Endothelial cell expression of hemoglobin a regulates nitric oxide signaling

Models of unregulated nitric oxide (NO) diffusion do not consistently account for the biochemistry of NO synthase (NOS)-dependent signaling in many cell systems. For example, endothelial NOS (eNOS) controls blood pressure, blood flow and oxygen delivery through its effect on vascular smooth muscle tone, but the regulation of these processes is not adequately explained by simple NO diffusion between endothelial and smooth muscle cells. Here, we report for the first time that hemoglobin (Hb) α is expressed in arteriolar endothelial cells, where it is in complex with eNOS. Moreover, we demonstrate that endothelial Hb α is enriched at the myoendothelial junction (MEJ), where it regulates the effect of eNOS on vascular reactivity. Surprisingly, this function is unique to Hb α and abrogated by its genetic depletion. Mechanistically, Hb α heme iron in the Fe3+ state permits active signaling across the MEJ, and this signaling is shut off when Hb α is reduced to the Fe 2+ state by endothelial cytochrome B5 reductase 3 (CytB5R3). Genetic and pharmacological inhibition of CytB5R3 increases NO bioavailability and decreases the reactivity of resistance vessels. These data reveal a novel paradigm by which the regulation of intracellular Hb oxidation controls NOS signaling in non-erythroid cells. This paradigm may be relevant to a broad range of somatic cells discovered to express Hb and known also to express NOS isoforms.