Interactive effects of temperature and hypoxia on heart rate and oxygen consumption of the 3-day old chicken embryo

In the chick embryo at day 3, gas exchange occurs by diffusion and oxygen consumption (V̇O2) does not depend on the cardiovascular convection of O2. Whether or not this is the case in hypoxia is not known and represents the aim of the study. The heart of chicken embryos at 72 h (stage HH18) was filmed through a window of the eggshell by a camera attached to a microscope. Stroke volume was estimated from the changes in heart silhouette between systole and diastole. V̇O2was measured by a closed system methodology. In normoxia, a decrease in temperature (T) from 39 to 31 °C had parallel depressant effects on V̇O2and HR. At 39 °C, a progressive decrease in O2 lowered V̇O2; HR was maintained until the O2 threshold of ∼ 15%. In severe hypoxia (4% O2) V̇O2and HR were, respectively, ∼ 12% and ∼ 62% of normoxia. At 32 °C the hypoxic threshold for HR was significantly lower. During constant hypoxia (7% O2) V̇O2did not respond to T, while the HR response was preserved. Stroke volume changed little with changes in T or O2, except at 6 and 4% O2, when it decreased by ∼ 20 and 30%. In embryos growth-retarded because of incubation in chronic hypoxia, V̇O2and HR responses to T and hypoxia were similar to those of normal embryos. We conclude that in the early embryo during hypoxia cardiovascular O2 convection is not responsible for the drop in V̇O2. The generalised hypometabolic response, in combination with the extremely small cardiac V̇O2, probably explains the minor effects of hypoxia on cardiac activity.