The present study used a novel mouse model with proximal tubule-specific knockout of AT _1a receptors in the kidney, PT- Agtr1a ^−/− , to test the hypothesis that intratubular Ang II (angiotensin II) and AT _1a receptors in the proximal tubules are required for maintaining normal blood pressure and the development of Ang II–induced hypertension. Twenty-six groups (n=6–15 per group) of adult male wild-type, global Agtr1a ^−/− , and PT- Agtr1a ^−/− mice were infused with Ang II (1.5 mg/kg per day, IP), or overexpressed an intracellular Ang II fusion protein in the proximal tubules for 2 weeks. Basal telemetry blood pressure were ≈15±3 mm Hg lower in PT- Agtr1a ^−/− than wild-type mice and ≈13±3 mm Hg higher than Agtr1a ^−/− mice ( P <0.01). Basal glomerular filtration was ≈23.9% higher ( P <0.01), whereas fractional proximal tubule Na ⁺ reabsorption was lower in PT- Agtr1a ^−/− mice ( P <0.01). Deletion of AT _1a receptors in the proximal tubules augmented the pressure-natriuresis response ( P <0.01) and natriuretic responses to salt loading or Ang III infusion ( P <0.01). Ang II induced hypertension in wild-type, PT- Agtr1a ^−/− and PT- Nhe3 ^−/− mice, but the pressor response was ≈16±2 mm Hg lower in PT- Agtr1a ^−/− and PT- Nhe3 ^−/− mice ( P <0.01). Deletion of AT _1a receptors or NHE3 (Na ⁺ /H ⁺ exchanger 3) in the proximal tubules attenuated ≈50% of Ang II–induced hypertension in wild-type mice ( P <0.01), but blocked intracellular Ang II fusion protein-induced hypertension in PT- Agtr1a ^−/− mice ( P <0.01). Taken together, the results of the present study provide new insights into the critical role of intratubular Ang II/AT ₁ (AT _1a )/NHE3 pathways in the proximal tubules in normal blood pressure control and the development of Ang II–induced hypertension.

Contrary to public perception, hypertension remains one of the most important public health problems in the United States, affecting 46% of adults with increased risk for heart attack, stroke, and kidney diseases. The mechanisms underlying poorly controlled hypertension remain incompletely understood. Recent development in the Cre/LoxP approach to study gain or loss of function of a particular gene has significantly helped advance our new insights into the role of proximal tubule angiotensin II (Ang II) and its AT₁ (AT_1a) receptors in basal blood pressure control and the development of Ang II-induced hypertension. This novel approach has provided us and others with an important tool to generate novel mouse models with proximal tubule-specific loss (deletion) or gain of the function (overexpression). The objective of this invited review article is to review and discuss recent findings using novel genetically modifying proximal tubule-specific mouse models. These new studies have consistently demonstrated that deletion of AT₁ (AT_1a) receptors or its direct downstream target Na⁺/H⁺ exchanger 3 (NHE3) selectively in the proximal tubules of the kidney lowers basal blood pressure, increases the pressure-natriuresis response, and induces natriuretic responses, whereas overexpression of an intracellular Ang II fusion protein or AT₁ (AT_1a) receptors selectively in the proximal tubules increases proximal tubule Na⁺ reabsorption, impairs the pressure-natriuresis response, and elevates blood pressure. Furthermore, the development of Ang II-induced hypertension by systemic Ang II infusion or by proximal tubule-specific overexpression of an intracellular Ang II fusion protein was attenuated in mutant mice with proximal tubule-specific deletion of AT₁ (AT_1a) receptors or NHE3. Thus, these recent studies provide evidence for and new insights into the important roles of intratubular Ang II via AT₁ (AT_1a) receptors and NHE3 in the proximal tubules in maintaining basal blood pressure homeostasis and the development of Ang II-induced hypertension.

In the present study, we tested the hypothesis that there are significant sex differences in angiotensin II (Ang II)-induced hypertension and kidney injury using male and female wildtype (WT) and proximal tubule-specific AT_1a receptor knockout mice (PT-Agtr1a^−/−). Twelve groups (n=8–12 per group) of adult male and female WT and PT-Agtr1a^−/− mice were infused with a pressor dose of Ang II via osmotic minipump for 2 weeks (1.5 mg/kg/day, i.p.) and simultaneously treated with or without losartan (20 mg/kg/day, p.o.) to determine the respective roles of AT_1a receptors in the proximal tubules versus systemic tissues. Basal systolic, diastolic, and mean arterial pressure were approximately 13 ± 3 mmHg lower (P<0.01), while basal 24-h urinary Na⁺, K⁺, and Cl⁻ excretion were significantly higher in both male and female PT-Agtr1a^−/− mice than WT controls (P<0.01) without significant sex differences between different strains. Both male and female WT and PT-Agtr1a^−/− mice developed hypertension (P<0.01), and the magnitudes of the pressor responses to Ang II were similar between male and female WT and PT-Agtr1a^−/− mice (n.s.). Likewise, Ang II-induced hypertension was significantly attenuated in both male and female PT-Agtr1a^−/− mice (P<0.01). Furthermore, losartan attenuated the hypertensive responses to Ang II to similar extents in both male and female WT and PT-Agtr1a^−/− mice. Finally, Ang II-induced kidney injury was attenuated in PT-Agtr1a^−/− mice (P<0.01). In conclusion, the present study demonstrates that deletion of AT_1a receptors in the proximal tubules of the kidney attenuates Ang II-induced hypertension and kidney injury without revealing significant sex differences.

Hypertension is well recognized to be the most important risk factor for cardiovascular diseases, stroke, and end-stage kidney failure. A quarter of the world’s adult populations and 46% of the US adults develop hypertension and currently require antihypertensive treatments. Only 50% of hypertensive patients are responsive to current antihypertensive drugs, whereas remaining patients may continue to develop cardiovascular, stroke, and kidney diseases. The mechanisms underlying the poorly controlled hypertension remain incompletely understood. Recently, we have focused our efforts to uncover additional renal mechanisms, pathways, and therapeutic targets of poorly controlled hypertension and target organ injury using novel animal models or innovative experimental approaches. Specifically, we studied and elucidated the important roles of intratubular, intracellular, and mitochondrial angiotensin II (Ang II) system in the development of Ang II-dependent hypertension. The objectives of this invited article are to review and discuss our recent findings that (a) circulating and intratubular Ang II is taken up by the proximal tubules via the (AT₁) AT_1a receptor-dependent mechanism, (b) intracellular administration of Ang II in proximal tubule cells or adenovirus-mediated overexpression of an intracellular Ang II fusion protein selectively in the mitochonria of the proximal tubules induces blood pressure responses, and (c) genetic deletion of AT₁ (AT_1a) receptors or the Na⁺/H⁺ exchanger 3 selectively in the proximal tubules decreases basal blood pressure and attenuates Ang II-induced hypertension. These studies provide a new perspective into the important roles of the intratubular, intracellular, and mitochondrial angiotensin II/AT₁ (AT_1a) receptor signaling in Ang II-dependent hypertensive kidney diseases.

The roles of angiotensin II (Ang II) AT₁ (AT_1a) receptors and its downstream target Na⁺/H⁺ exchanger 3 (NHE3) in the proximal tubules in the development of two-kidney, 1-clip (2K1C) Goldblatt hypertension have not been investigated previously. The present study tested the hypothesis that deletion of the AT_1a receptor or NHE3 selectively in the proximal tubules of the kidney attenuates the development of 2K1C hypertension using novel mouse models with proximal tubule-specific deletion of AT_1a receptors or NHE3. 2K1C Goldblatt hypertension was induced by placing a silver clip (0.12 mm) on the left renal artery for 4 weeks in adult male wild-type (WT), global Agtr1a^−/−, proximal tubule (PT)-specific PT-Agtr1a^−/− or PT-Nhe3^−/− mice, respectively. As expected, telemetry blood pressure increased in a time-dependent manner in WT mice, reaching a maximal response by Week 3 (p < 0.01). 2K1C hypertension in WT mice was associated with increases in renin expression in the clipped kidney and decreases in the nonclipped kidney (p < 0.05). Plasma and kidney Ang II were significantly increased in WT mice with 2K1C hypertension (p < 0.05). Tubulointerstitial fibrotic responses were significantly increased in the clipped kidney (p < 0.01). Whole-body deletion of AT_1a receptors completely blocked the development of 2K1C hypertension in Agtr1a^−/− mice (p < 0.01 vs. WT). Likewise, proximal tubule-specific deletion of Agtr1a in PT-Agtr1a^−/− mice or NHE3 in PT-Nhe3^−/− mice also blocked the development of 2K1C hypertension (p < 0.01 vs. WT). Taken together, the present study provides new evidence for a critical role of proximal tubule Ang II/AT₁ (AT_1a)/NHE3 axis in the development of 2K1C Goldblatt hypertension.