Time-resolved Equatorial X-ray diffraction studies of skinned muscle fibres during stretch and release

Equatorial X-ray diffraction patterns were recorded from small bundles of o ne to three chemically skinned frog sartorius muscle fibres (time resolutio n 250 mu s) during rapid stretch and subsequent release. In the relaxed sta te, the dynamic A-band lattice spacing change as a result of a 2% step stre tch (determined from the positions of the 10 and 11 reflections) resulted i n a 21% increase in lattice volume, while static studies of spacing and sar comere length indicated than an increase in volume of greater than or equal to 50% for the same length change. In rigor, stretch caused a lattice volu me decrease which was reversed by a subsequent release. In activated fibres (pCa 4.5) exposed to 10 mM 2,3-butanedione 2-monoxime (BDM), stretch was a ccompanied by a lattice compression exceeding that of constant volume behav iour, but during tension recovery, compression was partially reversed to le ave a net spacing change close to that observed in the relaxed fibre. In th e relaxed state, spacing changes were correlated with the amplitude of the length step, while in rigor and BDM states, spacing changes correlated more closely with axial force. This behaviour Is explicable in terms of two com ponents of radial force, one due to structural, constraints as seen in the relaxed state, and an additional component arising from cross-bridge format ion. The ratio of axial to radial force for a single thick filament resulti ng from a length step was four in rigor and BDM, but close to unity for the relaxed state. (C) 1999 Academic Press.