X-RAY-DIFFRACTION STUDIES ON THERMALLY-INDUCED TENSION GENERATION IN RIGOR MUSCLE

Muscle fibres in the rigor state and free of nucleotide contract if he ated above their physiological working temperature. Kinetic studies on the mechanism of this process, termed rigor contraction, indicate tha t it has a number of features in common with the contraction of maxima lly Ca2+ activated fibres. De novo tension generation appears to be as sociated with a single, tension sensitive, endothermic step in both sy stems. Rigor contraction differs in that steps associated with crossbr idge attachment and detachment are absent. We investigated structural changes associated with rigor contraction using X-ray diffraction. Ove rall changes in the low angle X-ray diffraction pattern were surveyed using a two-dimensional image plate. Reversible changes in the diffrac tion pattern included a 28% decrease in intensity of the 14.5 nm merid ional reflection, a 12% increase in intensity of 5.9 nm actin layer-li ne and a somewhat variable 34% increase in intensity of 5.1 nm actin l ayer-line in laser temperature-jump experiments. When fibres were heat ed with a temperature ramp, we found that a 70% decrease in intensity of the myosin-related meridional reflection at (14.5 nm)(-1) correlate d 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 a ctin and actin-bound myosin heads contribute to the 5.1 and 5.9 nm act in layer-lines. Our observed changes in intensity are interpreted as c ontraction-associated changes in crossbridge shape and/or position on actin.