LAUSR

Simple Summary In pulmonary artery hypertension, right ventricular (RV) afterload is increased, which requires the cardiomyocytes to contract with greater force against the additional pulmonary artery pressure. In response, RV cardiomyocytes increase contractile protein content to maintain greater workload, consuming larger amounts of energy (supplied by the mitochondria) on a beat-to-beat basis. Failing hearts have been described as an “engine out of fuel”, but it is unclear how the mitochondria match ATP supply to demand in hypertrophic hearts prior to failure. Therefore, our aims were (i) to measure beat-to-beat mitochondrial Ca2+ fluxes, and (ii) to determine mitochondrial abundance and function in hypertrophied cardiomyocytes prior to the onset of heart failure. To identify the early adaptive changes in energy supply prior to failure, we utilised a rat model of pulmonary artery hypertension to investigate RV cardiomyocytes during compensated hypertrophy in comparison to their normotensive controls. Mitochondrial Ca2+ fluxes were increased in hypertrophied cardiomyocytes, but no difference was found in oxidative phosphorylation between the groups. This suggests that the larger mitochondrial Ca2+ transients are a compensatory mechanism to match ATP supply to the increased energy demands of hypertrophic cardiomyocytes. Abstract Pulmonary artery hypertension causes right ventricular hypertrophy which rapidly progresses to heart failure with underlying cardiac mitochondrial dysfunction. Prior to failure, there are alterations in cytosolic Ca2+ handling that might impact mitochondrial function in the compensatory phase of RV hypertrophy. Our aims, therefore, were (i) to measure beat-to-beat mitochondrial Ca2+ fluxes, and (ii) to determine mitochondrial abundance and function in non-failing, hypertrophic cardiomyocytes. Male Wistar rats were injected with either saline (CON) or monocrotaline (MCT) to induce pulmonary artery hypertension and RV hypertrophy after four weeks. Cytosolic Ca2+ ([Ca2+]cyto) transients were obtained in isolated right ventricular (RV) cardiomyocytes, and mitochondrial Ca2+ ([Ca2+]mito) was recorded in separate RV cardiomyocytes. The distribution and abundance of key proteins was determined using confocal and stimulated emission depletion (STED) microscopy. The RV mitochondrial function was also assessed in RV homogenates using oxygraphy. The MCT cardiomyocytes had increased area, larger [Ca2+]cyto transients, increased Ca2+ store content, and faster trans-sarcolemmal Ca2+ extrusion relative to CON. The MCT cardiomyocytes also had larger [Ca2+]mito transients. STED images detected increased mitochondrial protein abundance (TOM20 clusters per μm2) in MCT, yet no difference was found when comparing mitochondrial respiration and membrane potential between the groups. We suggest that the larger [Ca2+]mito transients compensate to match ATP supply to the increased energy demands of hypertrophic cardiomyocytes.