Fish red blood cell carbon dioxide transport in vitro: A comparative study

Red blood cell (rbc) carbon dioxide transport was examined in vitro in three teleosts (Oncorhynchus mykiss, Anguilla anguilla, Scophthalmus maximus) and an elasmobranch (Scyliorhinus canicula) using a radioisotopic assay that measures the net conversion of plasma HCO3- to CO2. The experiments were designed to compare the intrinsic rates of rbc CO2 excretion and the impact of haemoglobin oxygenation/deoxygenation among the species. Under conditions simulating in vivo levels of plasma HCO3- and natural haematocrits, the rate of whole blood CO2 excretion varied between 14.0 mu mol ml(-1) h(-1) (S. canicula) and 17.6 mu mol ml(-1) h(-1) (O. mykiss). The rate of CO2 excretion in separated plasma was significantly greater in the dogfish, S. canicula. The contribution of the rbc to overall whole blood CO2 excretion was low in the dogfish (46 +/- 6%) compared to the teleosts (trout, 71 +/- 4%; turbot, 64 +/- 5%; eel, 55 +/- 3%). To eliminate the naturally occurring differences in haematocrit and plasma [HCO3-] as inter-specific variables, the rates of whole blood CO2 excretion were determined in blood that had been resuspended to constant [HCO3-] (5 mmol(-1)) and haematocrit (20%) in appropriate teleost and elasmobranch Ringer solutions. Under such normalized conditions, the rate of whole blood CO2 excretion was significantly higher in the turbot (22.4 +/- 1.3 mu mol ml(-1) h(-1)) in comparison to the other species (16.4-18.4 mu mol ml(-1) h(-1)) and thus revealed a greater intrinsic rate of rbc CO2 excretion in the turbot. To study the contribution of Bohr protons, the rates of whole blood CO2 excretion were assessed in blood subjected to rapid oxygenation during the initial phase of the 3 min assay period. Rapid oxygenation significantly enhanced the rate of CO2 excretion in the teleosts but not in the elasmobranch. The extent of the increase provided by the rapid oxygenation of haemoglobin was a linear function of the extent of the Haldane effect, as quantified in each species from in vitro CO2 dissociation (combining) curves. Under steady-state conditions, deoxygenated blood exhibited greater rates of CO2 excretion than oxygenated blood in the teleosts but not in the elasmobranch. As a consequence of the Haldane effect, rbc intracellular pH was increased in the teleosts by deoxygenation but was unaltered in the elasmobranch. The results, by extrapolation, suggest that the rates of CO2 excretion in vivo are influenced by the magnitude of the Haldane effect and the extent of haemoglobin oxygenation during gill transit in addition to the intrinsic rate at which the rbc converts plasma HCO3- to CO2.