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

Some teleost fishes exhibit a non-linear release of H+ with haemoglobin oxy gen 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 (pH(i)) and SO2 was evaluated in two teleost fishes, the carp (Cyprinus carpio) and tambaqui (C olossoma macropomum). Carp possess a relatively small Root effect within th e physiological pH range, while that in tambaqui is relatively large. It wa s therefore hypothesized that both species would possess a non-linear relea se of H+ with SO2 under in vivo conditions, but that the degree of non-line arity would be less pronounced in carp than in tambaqui. Red cell suspensio ns of tambaqui showed a marked non-linear relationship between pH(i) and SO 2 at in vivo pH values (pH(e)=7.66), where the majority of Bohr protons wer e released between 50 and 100% SO2. In whole blood of carp, the relationshi p between pH(i) and SO2 was almost linear at normal resting extracellular p H values (pH(e)=8.10) where Hills n(50) (cooperativity) was 1.3. Under acid ic conditions (pH(e)=7.11), n(50) decreased to 0.9 and the release of H+ wi th SO2 became slightly non-linear. The reduction in pH in carp blood was as sociated with the onset of the Root effect, and the oxygen tension at half saturation (P-50) 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 pH(e) 8.1 to being non-linear at pH(e) 7.1, consistent with the relationshi p between pH(i) and SO2. Thus, possession of a Root effect could be a prere quisite 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 transpo rt and acid-base homeostasis.