Aldosterone-independent regulation of ENaC by systemic salt

描述(申请人提供)：众所周知，醛固酮影响远端肾单位的ENaC活性，以调节钠平衡，从而调节血压。这项建议挑战了我们目前的观点，即ENaC活动如何通过确定这一调节的不依赖于醛固酮的方式来应对饮食盐摄入量的变化。具体地说，我们已经发现证据表明，刺激性Ang II和抑制性缓激肽(BK)级联之间的协调耦合对于ENaC活性正确适应饮食钠的变化是必要的。我们的初步工作有力地支持了我们的中心假设，即Ang II直接刺激哺乳动物对醛固酮敏感的远端肾单位(ASDN)由ENaC介导的Na+重吸收，并且这种调节对ENaC的醛固酮作用不是多余的。此外，我们假设在钠限制过程中Ang II级联的激活通过限制BK信号对ENaC的抑制作用而进一步增强ENaC的活性。相反，BK信号是在盐摄入量增加时抑制ENaC活性所必需的，在缺乏B2受体的小鼠中，BK信号的功能障碍导致由于ENaC活性增加而导致过度的Na+保存。重要的是，B2受体缺失的小鼠会患上盐敏感型高血压。为了验证这一假说，我们提出了3个具体目标：1)确定全身盐摄入量对ENaC的醛固酮非依赖性调节的生理重要性。明确血管紧张素II调节ENaC在这一过程中的作用。2)明确Ang II调节哺乳动物ASDN ENaC活性的作用机制，揭示Ang II调节ENaC活性的细胞信号通路。3)确定Ang II和BK信号通路之间的功能偶联如何控制ENaC活性，以响应系统盐的变化，并确定在B2-/-小鼠中破坏这一调节的病理生理后果。我们的实验方法范围从天然远端肾单位细胞的电生理学到整个动物的平衡研究，并将结合1)膜片钳测量分裂开放的小鼠远端肾单位的ENaC活性，以及通过Western blotting/RPPA和免疫组织化学检测ENaC的表达水平，以及2)利用分子遗传学工具评估小鼠肾脏Na+排泄的平衡研究，以确定醛固酮非依赖调节ENaC的生理作用和肾脏对钠的处理。从药理学的观点来看，这项工作的重要性将促使B2激动剂的开发，作为纠正远端肾单位钠处理以控制血压的工具。

DESCRIPTION (provided by applicant): It is well known that aldosterone affects ENaC activity in the distal nephron to regulate sodium balance and, consequently blood pressure. This proposal challenges our current view on how ENaC activity responds to variations in dietary salt intake by identifying aldosterone-independent means of this regulation. Specifically, we have developed evidence that coordinated coupling between stimulatory Ang II and inhibitory Bradykinin (BK) cascades is necessary for proper adaptation of ENaC activity to variations in dietary sodium. Our preliminary work strongly supports our central hypothesis that Ang II directly stimulates ENaC-mediated Na+- reabsorption in the mammalian aldosterone-sensitive distal nephron (ASDN) and this regulation is non- redundant to aldosterone actions on ENaC. In addition, we hypothesize that the activation of Ang II cascade during sodium restriction further augments ENaC activity by limiting inhibitory actions of BK signaling on ENaC. In contrast, BK signaling is necessary to suppress ENaC activity during elevated salt intake with its dysfunction in mice lacking B2 receptors causing excessive Na+ conservation as a result of elevated ENaC activity. Importantly, mice with deleted B2 receptors develop salt-sensitive hypertension. To test this hypothesis we address 3 specific aims: 1) Establish the physiological importance of aldosterone-independent regulation of ENaC by systemic salt intake. Define the role of Ang II regulation of ENaC in this process. 2) Define the mechanism of action and delineate the cellular signaling pathway of Ang II regulation of ENaC activity in mammalian ASDN. 3) Determine how functional coupling between Ang II and BK signaling cascades controls ENaC activity in response to changes in systemic salt and establish the patho-physiological consequences of disrupting this regulation in B2 -/- mice. Our experimental approach ranges from electrophysiology in native distal nephron cells to whole animal balance studies and will combine 1) patch clamp measurements of ENaC activity in the split-open murine distal nephrons with ENaC expression levels probed by western blotting/RPPA and immunohistochemistry, and 2) balance studies to assess renal Na+ excretion with molecular genetic tools in mice to define a physiological role of aldosterone-independent regulation of ENaC and sodium handling by the kidney. From a pharmacological standpoint, the importance of this work will urge the development of B2 agonists as tools for correction of sodium handling in the distal nephron to control blood pressure.