Regulation of Pulmonary Vascular Tone During Cirrhosis

控制肺血管张力调节的细胞机制是复杂和不完全了解的，特别是在病理生理条件下。一种这样的病理生理状况是肝肺综合征。肝肺综合征是一种临床三联症，包括晚期肝病(通常为肝硬变)、肺气体交换异常(如分流)，导致严重的全身动脉低氧血症，以及在没有内源性心肺疾病的情况下广泛的肺血管扩张。这种综合征发生在15%-30%的肝硬变患者中，并极大地使他们的治疗复杂化。一氧化氮(NO)被认为在肝肺综合征的发生发展中起中心作用。最近建立了一种大鼠肝肺综合征动物模型，为研究肝肺综合征的发病机制奠定了基础。这些动物有肺内分流和低氧血症。NO与肝肺综合征的发病机制尚未明确。这项建议采用体内和体外实验策略的综合方法来研究肝肺综合征的潜在机制。我们提供的初步数据表明，除了NO和eNOS升高外，肝硬变大鼠肺内血管收缩因子内皮素(ET-1)的表达减少。也有证据表明，在肝硬变期间，血管平滑肌钾通道被激活。这是有史以来的第一个数据，为肝硬变期间肺血管扩张和钝化的缺氧性升压反应提供了一种机制。更多的数据显示，在肝硬变期间，应激反应基因血红素氧合酶-1(HO-1)在肺和肝脏显著上调，而在肾脏显著降低。HO-1的酶活性释放了CO，这是一种已知的血管扩张剂，可以通过cGMP依赖和非依赖的途径发挥作用。因此，HO-1/CO轴的组织特异性调节可能参与了肝硬变时肺血管的扩张和肾血管的收缩。最后，为了研究NO在ET-1、钾通道和HO-1改变中的作用，对肝硬变大鼠进行了NO抑制剂的慢性治疗。这种治疗完全逆转了与肝硬变相关的基因表达变化。综上所述，我们的数据表明，在肝硬变过程中，NO不仅作为血管扩张剂，而且作为ET-1、钾通道和HO-1基因表达的调节，在肝肺综合征的发生发展中起着中心作用。我们将验证这样的假设：(1)肝硬变期间慢性NO升高通过直接的血管扩张作用和对基因表达的间接修饰而使肺循环对低氧无反应；(2)由肝硬变释放的因子调节肺血管张力；(3)HO-1衍生的CO参与了肝硬变期间的肺血管扩张。该项目不仅将确定肝肺综合征的细胞学基础，而且还将有助于我们在最基本的水平上了解肺血管张力是如何控制的。

The cellular mechanisms governing the regulation of pulmonary vascular tone are complex and incompletely understood, particularly during pathphysiological conditions. One such pathophysiological condition is hepatopulmonary syndrome. Hepatopulmonary syndrome is a clinical triad of advanced liver disease (usually cirrhosis), pulmonary gas exchange abnormalities (i.e. shunting) leading to severe systemic arterial hypoxemia, and widespread pulmonary vasodilations in the absence of intrinsic cardiopulmonary disease. This syndrome occurs in 15 - 30 percent of cirrhotic individuals and vastly complicates their treatment. Nitric oxide (NO) has been postulated to be central to the development of hepatopulmonary syndrome. An animal model of hepatopulmonary syndrome recently has been developed in rats that has proven useful for investigating to pathogenesis of hepatopulmonary syndrome. These animals have intrapulmonary shunting and hypoxemia. The mechanisms linking NO to the development of hepatopulmonary syndrome have not been defined. This proposal investigates the underlying mechanisms of hepatopulmonary syndrome using a comprehensive approach of in vivo and in vitro experimental strategies. We provide preliminary data demonstrating that in addition to elevated NO and eNOS, expression in lung of the vasocontrictor endothelin (ET-1) is decreased in cirrhotic rats. Evidence is also provided showing that vascular smooth muscle potassium channels are activated during cirrhosis. These are the first data ever, providing a mechanism for the pulmonary vasodilation and blunted hypoxic pressor response during cirrhosis. Additional data is shown demonstrating that during cirrhosis the stress response gene heme oxygenase-1 (HO-1) is significantly upregulated in lung and liver and decreased in kidney. HO-1 enzymatic activity liberates CO, a known vasodilator that can act via cGMP-dependent and -independent pathways. Therefore, it is possible that the tissue-specific regulation of the HO-1/CO axis contributes to the pulmonary vasodilation and renal vasoconstriction during cirrhosis. Finally, to investigate the role of NO in alterations to ET-1, potassium channels, and HO-1, cirrhotic rats were chronically treated with a NO inhibitor. This treatment resulted in a complete reversal of the cirrhotic associated changes to gene expression. Taken together, our data suggest that during cirrhosis, NO is central to the development of hepatopulmonary syndrome acting not only as a vasodilator but also as a regulator of gene expression of ET-1, potassium channels, and HO-1. We will test the hypotheses that: (1) chronic NO elevation during cirrhosis renders the pulmonary circulation unresponsive to hypoxia via direct vasodilatory actions and indirect modifications to gene expression; (2) factors released by the cirrhotic liver regulate pulmonary vascular tone; (3) HO-1 derived CO contributes to the pulmonary vasodilation during cirrhosis. This project will not only define the cellular basis for hepatopulmonary syndrome, but will also contribute to our understanding of how pulmonary vascular tone is controlled at the most basic level.