NFATc1 and slow-to-fast transition of myosin heavy chain isoforms in gravitational unloading of the rat soleus

An attempt was made to determine whether or not the concentration of NFATc1 (nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1) in nuclear and cytoplasmic extracts is related to an increase in the concentration of fibers containing type IIa myosin heavy chains under modeled gravitational unloading of m. soleus. Experiments were carried out on Wistar rats using the Morey-Holton tail suspension model. It was found that the soleus contains three isoforms of NFATc1 (140, 110, and 86 kDa). Under unloading, the 140-kDa isoform is translocated into the nucleus, the concentration of the 110-kDa isoform in the cytoplasmic extract decreases, and the concentration of the 86-kDa isoform in the nuclear extract increases. Under gravitational unloading of the muscle, the concentration of fibers containing type IIa myosin heavy chains increases. The increase in the concentration of the 140-and 86-kDa NFATc1 isoforms in the nucleus is accompanied by a decrease in the fraction of muscle fibers containing type I myosin heavy chains and an increase in the fraction containing type IIa chains.