Time course of red blood cell intracellular pH recovery following short-circuiting in relation to venous transit times in rainbow trout,Oncorhynchus mykiss

Accumulating evidence is highlighting the importance of a system of enhanced hemoglobin-oxygen (Hb-O 2 ) unloading for cardiovascular O 2 transport in teleosts. Adrenergically stimulated sodium-proton exchangers (β-NHE) create H + gradients across the red blood cell (RBC) membrane that are short-circuited in the presence of plasma-accessible carbonic anhydrase (paCA) at the tissues; the result is a large arterial-venous pH shift that greatly enhances O 2 unloading from pH-sensitive Hb. However, RBC intracellular pH (pH i ) must recover during venous transit (31–90 s) to enable O 2 loading at the gills. The halftimes ( t 1/2 ) and magnitudes of RBC β-adrenergic stimulation, short-circuiting with paCA and recovery of RBC pH i , were assessed in vitro, on rainbow trout whole blood, and using changes in closed-system partial pressure of O 2 as a sensitive indicator for changes in RBC pH i . In addition, the recovery rate of RBC pH i was assessed in a continuous-flow apparatus that more closely mimics RBC transit through the circulation. Results indicate that: 1) the t 1/2 of β-NHE short-circuiting is likely within the residence time of blood in the capillaries, 2) the t 1/2 of RBC pH i recovery is 17 s and within the time of RBC venous transit, and 3) after short-circuiting, RBCs reestablish the initial H + gradient across the membrane and can potentially undergo repeated cycles of short-circuiting and recovery. Thus, teleosts have evolved a system that greatly enhances O 2 unloading from pH-sensitive Hb at the tissues, while protecting O 2 loading at the gills; the resulting increase in O 2 transport per unit of blood flow may enable the tremendous athletic ability of salmonids.