Does blood flow limit acute hypoxia performance in larval zebrafish (Danio rerio)?

Oxygen uptake (ṀO2) in larval zebrafish prior to maturation of the gill relies on cutaneous O2 transfer. Under normoxic conditions, rates of cutaneous O2 transfer are unaffected by haemoglobin availability but are diminished in fish lacking a functional circulatory system, suggesting that internal convection is critically involved in setting the resting ṀO2 in zebrafish larvae, even when relying on cutaneous O2 transfer. The reliance of ṀO2 on blood circulation led to the first objective of the current study, to determine whether loss of internal convection would reduce acute hypoxia performance (as determined by measuring critical PO2; Pcrit) in larval zebrafish under conditions of moderate hypoxia (PO2 = 55 mmHg) at 28.5 and 34 °C. Internal convection was eliminated by preventing development of blood vessels using morpholino knockdown of vascular endothelial growth factor (VEGF); these fish are termed VEGF morphants. Breathing frequency (fV) and heart rate (fH) also were measured (at 28.5 °C) to determine whether any detriment in performance might be linked to cardiorespiratory dysfunction. Although ṀO2 was reduced in the VEGF morphants, there was no significant effect on Pcrit at 28.5 °C. Raising temperature to 34 °C resulted in the VEGF morphants exhibiting a higher Pcrit than the shams, suggesting an impairment of hypoxia tolerance in the morphants at the higher temperature. The usual robust increase in fV during hypoxia was absent or attenuated in VEGF morphants at 4 and 5 days post fertilization (dpf), respectively. Resting fH was reduced in the VEGF morphants and unlike the sham fish, the morphants did not exhibit hypoxic tachycardia at 4 or 5 dpf. The number of cutaneous neuroepithelial cells (presumptive O2 chemoreceptors) was significantly higher in the VEGF morphants and thus the cardiorespiratory impairment in the morphants during hypoxia was unlikely related to inadequate peripheral O2 sensing.