Identification of a Primary Renal AT2 Receptor Defect in Spontaneously Hypertensive Rats.

RATIONALE: Previous studies identified a defect in Ang III (angiotensin III [des-aspartyl1-angiotensin II])-elicited AT2R (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 AT2R signaling and renal sodium (Na+) retention by utilizing the selective AT2R agonist compound-21 (C-21). METHODS AND
RESULTS: Female 4-week-old Wistar Kyoto and SHR rats were studied after 24-hour systemic AT1R (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 AT2R 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 AT2R 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 AT2R-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 AT2R recruitment. In contrast, 8-bromo-cAMP administration had no effect on natriuresis or AT2R recruitment in SHR.
CONCLUSIONS: The results demonstrate a primary renal proximal tubule cell AT2R 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.