The conformational state of actin filaments was studied in the rat soleus muscle atrophying after denervation, recovering following reinnervation, hypertrophying following tenotomy of synergists and in intact muscle. Intrinsic (tryptophan residues of F-actin) and extrinsic (rhodamine-phalloidin or 1,5-IAEDANS attached to F-actin) polarized fluorescence was measured. In parallel, the influence of ATP or NEM on the state of F-actin was studied. The results show that the conformational state of F-actin is changed in all experimental muscles. These changes of the denervated muscle differ from those of the reinnervated and hypertrophying muscles. In the reinnervated muscle, beginning with the first days of recovery, the structure of F-actin seems to "recover" to the state in intact muscle. In the later stage of muscle recovery, the state of F-actin is similar to that in hypertrophying muscle. Differences between the mentioned muscles in the conformational state of actin monomers, in the orientation of monomers and in the flexibility of thin filaments are discussed.
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