The hypometabolic response to repeated or prolonged hypoxic episodes in the chicken embryo

• The hypometabolic response to hypoxia (HH) is common in embryos.
• We asked whether the degree of HH in chicken embryos varies with the hypoxic history.
• In general HH was independent of the preceding hypoxic exposures.
• Results are interpreted according to the embryo's costs of growth and maintenance.

Hypoxia (hx) in embryos causes a drop in oxygen consumption (V˙O2) that rapidly recovers upon return to normoxia. We asked whether or not this pattern varies with the embryo's hypoxic history. The V˙O2 of chicken embryos in the middle (E12) or at end-incubation (E19) was measured by an open-flow methodology during 15-min epochs of moderate (15% O2) or severe hx (10% O2). Each hx-epoch was repeated or alternated with air by various modalities (air-hx-air-hx-air-hx-air, air-2·hx-air-2·hx-air, air-5·hx-air), in randomized sequences. The hx drop in V˙O2 was larger with severe than with moderate hx; however, in either case, its magnitude was essentially independent of the preceding hx history. E19 embryos had hx drops in V˙O2 of the same magnitude whether their incubation was in air or in moderate hx from E4 to E19. A different protocol (air-12·hx-air) gave variable results; with moderate hx, the V˙O2 response was similar to that of the other hx regimes. Differently, with severe hx most embryos progressively decreased V˙O2 and eventually died. We interpret these data on the basis of what is known on the ‘compensatory partitioning' between costs of growth and maintenance. With moderate hx presumably each episode caused an energy shortfall absorbed entirely by the blunted growth. Hypoxic events of this type, therefore, should have no long-term functional effects other than those related to the small birth weight. Differently, the aerobic energy shortfall with severe hypoxia probably impinged on some maintenance functions and became incompatible with survival.