Non-linear release of Bohr protons with haemoglobin-oxygenation in the blood of two teleost fishes; carp (Cyprinus carpio) and tambaqui (Colossoma macropomum)

Abstract

Some teleost fishes exhibit a non-linear release of H+ with haemoglobin oxygen saturation (SO2) in whole blood that may be related to the Root effect and a low apparent cooperativity in oxygen binding. To further investigate this correlation, the relationship between red cell pH (pHi) and SO2 was evaluated in two teleost fishes, the carp (Cyprinus carpio) and tambaqui (Colossoma macropomum). Carp possess a relatively small Root effect within the physiological pH range, while that in tambaqui is relatively large. It was therefore hypothesized that both species would possess a non-linear release of H+ with SO2 under in vivo conditions, but that the degree of non-linearity would be less pronounced in carp than in tambaqui. Red cell suspensions of tambaqui showed a marked non-linear relationship between pHi and SO2 at in vivo pH values (pHe = 7.66), where the majority of Bohr protons were released between 50 and 100% SO2. In whole blood of carp, the relationship between pHi and SO2 was almost linear at normal resting extracellular pH values (pHe = 8.10) where Hills n50 (cooperativity) was 1.3. Under acidic conditions (pHe = 7.11), n50 decreased to 0.9 and the release of H+ with SO2 became slightly non-linear. The reduction in pH in carp blood was associated with the onset of the Root effect, and the oxygen tension at half saturation (P50) increased from 3.8 to 38.2 mmHg. The relationship between total CO2 and SO2 in carp whole blood changed from being almost linear at pHe 8.1 to being non-linear at pHe 7.1, consistent with the relationship between pHi and SO2. Thus, possession of a Root effect could be a prerequisite for a non-linear release of Bohr protons with oxygenation, but the expression in whole blood may depend upon a species-specific cooperativity threshold. The non-linear release of Bohr protons with oxygenation may be a general phenomenon in teleosts with important implications for gas transport and acid-base homeostasis.