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During hypoxia, fishes exhibit a characteristic hypoxic bradycardia, the functional significance of which remains debated. Here, we investigated the hypothesis that hypoxic bradycardia primarily safeguards cardiac performance. In preparations from the European eel (Anguilla anguilla), a decrease in stimulation frequency from 40 to 15 beats min(-1), which replicates hypoxic bradycardia in vivo, vastly improved cardiac performance during hypoxia in vitro. As eels display dramatic shifts in extracellular HCO3(-)/CO2, we further investigated the effect this has upon hypoxic cardiac performance. Elevations from 10 mmol l(-1) HCO3(-)/1% CO2 to 40 mmol l(-1) HCO3(-)/4% CO2 had few effects on performance; however, further, but still physiologically relevant, increases to 70 mmol l(-1) HCO3(-)/7% CO2 compromised hypoxia tolerance. We revealed a four-way interaction between HCO3(-)/CO2, contraction frequency, hypoxia and performance over time, whereby the benefit of hypoxic bradycardia was most prolonged at 10 mmol l(-1) HCO3(-)/1% CO2. Together, our data suggest that hypoxic bradycardia greatly benefits cardiac performance, but its significance may be context specific.

The effects of hypoxic bradycardia and extracellular HCO3−/CO2 on hypoxic performance in the eel heart