Diffusion Restrictions in Cardiomyocytes from Low-Performance Heart

Abstract

In adult mammalian cardiomyocytes, intracellular diffusion restrictions affect metabolic regulation. Despite extensive studies on rat cardiomyocytes, their cause and role in vivo is still unclear. Intracellular membrane structures may play a role. Previous studies suggest that rainbow trout permeabilized cardiac fibers also have diffusion restrictions. This is surprising because rainbow trout cardiomyocytes are thinner and have fewer intracellular membrane structures than adult rat cardiomyocytes. However, results from fibers may be affected by incomplete separation of the cells. The aim of this study was to verify the existence of diffusion restrictions in trout cardiomyocytes by comparing ADP-kinetics of mitochondrial respiration in permeabilized fibers, permeabilized isolated cardiomyocytes and isolated mitochondria from rainbow trout heart. We developed a new solution specific for trout cardiomyocytes, where they retained their shape and showed stable steady state respiration rates. The apparent ADP-affinity of permeabilized cardiomyocytes was different from that of fibers. It was higher, independent of temperature and not increased by creatine. However, it was still about ten times lower than in isolated mitochondria. This suggests that intracellular diffusion of ADP is indeed restricted in trout cardiomyocytes. The difference between fibers and cardiomyocytes suggest that results from trout cardiac fibers were affected by incomplete separation of the cells. The lack of a creatine effect indicates that trout heart lacks mitochondrial creatine kinase tightly coupled to respiration. These results from rainbow trout cardiomyocytes are similar to those from neonatal mammalian cardiomyocytes. Thus, it seems that metabolic regulation is related to cardiac performance. It is likely that rainbow trout can be used as a model animal for further studies of the localization and role of diffusion restrictions in low-performance hearts. Next step will be to identify the contribution of mitochondrial outer membrane and cytosolic factors in intracellular diffusion restriction.