Increased Endogenous Activity of the Renin-Angiotensin System Reduces Infarct Size in the Rats with Early Angiotensin II-dependent Hypertension which Survive the Acute Ischemia/Reperfusion Injury

Abstract

We investigated the role of the interaction between hypertension and the renin-angiotensin system in the pathophysiology of myocardial ischemia/reperfusion injury. We hypothesized that in the early phase of angiotensin II (ANG II)-dependent hypertension with developed left ventricular hypertrophy, cardioprotective mechanism(s) are fully activated. The experiments were performed in transgenic rats with inducible hypertension, noninduced rats served as controls. The early phase of ANG II-dependent hypertension was induced by five-days (5 days) dietary indole-3-carbinol administration. Cardiac hypertrophy, ANG II and ANG 1-7 levels, protein expression of their receptors and enzymes were determined. Separate groups were subjected to acute myocardial ischemia/reperfusion injury, and infarct size and ventricular arrhythmias were assessed. Induced rats developed marked cardiac hypertrophy accompanied by elevated ANG levels. Ischemia/reperfusion mortality was significantly higher in induced than noninduced rats (52.1 and 25%, respectively). The blockade of AT1 receptors with losartan significantly increased survival rate in both groups. Myocardial infarct size was significantly reduced after 5 days induction (by 11%), without changes after losartan treatment. In conclusion, we confirmed improved cardiac tolerance to ischemia/reperfusion injury in hypertensive cardiohypertrophied rats and found that activation of AT1 receptors by locally produced ANG II in the heart was not the mechanism underlying infarct size reduction.

Keywords: angiotensin II receptor antagonist; hypertension; infarct size; ischemia/reperfusion injury; renin-angiotensin system.

FIGURE 1

The ratio of left ventricle weight to tibia length (A) and heart angiotensin II (ANG II) levels (B) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM. *p < 0.05 vs. untreated noninduced rats; ^# p < 0.05 vs. untreated group at the same time point; ^@ p < 0.05 vs. untreated 12 h I3C-induced rats. Multiple-group comparisons were performed by regular two-way analysis of variance followed by Tukey‘s post hoc test.

FIGURE 2

Plasma angiotensin II (ANG II) levels (A) and kidney ANG II levels (B) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM. *p < 0.05 vs. untreated noninduced rats; ^# p < 0.05 vs. untreated group at the same time point; ^@ p < 0.05 vs. untreated 12 h I3C-induced rats. Multiple-group comparisons were performed by regular two-way analysis of variance followed by Tukey’s post hoc test.

FIGURE 3

Plasma angiotensin 1–7 (ANG 1–7) levels (A) and kidney ANG 1–7 levels (B) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM. *p < 0.05 vs. untreated noninduced rats; ^# p < 0.05 vs. untreated group at the same time point; ^@ p < 0.05 vs. untreated 12 h I3C-induced rats. Multiple-group comparisons were performed by regular two-way analysis of variance followed by Tukey’s post hoc test.

FIGURE 4

Protein expressions of renal AT1 (A) and Mas (B) receptors, ACE (C) and ACE2 (D) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM. Multiple-group comparisons were performed by regular two-way analysis of variance followed by Tukey’s post hoc test.

FIGURE 5

Mortality (A), myocardial area at risk normalized to the size of the left heart ventricle (B) and infarct size presented as per cent of the area at risk (C) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM. *p < 0.05 vs. untreated noninduced rats. Data were analyzed by Fischer’s exact test (mortality) and by One-way ANOVA with Student-Neumann-Keuls test (relative size of the area at risk and infarct size).

FIGURE 6

Ischemic arrhythmias: the score of ischemic arrhythmias (A), the incidence of ventricular fibrillation, both reversible and sustained (B) and, duration of reversible ventricular fibrillation (VF) (C) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM or as median with interquartile range. *p < 0.05 vs. untreated noninduced rats; #p < 0.05 vs. untreated group at the same time point. Data were analyzed by Fischer’s exact test (incidence of VF) and by one-way ANOVA or ANOVA for repeated measurements with Student-Neumann-Keuls test in normally distributed variables (score of ischemic arrhythmias). Differences in the duration of reversible VF between the groups were compared by the Kruskal–Wallis nonparametric test.

FIGURE 7

Reperfusion arrhythmias: the score of reperfusion arrhythmias (A) and the number of reperfusion premature ventricular complexes (PVCs) (B) in untreated and losartan-treated noninduced (NI) and I3C-induced Cyp1a1-Ren-2 transgenic rats. Values are expressed as means ± SEM or as a median with interquartile range. *p < 0.05 vs. untreated noninduced rats. Data were analyzed by one-way ANOVA or ANOVA for repeated measurements with Student-Neumann-Keuls test in normally distributed variables (score of reperfusion arrhythmias). Differences in the number of PVCs between the groups were compared by the Kruskal–Wallis nonparametric test.