Microvascular preservation and cardiomyocyte hyperplasia underlie adaptive right ventricle development in congenital heart disease-pulmonary arterial hypertension

Right ventricular (RV) failure is the primary cause of death among patients with pulmonary arterial hypertension (PAH). Patients with congenital heart disease-associated PAH (CHD-PAH) demonstrate improved outcomes compared with patients with other forms of PAH, which is related to the maintenance of an adaptively hypertrophied RV. In an ovine model of CHD-PAH, we aimed to elucidate the cellular, microvascular, and transcriptional adaptations to congenital pressure overload that support RV function. Fetal surgery was performed on late gestation lambs to insert an aortopulmonary graft, leading to a persistent congenital left-right shunt and RV pressure load. At 3 days and 4-6 wk of life, shunt RV myocardial structure, growth mechanisms, and transcriptomes were compared with age-matched control and unoperated fetal RV. At 4-6 wk of age, shunt lambs demonstrate significant RV enlargement (shunt 37.1 ± 7.6 g vs. control 15.9 ± 1.9 g, <i>P</i> < 0.001) but maintain stable microvascular density (fetal 3.0 ± 1.2 vs. shunt 2.9 ± 0.5 vs. control 3.1 ± 1.2 capillaries per 1,000 µm², <i>P</i> > 0.05). Shunt RV cardiomyocytes are significantly more numerous and smaller by cross-sectional area than age-matched controls (shunt 73.3 ± 11.0 µm² vs. control 99.2 ± 9.8 µm², <i>P</i> = 0.013). At 3 days, shunt RV cardiomyocytes show evidence of increased proliferative capacity and ongoing hyperplasia compared with controls. RNA sequencing analyses reveal a distinct transcriptomic profile in shunt RV consistent with a delay in terminal differentiation and metabolic adaptations to support adaptive function. This study provides novel insights into the roles of microvascular preservation and cardiomyocyte hyperplasia in the development of adaptive RV hypertrophy in CHD-PAH.<b>NEW & NOTEWORTHY</b> In this study, we utilize an ovine model of congenital heart disease-associated pulmonary arterial hypertension (CHD-PAH) to unveil the structural and transcriptional changes that underlie the maintenance of an adaptively hypertrophied right ventricle (RV). In CHD-PAH, the RV hypertrophies while maintaining microvascular density and expanding its cardiomyocyte population, structural and cellular adaptations that are critical to supporting RV function. These findings provide novel insight into myocardial growth mechanisms that are triggered by congenital pressure overload.