In vitro experiments were carried out to examine the interactions between oxygen and carbon dioxide transport in the blood of the sea lamprey. Oxygen dissociation curves for whole blood obtained from quiescent lampreys had Hill numbers (nH) ranging from 1.52 to 1.89. The Bohr coefficient for whole blood was -0.17 when extracellular pH (pHe) was considered, but was much greater (-0.63) when red blood cell pH (pHi) was considered. The pHi was largely dependent on haemoglobin oxygen- saturation (SO2) and the pH gradient across the red blood cell membrane was often reversed when PCO2 was increased and/or SO2 was lowered. The magnitude of the increase in pHi associated with the Haldane effect ranged from 0.169 pH units at 2.9 kPa PCO2 to 0.453 pH units at a PCO2 of 0.2 kPa. Deoxygenated red blood cells had a much greater total CO2 concentration (CCO2) than oxygenated red blood cells, but the nonbicarbonate buffer value for the red blood cells was unaffected by oxygenation. Plasma CCO2 was not significantly different under oxygenated or deoxygenated conditions. Partitioning of CO2 carriage in oxygenated and deoxygenated blood supports recent in vivo observations that red blood cell CO2 carriage can account for much of the CCO2 difference between arterial and venous blood. Together, the results also suggest that oxygen and carbon dioxide transport may not be tightly coupled in the blood of these primitive vertebrates. Finally, red cell sodium concentrations were dependent on oxygen and carbon dioxide tensions in the blood, suggesting that sodium-dependent ion transport processes may contribute to the unique strategy for gas transport in sea lamprey blood.

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