title: Thixotropy and rheopexy of muscle fibers probed using sinusoidal oscillations.
abstract: Length changes of muscle fibers have previously been shown to result in a temporary reduction in fiber stiffness that is referred to as thixotropy. Understanding the mechanism of this thixotropy is important to our understanding of muscle function since there are many instances in which muscle is subjected to repeated patterns of lengthening and shortening. By applying sinusoidal length changes to one end of single permeabilized muscle fibers and measuring the force response at the opposite end, we studied the history-dependent stiffness of both relaxed and activated muscle fibers. For length change oscillations greater than 1 Hz, we observed thixotropic behavior of activated fibers. Treatment of these fibers with EDTA and blebbistatin, which inhibits myosin-actin interactions, quashed this effect, suggesting that the mechanism of muscle fiber thixotropy is cross-bridge dependent. We modeled a half-sarcomere experiencing sinusoidal length changes, and our simulations suggest that thixotropy could arise from force-dependent cross-bridge kinetics. Surprisingly, we also observed that, for length change oscillations less than 1 Hz, the muscle fiber exhibited rheopexy. In other words, the stiffness of the fiber increased in response to the length changes. Blebbistatin and EDTA did not disrupt the rheopectic behavior, suggesting that a non-cross-bridge mechanism contributes to this phenomenon.
authors.affiliation: Department of Physics, Willamette University, Salem, Oregon, United States of America.
authors.forename: David
authors.initials: D
authors.lastname: Altman
authors.affiliation: Department of Kinesiology and Physical Education, McGill University, Montreal, Quebec, Canada.
authors.forename: Fabio C
authors.initials: FC
authors.lastname: Minozzo
authors.affiliation: Departments of Kinesiology and Physical Education, Physiology, and Physics, McGill University, Montreal, Quebec, Canada.
authors.forename: Dilson E
authors.initials: DE
authors.lastname: Rassier
mesh_terms: D000818:Animals; D018482:Muscle, Skeletal / Q000502:physiology*; D011817:Rabbits