MECHANICAL STUDY OF RAT SOLEUS MUSCLE USING CAGED-ATP AND X-RAY-DIFFRACTION - HIGH ADP AFFINITY OF SLOW CROSS-BRIDGES

1. The cross-bridges in slow- and fast-twitch fibres (taken, respectiv ely, from soleus and psoas muscles of rats) were examined in mechanica l experiments using caged ATP and X-ray diffraction, to compare their binding of ATP and ADP. 2. Caged ATP was photolysed in rigor fibres. W hen ADP was removed from pre-photolysis fibres, the initial relaxation (+/-Ca2+) in soleus was as fast as that in psoas fibres, whereas the subsequent contraction (+Ca2+) was slower in soleus than in psoas. The ATPase rate during the steady-state contraction was also slower in so leus fibres. 3. When ADP was added to pre-photolysis fibres (+/-Ca2+), tension developed even in the initial phase, the overall tension deve lopment being biphasic. Both initial and late components of the Ca2+-f ree contraction were enhanced when ADP was added before photolysis, al though pre-photolysis ADP was not a prerequisite for the late componen t. The effect of ADP was greater in soleus than in psoas fibres. Stati c experiments on rigor fibres revealed a higher ADP affinity in soleus fibres. 4. The intensity of the actin layer-line from ADP rigor soleu s fibres decreased rapidly on photorelease of ATP. We conclude that, d espite the tight ADP binding of the soleus crossbridge, its isometric reaction is not rate Limited by the 'off' rate of ADP.