Diffusional anisotropy is induced by subcellular barriers in skeletal muscle

The time- and orientational-dependence of phosphocreatine (PCr) diffusion w as measured using pulsed-field gradient nuclear magnetic resonance (PFG-NMR ) as a means of non-invasively probing the intracellular diffusive barriers of skeletal muscle. Red and white skeletal muscle from fish was used becau se fish muscle cells are very large, which facilitates the examination of d iffusional barriers in the intracellular environment, and because they have regions of very homogeneous fiber type. Fish were cold-acclimated (5 degre es C) to amplify the contrast between red and white fibers. Apparent diffus ion coefficients, D, were measured axially, D-parallel to, and radially, D- perpendicular to in small muscle strips over a time course ranging from 12 to 700 ms. Radial diffusion was strongly time dependent in both fiber types , and D decreased with time until a steady-state value was reached at a dif fusion time similar or equal to 100 ms. Diffusion was also highly anisotrop ic, with D-parallel to being higher than D-perpendicular to for all time po ints. The time scale over which changes in D-perpendicular to occurred indi cated that the observed anisotropy was not a result of interactions with th e thick and thin filament lattice of actin and myosin or restriction within the cylindrical sarcolemma, as has been previously suggested. Rather, the sarcoplasmic reticulum (SR) and mitochondria appear to be the principal int racellular structures that inhibit mobility in an orientation-dependent man ner. This work is the first example of diffusional anisotropy induced by re adily identifiable intracellular structures. Copyright (C) 1999 John Wiley & Sons, Ltd.