Transversal stiffness of fibers and desmin content in leg muscles of rats under gravitational unloading of various durations.

The aim of this research was the analysis of structural changes in various parts of the sarcolemma and contractile apparatus of muscle fibers by measuring their transversal stiffness by atomic force microscopy under gravitational unloading. Soleus, medial gastrocnemius, and tibialis anterior muscles of Wistar rats were the objects of the study. Gravitational unloading was carried out by antiorthostatic suspension of hindlimbs for 1, 3, 7, and 12 days. It was shown that the transversal stiffness of different parts of the contractile apparatus of soleus muscle fibers decreases during gravitational unloading in the relaxed, calcium-activated, and rigor states, the fibers of the medial gastrocnemius show no changes, whereas the transversal stiffness of tibialis anterior muscle increases. Thus the transversal stiffness of the sarcolemma in the relaxed state is reduced in all muscles, which may be due to the direct action of gravity as an external mechanical factor that can influence the tension on a membrane. The change of sarcolemma stiffness in activated fibers, which is due probably to the transfer of tension from the contractile apparatus, correlates with the dynamics of changes in the content of desmin.