Quantifying the potential for red blood cell β-adrenergic sodium-proton exchangers to protect oxygen transport in hypoxic and hypercapnic white seabass

White seabass ( Atractoscion nobilis ) are increasingly experiencing periods of low oxygen (O 2 ; hypoxia) and high carbon dioxide (CO 2 , hypercapnia) due to climate change and eutrophication of the coastal waters of California. Hemoglobin (Hb) is the principal O 2 carrier in the blood and in many teleost fishes Hb-O 2 binding is compromised at low pH; however, the red blood cells (RBC) of some species regulate intracellular pH with adrenergically-stimulated sodium-proton-exchangers (β-NHE). We hypothesized that RBC β-NHEs in white seabass are an important mechanism that can protect the blood O 2 -carrying capacity during hypoxia and hypercapnia. We determined the O 2 -binding characteristics of white seabass blood, the response of RBCs to adrenergic stimulation, and quantified the protective effect of β-NHE activity on Hb-O 2 saturation. White seabass had typical teleost Hb characteristics, with a moderate O 2 affinity (PO 2 at half-saturation; P 50 2.9 kPa) that was highly pH-sensitive (Bohr coefficient -0.92; Root effect 52%). The presence of RBC β-NHEs was confirmed by functional, molecular and bioinformatic data and super-resolution imaging revealed, for the first time, the subcellular location of β-NHE protein in vesicle-like structures and on the RBC membrane, and its translocation after adrenergic stimulation. The activation of RBC β-NHEs increased Hb-O 2 saturation by ∼8% in normoxia at 1 kPa PCO 2 , and by up to 20% in hypoxia. Our results confirm that RBC β-NHE activity in white seabass has great potential to protect arterial O 2 transport in environmentally relevant conditions of hypoxia and hypercapnia, but also reveal a potential vulnerability of fish to combinations of these stressors.