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

Abstract

In the chick embryo at day 3, gas exchange occurs by diffusion and oxygen consumption ( V̇ O 2 ) does not depend on the cardiovascular convection of O 2. 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. V̇ O 2 was measured by a closed system methodology. In normoxia, a decrease in temperature ( T) from 39 to 31 °C had parallel depressant effects on V̇ O 2 and HR. At 39 °C, a progressive decrease in O 2 lowered V̇ O 2 ; HR was maintained until the O 2 threshold of ∼ 15%. In severe hypoxia (4% O 2) V̇ O 2 and HR were, respectively, ∼ 12% and ∼ 62% of normoxia. At 32 °C the hypoxic threshold for HR was significantly lower. During constant hypoxia (7% O 2) V̇ O 2 did not respond to T, while the HR response was preserved. Stroke volume changed little with changes in T or O 2, except at 6 and 4% O 2, when it decreased by ∼ 20 and 30%. In embryos growth-retarded because of incubation in chronic hypoxia, V̇ O 2 and HR responses to T and hypoxia were similar to those of normal embryos. We conclude that in the early embryo during hypoxia cardiovascular O 2 convection is not responsible for the drop in V̇ O 2 . The generalised hypometabolic response, in combination with the extremely small cardiac V̇ O 2 , probably explains the minor effects of hypoxia on cardiac activity.