Defining the Contribution of ENaC to ADH-mediated Water and Sodium Excretion

 描述（由申请人提供）：抗利尿激素（ADH）通过控制肾脏水转运在调节水平衡中起关键作用。ADH与加压素-2受体（V2 R）结合，并增加集合管细胞中水通道蛋白2（AQP 2）水通道的表达，以增加肾小管水渗透性。髓质中陡峭的溶质皮质-髓质梯度为肾小管液中的水吸收提供了驱动力， 远端肾单位我们和其他人已经证明ADH也刺激集合管中上皮钠通道（ENaC）的活性，并且ENaC可能促进尿液浓缩。然而，ENaC传统上被认为是肾素-血管紧张素-醛固酮系统（RAAS）的末端效应物，并且是控制肾Na+排泄从而控制血压的最终元件。然而，我们已经表明，在某些低钠血症的情况下，如肾上腺切除术后，ENaC活性是惊人的强大。肾上腺切除术后，血清ADH水平升高，导致ADH介导的ENaC活性增加。这种ENaC活性的增加发生在醛固酮缺乏的情况下，提出了许多关于ADH调节ENaC的新问题。ENaC可以响应ADH和RAAS的概念与该通道仅控制肾Na+重吸收和血压的公认作用不一致。为了调和ADH在调节水和平衡方面的明显冲突作用，我们提出了一个统一的范式，将ENaC的贡献定义为水和Na+平衡的调节剂。我们推测ADH同时激活ENaC和AQP 2可促进尿液浓缩和血浆稀释。如果平行信号通路刺激ENaC活性达到足够高的水平，则ADH介导的ENaC激活将诱导Na+潴留超过尿液浓缩所需的量并导致高血压。我们将使用最先进的方法（例如，小鼠分离的开放集合管的膜片钳研究）和新试剂（连接节段/集合管特异性Nedd 4 -2敲除小鼠）来测试三个特定目的：1）测试ENaC是否有助于病理性肾水重吸收和低钠血症，2）测试V2 R激活是否可以诱导肾Na+潴留和高血压，（3）检测V2 R是否通过Nedd 4 -2信号传导刺激ENaC。该提案将提供ADH，AQP 2和ENaC如何相互作用并最终控制水和Na+稳态的机制见解。这些知识具有临床意义，因为它可能为临床医生提供治疗低钠血症和高血压的新策略。

 DESCRIPTION (provided by applicant): Antidiuretic hormone (ADH) plays a key role in regulating water balance by controlling renal water transport. ADH binds to the vasopressin-2 receptor (V2R) and increases expression of aquaporin 2 (AQP2) water channels in collecting duct cells to increase renal tubular water permeability. A steep solute corticomedullary gradient in the medullary interstitium provides the driving force for water absorption from tubular fluid in the distal nephron. We and others have demonstrated that ADH also stimulates activity of the epithelial sodium channel (ENaC) in the collecting duct and that ENaC possibly facilitates urine concentration. However, ENaC is traditionally regarded as the end-effector of the renin-angiotensin-aldosterone system (RAAS) and the final element that controls renal Na+ excretion and thus blood pressure. Yet we have shown that in some cases of hyponatremia, such as after adrenalectomy, ENaC activity is surprisingly robust. After adrenalectomy, serum ADH levels rise, leading to an increase in ADH-mediated ENaC activity. This increase in ENaC activity occurs in the absence of aldosterone, raising many new questions about ADH regulation of ENaC. The notion that ENaC can respond to both ADH and RAAS is at odds with the accepted role of this channel in merely controlling renal Na+ reabsorption and blood pressure. To reconcile the apparent conflicting roles of ADH in regulating both water and balance, we propose a unifying paradigm that defines the contribution of ENaC as a regulator of water and Na+ balance. We hypothesize that simultaneous activation of ENaC and AQP2 by ADH promotes urine concentration and plasma dilution. If parallel signaling pathways stimulate ENaC activity to a sufficiently high level, then ADH-mediated ENaC activation will induce Na+ retention beyond what is needed for urine concentration and lead to hypertension. We will use state-of-the-art methodologies (e.g., patch clamp studies of isolated split-open collecting duct in mice) and novel reagents (connecting segment/collecting duct-specific Nedd4-2 knockout mice) to test three Specific Aims: 1) Test whether ENaC contributes to pathologic renal water reabsorption and hyponatremia, 2) Test whether V2R activation can induce renal Na+ retention and high blood pressure, and 3) Test whether V2R signals through Nedd4-2 to stimulate ENaC in vivo. This proposal will provide mechanistic insights into how ADH, AQP2, and ENaC interact and ultimately control water and Na+ homeostasis. This knowledge is clinically significant because it may suggest new strategies, which are already currently available to clinicians, for the treatment of hyponatremia and hypertension.