X-ray diffraction studies on muscle during rapid shortening and their implications concerning crossbridge behaviour

In isometric contraction, a high proportion of crossbridges are always in the attached state and crossbridge cycling is slow. During shortening, crossbridges must be entering the detached state at a higher rate, as they come to the end of their working strokes. The size of the population of detached crossbridges will then depend on the re-attachment rate and it is therefore of some interest to find out whether a significant detached population can be detected. Observations on the equatorial X-ray diffraction pattern indicate that this is the case at higher speeds of shortening, for example at a speed where the detachment rate must be of the order of 200 per second. In a muscle under these conditions, the 59 A and 51 A actin layer line reflections decrease in intensity compared to their values during isometric contraction. This decrease does not appear to be associated with a change in structure of the actin-troponin-tropomyosin complex, since the second actin layer-line reflection remains virtually unchanged in intensity. Thus the change is likely to arise from either a different total number of attached crossbridges, or a different number of attached crossbridges in the tension generating state. The result provides some further evidence for specific helical labelling of the actin structure by crossbridges during contraction, as do some recent electronmicroscope studies of rapidly frozen contracting muscle.