During direct stimulation of m. Soleus by train of 5, 10 and 50 stimuli with a frequency of 20 Hz in the control (n=16) was observed a biphasic change in the amplitude of the last contractile responses (LCRN) depending on N, where N is number of individual contractile responses within the tetanus. Thus, an initial decrease of LCRN amplitude was replaced by their subsequent growth associated with a significant shortening of the half-relaxation time. Caffeine at concentrations of 5 mM (n=6) and 10 mM (n=4), while preserving the overall two phase character of the responses exacerbated LCR5 depression during the initial inhibitory phase. LCR50 half-relaxation time during the action of both caffeine concentrations remained still considerably shorter than the individual responses recorded either in the presence of caffeine or in control. In contrast to the control and caffeine effects, LCR5 and LCR10 amplitude during the action of 10 mkM of dantrolene (n=5) remained at the level close to the value of the first response, and LCR50 amplitude demonstrated a significantly smaller increase than was observed in the control muscle. Additionally, dantrolene enhanced muscle relaxation at rest. Caffeine (10 mM) restored the dynamics of changes of amplitude-time characteristics of the last contractile responses caused by dantrolene to values close to the control. These data can be interpreted to support the previously suggested theory about the participation of "Ca2+-dependent Ca2+ release" as an additional mechanism of excitation-contraction coupling in skeletal muscle under conditions of tetanic stimulation [1, 2]. This work is supported by RSF №15-15-20008.
dantrolene, caffeine, ryanodine receptors, skeletal muscles, Ca2+-ATPase of the sarcoplasmic reticulum, excitation-contraction coupling
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