Allosteric Control of Oxygen Binding by Haemoglobin During Embryonic Development in the Crocodile Crocodylus Porosus: The Role of Red Cell Organic Phosphates and Carbon Dioxide

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Abstract
The P50 of whole blood [30°C, (15.6mmHg)] decreases during embryonic development from approximately 6.7kPa (50mmHg) at 15 days to about half this value at hatching (86 days), paralleling a decrease in ATP from 100 to 5–10 μmol g−1 Hb. There is also a progressive changeover from embryonic to adult haemoglobin (HbA). A pulse of 2,3-diphosphoglycerate (2,3-DPG) (18μmol g−1 Hb) occurs late in embryonic life. It has no effect on whole-blood oxygen-affinity and falls rapidly at hatching to values typical of post-hatchling crocodilians in general (<1.0 μmol g−1 Hb). ATP has a marked effect on the oxygen affinity of embryonic haemoglobin (HbE) but not on HbA. 2,3-DPG has only very small effects on the oxygen affinities of HbE and HbA. CO2 has a small effect on the oxygen affinity of HbE but a marked effect on that of HbA. Values of measured in the chorio-allantoic artery [2.9kPa (22mmHg)] and vein [5.9kPa (52mmHg)] imply an increase in saturation from approximately 30% to more than 80%. Neither whole-blood oxygen-affinity nor ATP level was altered in response to an experimental 7-day exposure to low ambient oxygen levels [10.7kPa (80mmHg)]. The results do not lend themselves easily to the pan-selectionist paradigm in which all physiological traits are viewed as being adaptive.