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Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters and are enriched with mitochondria; therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na+ uptake evoked by acidic or low Na+ rearing conditions would result in corresponding changes in whole-body oxygen consumption (Ṁ O2 ) and/or cutaneous oxygen flux (J O2 ), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they displayed a lower Ṁ O2  and no difference in cutaneous J O2 Despite a higher Na+ uptake capacity in larvae reared under low Na+ conditions, there were no differences in Ṁ O2  and J O2  at 4 dpf. Furthermore, although Na+ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, Ṁ O2 and J O2  were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous J O2 Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na+, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost. © 2020. Published by The Company of Biologists Ltd.

Citation

Julian J Parker, Alex M Zimmer, Steve F Perry. Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio). The Journal of experimental biology. 2020 Sep 23;223(Pt 18)

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PMID: 32709624

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