Heme oxygenase-1 (HO-1) mediated respiratory responses to hypoxia in the goldfish, Carassius auratus

• Heme oxygenase-1 (HO-1) was present in the neuroepithelial cells (NECs; putative branchial O2 chemoreceptors) of 7 °C goldfish only.
• Hypoxia exposure increased gill HO-1 activity in hypoxic fish.
• Inhibition of HO-1 activity with zinc protoporphyrin IX (ZnPPIX) increased the ventilation frequency response to acute hypoxia (30 mmHg) in 7 °C goldfish.
• 7 °C fish injected with ZnPPIX did not significantly decrease breathing frequency during acute hyperoxia (∼500 mmHg).
• Inhibition of HO-1 activity did not affect the ventilatory response of 25 °C goldfish exposed to acute hypoxia or hyperoxia.

In this study we investigated the role of heme oxygenase-1 (HO-1) in modulating the hypoxic and hyperoxic ventilatory responses of goldfish (Carassius auratus) acclimated to 7 and 25 °C. HO-1 was present in the neuroepithelial cells (NECs; putative branchial O2 chemoreceptors) of fish acclimated to 7 °C only. Hypoxia exposure increased gill HO-1 activity in 7 °C fish (14.0 ± 1.4 to 42.5 ± 3.2 pmol bilirubin min−1 mg protein−1). Inhibition of HO-1 activity with zinc protophorphyrin IX (ZnPPIX) increased the ventilation frequency response to acute hypoxia (30 mmHg); frequency increased from 48.3 ± 5.1 to 137.4 ± 16.0 breaths per min (BPM) in hypoxic 7 °C fish treated with ZnPPIX compared to 46.2 ± 4.2 to 77.9 ± 5.3 BPM in control fish. Unlike in the control (untreated) 7 °C fish exposed to hyperoxia, fish injected with ZnPPIX did not significantly decrease breathing frequency. Inhibiting HO-1 activity was without effect on the hypoxic or hyperoxic ventilatory responses of fish acclimated to 25 °C. Based on these observations, we suggest that HO-1 plays an inhibitory role in regulating breathing frequency but only in goldfish acclimated to 7 °C.