Active Blood Acidification Greatly Enhances Oxygen Supply to the Teleost Retina

Most ray-finned fishes possess a unique system for retinal oxygen delivery, where intracellular acidification of red blood cells (RBCs) in the capillaries induces offloading from pH-sensitive hemoglobin. A countercurrent vascular network, choroid rete mirabile, posterior to retina augments this “oxygen-secretory” mechanism and confers PO2 values that can exceed 1300 mmHg some species. We hypothesized mirabile exerts an active role RBC facilitate secretion. show presence enzymatic activities extracellular carbonic anhydrase (CA) vacuolar-type H+-ATPase (VHA) lumen mirabile. These data are consistent with pathway, VHA secretes H+ into lumen, which combines HCO3– form CO2 (catalyzed by CA) rapidly diffuses facilitating hemoglobin offloading. Next, we validated model vivo injecting CA inhibitor (C18) dorsal aorta anesthetized rainbow trout electrode implanted retina. Here, inhibition resulted rapid reduction 517 226 within minutes. strongly support proposed Further, primarily diffusion inner segments involved signal-processing evolution pathway was tightly associated morphological expansion part Together, these suggest evolutionary origin secretion early history greatly enhanced capacity visual processing, potentially permitting improved prey detection colonization novel environments.