The angiotensin AT2 receptor modulates renal production of cyclic guanosine 3',5'-monophosphate (cGMP; J. Clin. Invest. 1996. 97:1978-1982). In the present study, we hypothesized that angiotensin II (Ang II) acts at the AT2 receptor to stimulate renal production of nitric oxide leading to the previously observed increase in cGMP. Using a microdialysis technique, we monitored changes in renal interstitial fluid (RIF) cGMP in response to intravenous infusion of the AT2 receptor antagonist PD 123319 (PD), the AT1 receptor antagonist Losartan, the nitric oxide synthase (NOS) inhibitor nitro--arginine-methyl-ester (-NAME), the specific neural NOS inhibitor 7-nitroindazole (7-NI), or Ang II individually or combined in conscious rats during low or normal sodium balance. Sodium depletion significantly increased RIF cGMP. During sodium depletion, both PD and -NAME caused a similar decrease in RIF cGMP. Combined administration of PD and -NAME decreased RIF cGMP to levels observed with PD or -NAME alone or during normal sodium intake. During normal sodium intake, Ang II caused a twofold increase in RIF cGMP. Neither PD nor -NAME, individually or combined, changed RIF cGMP. Combined administration of Ang II and either PD or -NAME produced a significant decrease in RIF cGMP compared with that induced by Ang II alone. Combined administration of Ang II, PD, and -NAME blocked the increase in RIF cGMP produced by Ang II alone. During sodium depletion, 7-NI decreased RIF cGMP, but the reduction of cGMP in response to PD alone or PD combined with 7-NI was greater than with 7-NI alone. During normal sodium intake, 7-NI blocked the Ang II-induced increase in RIF cGMP. PD alone or combined with 7-NI produced a greater inhibition of cGMP than did 7-NI alone. During sodium depletion, 7-NI (partially) and -NAME (completely) inhibited RIF cGMP responses to -arginine. These data demonstrate that activation of the renin- angiotensin system during sodium depletion increases renal nitric oxide production through stimulation by Ang II at the angiotensin AT2 receptor. This response is partially mediated by neural NOS, but other NOS isoforms also contribute to nitric oxide production by this pathway.

The existence of local or tissue-based renin-angiotensin-aldosterone systems (RAAS) is well documented and has been implicated as a key player in the pathogenesis of cardiovascular and renal diseases. The kidney contains all elements of the RAAS, and intrarenal formation of angiotensin II not only controls glomerular hemodynamics and tubule sodium transport, but also activates a number of inflammatory and fibrotic pathways. Experimental and clinical studies have shown that the intrarenal RAAS is activated early in diabetic nephropathy, the leading cause of chronic kidney disease (CKD). Although angiotensin-converting enzyme inhibitors and angiotensin receptor blockers decrease the rate of decline in kidney function in patients with diabetic and non-diabetic nephropathy, many patients still progress to end-stage renal disease or die from cardiovascular events. There is still a clear need for additional strategies to block the RAAS more effectively to reduce progression of CKD. The focus of this paper is to review the importance of the intrarenal RAAS in CKD and recent findings in renin-angiotensin biology pertinent to the kidney. We also discuss additional strategies to inhibit the RAAS more effectively and the potential impact of direct renin inhibition on the prevention and management of CKD.

—We evaluated the role of the renal angiotensin II type 2 (AT ₂ ) receptor in blood pressure regulation in rats with 2-kidney, 1 figure-8 wrap (Grollman) hypertension. Renal wrapping increased systolic blood pressure (SBP). Renal interstitial fluid (RIF) bradykinin (BK), nitric oxide end-products (NOX), and cGMP were higher in the contralateral intact kidney than in the wrapped kidney. In rats with Grollman hypertension, losartan normalized SBP and increased renal function, RIF BK, NOX, and cGMP only in contralateral kidneys. In contrast, PD 123319, a specific AT ₂ -receptor antagonist, significantly increased SBP and decreased RIF BK, NOX, and cGMP in both kidneys. Combined administration of losartan and PD 123319 prevented the decrease in SBP and the increase in RIF BK, NOX, and cGMP levels observed with losartan alone. BK-receptor blockade caused a significant increase in RIF BK and a decrease in RIF NOX and cGMP in both kidneys similar to that observed during administration of PD 123319. In rats that underwent sham operation, RIF BK increased in response to angiotensin II, an effect that was blocked by PD 123319. These data demonstrate that angiotensin II mediates renal production of BK, which, in turn, releases nitric oxide and cGMP via stimulation of AT ₂ receptors. The increase in blood pressure and the decrease in renal BK, nitric oxide, and cGMP during AT ₂ -receptor blockade suggests that the AT ₂ receptor mediates counterregulatory vasodilation in Grollman hypertension and prevents a further increase in blood pressure.

Angiotensin II type 2 (AT ₂ R) or bradykinin B ₂ (B ₂ R) receptor activation enhances NO production. Recently, we demonstrated enhancement of NO production when AT ₂ R and B ₂ R are simultaneously activated in vivo. However, the mechanism involved in this enhancement is unknown. Using confocal fluorescence resonance energy transfer microscopy, we report the distance between the AT ₂ R and B ₂ R in PC12W cell membranes to be 50±5 Å, providing evidence and quantification of receptor heterodimerization as the mechanism for enhancing NO production. The rate of AT ₂ R–B ₂ R heterodimer formation is largely a function of the degree of AT ₂ R–B ₂ R expression. The physical association between the dimerized receptors initiates changes in intracellular phosphoprotein signaling activities leading to phosphorylation of c-Jun terminal kinase, phosphotyrosine phosphatase, inhibitory protein κBα, and activating transcription factor 2; dephosphorylation of p38 and p42/44 mitogen-activated protein kinase and signal transducer inhibitor of transcription 3; and enhancing production of NO and cGMP. Controlling the expression of AT ₂ R–B ₂ R, consequently influencing their biologically active dimerization, presents a potential therapeutic target for the treatment of hypertension and other cardiovascular and renal disorders.

Both bradykinin B ₂ and angiotensin II type 2 (AT ₂ ) receptors are known to stimulate renal production of nitric oxide (NO). To evaluate the individual contributions of AT ₂ and B ₂ receptors to renal NO production, we monitored renal interstitial, stable NO metabolites and cGMP by a microdialysis technique in conscious, bradykinin B ₂ –null and wild-type mice (n=8 in each group) during low sodium intake alone or with the angiotensin AT ₁ or AT ₂ receptor blockers, valsartan (0.5 μg/min) or PD123319 (0.15 μg/min), or both. During normal salt intake, renal interstitial fluid NO and cGMP levels in B ₂ -null mice were not different from those of wild-type mice. Low sodium intake increased NO and cGMP in wild-type mice but not in B ₂ -null mice. Valsartan increased NO and cGMP in both wild-type and B ₂ -null mice but to a significantly greater degree in the wild-type than in B ₂ -null mice. PD123319 decreased NO and cGMP in both wild-type and B ₂ -null mice. Combined valsartan and PD123319 decreased NO and cGMP in both wild-type and B ₂ -null mice, but there was no significant difference during combined treatment from their levels after administration of PD123319 alone. Our results indicate that during ingestion of a low-salt diet, production of NO is mediated mainly via the AT ₂ -B ₂ receptor cascade. Blockade of the AT ₁ receptor enhances the production of NO via the AT ₂ receptor in both wild-type and B ₂ -null mice. We conclude that NO can be produced by 2 alternative pathways: directly through the AT ₂ receptor or indirectly from AT ₂ receptor stimulation of bradykinin via the B ₂ receptor.