STRAIN DEPENDENCE OF THE ELASTIC PROPERTIES OF FORCE-PRODUCING CROSS-BRIDGES IN RIGOR SKELETAL-MUSCLE

Stretch and release experiments carried out an skinned single fibers o f frog skeletal muscle under rigor conditions indicate that the elasti c properties of the fiber depend on strain. For modulation frequencies below 1000 Hz, the results show an increase in Young's modulus of 20% upon a stretch of 1 nm/half-sarcomere. Remarkably, the strain depende nce of Young's modulus decreases at higher frequencies to about 10% up on a 1-nm/half-sarcomere stretch at a modulation frequency of 10 kHz. This suggests that the cause of the effect is less straightforward tha n originally believed: a simple slackening of the filaments would resu lt in an equally large strain dependence at all frequencies, whereas s train-dependent properties of the actin filaments should show up mast clearly at higher frequencies. We believe that the reduction of the st rain dependence points to transitions of the cross-bridges between dis tinct force-producing states. This is consistent with the earlier obse rvation that Young's modulus in rigor increases toward higher frequenc ies