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Mechanical study of rat soleus muscle using caged ATP and X‐ray diffraction: high ADP affinity of slow cross‐bridges
Author(s) -
Horiuti K.,
Yagi N.,
Takemori S.
Publication year - 1997
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1997.433bk.x
Subject(s) - soleus muscle , chemistry , biophysics , biochemistry , anatomy , skeletal muscle , biology
1 The cross‐bridges in slow‐ and fast‐twitch fibres (taken, respectively, from soleus and psoas muscles of rats) were examined in mechanical experiments using caged ATP and X‐ray diffraction, to compare their binding of ATP and ADP. 2 Caged ATP was photolysed in rigor fibres. When ADP was removed from pre‐photolysis fibres, the initial relaxation (±Ca 2+ ) in soleus was as fast as that in psoas fibres, whereas the subsequent contraction (+Ca 2+ ) was slower in soleus than in psoas. The ATPase rate during the steady‐state contraction was also slower in soleus fibres. 3 When ADP was added to pre‐photolysis fibres (±Ca 2+ ), tension developed even in the initial phase, the overall tension development being biphasic. Both initial and late components of the Ca 2+ ‐free contraction were enhanced when ADP was added before photolysis, although pre‐photolysis ADP was not a prerequisite for the late component. The effect of ADP was greater in soleus than in psoas fibres. Static experiments on rigor fibres revealed a higher ADP affinity in soleus fibres. 4 The intensity of the actin layer‐line from ADP rigor soleus fibres decreased rapidly on photorelease of ATP. We conclude that, despite the tight ADP binding of the soleus cross‐bridge, its isometric reaction is not rate limited by the ‘off’ rate of ADP.