1 Introduction

Functional unloading or hypogravity can be consequence of multiple conditions like a prolong bed rest and spinal cord injury and may lead to the serious physiological changes in skeletal muscles [1]. These changes commonly appear as decrease of the muscle size and volume [2, 3]. Titin plays an important role in maintaining the muscle sacromere structure and its decrease affects the muscles contractile properties [4]. Understanding the role of titin in molecular mechanisms of the morfo-functional changes in muscles can lead to development of new rehabilitation therapies.

2 Material and Methods

All experiments were conducted on adult laboratory rats (180–200 g) according to the rules of the laboratory animals’ care and bioethical standards. As a model of gravitational unloading, we used a model of anti-orthostatic support described by Morey-Holton E.R et al. (2002) [5].

After 7 days of gravitational unloading, m. soleus (SM) (n = 12) and m. extensor digitorum longus (EDL) (n = 8) were prepared, and titin isomers were evaluated using DSN-gel electrophoresis according to Tatsumi R. and Hattori A. (1995) [6]. The quantity of a titin was estimated in relation to the heavy chains of a myosin (MHC) [7]. Immunoblotting of titin was conducted by method [8]. Statistical analysis was performed with t student criterion.

3 Results and Discussion

Decrease of N2A-izoform of a titin in comparison to MHC in 25 % (p < 0.05) was observed in SM after 7 days of gravitational unloading. Three times increase of T2 fragment (p < 0.05) was followed by reducing the relative amount of N2A-izoform of a titin that demonstrates change of a proteolysis of intact N2A-izoform of a titin in the conditions of gravitational unloading. Our results show that one to two molecules of intact N2A-izoform were disrupted in SM after unloading for 7 days (Fig. 1a).

Fig. 1
figure 1

Changes of isoform structure of a titin in m. soleus (a) and m. extensor digitorum longus (b) after 7 days of gravitational unloading. On an axis of ordinates the following are designated: the relations of number of N2A-izoform of a titin and T2 fragment in relation to heavy chains of a myosin by results of a densitometry of proteinaceous strips on the elekroforegram expressed in the relative units. The x-axis: the white columns—“control” and gray—“anti-orthostatic hanging.” The level of significance p < 0.05

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Decreasing of a titin of abundance of MHC in N2A-izoform was not observed in EDL after 7-day unloading (Fig. 1b).

We also found that content of a titin in SM was reduced during the first 7 days after unloading and did not change in EDL. Our findings were supported by the data of the other studies. It was shown that development of atrophic changes in skeletal muscles in animals and human was accompanied by decrease in titin and a nebulin, during hypogravity conditions [4, 9, 10], and during actual microgravity [11, 12]. It was confirmed that 7-day gravitational unloading led to an essential atrophy of postural, anti-gravitational SM, but not the fast EDL. Similar results were described in experiments conducted during the space flight when the atrophy was not observed in sural and forward tibial muscles at rats after 14 daily flights [13]. In model experiments, a 14-day hanging caused atrophy of soleus, but not sural and forward tibial muscles [14, 15]. It is shown that decrease in a sectional area of fiber in soleus muscle after a 14-day hanging was more in slow, than in fast fibers [16].

4 Conclusions

The reduction of titin in m. soleus after gravitational unloading, along with other changes observed in muscle, may contribute to the development of “hypogravitational muscle syndrome.” These data show that muscle atrophy can be related to the muscles activity, so the less used muscles, such as EDL, are less sensitive to gravitational changes while anti-gravitational muscles, such as SM, quickly develop atrophic changes.