Restricted ADP movement in cardiomyocytes: Cytosolic diffusion obstacles are complemented with a small number of open mitochondrial voltage-dependent anion channels
Päivo Simson, Natalja Jepihhina, Martin Laasmaa, Pearu Peterson, Rikke Birkedal, Marko Vendelin
J Mol Cell Cardiol, 2016 Aug; 97: 197-203.
- PMID: 27261153
- DOI: 10.1016/j.yjmcc.2016.04.012
Significance
Heart cells need to move energy-carrying molecules rapidly between mitochondria, which make ATP, and the proteins that use it. This study separates two sources of resistance to that movement: barriers in the cytoplasm and the outer membrane of mitochondria. Measurements and cell-specific models showed that both barriers restrict ADP and ATP to a similar extent, while only about 2% of mitochondrial VDAC channels are accessible to cytosolic ADP. The findings clarify how the internal organization of heart cells controls energy transfer and may help explain why impaired energy transport contributes to heart failure.
Abstract
Adequate intracellular energy transfer is crucial for proper cardiac function. In energy starved failing hearts, partial restoration of energy transfer can rescue mechanical performance. There are two types of diffusion obstacles that interfere with energy transfer from mitochondria to ATPases: mitochondrial outer membrane (MOM) with voltage-dependent anion channel (VDAC) permeable to small hydrophilic molecules and cytoplasmatic diffusion barriers grouping ATP-producers and -consumers. So far, there is no method developed to clearly distinguish the contributions of cytoplasmatic barriers and MOM to the overall diffusion restriction. Furthermore, the number of open VDACs in vivo remains unknown. The aim of this work was to establish the partitioning of intracellular diffusion obstacles in cardiomyocytes. We studied the response of mitochondrial oxidative phosphorylation of permeabilized rat cardiomyocytes to changes in extracellular ADP by recording 3D image stacks of NADH autofluorescence. Using cell-specific mathematical models, we determined the permeability of MOM and cytoplasmatic barriers. We found that only ~2% of VDACs are accessible to cytosolic ADP and cytoplasmatic diffusion barriers reduce the apparent diffusion coefficient by 6-10×. In cardiomyocytes, diffusion barriers in the cytoplasm and by the MOM restrict ADP/ATP diffusion to similar extents suggesting a major role of both barriers in energy transfer and other intracellular processes.
Keywords: Intracellular diffusion obstacles; Oxidative phosphorylation; Rat cardiomyocytes; Regulation; Voltage-dependent anion channel VDAC.