Modeling of calcium‐induced swelling in spatially divergent populations of cardiac mitochondria

Abstract

In addition to ATP synthesis, mitochondria are involved in high cytosolic calcium (Ca2+) waves necessary for the regulation of the cardiac excitation‐contraction coupling, a focal point in the determination of cardio‐pathophysiology. Depending on mitochondrial location within subcellular compartments, mitochondrial functions expands from the buffering of cytosolic Ca2+ necessary for cellular signaling to the initiation of cell death via necrosis or apoptosis depending on ATP availability. The exposure of mitochondria to Ca2+ plays a dual role in determining metabolic activity; the sequestering of low Ca2+ concentration ([Ca2+]) accompanied by a minor matrix swelling enhances ATP production through the upregulation in activity of TCA cycle enzymes, ETC complexes and oxidative phosphorylation. However, excessive matrix swelling at high [Ca2+] can induce the opening of non‐selective permeability transition pores (PTP), sustained (irreversible) opening of which allow for the release Ca2+ to propagate from one mitochondria to the other leading to the wide spread of mitochondrial swelling, thus initiating cell death. Studies have suggested that different mitochondrial sub‐population differ in functional properties such as membrane potential, calcium retention capacity (CRC), and ROS production. In this study, we designed a biophysical model that can explain the kinetics of the transition from a reversible to an irreversible Ca2+‐induced swelling state in heterogenic mitochondrial population including subsarcolemmal (SSM) and intermyofibrillar mitochondria (IFM) of cardiac cells.Model developmentIn this project, a heterogenic population of mitochondria were isolated and purified via differential centrifugation. CRC was determined by the application of numerous Ca2+ pulses until Ca2+ release via PTP opening measured through spectrophotometry. Kinetics model was developed utilizing the 4th order Runge‐Kutta approximation method and fitted to experimental mitochondrial swelling data using the least square method. Reversible and irreversible mitochondrial swelling induced by low and high [Ca2+] in isolated cardiac mitochondria have been measured. Reversibility of mitochondrial swelling was determined in the presence of EGTA, a Ca2+‐chelating agent. Based on experimental data, the development of a kinetic model that simulates the swelling dynamics at low and high [Ca2+] in cardiac mitochondria has been created with a correlation coefficient of r >0.95.ConclusionOur model describes transition from reversible to irreversible swelling in different subcellular populations of mitochondria that is important for understanding the role of mitochondrial heterogeneity in PTP‐induced cell death.Support or Funding InformationSupported by NHLBI NIH (SC1HL118669)