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 (Oncurhynchw mykiss, Anguilla unguilla, Scophtimus muximus) and an elasmobranch (Scyliurhinus cunicula) using a radioisotopic assay that measures the net conversion of plasma HC03to COz. The experiments were designed to compare the intrinsic rates of rbc CO, excretion and the impact of haemoglobin oxygenation/deoxygenation among the species. Under conditions simulating in viuo levels of plasma HCO,and natural haematocrits, the rate of whole blood CO1 excretion varied between 14.0 pmol ml-’ h-’ (S. canicula) and 17.6 prnol ml-’ h-l (0. mykiss). The rate of CO, excretion in separated plasma was significantly greater in the dogfish, S. canicula. The contribution of the rbc to overall whole blood CO, excretion was low in the dogfish (46 L 6%) compared to the teleosts (trout, 71+ 4%; turbot, 64 * 5%; eel, 55 c 3%). To eliminate the naturally occurring differences in haematocrit and plasma [HCO,-] as interespecific variables, the rates of whole blood CO, excretion were determined in blood that had been resuspended to constant [HCO,-] (5 mmol-‘) and haematocrit (20%) in appropriate teleosc and elasmobranch Ringer solutions. Under such normalized conditions, the rate of whole blood CO, excretion was significantly higher in the turbot (22.4 2 1.3 pmol ml-’ h-l) in comparison to the other species (16.4-18.4 pmol ml-’ h-l) and thus revealed a greater intrinsic rate of rbc CO1 excretion in the turbot. To study the contribution of Bohr protons, the rates of whole blood CO, 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 CO, 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 CO, dissociation (combining) curves. Under steady-state conditions, deoxygenated blood exhibited greater rates of CO, 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 CO* excretion in oivo 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 HCOjto COz. COMP BIOCHEM PHYSIOL 113;2:121-130, 1996.