Changes in the 5.9 nm actin layer-line on activation of frog skeletal muscles.

The intensity changes of the actin layer-lines during activation of sartorius and overstretched semitendinosus muscles were measured by using a two-dimensional X-ray detector. In both muscles, the intensity of the 5.9 nm layer-line increased without a detectable shift in the peak position. The intensity of the second layer-line increased in both muscles by a similar amount. In semitendinosus muscle, a large intensity decrease of the first actin layer-line was observed. Model calculations suggest that these results are difficult to explain in terms of a simple steric blocking model of regulation involving only a movement of tropomyosin on the actin helix.