Angiotensin II receptor imbalance associated with neonatal cardiac growth restriction is a prelude to adult cardiac hypertrophy

The Hypertrophic Heart Rat (HHR) displays spontaneous cardiomyocyte hypertrophy in association with an apparent reduction in myocyte number in adulthood. This suggests the possibility of reduced hyperplasia or increased apoptosis during early cardiac development. The angiotensin AT1 and AT2 receptor subtypes have been implicated in both cellular growth and apoptosis, but the precise mechanisms are unclear. The aim of this study was to determine the relationship between cardiac AngII receptor expression levels and neonatal cardiomyocyte growth and apoptotic responses in the HHR compared with the Normal Heart Rat (NHR) control strain. Cardiac tissues were freshly harvested from male HHR and NHR at several developmental stages (p2 and 4, 6, 8, 12wks). HHR cardiac weight indices were considerably smaller than NHR at day 2 (4.330.19 vs 5.010.08 mg/g), but ‘caught-up’ to NHR by 4 weeks (5.100.15 vs 5.160.11 mg/g). By 12 weeks, HHR hearts were 27% larger than NHR. Tissue AT1A and AT2 mRNA expression levels were quantified by real-time RT-PCR. Relative to NHR, HHR neonatal hearts exhibited a 4.6-fold higher AT2/AT1 mRNA expression ratio. Cultured neonatal cardiomyocytes were infected with AT1A and/or AT2 receptor-expressing adenoviruses to achieve a physiological level of receptor expression (150 fmol receptor protein/mg total cell protein). In addition, to emulate receptor expression in neonatal HHR hearts, cells were co-infected with AT1A and AT2 receptors at a 4:1 ratio. Apoptosis incidence was studied by morphological analysis after 72 hours exposure to 0.1 M AngII. When infected with the AT1A receptor alone, a higher proportion of HHR myocytes appeared apoptotic than NHR (22.7 4.1% vs 1.1 0.6%, P 0.001). This implies that intrinsic differences predispose HHR cells to accentuated AT1-mediated apoptosis. Interestingly, the bax-1/bcl-2 mRNA expression ratio was significantly higher (50%) in HHR neonatal hearts. When cells were co-infected with AT1A and AT2 receptors, evidence of apoptosis in HHR cells virtually disappeared (0.4 0.1%). These findings suggest a novel capacity of AT2 receptors to counteract accentuated AT1A receptor-induced apoptosis in the HHR in early cardiac growth.