Hypoxic hypometabolism in chicken embryos: conformism and downregulation

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Postnatally, during hypoxia the decrease in oxygen consumption (V̇O2) can exceed what expected from the limitation in O2 availability, meaning that V̇O2 -downregulation exceeds O2-conformism. We questioned whether a similar phenomenon could occur prenatally, in chicken embryos at mid- (E11, out of 20.5 days) or near end- (E18) incubation. V̇O2 was measured with an open-flow system in the sequence of normoxia-normothermia (21% O2, 37 °C, 30 min), hypoxia in normothermia (Hx-NT, either 18, 15, 12 or 9% O2, 37 °C, 1 hour), hypoxia in hyperthermia (Hx-HT, up to 43 °C, 1 hour) and return to normoxia-normothermia (30 min). During Hx-NT V̇O2 invariably decreased in a [O2]-dependent fashion. The hypoxic drop in V̇O2 did not require a post-hypoxic payment of the O2-debt, implying that the decrease in V̇O2 reflected hypometabolism. V̇O2 did not differ significantly between Hx-HT and Hx-NT for [O2] = 15% or less, as expected by O2-conformism. Differently, with milder hypoxia (18% O2), V̇O2 during Hx-HT significantly exceeded that in Hx-NT, meaning that the value of V̇O2 in Hx-NT was not limited by O2 supply. We conclude that a phenomenon of hypoxic V̇O2 downregulation like that observed in postnatal mammals can occur also prenatally, in the chicken embryos. The mechanisms at the basis of the downregulation remain unresolved and could combine physiological and cellular processes.