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.
The problem of high blood pressure is particularly important since more than 20% of all Americans suffer from high blood pressure. Appropriate salt handling by the kidneys is pivotal to blood pressure control. This proposal identifies new signaling cascade that is important for regulation of sodium handling in the distal nephron by dietary salt intake.
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