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 (V˙O2) 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 V˙O2 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 V˙O2 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.

► During avian embryogenesis, the hypoxic drop in V˙O2 did not cause a payment back of the O2 debt. ► At age E3, hypoxic hypometabolism was more prominent than at E5. ► The opposite was the case for heart rate (HR). ► In early embryos the O2 saving during hypometabolism can protect cardiac activity.