Does hypoxia-inducible factor 1α play a role in regulating cutaneous oxygen flux in larval zebrafish (Danio rerio)?

Previous studies have demonstrated that hypoxia tolerance is improved in zebrafish (Danio rerio) larvae after prior exposure to lowered ambient O₂ levels. Such improved hypoxia performance was attributed in part, to increased levels of hypoxia-inducible factor 1α (Hif-1α) exerting downstream effects on various physiological processes including promotion of trunk skin angiogenesis. Since O₂ uptake ([Formula: see text]) in larvae is facilitated largely by O₂ diffusion across the skin, enhanced cutaneous vascularization is expected to enhance [Formula: see text] during hypoxia and thus contribute to improved hypoxia tolerance. In this study, we used the scanning micro-optrode technique together with quantification of cutaneous vascularity in wild types (WT) and Hif-1α knockouts (hif1aa^-/-ab^-/-) to test the hypothesis that improved hypoxia tolerance after hypoxia acclimation in larvae at 4 or 7 days post-fertilization (dpf) was associated with Hif-1α-dependent increases in skin vascularity and regional cutaneous O₂ fluxes (JO₂). Hypoxia tolerance, as determined by measurements of critical PO₂ (P_crit), was unaltered by hypoxia pre-exposure in larvae at 4 dpf and there were no significant differences in P_crit between WT and hif1aa^-/-ab^-/- larvae at this developmental stage. However, at 7 dpf there was a significant effect of genotype with WT larvae showing a lower P_crit than hif1aa^-/-ab^-/- larvae, an effect that was being driven by a reduced P_crit in the WT larvae after hypoxia pre-exposure (19.2 ± 1.9 mmHg) compared to hif1aa^-/-ab^-/- fish (35.5 ± 3.5 mmHg). Regardless of genotype, pre-exposure status or developmental age, JO₂ decreased along the body in the anterior-to-posterior direction. Neither hypoxia pre-exposure nor genotype affected JO₂ at any region along the body. The lack of any effect of hypoxia pre-exposure or genotype on JO₂ was consistent with the lack of any effect on skin vascularity as measured in Tg(fli1:EGFP)^yl transgenic larvae. Thus, the decreased hypoxia performance (increased P_crit) at 7 dpf in the hif1aa^-/-ab^-/- larvae did not appear to be reliant on changes in trunk vascularity or cutaneous O₂ diffusion.