Angiotensin II type 1 receptor-mediated reduction of angiotensin-converting enzyme 2 activity in the brain impairs baroreflex function in hypertensive mice

Abstract

Angiotensin-converting enzyme 2 (ACE2), a new component of the brain renin-angiotensin system, has been suggested to participate in the central regulation of blood pressure (BP). To clarify the relationship between ACE2 and other brain renin-angiotensin system components, we hypothesized that central angiotensin II type 1 receptors reduce ACE2 expression/activity in hypertensive mice, thereby impairing baroreflex function and promoting hypertension. To test this hypothesis, chronically hypertensive mice (RA) with elevated angiotensin II levels were treated with losartan (angiotensin II type 1 receptor blocker) or PD123319 (angiotensin II type 2 antagonist; 10 mg/kg per day, SC) for 2 weeks. Baseline spontaneous baroreflex sensitivity and brain ACE2 activity were dramatically decreased in RA compared with nontransgenic mice, whereas peripheral ACE2 activity/expression remained unaffected. Losartan, but not PD123319, increased central ACE2 activity, spontaneous baroreflex sensitivity, and normalized BP in RA mice. To confirm the critical role of central ACE2 in BP regulation, we generated a triple-transgenic model with brain ACE2 overexpression on a hypertensive RA background. Triple-transgenic-model mice exhibit lower BP and blunted water intake versus RA, suggesting lower brain angiotensin II levels. Moreover, the impaired spontaneous baroreflex sensitivity, parasympathetic tone, and increased sympathetic drive, observed in RA, were normalized in triple-transgenic-model mice. These data suggest that angiotensin II type 1 receptors inhibit ACE2 activity in RA mice brain, thus contributing to the maintenance of hypertension. In addition, overexpression of ACE2 in the brain reduces hypertension by improving arterial baroreflex and autonomic function. Together, our data suggest that angiotensin II type 1 receptor-mediated ACE2 inhibition impairs baroreflex function and support a critical role for ACE2 in the central regulation of BP and the development of hypertension.

Figure 1

Losartan normalizes BP and SBRS in RA mice. A, Typical BP recording traces in conscious NT and RA mice. B, Baseline MAP was significantly higher in RA vs NT mice (n=12; ***P<0.001); chronic infusion of the selective AT₁R antagonist losartan (10 mg/kg per day SC) normalized MAP in RA (n=12; ‡P<0.001 vs RA+saline) and slightly decreased MAP in NT mice (n=12; †P<0.05 vs NT+saline). The AT₂R antagonist PD123319 (10 mg/kg per day SC) did not affect MAP in any strain (n=12; P>0.05 vs saline). C, Baseline SBRS was significantly lower in RA vs NT mice (n=12; ***P<0.001). Chronic infusion of losartan restored SBRS only in RA mice (‡P<0.001 vs RA+saline), whereas AT₂R blockade did not affect SBRS in NT or RA mice.

Figure 2

ACE2 expression and activity in RA mice. Western blotting showed no changes in mACE2 protein expression in the brain (A) or kidney (B) of RA vs NT mice (n=4; P>0.05). ACE2 activity was significantly decreased in the brain of RA (C) (n=4; *P<0.05) but not altered in the kidney (D; n=4; P>0.05) when compared with NT littermates.

Figure 3

Effects of ATR blockade on ACE2 protein expression and activity in the brain of RA mice. A, Typical Western blot and quantified data showing that protein expression of mACE2 in the brain of RA mice was not affected by losartan or PD123319 (10 mg/kg per day sc, 14 days; n=4; P>0.05 vs RA). B, ACE2 activity in the brain of RA mice was significantly increased after AT₁R blockade (n=4; *P<0.05 vs RA) but not altered by the AT₂R antagonist PD123319 (n=4; P>0.05 vs RA).

Figure 4

ACE2 overexpression attenuates Ang II-mediated responses in SARA mice. The dramatic BP (A) and water intake (B) increases observed in RA (n=12; ***P<0.001 vs NT) were significantly attenuated in SARA mice (n=12; ‡P<0.01 vs RA). Moreover, the impaired SBRS (C), reduced parasympathetic tone (D), and enhanced sympathetic drive (E) observed in RA (n=12; ***P<0.001 and **P<0.01 vs NT) were fully normalized by ACE2 overexpression in the brain (n=12; †P<0.05 vs RA). Intrinsic heart rate (F) shows no difference between genotypes (n=12; P>0.05).