Résumé
The Root effect is a pH-dependent reduction in hemoglobin-O-2 carrying capacity. Specific to ray-finned fishes, the Root effect has been ascribed specialized roles in retinal oxygenation and swimbladder inflation. We report that when rainbow trout are exposed to elevated water carbon dioxide (CO2), red muscle partial pressure of oxygen (PO2) increases by 65%-evidence that Root hemoglobins enhance general tissue O-2 delivery during acidotic stress. Inhibiting carbonic anhydrase (CA) in the plasma abolished this effect. We argue that CA activity in muscle capillaries short-circuits red blood cell (RBC) pH regulation. This acidifies RBCs, unloads O-2 from hemoglobin, and elevates tissue PO2, which could double O-2 delivery with no change in perfusion. This previously undescribed mechanism to enhance O-2 delivery during stress may represent the incipient function of Root hemoglobins in fishes.