Rapid and reversible modulation of blood haemoglobin content during diel cycles of hypoxia in killifish (Fundulus heteroclitus)

Abstract

We investigated whether fish can make dynamic haematological adjustments to support aerobic metabolism during repeated cycles of hypoxia-reoxygenation. Killifish were acclimated to normoxia, constant hypoxia (2 kPa O2), or intermittent cycles of nocturnal hypoxia (12 h of normoxia: 12 h of 2 kPa O2 hypoxia) for 28 days. Normoxia-acclimated fish were sampled in the daytime in normoxia and after exposure to a single bout of nocturnal hypoxia. Each hypoxia acclimation group were sampled at the PO2 experienced during acclimation during both the day and night. All acclimation groups had increased blood haemoglobin content and haematocrit and reduced spleen mass during nocturnal hypoxia compared to normoxic controls. Blood haemoglobin content was negatively correlated with spleen mass at both the individual and group level. Fish acclimated to intermittent hypoxia rapidly reversed these changes during diurnal reoxygenation. The concentrations of haemo- globin, ATP, and GTP within erythrocytes did not vary substantially between groups. We also measured resting O2 consumption rate (MO2) and maximum MO2 (induced by an exhaustive chase) in hypoxia in each acclimation group. Fish acclimated to intermittent hypoxia maintained higher resting MO2 than other groups in hypoxia, comparable to the resting MO2 of normoxia-acclimated controls measured in normoxia. Differences in resting MO2 in hypoxia did not result from variation in O2 transport capacity, because maximal MO2 in hypoxia always exceeded resting MO2. Therefore, reversible modulation of blood haemoglobin content along with metabolic adjustments help killifish cope with intermittent cycles of hypoxia in the estuarine environment.