Analysis of molecular movement reveals latticelike obstructions to diffusion in heart muscle cells

Illaste A, Laasmaa M, Peterson P, Vendelin M

Biophys. J. 2012 Feb;102(4):739-48

PMID: 22385844

Abstract

molecular_movement_in_cardiomyocyte

Red and green vehicles represent molecules with different diffusion coefficients: small molecules are represented by a fast red car and big molecules by a large green truck. In solution surrounding the cell (on the highway), diffusion is unhindered and the diffusion coefficient of smaller molecules (fluorescent ATP in this case) is almost 4 times higher than of a larger molecule (fluorescent Dextran 10K). In the cardiomyocyte, diffusion restrictions imposed by intracellular structures lead to considerably smaller difference between estimated diffusion coefficients (1.7 times). (Author of image: Erik Illaste)

Intracellular diffusion in muscle cells is known to be restricted. Although characteristics and localization of these restrictions is yet to be elucidated, it has been established that ischemia-reperfusion injury reduces the overall diffusion restriction. Here we apply an extended version of raster image correlation spectroscopy to determine directional anisotropy and coefficients of diffusion in rat cardiomyocytes. Our experimental results indicate that diffusion of a smaller molecule (1127 MW fluorescently labeled ATTO633-ATP) is restricted more than that of a larger one (10,000 MW Alexa647-dextran), when comparing diffusion in cardiomyocytes to that in solution. We attempt to provide a resolution to this counterintuitive result by applying a quantitative stochastic model of diffusion. Modeling results suggest the presence of periodic intracellular barriers situated ∼1 μm apart having very low permeabilities and a small effect of molecular crowding in volumes between the barriers. Such intracellular structuring could restrict diffusion of molecules of energy metabolism, reactive oxygen species, and apoptotic signals, enacting a significant role in normally functioning cardiomyocytes as well as in pathological conditions of the heart.