[PROPERTIES OF INDIVIDUAL CONTRACTILE RESPONSES WITHIN TETANUS OF RAT SLOW MUSCLE UNDER CONDITIONS OF MODULATION OF SARCOPLASMIC RETICULUM Ca²⁺ RELEASE].

During direct stimulation of m. Soleus by trains of 5, 10 and 50 stimuli with a frequency of 20 Hz in control experiments (n = 16) a biphasic change was observed in the amplitude of the last contractile responses (LCRN) depending on N, where N is the number of individual contractile responses within the te- tanus. Thus, an initial decrease of LCRN amplitude (up to 54 ± 8 % for LCR₅) was replaced by their subsequent growth (up 218 ± 14 % for LCR5o) associated with a significant shortening of their half-relaxation time relative to the initial response (to 44 ± 8 % for LCR₅₀). Caffeine at concentrations of 5 mM (n = 6) and 10 mM (n = 4), at the background of developing characteristic stationary contracture respon- ses, increased LCR5 depression during the initial inhibitory phase (31 ± 8 % and 15 ± 4 %, respectively). The subsequent growth of LCRN amplitude was significantly lower than in the control (114 ± 18 % and 46 ± 9 % for LCR₅₀ at 5 and 10 mM caffeine, respectively). LCR₅₀ half-relaxation time during the action of both caffeine concentrations remained still considerably shorter than the individual responses recorded both in the presence of caffeine and in control. In contrast to the control and caffeine effects, LCR5 and ₁₀ (to 143 ± 14 %) than was observed in the control muscle. Additionally, dantrolene enhanced muscle relaxation at rest. Caffeine (10 mM), at the background of dantrolene, restored the dynamics of changes of amplitude time characteristics of the last contractile responses to values close to the control. The time-amplitude characteristics of the extracellular AP recorded in individual muscle fibers in m. Soleus did not change significantly during tetanic stimulation under protocol similar to that used for mechanografical experiments. These data can be interpreted to support the previously suggested theory about the participation of <> as an additional mechanism of excitation-contraction coupling in skeletal muscle under conditions of tetanic stimulation [1, 2].