The effects of CO₂ and fixed acid on the O₂-Hb affinity of rabbit and cat blood

Abstract

The action of respiratory and metabolic acid-base disturbances on the O₂-Hb affinity was studied in rabbits and cats. Blood samples of both species were exposed to in vitro pH-changes, which were either achieved by variation ofPCO₂ (2.8–8.3 kPa) at constant lactic acid concentration, or by addition of lactic acid (5–14 mmol·l⁻¹) at constantPCO₂. ThePO₂ at halfsaturation (P₅₀) and the Hill'sn were determined from O₂-Hb dissociation curves (ODC) in a range between 20 and 80% SO₂. Under standard conditions (T=331 K,PCO₂=5.33 kPa, pH=7.4), the average P₅₀ value was 4.66±0.05 kPa in rabbits, that is slightly higher than reported by others, and 5.17±0.03 kPa in cats. The average values of Hill'sn were 2.91±0.02 and 2.95±0.03 for rabbits and cats, respectively.

When plasma pH was varied by CO₂, the resulting classical CO₂ Bohr factor φCO₂=ΔlgP₅₀/ΔpH₅₀ was distinctly higher in cats (−0.560±0.006,n=25) than in rabbits (−0.504±0.014,n=22), although in the latter species being even higher than reported elsewhere. Concomitant metabolic acidosis did not significantly affect φCO₂, but shifted the P₅₀ at a given plasma pH to lower values. Substitution of lactic acid with equimolar amounts of sodium lactate left both φCO₂ and P₅₀ unchanged.

When plasma pH was varied by lactic acid (LA), the resulting fixed acid Bohr factors φLA were similar for cats (−0.420±0.016,n=21) and rabbits (−0.427±0.010,n=18), and not influenced by additional respiratory acidosis. Concomitant respiratory acidosis, however, shifted the P₅₀ at a given plasma pH to higher values.

The deviation of lgP₅₀ from that under standard conditions (lgP₅₀st) induced by both, respiratory and metabolic acid-base changes, can generally be expressed by Eqs. (1) or (2)

$$1gP_{50} - 1gP_{50} st = \varphi CO_2 \left( {pH - 7.4} \right) - \left( {\varphi CO_2 - \varphi LA} \right) \cdot \left[ {pH\left( {LA} \right) - 7.4} \right]$$

((1))

$$1gP_{50} - 1gP_{50} st = \varphi LA\left( {pH - 7.4} \right) + \left( {\varphi CO_2 - \varphi LA} \right) \cdot \left[ {pH\left( {CO_2 } \right) - 7.4} \right].$$

((2))

Thereby, the first term considers the total pH-deviation, irrespective of its origin, and the second term represents the metabolic (1) or the respiratory (2) correction.

For practical use and within the physiological range of plasma pH (7.2–7.6), φCO₂ and φLA are assumed to be additively linked and to be independent of pH. Once these factors are determined for a given species, together with the buffer capacity for CO₂, they provide sufficient data to estimate fairly accurately the position of the ODC. This is particularly important for in vivo conditions where respiratory and metabolic acid-base disturbances often occur simultaneously.