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.
Relaxed Fiber Before Treatment 1
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052212_f3_A.dat
Relaxed Fiber Before Treatment 2
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052212_f4_A.dat
Relaxed Fiber Before Treatment 3
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052312_f1_A.dat
Relaxed Fiber Before Treatment 4
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052412_f1_A.dat
Relaxed Fiber Before Treatment 5
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052412_f3_A.dat
Relaxed Fiber Before Treatment 6
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations
DA_052512_f1_A.dat
Activated Fiber Before Treatment 1
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052212_f2_A.dat
Activated Fiber Before Treatment 2
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052212_f3_B.dat
Activated Fiber Before Treatment 3
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052212_f4_B.dat
Activated Fiber Before Treatment 4
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052312_f1_B.dat
Activated Fiber Before Treatment 5
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052412_f1_B.dat
Activated Fiber Before Treatment 6
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052412_f3_B.dat
Activated Fiber Before Treatment 7
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052512_f2_A.dat
Activated Fiber Before Treatment 8
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations
DA_052512_f4_A.dat
Activated Fiber After Treatment 1
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations following treatment with blebbistatin and EDTA
DA_052512_f4_G.dat
Activated Fiber After Treatment 2
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for an activated muscle fiber experiencing sinusoidal oscillations following treatment with blebbistatin and EDTA
DA_052312_f1_H.dat
Relaxed Fiber After Treatment 1
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations following treatment with blebbistatin and EDTA
DA_052512_f4_H.dat
Relaxed Fiber After Treatment 2
This is data from the 600A Digital Controller (Aurora Scientific Inc.) for a relaxed muscle fiber experiencing sinusoidal oscillations following treatment with blebbistatin and EDTA
DA_052312_f1_I.dat