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The evolution of proton-triggered oxygen pumps : root effect haemogobins in basal actinopterygian fishes Regan, Matthew D.


It has recently been proposed that the Root effect (reduction of blood oxygen carrying capacity at low pH) evolved in the basal actinopterygian lineage of fishes in the absence of red blood cell (RBC)pH1-protecting βNHE activity. Consequently, there is the potential for these species to experience a reduction in blood 0₂ carrying capacity, and thus 0₂ uptake at the gills, during a generalized acidosis associated with exercise, hypoxia, and hypercarbia. I analyzed the haemoglobins (Hbs) of seven species within the basal actinopterygian lineage (from basal to derived: American paddlefish (Polyodon spathula), white sturgeon (Acipenser transmontanus), spotted gar (Lepisosteus oculatus), alligator gar (Atractosteus spatula), bowfin (Amia calva), mooneye (Hiodon tergisus), and pirarucu (Arapaima gigas)) based on the hypothesis that RBC pH1 is not reduced to low enough levels in the general circulation to activate their Root effects. This may result from either RBC buffering by Rb, or onset pH values of the Root effect that are lower than those produced by these generalized acidoses. The former was investigated via Hb titrations, which simultaneously assessed intrinsic Hb buffer capacities and oxylabile Haldane effects, while the latter was investigated by spectrophotemetric analyses of Hb 0₂ saturation over a pH spectrum of pH 5.5 to 8.5. The results suggest that the Rb proton-binding properties of these species’ Hbs unlikely play a major role in the buffering of the intracellular compartment of the RBC when compared with those Hb proton-binding properties of Root effect species with βNHE. However, the onset pH values of these seven species’ variably-sized Root effects were considerably lower than those expected to be produced by a hypoxia- or exercise-induced generalized acidosis. There was also a correlation between the presence of countercurrent choroid retia and low Hb buffer values, large Bohr/Haldane effects, large Root effects, and high Root effect onset pH values, which collectively suggest a different process of Root effect evolution than is currently assumed. Taken together, it appears as though 0₂ uptake is not jeopardized in these early Root effect species, despite their relatively low Hb proton binding properties and lack of βNHE.

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