Hypoxic hypometabolism in chicken embryos: conformism and downregulation.

Postnatally, during hypoxia the decrease in oxygen consumption ( [Formula: see text] ) can exceed what expected from the limitation in O availability, meaning that [Formula: see text] -downregulation exceeds O-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. [Formula: see text] was measured with an open-flow system in the sequence of normoxia-normothermia (21% O, 37 °C, 30 min), hypoxia in normothermia (Hx-NT, either 18, 15, 12 or 9% O, 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 [Formula: see text] invariably decreased in a [O]-dependent fashion. The hypoxic drop in [Formula: see text] did not require a post-hypoxic payment of the O-debt, implying that the decrease in [Formula: see text] reflected hypometabolism. [Formula: see text] did not differ significantly between Hx-HT and Hx-NT for [O] = 15% or less, as expected by O-conformism. Differently, with milder hypoxia (18% O), [Formula: see text] during Hx-HT significantly exceeded that in Hx-NT, meaning that the value of [Formula: see text] in Hx-NT was not limited by O supply. We conclude that a phenomenon of hypoxic [Formula: see text] 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.