Properties of intratetanic individual contractile responses in rat slow skeletal muscles during modulation of sarcoplasmic reticulum Ca2+ release

In control experiments (n = 16), during direct stimulation of m. Soleus by trains of 5, 10 and 50 stimuli at a rate of 20 Hz a biphasic change was detected in the amplitude of the last contractile responses (LCRN) depending on N, where N is the number of individual contractile responses in the tetanus. Thus, an initial decrease in LCRN amplitudes (down to 54 ± 8% for LCR5) was followed by a subsequent increase (up to 218 ± 14% for LCR50) and significant shortening of their half-relaxation time compared to the initial response (down to 44 ± 8% for LCR50). Caffeine at concentrations of 5 (n = 6) and 10 (n = 4) mM exacerbated LCR5 depression (down to 31 ± 8% and 15 ± 4%, respectively) against the background of arising characteristic stationary contracture responses. The subsequent increase in the LCRN amplitude was substantially lower than in control experiments (114 ± 18% and 46 ± 9% for LCR50 compared to the initial response at 5 and 10 mM of caffeine, respectively). The LCR50 half-relaxation time during the effect of caffeine at both concentrations also remained considerably shorter than that of individual responses recorded both in the presence of caffeine and in control experiments. In contrast to the control and caffeine effects, LCR5 and LCR10 amplitudes during the effect of 10 μM of dantrolene (n = 5) remained at the level close to that of the first response (102 ± 7% and 106 ± 8%, respectively), while the LCR50 amplitude displayed a considerably smaller increase (to 143 ± 14%) than observed in control muscles. Besides, dantrolene further enhanced muscle relaxation at rest. Caffeine (10 mM), as applied in the presence of dantrolene, restored the dynamics of changes in amplitude–temporal characteristics of last contractile responses to values approximating those in control. The amplitude–temporal characteristics of action potentials recorded extracellularly in individual m. Soleus muscle fibers did not change significantly during the transition from single to train stimulation under the same protocol as in mechanographic experiments. These data may be interpreted in support of the previously advanced hypothesis on the implication of Ca2+-induced Ca2+ release in skeletal muscles under their tetanic stimulation as an additional mechanism of excitation–contraction coupling [1, 2].