Mechanical study of rat soleus muscle using caged ATP and X‐ray diffraction: high ADP affinity of slow cross‐bridges

Abstract

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. Caged ATP was photolysed in rigor fibres. When 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 soleus fibres. When ADP was added to pre‐photolysis fibres (±Ca2+), tension developed even in the initial phase, the overall tension development being biphasic. Both initial and late components of the Ca2+‐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. 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.

Insight into the actin-myosin motor from x-ray diffraction on muscle

“…Early x-ray experiments with flash photolysis of caged-ATP were focused on relaxation kinetics of the equatorial intensities (10,(133)(134)(135). Later with the use of more advanced radiation sources and 2D x-ray detectors the intensities of the M3 meridional reflection and some layer lines were also resolved in the absence and in the presence of Ca 2+ (11,136,137,(12)(13)(14)(15). It was found that the decay of the rigor structure of actin filaments decorated by strongly bound myosin heads has a time course similar to that found in solution if the rate of ATP liberation and binding of caged-ATP to myosin are taken into account (15).…”

Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle

“…A study of rat SOL and EDL, under conditions designed to phosphorylate or dephosphorylate the regulatory light chains, showed little to no lattice sampling on ML1 of SOL, and made no conclusion about the type of myosin filament lattice present (Yamaguchi et al, 2016); the conditions used for these experiments may have caused significant disordering of the myosin heads, thus weakening the myosin layer lines and their sampling. A comparison of rat SOL and psoas muscles focused on cross-bridge behavior and made no mention of lattice structure (Horiuti et al, 1997). Likewise, a comparison of chicken slow (anterior latissimus dorsi) and fast (posterior latissimus dorsi) muscles centered on myosin head proximity to thin filaments but did not study myosin layer lines or comment on the type of filament lattice (Matsubara et al, 1991).…”

Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle

“…A study of rat SOL and EDL, under conditions designed to phosphorylate or dephosphorylate the regulatory light chains, showed little to no lattice sampling on ML1 of SOL, and made no conclusion about the type of myosin filament lattice present [30]; the conditions used for these experiments may have caused significant disordering of the myosin heads, thus weakening the myosin layer lines and their sampling. A comparison of rat SOL and psoas muscles focused on crossbridge behavior and made no mention of lattice structure [31]. Likewise, a comparison of chicken slow (anterior latissimus dorsi -ALD) and fast (posterior latissimus dorsi -PLD) muscles centered on myosin head proximity to thin filaments, but did not study myosin layer lines or comment on the type of filament lattice [32].…”