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理解的，特别是在病理生理条件下。肝肺综合征就是这样一种病理生理状况。肝肺综合征是晚期肝病（通常为肝硬化）、肺气体交换异常（即分流）导致严重的全身动脉低氧血症和无内在心肺疾病时广泛的肺血管扩张的临床三联征。这种综合征发生在15% - 30%的肝硬化患者中，并使其治疗大大复杂化。一氧化氮（NO）被认为是肝肺综合征发展的核心。近年来建立的大鼠肝肺综合征动物模型对探讨肝肺综合征的发病机制具有重要意义。这些动物有肺内分流和低氧血症。一氧化氮与肝肺综合征发展的机制尚未明确。本研究采用体内和体外实验的综合方法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，肝硬化大鼠肺中血管收缩素（ET-1）的表达也下降。也有证据表明血管平滑肌钾通道在肝硬化期间被激活。这是迄今为止的第一个数据，为肝硬化期间肺血管舒张和缺氧反应钝化提供了机制。其他数据显示，在肝硬化期间，应激反应基因血红素加氧酶-1 （HO-1）在肺和肝脏中显著上调，在肾脏中显著降低。HO-1酶活性释放CO，一种已知的血管扩张剂，可以通过cgmp依赖性和非依赖性途径起作用。因此，HO-1/CO轴的组织特异性调节可能有助于肝硬化期间肺血管扩张和肾血管收缩。最后，为了研究NO在改变ET-1、钾通道和HO-1中的作用，我们给肝硬化大鼠长期服用NO抑制剂。这种治疗导致了肝硬化相关基因表达改变的完全逆转。综上所述，我们的数据表明，在肝硬化期间，NO是肝肺综合征发展的核心，不仅作为血管扩张剂，而且作为ET-1、钾通道和HO-1基因表达的调节剂。我们将验证以下假设：(1)肝硬化期间慢性NO升高通过直接的血管扩张作用和间接的基因表达修饰使肺循环对缺氧无反应；(2)肝硬化释放的因子调节肺血管张力；(3)肝硬化期间HO-1来源的CO参与肺血管扩张。这个项目不仅将定义肝肺综合征的细胞基础，而且将有助于我们了解肺血管张力是如何在最基本的水平上控制的。

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.