Oxygen consumption of the chicken embryo: interaction between temperature and oxygenation

Abstract

We measured the effects of hypoxia and changes in ambient temperature (T) on the oxygen consumption (${\dot{V}}_{{\text{O}}_2}$) of chicken embryos at embryonic days 11, 16 and 20 (E11, E16 and E20, respectively), and post-hatching day 1 (H1). Between 30 and 39 °C, at E11 and E16, ${\dot{V}}_{{\text{O}}_2}$ changed linearly with T, as in ectothermic animals, with a Q₁₀ of about 2.1. At E20, ${\dot{V}}_{{\text{O}}_2}$ did not significantly change with T, indicating the onset of endothermy. At H1, a drop in T increased ${\dot{V}}_{{\text{O}}_2}$, a clear thermogenic response. Hypoxia (11% O₂ for 30 min) decreased ${\dot{V}}_{{\text{O}}_2}$, by an amount that varied with T and age. At H1, hypoxia lowered ${\dot{V}}_{{\text{O}}_2}$ especially at low T. At E20, hypoxic hypometabolism was similar at all T. At E11 and E16, hypoxia lowered ${\dot{V}}_{{\text{O}}_2}$ only at the higher T. In fact, at E11, with T = 39 °C even a modest hypoxia (15–18% O₂) decreased ${\dot{V}}_{{\text{O}}_2}$. Upon return to normoxia after 40 min of 11% O₂, ${\dot{V}}_{{\text{O}}_2}$ did not rise above the pre-hypoxic level, indicating that the hypometabolism during hypoxia did not generate an O₂ debt. At E11, during modest hypoxia (16% O₂) at 36 °C, the drop in ${\dot{V}}_{{\text{O}}_2}$ was lifted by raising the T to 39 °C, suggesting that the hypoxic hypometabolism at 36 °C was not due to O₂-supply limitation. In conclusion, the hypometabolic effects of hypoxia on the chicken embryo's ${\dot{V}}_{{\text{O}}_2}$ depend on the development of the thermogenic ability, occurring predominantly at high T during the early (ectothermic phase) and at low T during the late (endothermic) phase. At E11, both low T and low oxygen force ${\dot{V}}_{{\text{O}}_2}$ to drop. However, at a near-normal T, modest hypoxia promotes a hypometabolic response with the characteristics of regulated O₂ conformism.

Introduction

In the chicken embryo, during a large portion of development, oxygen consumption (${\dot{V}}_{{\text{O}}_2}$) varies with ambient temperature (T) according to the Arrhenius factor (or Q₁₀), as expected for ectothermic animals. Differently, in the last phases of incubation, when confronted by a drop in T, the embryo attempts to increase ${\dot{V}}_{{\text{O}}_2}$ as a means of controlling body temperature, in what represents the early manifestation of endothermy. Hence, the embryo gradually shifts from the poikilothermic pattern that characterizes a large portion of its development to the homeothermic behavior, which will be fully manifest post-natally (Whittow and Tazawa, 1991, Nichelmann and Tzschentke, 2003).

As for T, changes in O₂ availability also cause corresponding changes in the embryo's ${\dot{V}}_{{\text{O}}_2}$ (Bartels and Baumann, 1972, Stock et al., 1985, Bjønnes et al., 1987, Ar et al., 1991, Tazawa et al., 1992). The interaction between T and oxygenation in setting the embryo's metabolic rate has not been studied with specific experiments. One may expect that the effects of the interaction should vary depending on whether the ectothermic or the endothermic pattern is prevailing. At low T, during the ectothermic phase of the embryonic development, because ${\dot{V}}_{{\text{O}}_2}$ is small, changes in oxygenation should have less impact on ${\dot{V}}_{{\text{O}}_2}$ than at high T. Conversely, during the late phases of development, the embryo is likely to be more O₂ dependent at lower, than at higher T, because the low T promotes a thermogenic effort with the rise in ${\dot{V}}_{{\text{O}}_2}$. Therefore, one may hypothesize that, in the growing chicken embryo, hypoxia may pose a limit to ${\dot{V}}_{{\text{O}}_2}$ at high T during the earlier phases and at low T during the late phases of development.

The hypometabolic response during hypoxia could result from the scarce O₂ supply, like it would happen in any reaction with substrate limitation. Hence, the ${\dot{V}}_{{\text{O}}_2}$ level attained during the hypoxia-induced hypometabolism would be the maximal possible for that degree of oxygenation. In such a situation, a drop in T (during the endothermic phase of development) or an increase in T (during the ectothermic phase) would not be able to raise hypoxic ${\dot{V}}_{{\text{O}}_2}$, since hypoxic ${\dot{V}}_{{\text{O}}_2}$ is O₂-limited. Conversely, the hypometabolic response could reflect a regulated phenomenon, triggered by hypoxia but not necessarily forced upon by the O₂ limitation. If this was the case, the level of hypometabolism would not be necessarily the maximal attainable and for a constant level of hypoxia, the ${\dot{V}}_{{\text{O}}_2}$ level could still change with changes in T. A regulated hypometabolic response to hypoxia, or regulated O₂-conformism, seems to be a wide-spread phenomenon observed in invertebrates (Paul et al., 1997, Wiggins and Frappell, 2000, Hicks and McMahon, 2002, Alexander and McMahon, 2004), in reptiles (Hicks and Wang, 1999) and in adult and neonatal mammals (Saiki and Mortola, 1997, Rohlicek et al., 1998). During the embryonic period, the possibility of hypoxic hypometabolism being a regulated phenomenon, rather than O₂ limitation, has never been experimentally considered in any class of animals.

The primary purpose of this study was to examine the effect of hypoxia on the ${\dot{V}}_{{\text{O}}_2}$ response to changes in T in the chicken embryo at various stages of its development. Further, the possibility that the hypometabolic response to hypoxia is a regulated, rather than passive (i.e., O₂ supply-limited), response has been addressed by examining the effect of changes in T on ${\dot{V}}_{{\text{O}}_2}$ at various levels of hypoxia.