ABSTRACT
In water-breathing fishes, the hypoxic ventilatory response (HVR) represents an increase in water flow over the gills during exposure to lowered ambient O2 levels. The HVR is a critical defense mechanism that serves to delay the negative consequences of hypoxia on aerobic respiration. However, the physiological significance of the HVR in larval fishes is unclear as they do not have a fully developed gill and rely primarily on cutaneous gas transfer. Using larval zebrafish (4, 7, 10 and 15 days post-fertilization; dpf), we examined HVR under three levels of hypoxia (25, 45 and 60 mmHg). The larvae exhibited widely different HVRs as a function of developmental age and level of the hypoxia. Yet, critical O2 tensions (Pcrit) remained constant (30–34 mmHg) over the same period of development. Micro-optrode O2 sensors were used to measure a significant decrease in buccal cavity water O2 tensions in 4 and 7 dpf larvae compared with the water they inspired, demonstrating significant extraction of O2 from the buccal cavity. To assess the physiological significance of the HVR, ventilatory water flow was prevented in larvae at 4 and 7 dpf by embedding their heads in agar. An increase in Pcrit was observed in larvae at 7 dpf but not 4 dpf, suggesting that buccal ventilation is important for O2 extraction by 7 dpf. Combined, these data indicate that branchial/buccal gas transfer plays a significant role in O2 uptake during hypoxia, and supports a physiological benefit of the HVR in early life stages of zebrafish.
FOOTNOTES
Competing interests
The authors declare no competing or financial interests.
Author contributions
Conceptualization: Y.K.P., M.M., S.F.P.; Methodology: Y.K.P., M.M.; Validation: Y.K.P., M.M., A.M.Z., S.F.P.; Formal analysis: Y.K.P., M.M., A.M.Z.; Investigation: Y.K.P., M.M., A.M.Z.; Writing - original draft: Y.K.P.; Writing - review & editing: Y.K.P., M.M., A.M.Z., S.F.P.; Visualization: Y.K.P., M.M.; Supervision: S.F.P.; Project administration: Y.K.P., M.M.; Funding acquisition: S.F.P.
Funding
This work was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery and RTI grants to S.F.P. (G13017). M.M. and A.M.Z. were recipients of NSERC postdoctoral fellowships (PDF-502414-2017 to A.M.Z. and PDF-471707-2015 to M.M.).
Supplementary information
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.204800.supplemental
- Received April 11, 2019.
- Accepted June 6, 2019.
- © 2019. Published by The Company of Biologists Ltd
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