Oxygen uptake and chorioallantoic blood flow changes during acute hypoxia and hyperoxia in the 16 day chicken embryo

Oxygen consumption rate (MO2) of hen eggs was measured on incubation day 16 (37.8 degrees C, 55% humidity) during acute exposure (90 min) to ambient hyperoxia (FI02 = 0.42) or hypoxia (FIO2 = 0.105). During the last part of these exposures, an H2 washout method was used to estimate relative changes in chorioallantoic membrane (CAM) blood flow, taking as an index the net change in the H2 washout rate constant between any experimental condition and the circulation arrested egg. Doubling normoxic FIO2 increased MO2 to an asymptotic value which was 4% above the normal (P less than 0.05; MO2 in normoxia = 890 mumols/h) even after correcting for the normoxic increase in MO2 with time during development (delta MO2/delta t = 21.5 mumols/h2; P less than 0.001). Halving FIO2 reduced MO2 calculated in the same way to 388 mumols/h. The estimate of the CAM blood flow, relative to normoxia, was 1.12 in hyperoxia (not significant, P = 0.05) and 0.68 in hypoxia (P less than 0.001). The limited changes in CAM blood flow and MO2 during hyperoxia indicate that they are both already close to their maximal values in normoxia. During acute hypoxia the 16 day embryo behaves as an oxygen-conformer; however, the small relative decrease in MO2 per unit of the flow index observed during hyperoxia suggests that the embryo can regulate its CAM blood flow to a small extent. The survival of the embryo and its recovery from hypoxia without a detectable O2 repayment suggest small if any anaerobic regulatory pathways and indicate a true metabolic depression.