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

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.