Does chronic hypoxia cause an increased reliance on lipid metabolism in freshwater fish?

Abstract

Aquatic hypoxic zones around the world are increasing primarily due to eutrophication and climate change. The severity and duration of hypoxic exposures is also increasing, leading to major challenges for aquatic organisms. These organisms must find ways to survive these conditions, and many rely on a reduction in energy demands or metabolic reorganization. During an acute hypoxic exposure, there is a switch from aerobic ATP production to anaerobic glycolysis, which relies on adequate glycogen stores. We do know that glycogen stores can become depleted following chronic hypoxia, therefore, these organisms must rely on an alternative source of fuel. These may be lipids, amino acids, or ketone bodies, but it is currently unclear which is being used. There have been few studies on the potential use of lipids as a fuel source as there is evidence to suggest they are mobilized during chronic hypoxia. To understand if lipids are being used during chronic hypoxia, brook trout, a hypoxia sensitive species, were exposed to acute (24 hours) and chronic (9 days) hypoxia. Following the treatment, we examined the levels of glycogen in heart, liver, and muscle tissue and found it to be decreased in muscle and liver tissues. Seeing these declines in glycogen stores suggests that another fuel is being used. We examined plasma triglycerides and found decreases in the plasma suggesting that they are being taken up by the tissues or lipids are being circulated in another form during hypoxia. Finally, we examined lipid ß-oxidation in the tissues and found it to be increased in the liver where glycogen stores had been depleted. The results show some evidence for the use of lipids as an energy source during chronic hypoxia.