Differential mechanisms of ang (1-7)-mediated vasodepressor effect in adult and aged candesartan-treated rats

Abstract

Angiotensin (1-7) (Ang (1-7)) causes vasodilator effects in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) via angiotensin type 2 receptors (AT(2)R). However, the role of vascular AT(2)R in aging is not known. Therefore, we examined the effect of aging on Ang (1-7)-mediated vasodepressor effects and vascular angiotensin receptor localization in aging. Blood pressure was measured in conscious adult (~17 weeks) and aged (~19 months) normotensive rats that received drug combinations in a randomised fashion over a 4-day protocol: (i) Ang (1-7) alone, (ii) AT(1)R antagonist, candesartan, alone, (iii) Ang (1-7) and candesartan, or (iv) Ang-(1-7), candesartan, and the AT(2)R antagonist, PD123319. In a separate group of animals, the specific MasR antagonist, A779, was administered in place of PD123319. Receptor localisation was also assessed in aortic sections from adult and aged WKY rats by immunofluorescence. Ang (1-7) reduced blood pressure (~15 mmHg) in adult normotensive rats although this effect was dependant on the background dose of candesartan. This depressor effect was reversed by AT(2)R blockade. In aged rats, the depressor effect of Ang (1-7) was evident but was now inhibited by either AT(2)R blockade or MasR blockade. At the same time, AT(2)R, MasR, and ACE2 immunoreactivity was markedly elevated in aortic sections from aged animals. These results indicate that the Ang (1-7)-mediated depressor effect was preserved in aged animals. Whereas Ang (1-7) effects were mediated exclusively via stimulation of AT(2)R in adult WKY, with aging the vasodepressor effect of Ang (1-7) involved both AT(2)R and MasR.

Figure 1

Effect of the AT₁R Ang (1-7) (15 pmol/kg/min; 4-hour infusion depicted by full line), AT₁R antagonist, candesartan (0.01 mg/kg bolus IV; depicted by an arrow), saline (0.1 mL/kg 0.9% NaCl IV for 4 hours), and Ang (1-7) + candesartan on MAP in (a) adult WKY rats (n = 4) and (b) aged WKY rats (n = 7). Values represent mean ± SEM. ***P < 0.001, for treatment effect of Ang (1-7) + candesartan versus all other treatments (2-way RM ANOVA).

Figure 2

Effect of Ang (1-7) (15 pmol/kg/min; 4-hour infusion depicted by full line), AT₁R antagonist, candesartan (0.1 mg/kg bolus IV; depicted by an arrow), Ang (1-7) + candesartan, and Ang (1-7) + candesartan + AT₂R antagonist, PD123319 (50 μg/kg/min for 2 hours; depicted by dashed line), on MAP in (a) adult WKY rats (n = 6) and (b) aged WKY rats (n = 7). Values represent mean ± SEM. *P < 0.05; **P < 0.01; ***P < 0.001, for treatment effect of Ang (1-7) + candesartan versus all other treatments as indicated (2-way RM ANOVA).

Figure 3

Effect of Ang (1-7) (15 pmol/kg/min; 4-hour infusion depicted by full line), AT₁R antagonist, and candesartan in (a) adult WKY rats (0.1 mg/kg bolus IV; depicted by an arrow) and (b) aged WKY rats (0.01 mg/kg bolus IV; depicted by an arrow), together with Ang (1-7) + candesartan and Ang (1-7) + candesartan + MasR antagonist A779 (15 pmol/kg/min for 2 hours; depicted by dashed line), on MAP (n = 8 for both groups). Values represent mean ± SEM. (a) ***P < 0.001, for treatment effect of Ang (1-7) + candesartan versus Ang (1-7) or candesartan alone (2-way RM ANOVA), ^†<0.05; ^†††<0.001, for treatment effect of Ang (1-7) + candesartan +A779 versus candesartan or Ang (1-7) alone (2-way RM ANOVA). (b) **P < 0.01; ***<0.001 for treatment effect of Ang (1-7) + candesartan versus Ang (1-7), candesartan, or Ang (1-7) + candesartan + A779 (2-way RM ANOVA).

Figure 4

(a) Representative immunolocalisation images of AT₁R, AT₂R, MasR, and ACE2 in adult WKY rats and aged WKY rats. Mean data for aortic expression of the (b) AT₁R, (c) AT₂R, (d) MasR, and (e) ACE2 expressed as relative fluorescent units in adult (n = 5) and aged (n = 4) WKY rats. ***P < 0.001 versus adult WKY rats.

Figure 5

Ratios of (a) AT₂R : AT₁R, (b) MasR : AT₁R, and (c) ACE2 : AT₁R in adult and aged WKY rats. **P < 0.01; ***P < 0.001 versus adult WKY rats.

Figure 6

Schema depicting differential mechanisms of Ang (1-7)-mediated vasodepressor effect in adult and aged candesartan-treated rats. AT₁R expression was similar in aortae from adult and aged rats whereas there was upregulation of AT₂R, MasR, and ACE2. Therefore, a lower level of AT₁R blockade with candesartan (X) was required in aged animals (b) compared with adult animals (a) in order to unmask the vasodilator axis. MasR was not functionally active in adult rats.