OXYGEN-TRANSPORT BY RABBIT EMBRYONIC BLOOD - HIGH COOPERATIVITY OF HEMOGLOBIN OXYGEN-BINDING

Oxygen equilibrium curves (OECs) have been determined in brood of embryonic and adult rabbits (Oryctolagus cuniculus) using a thin firm method (modified Hemoscan). The gestational age of the rabbits was from 12 days to 14 days, the end of the embryonic period. Measurements were made at 37 degrees C and a P-CO2 of 21, 42, or 71 mmHg. The most striking finding was an embryonic OEC which was steep above 50% saturation. The Hill plot in this upper region gave a mean n(H) value of 5.3 in the embryos, the first finding of n(H) significantly greater than 4 in any normal eutherian mammal. On hemolysis, n(H) dropped below 4. Oxygen affinity of 14 day embryonic blood was higher than that of adult blood: respective P-50 values at P-CO2 = 42 mmHg were 29.6 mmHg and 32.5 mmHg. The P-50s were somewhat lower at earlier stages. The Bohr effect was measured (as Delta log P-50/Delta log P-CO2) in 14 day embryos. Its value was 0.26, about 10% lower than in adults. The results show an apparent aggregation of Hb tetramers, as found in marsupials, but no right shifting of the embryonic OEC compared to the adult OEC.