Submillisecond changes in myosin lattice spacing resulting from rapid length changes

The speed of the myofilament lattice spacing response to rapid changes in l oad or length of single, intact muscle fibres of the frog, was investigated during isometric tetani. During ramp releases at close to V-max and during step length changes (completed within 250 mu s), lattice spacing was calcu lated from the equatorial X-ray diffraction pattern (sampled at 250 mu s ti me resolution using synchrotron radiation). Ramp releases (total shortening = 1.39 %) caused a spacing increase, described with an exponential functio n (alpha = 271 s(-1), amplitude = 1.15 nm) plus an elastic component having the time course of discharge of axial tension (amplitude 0.28 nm). For a s tep release (amplitude = 0.87%), lattice expansion could be described with an exponential (alpha = 1005 s(-1), amplitude = 0.56 nm) plus an elastic co mponent of 0.25 nm amplitude. Lattice compression was associated with a ste p stretch (amplitude = 0.62%), and was also quasi-exponential (alpha = 367 s(-1), amplitude = 0.74nm), with an elastic component of 0.28 nm. The spaci ng change time course for length steps resembled that of the accompanying q uick recovery of axial tension and the associated change in the meridional 14.5 nm reflection intensity, which are both believed to be determined by t he kinetics of the molecular power stroke. Therefore, this shows that latti ce spacing changes, arising from radial forces exerted by attached crossbri dges, are fast enough to occur during the power stroke event. (C) 1999 Acad emic Press.