Transnitrosation signals oxyhemoglobin desaturation.

Erythrocytes dilate peripheral blood vessels as a function of oxyhemoglobin desaturation.1 This effect increases regional blood flow to hypoxic tissues. The mechanisms underlying the peripheral vasodilatory effects of desaturating erythrocytes are incompletely understood but do not involve activation of local, endothelial NO synthase (eNOS). Indeed, eNOS-derived NO itself primarily relaxes large vessels and does that primarily only in the absence of blood. In this issue of Circulation Research, Diesen et al confirm that thiols carrying a nitrosonium (NO ) equivalent signal cyclic GMP-dependent vascular smooth muscle relaxation during erythrocytic oxyhemoglobin desaturation.2 These data support paradigm-changing work demonstrating that nitrogen oxides are transported by circulating erythrocytes to signal oxyhemoglobin desaturation through serial NO/NO thiol equilibria and transfer reactions (transnitrosation) and that these reactions normally take place at sites remote from NOS activity.1–3 These new data show clearly that this signaling is independent of local NOS activity, of cyclooxygenase, of ATP, and of the effects of hypoxia itself on vascular smooth muscle. Erythrocytes are endogenously “preloaded” with nitrogen oxides for delivery to vessels in conditions of oxyhemoglobin desaturation.1–3 This signaling links delivery of erythrocytic NO/NO groups to oxyhemoglobin desaturation, permitting augmented blood flow to hypoxic tissue. Three mechanisms have been proposed by which transitions in hemoglobin (Hb) conformation may result in nitrogen oxide transfer to blood vessels. (1) In the originally proposed mechanism, Hb deoxygenation (R-to-T transition) permits transnitrosation from Hb -chain cysteine 93 ( Cys93) to erythrocytic carrier thiols (Figure).1–3 This concept is supported by the data obtained by Diesen et al.2 (2) At one time, NO radical was proposed to diffuse away from the Fe (II) heme iron in T state Hb. Although there was an initial enthusiasm for this construct as an NO radical–based alternative to nitrosothiolbased NO transfer, it is essentially impossible kinetically; it is now acknowledged to be irrelevant to physiology.1,4,5 (3) More recently, it has been proposed that deoxyhemoglobin serves as an NO2 reductase, forming an NO radical that, again, escapes autocapture by Fe(II) heme to diffuse into (and activate guanylate cyclase in) smooth muscle cells.6 This mechanism is also essentially impossible kinetically but is still referenced. The data from the study by Diesen et al argue against this construct, which is discussed further below. It had been suggested that NO transfer from Cys93 in deoxygenating Hb might not be relevant to physiology because mice genetically engineered to be deficient in the Cys93 were once suggested to not have had abnormalities in hypoxic responses. However, it turns out that none of the physiological responses to Hb desaturation demonstrated to hinge on transnitrosation has actually been tested in the Cys93-deficient mouse. Specifically, there are 3 such responses; none of the 3 was studied. These are: (1) pulmonary vascular remodeling in chronic hypoxemia7; (2) altered blood flow in vascular beds in response to altered oxyhemoglobin saturation1,8,9; and (3) augmented minute ventilation in response to hypoxia.10 Even assuming that humanized Hb expressed in these mice behaves identically to murine Hb (which it likely does not), the mice were studied only with regard to overall blood pressure responses.6 Blood pressure is not a surrogate for regional blood flow distribution. Furthermore, the work by Diesen et al provides insight into the negative vascular ring study using erythrocytes from these mice. Specifically, Cys93 S-nitrosothiol bonds can be rapidly depleted in wild-type red cells,2,11,13 ablating their ability to relax vascular smooth muscle with Hb desaturation. The pulmonary vascular ring responses in the studies of the Cys93-deficient mouse were minimal: either the control RBCs were S-nitrosothiol depleted (rendering them functionally indistinguishable from the RBCs of Cys93-deficient RBCs) or the vascular rings were essentially unresponsive. Diesen et al confirm previous studies that transnitrosation from erythrocytic S-nitrosothiols forms low-mass S-nitrosothiols that can signal vascular effects (Figure).4–6,8–10,13 Chronic in vivo transnitrosation from hypoxic erythrocytes to N-acetylcysteine (forming S-nitroso-N-acetyl cysteine) causes pulmonary vascular remodeling through upregulation of hypoxiaand S-nitrosothiol–sensitive genes in the pulmonary endothelium.7 Diesen et al now show that erythrocytic transnitrosation to N-acetyl-cysteine and other low-mass thiols potentiates acute vascular smooth muscle effects of erythrocytic S-nitrosothiols in the context of Hb deoxygenation. We now see a picture emerge of elegant regulation: NO transfer from deoxyhemoglobin thiols to erythrocytic lowmass and membrane14 thiols permits NO/NO transfer to target proteins in the vasculature. Thus, acute hypoxic effects are actually signaled by oxyhemoglobin desaturation rather than low PO2. These acute effects include increased minute ventilation and cGMP-dependent vascular smooth muscle relaxation.1,2,10 However, excessive NO/NO transfer reactions can signal counterregulatory, protective effects in the The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Pediatric Respiratory Medicine (N.M., B.G.), University of Virginia School of Medicine, Charlottesville; and Department of Pediatric Critical Care (A.D.), Washington University School of Medicine, St Louis, Mo. Correspondence to Benjamin Gaston, MD, Box 800386, University of Virginia School of Medicine, Charlottesville, VA 22908. E-mail [email protected] (Circ Res. 2008;103:441-443.) © 2008 American Heart Association, Inc.