TIME-RESOLVED X-RAY-DIFFRACTION STUDIES OF MYOSIN HEAD MOVEMENTS IN LIVE FROG SARTORIUS MUSCLE DURING ISOMETRIC AND ISOTONIC CONTRACTIONS

Using the facilities at the Daresbury Synchrotron Radiation Source, me ridional diffraction patterns of muscles at ca 8 degrees C were record ed with a time resolution of 2 or 4 ms. In isometric contractions teta nic peak tension (P-0) is reached in ca 400 ms. Under such conditions, following stimulation from rest, the timing of changes in the major r eflections (the 38.2 nm troponin reflection, and the 21.5 and 14.34/14 .58 nm myosin reflections) can be explained in terms of four types of time courses: K-1,K-2,K-3, and K-4. The onset of K-1 occurs immediatel y after stimulation, but that of K-2, K-3, and K-4 is delayed by a lat ent period of ca 16 ms. Relative to the end of their own latent period s the half-times for K-1, K-2, K-3 and K-4 are 14-16, 16, 32 and 52 ms , respectively. In half-times, K-1, K-2, K-3 lead tension rise by 52, 36 and 20 ms, respectively. K-4 parallels the time course of tension r ise. From an analysis of the data we conclude that K-1 reflects thin f ilament activation which involves the troponin system; K-2 arises from an order-disorder transition during which the register between the fi laments is lost; K-3 is due to the formation of an acto-myosin complex which (at P-0) causes 70% or more of the heads to diffract with actin -based periodicities; and K-4 is caused by a change in the axial orien tation of the myosin heads (relative to thin filament axis) which is e stimated to be from 65-70 degrees at rest to ca 90 degrees at P-0. Iso tonic contraction experiments showed that during shortening under a lo ad of ca 0.27 P-0, at least 85% of the heads (relative to those formin g an acto-myosin complex at P-0) diffract with actin-based periodiciti es, whilst their axial orientation does not change from that at rest. During shortening under a negligible load, at most 5-10% of the heads (relative to those forming an acto-myosin complex at P-0) diffract wit h actin-based periodicities, and their axial orientation also remains the same as that at rest. This suggests that in isometric contractions the change in axial orientation is not the cause of active tension pr oduction, but rather the result of it. Analysis of the data reveals th at independent of load, the extent of asynchronous axial motions execu ted by most of the cycling heads is no more than 0.5-0.65 nm greater t han at rest. To account for the diffraction data in terms of the conve ntional tilting head model one would have to suppose that a few of the heads, and/or a small part of their mass perform the much larger moti ons demanded by that model. Therefore we conclude either that the requ ired information is not available in our patterns or that an alternati ve hypothesis for contraction has to be developed.