Although regional synthesis of nitric oxide (NO) by the endothelium contributes importantly to local vasodilator tone, we have previously shown that NO bioactivity may be transported in blood, and have biological effects at a distance from the site of entry into the circulation. These endocrine effects of NO are mediated by intravascular NO-stores candidates are protein and heme-bound NO species (RXNO) in plasma or erythrocytes and the oxidative NO-metabolite nitrite. Cumulating evidence suggests that nitrite may serve as a major intravascular storage pool for NO. Recent studies by our group show that regional, intra-arterial infusion of nitrite elicits downstream vasodilator response. The mechanism of nitrite reduction in vivo may involve a number of pathways. We have observed that nitrite is reduced to NO by its reaction with deoxyheme proteins. This chemistry suggests a role for hemoglobin and other heme proteins as an oxygen dependent nitrite reductase and further that nitrite ions might contribute to systemic hypoxic vasodilation. Other research groups have proposed pH-dependent mechanisms, the involvement of electron donors such as ascorbic acid, or xanthinoxidase as a reducing agent. The present study was to be conducted in two stages (Parts A and B) with the following objectives: Part A will determine 1) whether systemic (intravenous) infusion of nitrite modulates vascular tone in the systemic circulation, 2) whether oxypurinol (a potent inhibitor of xanthine oxidase activity) lowers or ascorbic acid potentiates nitrite-induced vasodilation, 3) elucidate the pharmacokinetic profile of nitrite application in humans and 4) determine phase I data for dosing nitrite in human disease. Part B was to determine whether the systemic and pulmonary vascular responses to nitrite infusion are potentiated under hypoxic conditions and are mediated by NO gas per se as measured by NO content in exhaled breath. ? ? This study was approved on January 13, 2005 and the first volunteer was enrolled on February 18, 2005. We only enrolled volunteers in Part A of the study. Twenty volunteers completed Part A of the study. We performed biochemical, physiological, and pharmacological studies using nitrite infusion protocols in 20 normal human volunteers and in nonhuman primates to answer these questions, and we specifically tested 3 proposed mechanisms of bioactivation: reduction to nitric oxide by xanthine oxidoreductase, nonenzymatic disproportionation, and reduction by deoxyhemoglobin. We found that (1) nitrite is a relatively potent and fast vasodilator at near-physiological concentrations; (2) nitrite functions as an endocrine reservoir of nitric oxide, producing remote vasodilation during first-pass perfusion of the opposite limb; (3) nitrite is reduced to nitric oxide by intravascular reactions with hemoglobin and with intravascular reductants (ie, ascorbate); (4) inhibition of xanthine oxidoreductase with oxypurinol does not inhibit nitrite-dependent vasodilation but potentiates it; and (5) nitrite does not induce tolerance as observed with the organic nitrates.? ? The primary analysis for this part of the study will be the comparison of forearm blood flow (FBF) response before nitrite infusion with FBF after nitrite infusion by a two-sided paired Students t-test at the 5% significance level. Secondary analysis of Part A of the protocol will determine whether there are significant differences of nitrite induced increases in FBF of the ipsilateral forearm between saline, ascorbic acid, and oxypurinol coinfusion into human brachial artery.? ? Th protocol is open only for data and specimen analysis and we have decided not to pursue part B.
| Dejam, Andre; Hunter, Christian J; Gladwin, Mark T (2007) Effects of dietary nitrate on blood pressure. N Engl J Med 356:1590;author reply 1590 |
| Dejam, Andre; Hunter, Christian J; Tremonti, Carole et al. (2007) Nitrite infusion in humans and nonhuman primates: endocrine effects, pharmacokinetics, and tolerance formation. Circulation 116:1821-31 |
