Project Summary Angiotensin converting enzyme (ACE) inhibitors are regularly given to patients during early stages of chronic kidney disease (CKD). However, the benefits of slowing the progression of CKD are often overshadowed by dangerous episodes of hyperkalemia. In moderate CKD (stage 3; MCKD), the surviving nephrons adapt to high K diets with an extremely enhanced rate of K secretion per nephron. These proposed studies are designed to determine the importance of angiotensin II and why ACE inhibitors have such dangerous hyperkalemic effects in renal failure beginning in stage 3. This information should provide alternative therapeutic choices designed to prevent hypertension and hyperkalemia with high K consumption in moderate stages of renal failure. Aldosterone responds to chronically elevated plasma [K] and enhances expression of the epithelial Na channel (ENaC) and the Na-K-ATPase in the distal nephron. However, it is just as important to maintain a high rate of Na delivery to ENaC and the Na-K-ATPase. It has been known for decades that a high K diet (HK) causes a large natriuretic response. However, the signaling pathway is not understood. Based on our preliminary results and the literature, we hypothesize that a high K diet stimulates urinary ANGII, which activates AT2 receptors, thereby reducing Na reabsorption in the proximal tubule and increasing Na delivery to stimulate K secretion from K channels in the distal nephron. Aim #1 will determine whether liver- or PT-generated ANGII enhances K excretion in normal mice on HK and the 5/6 nephrectomized mouse model of renal failure. For this Aim, we will employ liver only angiotensinogen knockout mice (LKO) and double liver plus PT angiotensinogen knockout mice (DKO). Our preliminary results show that urinary ANGII is elevated by 5 to 8 fold in WT and LKO mice on HK (5%) but is not elevated in DKO on HK. When placed on HK, DKO, but not WT or LKO, exhibit severe hyperkalemia and a reduced urinary [K]/[creatinine]. We also find that PD123319, a blocker of ANGII 2 receptors (AT2), reduce HKIN in WT mice. For Aim #2, we will determine whether the AT2 receptor-cGMP-kinase pathway causes HKIN by inhibiting the Na-H exchanger 3 (NHE3) in the PT. Mice with a knock-out of the renal outer medullary K channel (ROMK-KO) are a model of Bartters syndrome but also are absent ROMK-mediated K secretion. ROMK-KO exhibit MCKD with GFR of approximately 55% of WT. Despite the compromised GFR and lack of ROMK-mediated K secretion, ROMK-KO maintain K balance on HK. Our preliminary results indicate that ROMK-KO compensate for the lack ROMK with enhanced HKIN that presumably stimulates large, Ca-activated K channels (BK). The enhanced HKIN may result from our finding of increased renal guanylin expression in ROMK-KO on HK. For Aim 3, we will explore whether enhanced guanylin-cGMP signaling of HK fed ROMK-KO inhibits Na reabsorption in the PT and increases Na delivery to stimulate K secretion via BK.