Currently, there are no tailored therapies available for the treatment of right ventricular (RV) hypertrophy, and the cellular mechanisms that underlie the disease are poorly understood. We investigated the cellular changes that occur early in the progression of the disease, when RV hypertrophy is evident, but prior to the onset of heart failure. Intracellular Ca2+ ([Ca2+]i) handling was examined in a rat model of monocrotaline (MCT)-induced pulmonary hypertension and subsequent RV hypertrophy. [Ca2+]i and stress production were measured in isolated RV trabeculae under baseline conditions (1-Hz stimulation, 1.5 mM [Ca2+]o, 37 °C), and in response to inotropic interventions (5-Hz stimulation or 1-μM isoproterenol). Under baseline conditions, MCT trabeculae had impaired Ca2+ release in response to stimulation with a 45% delay in the time-to-peak Ca2+, but there was no difference in the amplitude and decay of the Ca2+ transient, or active stress relative to RV trabeculae from normotensive hearts (CON). Increasing stimulation frequency from 1 to 5 Hz increased stress in CON, but not MCT trabeculae. Similarly, β-adrenergic stimulation with isoproterenol increased Ca2+ transient amplitude and active stress in CON, but not in MCT trabeculae, despite accelerating Ca2+ transient decay in trabeculae from both groups. During isoproterenol treatment, MCT trabeculae showed increased diastolic Ca2+ leak, which may explain the blunted inotropic response to β-adrenergic stimulation. Confocal imaging of trabeculae fixed following functional measurements showed that myocytes were on average wider, and transverse-tubule organisation was disrupted in MCT which provides a mechanism to explain the observed slower release of Ca2+.
Keywords: Intracellular calcium; Pulmonary hypertension; Right ventricular hypertrophy; Transverse-tubules; β-adrenergic stimulation.