The time-and orientational-dependence of phosphocreatine (PCr) diffusion was 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 because fish muscle cells are very large, which facilitates the examination of diffusional barriers in the intracellular environment, and because they have regions of very homogeneous fiber type. Fish were cold-acclimated (5°C) to amplify the contrast between red and white fibers. Apparent diffusion coefficients, D, were measured axially, D k , and radially, D c , 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 diffusion time 9100 ms. Diffusion was also highly anisotropic, with D k being higher than D c for all time points. The time scale over which changes in D c occurred indicated that the observed anisotropy was not a result of interactions with the 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 intracellular structures that inhibit mobility in an orientation-dependent manner. This work is the first example of diffusional anisotropy induced by readily identifiable intracellular structures.
The time‐ and orientational‐dependence of phosphocreatine (PCr) diffusion was 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 because fish muscle cells are very large, which facilitates the examination of diffusional barriers in the intracellular environment, and because they have regions of very homogeneous fiber type. Fish were cold‐acclimated (5°C) to amplify the contrast between red and white fibers. Apparent diffusion coefficients, D, were measured axially, D∥, and radially, D⟂, 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 diffusion time ≊ 100 ms. Diffusion was also highly anisotropic, with D∥ being higher than D⟂ for all time points. The time scale over which changes in D⟂ occurred indicated that the observed anisotropy was not a result of interactions with the 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 intracellular structures that inhibit mobility in an orientation‐dependent manner. This work is the first example of diffusional anisotropy induced by readily identifiable intracellular structures. Copyright © 1999 John Wiley & Sons, Ltd.
The structure of polyacrylamide gels was studied using proton spin-lattice relaxation and PFG diffusion methods. Polyacrylamide gels, with total polymer concentrations ranging from 0.25 to 0.35 g/ml and crosslinker concentrations from 0 to 10% by weight, were studied. The data showed no effect of the crosslinker concentration on the diffusion of water molecules. The Ogston-Morris and Mackie-Meares models fit the general trends observed for water diffusion in gels. The diffusion coefficients from the volume averaging method also fit the data, and this theory was able to account for the effects of water-gel interactions that are not accounted for in the other two theories. The averaging theory also did not require the physically unrealistic assumption, required in the other two theories, that the acrylamide fibers are of similar size to water molecules. Contrary to the diffusion data, T 1 relaxation measurements showed a significant effect of crosslinker concentration on the relaxation of water in gels. The model developed using the Bloch equations and the volume averaging method described the effects of water adsorption on the gel medium on both the diffusion coefficients and the relaxation measurements. In the proposed model the gel medium was assumed to consist of three phases (i.e., bulk water, uncrosslinked acrylamide fibers, and a bisacrylamide crosslinker phase). The effects of the crosslinker concentration were accounted for by introducing the proton partition coefficient, K eq , between the bulk water and crosslinker phase. The derived relaxation equations were successful in fitting the experimental data. The partition coefficient, K eq , decreased significantly as the crosslinker concentration increased from 5 to 10% by weight. This trend is consistent with the idea that bisacrylamide tends to form hydrophobic regions with increasing crosslinker concentration.
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