The time course of changes in the equatorial diffraction patterns from different muscle types on photolysis of caged-ATP.

Using the synchrotron X-ray source at DESY, Hamburg, we have measured the time courses of changes in the strongest equatorial reflections from small bundles of chemically skinned fibres from insect flight muscle, Limulus muscle and rabbit psoas and soleus muscles following the photolytic release of ATP. In all preparations the release of ca. 2 mM ATP caused the tension to relax with a complex time course, the final relaxation rates in insect and rabbit psoas fibres being ca. 10 times faster than those measured in rabbit soleus and Limulus fibres (ca. 50 ms cf. ca. 500 ms half times). However, in all fibre types there was a very rapid change in equatorial intensities towards relaxed values (half time less than 5 ms), the extent of this change and the occurrence of a slower phase of intensity change being dependent on the preparation. In insect the 1.0 and 2.0 intensities change rapidly to their relaxed values; in Limulus the 1.0 and 1.1 intensities change rapidly to within ca. 10% of their relaxed values; in rabbit psoas the initial, rapid 1.1 intensity fall is to within 30-40% of its relaxed value and is followed by a slower fall which appears to correlate with the rate of the final tension relaxation, e.g. phosphate ions accelerate both rates; in rabbit soleus the equatorial response is very similar to that of psoas fibres. These results are discussed in terms of the model of Goldman et al. in which a rapid ATP induced dissociation of rigor crossbridges is followed by the transient cooperative reattachment of some bridges which may proceed through at least part of the cross-bridge cycle.