Intracellular Diffusion Restrictions In Trout Cardiomyocytes

Abstract

Rat cardiomyocytes are compartmentalized by barriers that restrict intracellular diffusion of adenine nucleotides. The exact localization of these diffusion barriers is unknown. Some possible candidates for diffusion restriction are t-tubules, sarcoplasmic reticulum (SR) and outer mitochondrial membrane. Further, rat cardiomyocytes have several parallel rows of mitochondria and myofilaments wrapped in SR, and it is possible that peripheral mitochondria and SR restrict diffusion to more central parts of the cell. Diffusion is facilitated by the creatine kinase system. Trout cardiomyocytes lack t-tubules and have a much more sparse SR. Additionally, single cardiomyocytes have only one layer of myofilaments surrounding a central core of mitochondria. We take advantage of the structural differences between rat and trout cardiomyocytes to study intracellular diffusion restrictions further. We measured the apparent ADP-affinity of trout skinned ventricular fibres at different temperatures to cover the physiological range for rainbow trout. Measurements were performed in the absence and presence of creatine to test whether diffusion is facilitated by the creatine kinase system. Our results show that trout cardiomyocytes are characterized by a low ADP-affinity. The affinity is temperature-dependent and increases with temperature. Creatine increases affinity at all temperatures, but the affinity in the presence of creatine is also temperature-dependent. The low ADP-affinity suggests that diffusion restrictions also exist in trout cardiomyocytes despite their structural difference with much more sparse membrane structures. This makes trout cardiomyocytes a useful model to study intracellular diffusion restrictions further.