Soluble adenylyl cyclase is an acid‐base sensor in rainbow trout red blood cells that regulates intracellular pH and haemoglobin–oxygen binding

Aim To identify the physiological role of the acid‐base sensing enzyme, soluble adenylyl cyclase (sAC), in red blood cells (RBC) of the model teleost fish, rainbow trout. Methods We used: (i) super‐resolution microscopy to determine the subcellular location of sAC protein; (ii) live‐cell imaging of RBC intracellular pH (pH i ) with specific sAC inhibition (KH7 or LRE1) to determine its role in cellular acid‐base regulation; (iii) spectrophotometric measurements of haemoglobin–oxygen (Hb‐O 2 ) binding in steady‐state conditions; and (iv) during simulated arterial‐venous transit, to determine the role of sAC in systemic O 2 transport. Results Distinct pools of sAC protein were detected in the RBC cytoplasm, at the plasma membrane and within the nucleus. Inhibition of sAC decreased the setpoint for RBC pH i regulation by ~0.25 pH units compared to controls, and slowed the rates of RBC pH i recovery after an acid‐base disturbance. RBC pH i recovery was entirely through the anion exchanger (AE) that was in part regulated by HCO 3 − ‐dependent sAC signaling. Inhibition of sAC decreased Hb‐O 2 affinity during a respiratory acidosis compared to controls and reduced the cooperativity of O 2 binding. During in vitro simulations of arterial‐venous transit, sAC inhibition decreased the amount of O 2 that is unloaded by ~11%. Conclusion sAC represents a novel acid‐base sensor in the RBCs of rainbow trout, where it participates in the modulation of RBC pH i and blood O 2 transport though the regulation of AE activity. If substantiated in other species, these findings may have broad implications for our understanding of cardiovascular physiology in vertebrates.