Abstract
Two major strategies are used by most fish to maintain energy homeostasis under hypoxia. One is to utilize alternative metabolic pathways to increase energy production, and the other is to limit energy expenditure by suppressing energy-consuming processes, especially ionoregulation. Some anabantoid fishes live in tropical rivers, where hypoxic environments occur frequently. We previously found that under ambient hypoxia, anabantoid fishes do not downregulate Na+/K+-ATPase (NKA) activity to conserve energy in gills but instead increase the frequency of air-breathing respiration (ABR). In addition to the hypoxic condition, another factor that may cause cellular hypoxia in fish is abnormally high environmental temperatures. The frequency of such extreme thermal events has increased due to global climate change. In the present study, we examined whether the anabantoid fish, Macropodus opercularis employs the two strategies mentioned above to resist both ambient hypoxic and elevated thermal (cellular hypoxic) conditions. Results indicate that neither glucose metabolism nor gill NKA activity were altered by hypoxia (DO = 1.5 ± 1 mg/L), but glucose metabolism was increased by thermal stress (34 ± 1°C). NH4+ excretion and ABR frequency were both increased under hypoxia, thermal or hypoxic-and-thermal treatments. In fish that were restricted from breathing air, increased mortality and glucose metabolism were observed under hypoxic or thermal treatments. These results suggest that for M. opercularis, increasing ABR is an important strategy for coping with unmet oxygen demand under hypoxic or thermal stress. This behavioral compensation allows anabantoid fish to physiologically withstand hypoxic and thermal stresses, and constitutes a mechanism of stress resistance that is unavailable to water-breathing fishes.
Highlights
The availability of oxygen is directly related to the capacity for energy production that drives metabolic processes
Hypoxic stress leads to a decrease in metabolism rate, while, thermal situation causes a rise in metabolic rates (Diaz, 2001; Barnes et al, 2011)
The increased glucose mobilized for energy-consuming mechanism only appeared under thermal stress implies that different response mechanisms are activated for the two stressors
Summary
The availability of oxygen is directly related to the capacity for energy production that drives metabolic processes. The first strategy is to meet energy production requirements by adjusting flux through metabolic pathways, while the other is to reduce energy demand (Bickler and Buck, 2007; Richards et al, 2009). The fuel of carbohydrate metabolism, is produced by glycogenolysis from glycogen store (Moyes and Schulte, 2008). Glucose participates into circulation for energy fuel (ATP) production by aerobic or anaerobic metabolic pathways (Moyes and Schulte, 2008). Glucose used in energy metabolism related to fish hypoxic-resistance (Chippari-Gomes et al, 2005; Richards et al, 2009). It has been shown that fish slow down amino acid metabolism to reduce metabolic costs of ammonia excretion under hypoxic stress (Wood et al, 2007)
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