CHANGES IN THE X-RAY-DIFFRACTION PATTERN FROM SINGLE, INTACT MUSCLE-FIBERS PRODUCED BY RAPID SHORTENING AND STRETCH

Changes in the x-ray diffraction patterns produced by 100-mu s-length steps imposed during tetanic stimulation were recorded from single int act fibers of frog tibialis anterior muscle. For shortening steps, a s taircase length change was applied, with a 20-ms interval between step s, For stretches, each 20-ms cycle started with a stretch, followed af ter 4 ms by shortening to the original length, Each shortening step in the staircase and each stretch in the stretch/shortening protocol pro duced a response similar to that of a single step from the isometric s teady state. The intensity of the 14.5-nm x-ray reflection arising fro m the axial repeat of the myosin heads along their filaments decreased after both shortening and stretch; this decrease was not accompanied by broadening along or across the meridian. The relationship between t he intensity after the length step and step amplitude was approximatel y linear for both stretches and shortening steps, extrapolating to zer o intensity for 11-nm stretches and 13-nm shortening steps, but there was no significant intensity change for the first similar to 2 nm of s hortening. These results are broadly consistent with conventional mode ls of muscle contraction in which myosin heads move through about 10 n m during the working stroke in the shortening direction, but an additi onal distortion of the myosin heads may be produced by a stretch.