Mechanisms of Vasovagal Syncope

描述（由申请人提供）：血管迷走神经性晕厥（VVS，单纯性昏厥）是一过性意识丧失的最常见原因，是直立性不耐受（OI）的急性发作形式。体位性心动过速综合征（POTS）是成骨不全常见的慢性形式。两者都是由衰弱的症状和体征来定义的，而直立时可以通过平卧来缓解。尽管我们过去的研究表明，过度的直立性中央低血容量是由脏器动脉和静脉收缩缺陷引起的脏器血池引起的，但病理生理机制仍然难以理解。初步数据支持这样的假设，即在这些患者中一氧化氮（NO）的产生增加，导致交感神经在交界前和交界后部位的去肾上腺素能神经传递减少。我们的方法是双重的：1)我们将使用皮内微透析和激光多普勒血流仪（LDF）来描述NO调节去甲肾上腺素能神经传递的微血管机制，而不存在混淆的全身反射变化。2)我们将系统地将这一机制应用于一个直立应激、下体负压（LBNP）模型，同时通过惰性气体再呼吸测量心输出量、区域血容量和区域血流量，使用以内脏变化为重点的体积描记技术测量心输出量，并通过腓神经微神经摄影测量肌肉交感神经活动。将招募年龄在14-29岁的VVS、POTS和对照受试者各30人。我们将采用化学发光法测定微透析液中的NO，以检测其在OI中是否升高。使用高效液相色谱分析微透析物去甲肾上腺素（NE），通过逐级酪胺剂量评估结前神经传递。内源性溴胺阻断后，LDF对NE逐步增加的反应将评估结后神经传递。将评估基础和局部热刺激NO。我们将通过重复研究硝基- l -精氨酸（NLA）抑制NOS后和NLA+硝普钠（SNP）灌注过程中补充NO后的神经传递抑制是否为NO特异性。我们将进行皮肤活组织检查NOS异构体蛋白和表达。我们将确定系统性NO是否会抑制成骨不全患者突触前和突触后的肾上腺素能活性，从而改变对LBNP的反应。突触前神经传递将通过逐步增加LBNP和测量血浆NE反应来评估。突触后神经传递将通过逐步增加苯肾上腺素剂量的全身反应来评估。为了确认NO的特异性，将在L-NG-monomethyl Arginine（L-NMMA）抑制NOS后、L-NMMA +SNP灌注NO后、低剂量苯肾上腺素（phenylephrine）作为MSNA基线偏移和血管收缩的对照后重复研究。L-NMMA在数小时内缓慢进入中枢神经系统。为了确定其中心效应，我们将在实验结束时测量MSNA。研究将在皮肤人类替代模型中建立常见和衰弱型成骨不全的分子机制，并将这些知识应用于成骨不全的综合生理学。这项研究将导致有针对性和有效的药物治疗

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.