Mechanisms of Vasovagal Syncope

描述(由申请人提供)：血管迷走性晕厥(VVS，单纯性昏厥)是导致一过性意识丧失的最常见原因，是立位不耐受(OI)的急性发作性形式。体位性心动过速综合征(POTS)是OI常见的慢性形式。两者都是由衰弱的症状和体征来定义的，而直立则通过卧位来缓解。尽管我们过去的工作表明过度的直立性中枢性低血容量是由于内脏动脉和静脉收缩缺陷导致内脏血液汇集所致，但其病理生理机制仍不清楚。初步数据支持这一假设，即在这些患者中，一氧化氮(NO)的产生增加，导致交感神经交感去甲肾上腺素能神经传递在连接前和连接后部位减少。我们的方法有两个方面：1)我们将使用皮内微透析和激光多普勒血流仪(LDF)来描绘NO调节去甲肾上腺素能神经传递的微血管机制，而不会出现混杂的全身反射变化。2)我们将系统地将这一机制应用于立位应激、下体负压(LBNP)模型，同时通过惰性气体重复呼吸来测量心输出量，使用侧重内脏变化的体积描记技术来测量局部血流量和局部血流量，并通过腓骨显微神经造影术来测量肌交感神经活动。30名年龄在14-29岁的VVS、POTS和对照受试者将被招募。我们将用化学发光法测定微透析液中的NO，以检测其在OI中是否增加。微透析液去甲肾上腺素(NE)的高效液相色谱分析将通过逐步递增的酪胺剂量来评估连接前的神经传递。内源性阻滞剂Bretylium阻断后，通过LDF对NE递增的反应来评估连接后的神经传递。基础和局部热刺激的NO将被评估。在用硝基L精氨酸(NLA)抑制一氧化氮合酶(NOS)后和用NLA硝普钠(SNP)灌流期间恢复NO后，我们将通过重复研究来确定神经传递抑制是否是NO专一性的。我们将获得皮肤活检的一氧化氮合酶亚型蛋白和表达。我们将确定全身性一氧化氮是否抑制OI突触前和突触后的肾上腺素能活动，从而改变对LBNP的反应。将通过逐步增加下体负压和测量血浆NE反应来评估突触前神经传递。突触后神经传递将通过对逐步增加的苯肾上腺素剂量的全身反应来评估。为确定NO的特异性，在用L-NG-单甲基精氨酸(L-NMMA)抑制一氧化氮合酶(NOS)后，在L-NMMA SNP灌流过程中恢复NO，并以小剂量苯肾上腺素作为对照，观察MSNA的基线移位和血管收缩。L-NMMA在许多小时内缓慢进入中枢神经系统。为了确定其核心影响，我们将在实验结束时测量MSNA。研究将在皮肤人类代理模型中建立OI常见和衰弱形式的分子机制，并将这一知识应用于OI的综合生理学。这项研究将导致有针对性和有效的医学治疗。 适用于重要的OI患者群体。

DESCRIPTION (provided by applicant): Vasovagal syncope (VVS, simple faint) is the most common cause of transient loss of consciousness and is the acute episodic form of orthostatic intolerance (OI). Postural tachycardia syndrome (POTS) is the common chronic form of OI. Both are defined by debilitating symptoms and signs while upright relieved by recumbency. Pathophysiological mechanisms have remained elusive although our past work shows that excessive upright central hypovolemia results from splanchnic blood pooling due to defective splanchnic arterial and venous constriction. Preliminary data support the hypothesis that production of nitric oxide (NO) is enhanced in these patients resulting in reduced sympathetic noradrenergic neurotransmission at pre-junctional and post-junctional sites. Our approach is two-fold: 1) We will use intradermal microdialysis and laser Doppler flowmetry (LDF) to delineate the microvascular mechanisms of NO modulation of noradrenergic neurotransmission free of confounding systemic reflex changes. 2) We will systemically apply this mechanism to a model of orthostatic stress, lower body negative pressure (LBNP), while measuring cardiac output by inert gas rebreathing, regional blood volume, and regional blood flow using plethysmographic techniques focusing on splanchnic changes, and muscle sympathetic nerve activity by peroneal microneurography. 30 each of VVS, POTS and control subjects ages 14-29 years will be recruited. We will use chemiluminescence to measure NO in microdialysate to test whether it is increased in OI. Pre-junctional neurotransmission will be assessed by step wise tyramine doses using HPLC analysis of microdialysate norepinephrine (NE). Post-junctional neurotransmission will be assessed by LDF responses to step-wise increases of NE after endogenous blockade with bretylium. Basal and local heat stimulated NO will be assessed. We will determine whether inhibition of neurotransmission is NO-specific by repeating studies after NOS inhibition with nitro-L-arginine (NLA) and after repletion of NO during perfusion with NLA+sodium nitroprusside (SNP). We will obtain skin biopsies for NOS isoform protein and expression. We will determine if systemic NO inhibits pre and post synaptic adrenergic activity in OI, altering the response to LBNP. Presynaptic neurotransmission will be assessed by step-wise increasing LBNP and measuring plasma NE response. Post-synaptic neurotransmission will be assessed by the systemic response to stepwise increasing phenylephrine doses. To confirm NO-specificity, studies will be repeated after NOS inhibition with L-NG-monomethyl Arginine(L-NMMA), after repletion of NO during perfusion with L-NMMA +SNP, and after low dose phenylephrine as a control for baseline shifts of MSNA and vasoconstriction. L-NMMA enters the CNS slowly over many hours. To ascertain its central effects we will measure MSNA at the end of experiments. Studies will establish the molecular mechanism for common and debilitating forms of OI in the cutaneous human surrogate model, and apply this knowledge to the integrative physiology of OI. This research will lead to targeted and effective medical therapy for important groups of OI patients.