Time-resolved X-ray diffraction studies of myosin head movements in live frog sartorius muscle during isometric and isotonic contractions

Summary

Using the facilities at the Daresbury Synchrotron Radiation Source, meridional diffraction patterns of muscles at ca 8°C were recorded with a time resolution of 2 or 4 ms. In isometric contractions tetanic peak tension (P ₀) is reached in ca 400 ms. Under such conditions, following stimulation from rest, the timing of changes in the major reflections (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 immediately after stimulation, but that of K ₂, K ₃ and K ₄ is delayed by a latent period of ca 16 ms. Relative to the end of their own latent periods 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 rise. From an analysis of the data we conclude that K ₁ reflects thin filament activation which involves the troponin system; K ₂ arises from an order-disorder transition during which the register between the filaments 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 orientation of the myosin heads (relative to thin filament axis) which is estimated to be from 65–70° at rest to ca 90° at P ₀. Isotonic contraction experiments showed that during shortening under a load of ca 0.27 P ₀, at least 85% of the heads (relative to those forming an acto-myosin complex at P ₀) diffract with actin-based periodicities, 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 with 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 production, but rather the result of it. Analysis of the data reveals that independent of load, the extent of asynchronous axial motions executed by most of the cycling heads is no more than 0.5–0.65 nm greater than at rest. To account for the diffraction data in terms of the conventional 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 motions demanded by that model. Therefore we conclude either that the required information is not available in our patterns or that an alternative hypothesis for contraction has to be developed.