidative stress and the expression of the antioxidant protein peroxiredoxin 6 were reported for FW milkfish livers under hypothermal stress. In contrast, SW milkfish under low-temperature stress presented with stable aerobic metabolic rates, anabolic demand, antioxidant responses 31 , and glycogen metabolic rates 24 . FW was more stressful than SW to milkfish livers since that milkfish is a marine euryhaline teleost 23,31 .…”

“…In addition, different cold tolerance ability between FW and SW milkfish 23 suggested that the roles of their livers, the energy supply and de-toxic organ, might be salinity-dependent under hypothermal stress. Glycogen breakdown differed between SW- and FW-acclimated milkfish under hypothermal stress 24 also implied that the metabolic patterns in their hepatic mitochondria at low temperatures varied with environmental salinities. To elucidate the roles of liver mitochondria in maintaining energy supply in response to low-temperature stress, oxygen consumption was measured in SW- and FW-acclimated milkfish subjected to 18 °C.…”

Effects of Acute Cold Stress on Liver O-GlcNAcylation and Glycometabolism in Mice

“…Glycogen, a polymer of glucose residues, is considered an important form of energy storage, particularly in the liver [ 28 ]. In the liver, glycogen synthesis and degradation play major roles in regulating blood glucose homeostasis as well as supplying energy to other tissues [ 29 , 30 ].…”