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

RATIONALE

Previous studies identified a defect in Ang III (angiotensin III [des-aspartyl-angiotensin II])-elicited ATR (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 ATR signaling and renal sodium (Na) retention by utilizing the selective ATR agonist compound-21 (C-21).

METHODS AND RESULTS

Female 4-week-old Wistar Kyoto and SHR rats were studied after 24-hour systemic ATR (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 (=0.008, <0.0001, and <0.0001, respectively) and renal interstitial fluid levels of ATR 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 (=nonsignificant, <0.0001, and <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 ATR translocation to apical plasma membranes of renal proximal tubule cells, internalization/inactivation of NHE-3 (sodium-hydrogen exchanger-3) and Na/KATPase (sodium-potassium-atpase) and phosphorylation of ATR-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 ATR recruitment. In contrast, 8-bromo-cAMP administration had no effect on natriuresis or ATR recruitment in SHR.

CONCLUSIONS

The results demonstrate a primary renal proximal tubule cell ATR 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.