X-ray diffraction studies on thermally induced tension generation in rigor muscle

Abstract

Muscle fibres in the rigor state and free of nucleotide contract if heated above their physiological working temperature. Kinetic studies on the mechanism of this process, termed rigor contraction, indicate that it has a number of features in common with the contraction of maximally Ca2+ activated fibres. De novo tension generation appears to be associated with a single, tension sensitive, endothermic step in both systems. Rigor contraction differs in that steps associated with crossbridge attachment and detachment are absent. We investigated structural changes associated with rigor contraction using X-ray diffraction. Overall changes in the low angle X-ray diffraction pattern were surveyed using a two-dimensional image plate. Reversible changes in the diffraction pattern included a 28% decrease in intensity of the 14.5 nm meridional reflection, a 12% increase in intensity of 5.9 nm actin layer-line and a somewhat variable 34% increase in intensity of 5.1 nm actin layer-line in laser temperature-jump experiments. When fibres were heated with a temperature ramp, we found that a 70% decrease in intensity of the myosin-related meridional reflection at (14.5 nm)-1 correlated with tension generation. A similar decrease in intensity of the 14.5 nm reflection is seen during tension recovery following a step change in the length of maximally Ca2+ activated fibres. Signals both from actin and actin-bound myosin heads contribute to the 5.1 and 5.9 nm actin layer-lines. Our observed changes in intensity are interpreted as contraction-associated changes in crossbridge shape and/or position on actin.

Structural Changes in the Actin–Myosin Cross-Bridges Associated with Force Generation Induced by Temperature Jump in Permeabilized Frog Muscle Fibers

“…The absence of temperature-induced changes in the equatorial x-ray intensities in rigor (Fig. 14) agrees with the observations of Rapp and Davis (1996), who also did not see changes unless temperature was increased to Ͼ50°C where dramatic disorder of the filament lattice was induced. The highest temperature in our experiments was 33°C to avoid gross damage to the mechanical and structural properties of muscle fibers.…”

Mechanics and models of the myosin motor

“…The absence of an increase of sti¡ness following the temperature jump shows that the heads must have been already attached to actin but disorientated in azimuth. An increase in the intensity of the sixth and seventh actin layer lines (5.9 and 5.1nm) had been observed during the rise of tension when muscle in rigor is heated to about 50 8C (Rapp & Davis 1996), again indicating an increase in stereospeci¢c binding with temperature.…”