Functional properties of primitive and definitive red cells from chick embryo: oxygen-binding characteristics, pH and membrane potential, and response to hypoxia.

We compare properties of primitive and definitive red cells in three areas effect of different hemoglobin patterns on oxygen affinity cellular response to hypoxia, and red cell membrane potential and cell pH since during incubation blood pH decreases by 0.5 pH. We find that the first population of definitive red cells has many properties in common with the preceding primitive red cells. The oxygen-binding curves are nearly identical despite the fact that the definitive red cells contain only the adult hemoglobins A and D; in particular, n values greater than 4 are recorded in the upper saturation range. Within the definitive red cell population there is a drastic increase in the oxygen affinity between days 7 to 14. Hypoxia causes a series of coordinated changes in definitive but not primitive red cells, involving the phosphate pattern (2,3 DPG, ATP) as well as changes of carbonic anhydrase activity and oxygen affinity. The results suggest that the oxygen pressure controls part of the differentiation program of the definitive cell population and that control is dependent on plasma factors of high molecular weight. The membrane potential of primitive and early definitive red cells is a proton diffusion potential. During development proton conductivity decreases, and at day 16 proton and chloride distribution can be adequately described by the Donnan equilibrium. The linkage between membrane potential and proton distribution allows maintenance of red cell pH at about 7.2, irrespective of external pH, which greatly facilitates the adjustment of embryonic oxygen affinity.