Nuclear magnetic resonance transverse relaxation times of water protons in skeletal muscle.

The observation of the spin-echo decay in a long time domain has revealed that there exist at least three different fractions of non- (or slowly) exchanging water in the rat gastrocnemius muscle. These fractions of water are characterized with different nuclear magnetic resonance (NMR) relaxation times and are identified with the different parts of tissue water. The water associated with the macromolecules was found to be approximately 8% of the total tissue water and not to exchange rapidly with the rest of the intracellular water. The transverse relaxation time (T(2)) of the myoplasm is 45 ms which is roughly a 40-fold reduction from that of a dilute electrolyte solution. This fraction of water accounts for 82% of the tissue water. The reduced relaxation time is shown neither to be caused by fast exchange between the hydration and myoplasmic water nor by the diffusion of water across the local magnetic field gradients which arise from the heterogeneity in the sample. About 10% of the tissue water was resolved to be associated with the extracellular space, the relaxation time of which is approximately four times that of the myoplasm. Mathematical treatments of the proposed mechanisms which may be responsible for the reduction of tissue water relaxation times are given in this paper. The results of our study are consistent with the notion that the structure and/or motions of all or part of the cellular water are affected by the macromolecular interface and this causes a change in the NMR relaxation rates.