Identification of a Primary Renal AT 2 Receptor Defect in Spontaneously Hypertensive Rats

Abstract

Rationale: Previous studies identified a defect in Ang III (angiotensin III [des-aspartyl 1 -angiotensin II])–elicited AT 2 R (Ang type-2 receptor)-mediated natriuresis in renal proximal tubule cells of spontaneously hypertensive rats (SHR). Objective: This study aimed to delineate in prehypertensive SHR kidneys the receptor or postreceptor defect causing impaired AT 2 R signaling and renal sodium (Na + ) retention by utilizing the selective AT 2 R agonist compound-21 (C-21). Methods and Results: Female 4-week-old Wistar Kyoto and SHR rats were studied after 24-hour systemic AT 1 R (Ang II type-1 receptor) blockade. Left kidneys received 30-minute renal interstitial infusions of vehicle followed by C-21 (20, 40, and 60 ng/[kg·min], each dose 30 minutes). Right kidneys received vehicle infusions. In Wistar Kyoto, C-21 dose-dependently increased urine Na + excretion from 0.023±0.01 to 0.064±0.02, 0.087±0.01, and 0.089±0.01 µmol/min ( P =0.008, P <0.0001, and P <0.0001, respectively) and renal interstitial fluid levels of AT 2 R downstream signaling molecule cGMP (cyclic guanosine 3',5' monophosphate) from 0.91±0.3 to 3.1±1.0, 5.9±1.2 and 5.3±0.5 fmol/mL ( P =nonsignificant, P <0.0001, and P <0.0001, respectively). In contrast, C-21 did not increase urine Na + excretion or renal interstitial cGMP in SHR. Mean arterial pressure was slightly higher in SHR but within the normotensive range and unaffected by C-21. In Wistar Kyoto, but not SHR, C-21 induced AT 2 R translocation to apical plasma membranes of renal proximal tubule cells, internalization/inactivation of NHE-3 (sodium-hydrogen exchanger-3) and Na + /K + ATPase (sodium-potassium-atpase) and phosphorylation of AT 2 R-cGMP downstream signaling molecules Src (Src family kinase), ERK (extracellular signal-related kinase), and VASP (vasodilator-stimulated phosphoprotein). To test whether cGMP could bypass the natriuretic defect in SHR, we infused 8-bromo-cGMP. This restored natriuresis, Na + transporter internalization/inactivation, and Src and VASP phosphorylation, but not apical plasma membrane AT 2 R recruitment. In contrast, 8-bromo-cAMP administration had no effect on natriuresis or AT 2 R recruitment in SHR. Conclusions: The results demonstrate a primary renal proximal tubule cell AT 2 R natriuretic defect in SHR that may contribute to the development of hypertension. Since the defect is abrogated by exogenous intrarenal cGMP, the renal cGMP pathway may represent a viable target for the treatment of hypertension. Visual Overview: An online visual overview is available for this article.