Nitroglycerin as a Modulator of Hemoglobin Function.

Recent evidence suggests that oxygen acts as an allosteric modulator of nitric oxide (NO) binding to hemoglobin. This may have important physiological implications. Those effects, thought by some to be mediated by nitrosylation of the conserved cysteine β93 residues (S-nitrosohemoglobin; SNO-Hb), may also have therapeutic relevance in sickle cell disease and potentially be a useful strategy in controlling vasoactive effects of hemoglobin-based blood substitutes. Nitroglycerin (NTG), the classic vasodilator drug, acts in part through metabolism to NO. Normal (AA) and sickle (SS) blood suspensions were treated with nitroglycerin from 0.03 to 300 μM for periods up to 3.25 hours. Such treatment significantly lowered the oxygen affinity (increases P50, PO2 at which hemoglobin is half-saturated with O2) of both AA and SS red cells. The decrease in oxygen affinity is greater when oxygenated cells are treated than deoxygenated cells, with changes in P50 as great as 7.4 mm Hg in the AA cells and 9.9 mm Hg in the SS cells. Those shifts occur within 15 minutes, before significant increases in methemoglobin levels. This is opposite to what has been seen with other NO donors (2-(N,N-diethylamino)-diazenolate-2-oxide, S-nitrosocysteine, and sodium trioxodinitrate), where increased oxygen affinity occurs. Other classic NO donors also yielded significant methemoglobin formation within 15 minutes. In vivo effects of NTG infusion up to 1 μg/kg/min fail to demonstrate any change in P50 or methemoglobin in study subjects with normal AA erythrocytes. (Shigihara A. Masui1994;43:222) In addition, NTG has been shown to enhance oxygen release from erythrocytes without significantly increasing tissue blood flow in the hepatic microcirculation in a rat model. (Kosaka H, et al. Nat Med1997;3:456) Furthermore, a study in a dog model demonstrated NTG-induced enhanced oxygen unloading in the ischemic microcirculation. (Bin JP, et al. Circulation2006;113:2502) Elevated tandem erythrocyte S-nitrosothiol content, reflecting mainly SNO-Hb, was also noted. However, SNO-Hb increases oxygen affinity (Bonaventura C, et al. J Biol Chem1999;274:24742) and cannot explain the increased oxygen unloading. The effect of NTG on erythrocytes in our in vitro study model mimics in vivo results in the microcirculation (as opposed to the macrovascular circulation) and demonstrates that the reported enhanced oxygen unloading is due to a decrease in oxygen affinity of hemoglobin. Other biotransformation products of NTG may account for these effects as opposed to SNO-Hb. This study demonstrates that selective NO donors (i.e., NTG) may target erythrocytes for enhanced R (oxyhemoglobin) to T (deoxyhemoglobin) transformations specifically in the microcirculation and thereby improve tissue oxygenation.