Temperature and the oxygen-hemoglobin dissociation curve of the harbor seal, Phoca vitulina

To determine the effect of temperature and pH on oxygen-hemoglobin affinity of the harbor seal, we measured 61 biotonometric oxygen-hemoglobin dissociation curves on blood from 5 seals at 3 temperatures and a range of pH values. The average (± SEM) hemoglobin concentration was 3.44 ± 0.15 mM, nearly 50% greater than found in normal humans. At pH 7.4 the P₅₀ (partial pressure of O₂ at 50% hemoglobin saturation) ± SEM values were 22.4 ± 0.6, 25.3 ± 0.5, and 28.5 ± 0.4 Torr at 33,37 and 41°C, respectively. The effect of temperature on oxygen-hemoglobin affinity, (Δlog P₅₀/ΔT) was 0.014 ± 0.001 at pH 7.4, significantly lower than that observed in human and dog blood. This low temperature sensitivity may facilitate oxygen off-loading from hemoglobin when temperature gradients exist within the animal or as tissue temperature decreases during a dive. Temperature did not significantly affect the Hill coefficient 'n' (shape) of the dissociation curve which averaged 2.43 ± 0.04 at 37°C. The fixed-acid Bohr coefficient (Δlog P₅₀/ΔpH) was -0.606 ± 0.032 at 37°C and increased with temperature. This relatively large value for the Bohr coefficient was similar to those previously reported for the Northern Elephant, Bladdernose, and Weddell Seals, and may facilitate oxygen off-loading as acidosis develops during a dive.