1H-nuclear magnetic resonance evidence for acto-myosin-dependent structural changes of the intracellular water of frog skeletal muscle fiber.

The proton-spin relaxation process of the intracellular water in intact-relaxed and skinned-rigor fibers of frog skeletal muscle was studied for slack and stretched fibers by use of 1H-nuclear magnetic resonance technique. The longitudinal and transverse proton-spin relaxation processes of the intracellular water of intact-relaxed and skinned-rigor fibers were composed of a single- and multi-exponential processes, respectively. The longitudinal relaxation process was almost the same in slack as well as in stretched fibers for both intact-relaxed and skinned-rigor fibers. On the other hand, the transverse relaxation process was slightly but significantly faster in stretched than in slack fibers in the case of skinned-rigor fibers while it was almost the same in slack and in stretched fibers in the case of intact-relaxed fibers. As the overlap between actin and myosin filaments is maximal in slack fibers and minimal in stretched fibers, these results indicate that the intracellular water located in the overlap region is less structured in rigor fibers than that in relaxed fibers. This suggests that the rigor crossbridge formation disrupts the structured water bound to myosin and actin filaments in muscle fiber.