Metabolic and heart rate responses to hypoxia in early chicken embryos in the transition from diffusive to convective gas transport.

Measurements of normoxic, hypoxic (15% or 10% O₂) and post-hypoxic oxygen consumption (VO₂) were conducted in chicken embryos every other day between embryonic day 3 (E3) and day 19 (E19), out of a total embryogenic period of 20.5 days. The results indicated that, irrespective of age, hypoxia lowered VO₂ throughout embryogenesis without any contraction of an O₂ debt. Hypoxic hypometabolism was more prominent at E3 than at E5, probably because of the differences in O₂ sensitivity during the developmental transition from O₂ diffusion to O₂ convection forms of gas transport. Further measurements at these two ages with either progressively increasing hypoxia or a sudden drop to 8% O₂ indicated that, at E5, the less pronounced hypometabolism was accompanied by a greater drop in heart rate (HR) than at E3. It was postulated that a functional causative link existed between these two phenomena, the decrease in whole-embryo [Formula: see text] favouring O₂ availability to the heart. Indeed, when O₂ demands were decreased by cold exposure, the hypoxic effects on HR became similar between E3 and E5. We conclude that hypometabolism with no major reliance on anaerobic sources is the common response to hypoxia throughout embryogenesis. In the earliest phases of embryogenesis, when diffusion is the primary form of gas transport and despite the absence of neural regulation, the possibility of using some of the O₂ saved in favour of the heart adds further value to hypometabolism as a survival strategy against hypoxia.