Changes in the filament lattice of skeletal muscle fibres induced by changes in osmotic pressure, sarcomere length and the number of crossbridges

Abstract

Equatorial small-angle X-ray diffraction patterns offer the possibility of obtaining detailed knowledge about interfilament distance and mass distribution of the myofilament lattice of skeletal muscle. These patterns of skinned frog-muscle fibres have been studied as a function of changes in the incubation solution using the Synchrotron Radiation Source in Daresbury (England). The results of these X-ray diffraction measurements and the results of measurements of the elastic properties of single muscle fibres, with microsecond time resolution, enabled an insight to be gained into the mechanism of muscle contraction. Interfilament distance decreased with increasing osmotic pressure in the absence of free calcium, whereas the mass distribution within the filament lattice did not change. Similar changes in the interfilament distance could be obtained either by decreasing the ionic strength in the absence of free calcium or by increasing the concentration of free calcium of the incubation solution. In both cases, however, mass distribution within the filament lattice also changed, which indicates that under these conditions the number of operating actomyosin complexes in the muscle fibre (crossbridges) tends to increase. The changes in interfilament distance and mass distribution that occur which depend on the free calcium concentration can be explained by a changed number of force-generating crossbridges. In the case of changes induced by ionic strength two effects interfere. Only part of the change in lattice spacing can be explained by an increased number of weakly attached crossbridges; the remaining part is caused by a process similar to that in the case of osmotic pressure. The effect of weakly attached crossbridges in the absence of free calcium under normal ionicstrength conditions is small. Therefore, the large decrement in interfilament distance due to an increased sarcomere length, in the absence of free calcium, is not caused by weakly attached crossbridges. Changes in sacromere length, in the presence of free calcium, resulted in changes in the number of force-generating crossbridges. However, the observed effect on interfilament distance cannot be explained solely by the variation of the number of forcegenerating crossbridges. The results of X-ray diffraction patterns and stiffness measurements of fibres in rigor solutions [absence of ATP (adenosine triphosphate) and calcium] and fibres in calcium solutions indicated that the (force-generating) crossbridges in the latter differ from crossbridges in rigor fibres.