Distinct metabolic adjustments arise from acclimation to constant hypoxia and intermittent hypoxia in estuarine killifish (Fundulus heteroclitus)

ABSTRACT Many fish experience daily cycles of hypoxia in the wild, but the physiological strategies for coping with intermittent hypoxia are poorly understood. We examined how killifish adjust O 2 supply and demand during acute hypoxia, and how these responses are altered after prolonged acclimation to constant or intermittent patterns of hypoxia exposure. We acclimated killifish to normoxia (∼20 kPa O 2 ), constant hypoxia (2 kPa) or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia) for 28 days, and then compared whole-animal O 2 consumption rates ( Ṁ O 2 ) and tissue metabolites during exposure to 12 h of hypoxia followed by reoxygenation in normoxia. Normoxia-acclimated fish experienced a pronounced 27% drop in Ṁ O 2  during acute hypoxia, and modestly increased Ṁ O 2  upon reoxygenation. They strongly recruited anaerobic metabolism during acute hypoxia, indicated by lactate accumulation in plasma, muscle, liver, brain, heart and digestive tract, as well as a transient drop in intracellular pH, and they increased hypoxia inducible factor (HIF)-1α protein abundance in muscle. Glycogen, glucose and glucose-6-phosphate levels suggested that glycogen supported brain metabolism in hypoxia, while the muscle used circulating glucose. Acclimation to constant hypoxia caused a stable ∼50% decrease in Ṁ O 2  that persisted after reoxygenation, with minimal recruitment of anaerobic metabolism, suggestive of metabolic depression. By contrast, fish acclimated to intermittent hypoxia maintained sufficient O 2 transport to support normoxic Ṁ O 2 , modestly recruited lactate metabolism and increased Ṁ O 2  dramatically upon reoxygenation. Both groups of hypoxia-acclimated fish had similar glycogen, ATP, intracellular pH and HIF-1α levels as normoxic controls. We conclude that different patterns of hypoxia exposure favour distinct strategies for matching O 2 supply and O 2 demand.