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

Abstract

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 per minute, 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 mM HCO3−/ 1% to 40 mM HCO3−/ 4% CO2 had few effects on performance, however further, but still physiologically relevant, increases to 70 mM 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 mM HCO3−/ 1% CO2. Together, our data suggest that hypoxic bradycardia greatly benefits cardiac performance, but its significance may be context-specific.