Role of nitrite and hemoglobin in vascular NO homeostasis in the fetus and adult

描述（由申请人提供）：氧气输送与需求相匹配具有基本的生理重要性，尤其是在胎儿氧气供应经常受损的围产期。一氧化氮（NO）是对抗缺氧/缺血的关键内源性防御，其通过1）血管舒张动脉以增加氧递送和2）在线粒体水平降低氧消耗并因此暂时减少氧需求。最近令人信服的证据表明，亚硝酸盐，NO2-，一种普遍存在于哺乳动物血液中的阴离子，可以通过与脱氧血红蛋白反应转化为NO。许多实验表明，这种反应构成了缺氧/缺血期间NO的重要来源。由于胎儿的氧储备较低，并且比成人更容易受到氧供应中断的影响，因此在围产期期间，对缺氧/缺血的保护机制似乎更明显。从亚硝酸盐和脱氧血红蛋白之间的反应中产生NO是有利的亚硝酸盐，血红蛋白和H+的浓度增加，所有这些在缺氧胎儿的血液中升高超过成人。亚硝酸盐还原为NO的速率也受血红蛋白构象的影响，当血红蛋白为40%至60%氧饱和时最大，这一范围通常发生在胎儿中，但仅在成人缺氧期间发生。这也表明，与成人相比，胎儿中亚硝酸盐和脱氧血红蛋白的NO生成速率可能会升高。这个项目将解决的一般假设，亚硝酸盐/脱氧血红蛋白途径是一个更普遍的来源NO在胎儿比成人。假设的四个方面将通过四个具体目标进行检验。第一个将使用慢性仪器近足月胎羊，以确定循环亚硝酸盐对胎儿心血管和氧消耗的影响，以及这些影响是否是由缺氧增强。第二个将检查是否胎儿铁-亚硝酰血红蛋白，在脱氧血液中的亚硝酸盐代谢的副产品，释放NO更迅速地在体内比成人血液，因此是一个更容易的NO来源。在第三个目标，在体外研究中提出，这将确定是否胎儿血液是更有效的比成人血液在生产血管活性量的NO从亚硝酸盐。最后，我们将调查最近发现的亚硝酸盐，NO和高铁血红蛋白之间的反应的生理重要性，这可以解释红细胞是如何能够释放血管活性量的NO。这些研究的结果将提供一个新的和彻底的评估胎儿缺氧/缺血反应过程中的NO的来源与脱氧血红蛋白的亚硝酸盐的反应。此外，他们将提供洞察亚硝酸盐作为治疗缺氧/缺血性应激的工具的治疗潜力。公共卫生相关性：亚硝酸盐通过与脱氧血红蛋白反应转化为一氧化氮（NO），被认为是缺氧/缺血保护性反应的关键介质。如果是这样的话，亚硝酸盐具有作为治疗剂用于治疗涉及缺氧/缺血应激的疾病和损伤的广泛潜力。

DESCRIPTION (provided by applicant): Matching oxygen delivery with need is of fundamental physiological importance, especially during the perinatal period when oxygen supply to the fetus is often compromised. Nitric oxide (NO) is a key endogenous defense against hypoxia/ischemia by 1) vasodilating arteries to increase oxygen delivery and 2) decreasing oxygen consumption at the level of the mitochondria and hence temporarily reducing oxygen need. Compelling recent evidence indicates that nitrite, NO2-, a ubiquitous anion in the blood of mammals, can be converted to NO by reaction with deoxyhemoglobin. Many experiments suggest that this reaction constitutes a significant source of NO during hypoxia/ischemia. Because the fetus has lower oxygen reserves and is more subject to interrupted oxygen supply than the adult, it would seem logical for protective mechanisms against hypoxia/ischemia to be more pronounced during perinatal life. The production of NO from the reaction between nitrite and deoxyhemoglobin is favored by increased concentrations of nitrite, hemoglobin and H+, all of which are elevated in the blood of hypoxic fetuses more than the adult. The rate of nitrite reduction to NO is also affected by hemoglobin conformation, being maximal when hemoglobin is 40 to 60% oxygen saturated, a range that occurs normally in the fetus but only during hypoxia in the adult. This also suggests the rate of NO production from nitrite and deoxyhemoglobin may be elevated in the fetus compared to the adult. This project will address the general hypothesis that the nitrite/deoxyhemoglobin pathway is a more prevalent source of NO in the fetus than in the adult. Four aspects of the hypothesis will be tested by four specific aims. The first will use chronically instrumented near-term fetal sheep to determine the effects of circulating nitrite on the fetal cardiovasculature and oxygen consumption, and whether these effects are potentiated by hypoxia. The second will examine whether fetal iron-nitrosyl hemoglobin, a byproduct of nitrite metabolism in deoxygenated blood, releases NO more rapidly in vivo than that of adult blood, and hence is a more ready source of NO. In the third aim, in vitro studies are proposed which will determine whether fetal blood is more effective than adult blood at producing vasoactive quantities of NO from nitrite. Finally, we will investigate the physiological importance of a recently discovered reaction between nitrite, NO, and methemoglobin, which may explain how the erythrocyte is capable of releasing vasoactive amounts of NO. The results of these studies will provide a novel and thorough assessment of the reaction of nitrite with deoxyhemoglobin as a source of NO during fetal responses to hypoxia/ischemia. Furthermore, they will provide insight into the therapeutic potential of nitrite as a tool in the treatment of hypoxic/ischemic stress. PUBLIC HEALTH RELEVANCE: Project Narrative Nitrite, upon conversion to nitric oxide (NO) by reaction with deoxyhemoglobin, has been proposed as a key mediator of protective responses to hypoxia/ischemia. If so, nitrite holds wide- ranging potential as a therapeutic agent for the treatment of diseases and injury involving hypoxic/ischemic stress.