The results of nitric oxide (NO) infusions in normal volunteers and NO infusions and inhalation in experimental animals confirms that NO can be transported as a hormone and thus has the potential to be a pharmacological agent (i.e., a drug). We believe that the lack of vascular effects in our sickle cell patients is due to the presence of circulating hemoglobin and that this contributes to the pathophysiology of this and other chronic and acute hemolytic syndromes, especially the pulmonary hypertension complications which we have found to be severe and of high frequency in older patients. In recent studies we have infused nitrite into the brachial arteries of normal human volunteers and have shown that this increases blood flow, suggesting that nitrite could function physiologically as a source of NO and could be used pharmacologically. However, we find that the effects of nitrite infusion, both on vascular properties and on methemoglobin formation, are relatively long lasting and suggest partition of the nitrite into various tissues. We find that in vitro deoxyerythrocytes and nitrite cause aortic ring preparations to dilate, suggesting a mechanism of nitrite activation by deoxyheme proteins. We also find that nitrite inhalalation in hypoxic newborn sheep lead to decreased pulmonary artery pressures and exhalation of NO; nitrite infusions in these animals leads to decreases in mean arterial blood pressure. We are currently studying the formation and compartmentalization of nitrite in the blood, in erythrocytes in particular, and whether nitrite levels may be a marker of cardiovascular risk in humans. These studies are designed to allow us to initiate nitrite infusions in normal human subjects and those with a variety of ischemic (including sickle cell anemia) diseases. We have shown that the maximum production of NO from nitrite occurs near the p50 of hemoglobin and is dependent on the allosteric conformation of hemoglobin. We have also developed methods to measure nitrite levels precisely in human blood and have found that most of blood nitrite is contained in the red cells. In the current reporting period, we have measured the uptake of nitrite into the human red cell as a function of oxygent tension, pH, temperature and other variables. Such uptake appears to be an important factor controlling the overall rates of conversion of nitrite to NO intravascularly. Further, we have shown that dehydroascorbic acid in red cells can catalyze the oxidation of iron-nitrosylhemoglobin (NbNO) to release NO and that this NO is then converted to nitite but not nitrate intracellularly. This reaction may be the major mechanism for the formation of red cell nitrite, which we believe is one of the major storage sites for bioactive NO in the body, and has led us to develop a model of the interaction of the ascorbic acid/dehyroascorbic acid and the NO/nitrite cycles inside the erythrocyte. We have also examined the hypothesis that in hypoxia cell-free hemoglobin could serve as an additional endocrine source of NO. In this study, we constructed a multicellular model to characterize the amount of NO delivered by the reaction of nitrite with both intraerythrocytic and cell-free hemoglobin and find that levels of NO production by this mechanism may be quite significant, comparable to production by the vascular wall itself. We have also examined in detail the reaction mechanism of nitrite with oxy- and deoxy-hemoglobin and find that at conditions likely to obtain physiologically and pharmacologically-especially with regard to the ratio of nitrite ions to hemoglobin molecules-the formation of free radicals (including that of the ferryl form of hemoglobin) is not likely to occur to a major extent. This is important because the formation of free radicals had been of concern with regard to the development of nitrite (or NO) pharmacology. We believe that the above studies should contribute to our understanding of the role of the human erythrocyte in modulating NO bioactivity, especially via a nitrite intermediate, and also facilitate the development of nitrite as a useful drug for cardiovascular pathology.
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