Effects of chronic hypobaric hypoxia on blood oxygen binding in pigeons

Abstract

Effects of chronic hypobaric hypoxia on blood O2 transport were studied using two groups of domestic pigeons (Columba livia). High-altitude (HA) birds were acclimated for 6 wk at 7 km simulated altitude (308 Torr, 25°C); sea-level (SL) pigeons were maintained for 5-6 wk at 758 Torr (25°C). Whole blood O2 equilibrium curves (O2ECs) were generated for HA and SL pi- geons at 37°C and 41°C using thin film techniques. At pH 7.50 and 41°C, half-saturation PO2s (P50) for HA and SL birds were 32.9 ± 1.4 (6); (mean ± S.E.M. (N)) and 35.6 ± 0.6 (6) Torr, respectively. At pH 7.56 and 37°C, the corresponding P50s were 26.5 ± 1.8 (4) and 29.0 ± 0.6 (5) Torr, respectively. HA and SL pigeons exhibited similar Hb-O2 temperature coefficients (ΔH = -9.8 to -10.6 kcal/mole O2) and CO2 (Bohr slopes (Δlog P50/ΔpH = -.43 to -.44 at 37°C; -.41 at 41°C). Hill's n increased with increasing saturation for both animal groups at 37°C and 41°C. Hypoxic acclimation elicited a significant increase in O2 capacity; %Hct and (Hb) increased by 38% and 43%, respectively. Chronic hypoxia had no effect on isohemoglobin composition; both groups exhibited two isoHb fractions in a 9:1 molar ratio. At PaO2 = 30 Torr, calculated in vivo CaO2 for HA (39.2°C, pH 7.53) and SL (41°C, pH 7.50) birds were 14.2 vol % and 7.6 vol %, respectively. Results suggest that altitude tolerance of pigeons is enhanced by the increased blood O2 capacity and the effect of hypoxia- induced hypothermia on Hb-O2 affinity. J. Exp. Zool. 277:293-300, 1997. © 1997 Wiley-Liss, Inc.