Objective: Renin-angiotensin system inhibitors transiently induce apoptosis at the onset of cardiac hypertrophy regression in spontaneously hypertensive rats (SHRs). The focus of this study is to evaluate the cell selectivity of this response.
Methods: SHRs were treated with valsartan or enalapril (30 mg kg(-1) day(-1)) or placebo for 1 to 4 weeks. Stereological and morphological data were obtained from immunohistological analyses. Apoptosis was quantified by DEVDase (caspase-3-like) activity assay and immunoblot analysis of apoptosis-regulatory proteins (Bax and Bcl-2). Identification of the apoptotic cell type was conducted using in situ TUNEL labeling, in conjunction with alpha-sarcomeric actin or lectin immunoreactivity as markers for cardiomyocytes and endothelial cells, respectively.
Results: Stereological analysis of the left ventricle revealed significant non-cardiomyocyte hyperplasia in placebo-treated SHRs (239+/-29x10(6) nuclei) as compared to untreated age-matched normotensive Wistar-Kyoto (WKY) rats (107+/-12x10(6)). In contrast, the number of cardiomyocyte nuclei was comparable between untreated SHRs (48+/-4x10(6)) and WKY rats. After 4 weeks of valsartan or enalapril treatment, SHRs showed significant reductions in systolic blood pressure (>28%), left ventricular hypertrophy (>9%) and cardiomyocyte cross-sectional area (>17%). Moreover, these treatments abolished non-cardiomyocyte hyperplasia in SHR left ventricle without affecting cardiomyocyte number, capillary density or number of capillary per cardiomyocyte nucleus. As a mechanism of cell deletion consistent with apoptosis induction, ventricles showed increased caspase-3 activation (>4.5-fold) as well as Bax to Bcl-2 protein ratio (>3.2-fold) within 2 weeks of valsartan or enalapril treatment. Immunohistological analysis revealed a significant increase in TUNEL-positive, lectin-negative non-cardiomyocytes, suggesting a rise in apoptotic interstitial fibroblasts in the left ventricle within 2 weeks of treatment with valsartan or enalapril (>63%), with a return to baseline (0.033+/-0.003%) at 4 weeks. Treatments did not affect right ventricular mass, apoptosis or cellularity.
Conclusion: Cardiac apoptosis induction during regression of left ventricular hypertrophy reverses interstitial fibroblast hyperplasia in SHRs treated with inhibitors of the renin-angiotensin system.