## 4 Discussion  
It is known that both long-lived (O\({}_{3}\) and H\({}_{2}\)O\({}_{2}\)) and short-lived (\({}^{\bullet}\)OH and O\({}_{2}\)\({}^{\bullet}\)) ROS may be produced under various physiological (e.g., the natural process of aging) as well as pathological conditions (e.g., cerebellar ischemia, atherosclerosis and sunburns). The excessive generation of ROS leads to the development of oxidative stress, which causes damage to cells and cell death. However, some reports also indicate the crucial role of ROS in the regulation of the metabolism, proliferation and cell differentiation [20; 21; 22]. Figure 6: Changes in the relative expression of caspase 3 (**A**) and JNK (**B**) in the BMSC culture at different times after LTP exposure (relative to bAct, shown by the blue bars). The green bars indicate no exposure to plasma (i.e., the control). The values are given as means \(\pm\) SEM (\(n=3\)). \({}^{\star}p<0.05\) compared to the control. Figure 7: Evaluation of the fibroblast growth factor (FGFPb) level in BMSC at different times after LTP exposure (blue bars). Data are represented as the means \(\pm\) SEM (\(n=3\)). The green bars indicate no exposure to plasma. LTP was found to be accompanied by the formation of various types of ROS exhibiting biological activity. Depending on the concentration, ROS can both damage biological macromolecules and play a significant regulatory role. For example, in the case of tissue rupture, ROS can, on one hand, contribute to the wound's healing process, and, on the other hand, lead to the inactivation of pathogenic microorganisms. It is important to note that these biological effects of LTP were observed mainly in a liquid medium, which indicates that chemically active substances produced by the action of plasma on the gas phase can be transported by diffusion into liquid media. The number of various forms of ROS generated by LTP largely depends on the composition of the liquid medium, the plasma parameters and the exposure time [39]. As established in the present work, a significant amount of hydrogen peroxide was actually formed in the aqueous solution when exposed to LTP (Figure 2A). Plasma was formed by the ionization of water vapor molecules. The most likely ionization mechanism was the dissociation of water molecules through dissociative electron attachment [40], resulting in the formation of negatively charged H\({}_{2}\)O\({}^{-}\). The latter is unstable and decomposes into a neutral hydroxyl radical and a negatively charged hydrogen ion. The main reactions in plasma proceed according to the following scheme:  
\[\mathrm{H_{2}O+e\to H_{2}O^{-}\to OH^{\bullet}+H^{-}}\]  
\[\mathrm{H^{-}+M^{\prime}\to H+M+e}\]  
\[\mathrm{H^{-}+H^{\bullet}\to H_{2}+e}\]  
\[\mathrm{H^{-}+e\to H^{\bullet}+2e}\]  
\[\mathrm{OH^{\bullet}+H_{2}O^{\bullet}\to H_{2}O_{2}+H}\]  
In this case, multiple use of one electron occurs, which ensures a chain process in the proceeding reaction. The acute effect of ROS within 1 h after LTP exposure led to a significant increase in the level of DC and MDA in the BMSC culture (Figure 2B,C), which is evidence of the development of lipid peroxidation of the cell membranes and oxidative stress. In this case, the level of LPO products strongly correlated with the level of H\({}_{2}\)O\({}_{2}\) in the medium (R\({}^{2}\) = 0.96) and also showed a tendency to decrease to the control values by the third day of incubation. Hence, one could assume that the acute effect of LTP-induced ROS is associated with the development of oxidative processes, including lipid peroxidation of the BMSC membranes. It is quite obvious that the highest local concentrations of ROS were observed in the plasma exposure zone (near the needle of the electrode). One can suppose that the primary effects of LTP on BMSC are caused by the direct action of plasma through the generation of ions, electrons and short-living ROS, formed at the moment of action of LTP and acting in close proximity of the electrode.