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 ((V) over dot(O2)) 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. (V) over dot(O2) was measured by a closed system methodology. In normoxia, a decrease in temperature (T) from 39 to 31 degrees C had parallel depressant effects on (V) over dot(O2) and HR. At 39 degrees C, a progressive decrease in O-2 lowered (V) over dot(O2); HR was maintained until the O-2 threshold of similar to 15%. In severe hypoxia (4% O-2) (V) over dot(O2) and HR were, respectively, similar to 12% and similar to 62% of normoxia. At 32 degrees C the hypoxic threshold for HR was significantly lower. During constant hypoxia (7% O-2) (V) over dot(O2) 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 similar to 20 and 30%. In embryos growth-retarded because of incubation in chronic hypoxia, (V) over dot(O2). 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 (V) over dot(O2). The generalised hypometabolic response, in combination with the extremely small cardiac (V) over dot(O2), probably explains the minor effects of hypoxia on cardiac activity. (C) 2009 Elsevier Inc. All rights reserved.