[TRANSFORMATION OF INDIVIDUAL CONTRACTILE RESPONSES DURING TETANUS IN FAST AND SLOW RAT SKELETAL MUSCLES]

The last contractile responses (LCRN), where N is the number of individual contractile responses within tetanus, were separated from the integral tetanic responses of fast, m. Extensor digitorum longus (m. EDL), and slow, m. Soleus, rat muscles using a computer-graphic technique. The average amplitude of LCR5 in m. Soleus at a 20 Hz stimulation rate decreased to 64 ± 9 % re the amplitude of a single contraction. As N was increasing, a restoration of LCRN was observed with their subsequent rise to values almost twofold exceeding the initial single contractile responses of that muscle (up to 211 ± 10 % for LCR50). Simultaneously, against the background of rise of individual contractile responses of these muscles, a considerable shortening of their half-life time (to ≈ 50%) and formation of a stationary plateau within LCRN were observed. In m. EDL at a 50 Hz stimulation rate only single-phase rise of LCRN was observed (up to 165 ± 18% for LCR50) without change of their half-life time and plateau formation. After the end of tetanic responses in muscles of both types a prolonged (up to 30 s) "hyper-relaxation effect was shown to develop manifested as a decrease of muscle tension with its subsequent restoration to the initial values. Possible mechanisms of these effects are discussed. It is supposed that transformation of individual contractile responses in skeletal muscles may be executed at the expense of specialized microdomains in muscle fibers regulating accumulation and extrusion levels of Ca2+ ions during tetanic activity. The possible involvement of an additional, Ca(2+)-induced Ca2+ release (CICR), in the basic, depolarization-induced Ca2+ release (DICR), is analyzed.