Kinetics of Chloride Transport Across Fish Red Blood Cell Membranes

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Abstract
The continuous flow tube method was used to investigate the kinetics of chloride transport, and its potential oxygenation-dependency, in red blood cells (RBCs) from four teleost fish species and man. A significant interspecific variation in Cl- transport kinetics was found. At 15 °C, the rate constant k for unidirectional 36Cl- efflux was significantly lower in RBCs from eel and carp than in RBCs from rainbow trout and Atlantic cod. The values of k of cod RBCs at 15 °C and of human RBCs at 37 °C were not significantly different. The volume and surface area of the RBCs were evaluated and used to calculate the apparent membrane permeability to Cl- (PCl). The magnitude of PCl followed the sequence: eel<carp<trout¾cod. PCl values in trout and cod at 15 °C were similar to human values at 37 °C. An extrapolation of human values to 15 °C revealed that the Cl- shift at this temperature was considerable faster in all four teleosts than in man. This illustrates appropriate adaption of band-3-mediated anion transport to the different temperature regimes encountered by fish and mammals. The Cl- transport kinetics did not differ significantly between oxygenated and deoxygenated RBCs in any of the species examined. The apparent absence of any effect of a change in haemoglobin oxygen-saturation may be related to the presence of a flexible link which results in minimal interaction between the membrane domain (mediating Cl- transport) and the cytoplasmic domain (to which oxygenation-dependent haemoglobin binding occurs) of band 3. In carp, Cl- transport kinetics were not influenced by pH over the extracellular pH (pHe) range 7.6­8.36, which spans the in vivo pHe range. The data are discussed in relation to the rate-limiting role of red blood cell HCO3-/Cl- exchange for CO2 excretion.