Comment on Crawford et al, page 566
Crawford and colleagues suggest that hypoxic red blood cells mediate vasodilatation by reducing circulating nitrite anions to nitric oxide.
Regulation of microcirculation is a dynamic process in which the need of the tissues is communicated to the vasculature, enabling the appropriate matching of oxygen supply to demand. A hypothesis has been put forward claiming that red blood cells (RBCs) sense hypoxia and then mediate an instantaneous vasodilatory response.1,2 In this process, the unloading of oxygen from hemoglobin (Hb) is coupled to the release of a vasodilator from RBCs. The primary candidates for mediating red cell–induced vasodilation are ATP and NO. Stamler and colleagues (Jia et al2 ) originally suggested a role for a thiol (SH) group in Hb as a carrier and releaser of NO. According to this theory, the binding (formation of S-nitrosohemoglobin, SNOHb) and release of NO from Hb are allosterically regulated so that NO release occurs when Hb is deoxygenated.2
We have suggested a physiologic role for the nitrite anion (NO2–) in NO-mediated metabolic vasoregulation.3,4 This involves NO synthase–independent reduction of nitrite to NO, a reaction that is greatly enhanced under hypoxic/ischemic conditions. Cosby et al elegantly expanded on this and showed that nitrite is reduced to vasodilatory NO in vivo by deoxyhemoglobin in RBCs.5 In this issue of Blood, Crawford and colleagues have carefully examined the role of RBCs and hemoglobin in relation to hypoxic vasodilatation. In their interesting study, they report that RBCs handle nitrite differently depending on the Hb oxygen saturation. When Hb is fully oxygenated, the primary reaction is oxidation of nitrite into biologically inert nitrate. As oxygen saturation falls along the vascular tree, Hb gradually turns into a “reductase” and starts to reduce nitrite into vasodilatory NO. The maximal nitrite reduction is observed when Hb is approximately 50% oxygenated (P50). According to the authors, this is ideal for the purpose of hypoxic vasoregulation, as 50% oxygenation prevails in the critical arterioles where blood flow and oxygen delivery is controlled.
Of interest, the physiologic effects of nitrite seem to extend beyond regulation of blood flow.6 In heart and liver tissue, nitrite protects against ischemia-reperfusion injury, and in the gastric mucosa it serves protective functions via acidic reduction to NO. To better understand the overall physiology of nitrite, we need to carefully examine its origin. An obvious source is the NO synthases, given that nitrite is a major oxidation product of NO. In addition to this, the diet is also a large contributor to systemic nitrite levels mainly via bioconversion of dietary nitrate (found mainly in vegetables) to nitrite.6 This implies that the physiologic regulatory role of nitrite described here and elsewhere is influenced by what we eat. Thus, ingestion of nitrate-rich food results in a build-up of “nitrite reserves” to be used in critical situations of hypoxia.
There is an intense ongoing debate as to which pathway for NO formation by RBCs is the most significant: the SNOHb pathway or the nitrite pathway. The SNOHb theory has been questioned lately mainly because many groups have been unable to detect SNOHb in human blood. Crawford and colleagues now convincingly show that at least nitrite reduction to NO does not require SNOHb as an intermediate. An inherent problem with both models is the mechanism by which NO is released from RBCs. How can NO escape without being captured and destroyed by the abundant oxyHb? The present study nicely demonstrates NO gas formation from hypoxic RBCs and nitrite (albeit at unphysiologically high concentrations) and with maximum levels obtained at the P50. Nevertheless, I believe the final piece of evidence for a role of red cell–derived NO in physiologic regulation of blood flow is still lacking. What we need now is for different laboratories to start pulling together to design the missing crucial in vivo experiments. Nevertheless, it is a truly fascinating story we are witnessing. In just one decade, Hb has gone from being merely a NO scavenger to NO carrier and now NO generator. ▪