Transformation of individual contractile responses during tetanus in rat fast and slow skeletal muscles

Using a computer graphics approach, the last contractile responses (LCR_N, where N is a number of elementary contractile responses in tetanus) were separated from integral tetanic responses of rat fast muscles, m. Eхtensor digitorum longus (m. EDL), and slow muscles, m. Soleus, evoked by trains of 5, 10 and 50 stimuli. In m. Soleus, at a stimulation frequency of 20 Hz, the LCR₅ average amplitude decreased to 64 ± 9% compared to the single contraction amplitude. As N increased, LCR_N recovered and then rose to the values exceeding almost twofold initial elementary contractile responses (up to 211 ± 10% for LCR₅₀). Simultaneously, against the background of rising elementary contractile responses, a significant shortening of their half-decay time (～by 50%) and the formation of a stationary plateau within LCR_N was observed. In m. EDL, at a stimulation frequency of 50 Hz, there was only a single-phase LCR_N rise (up to 165 ± 18% for LCR₅₀) without changes in half-decay time and plateau formation. In skeletal muscles of both types, the prolonged (up to 30 s) ‘hyper-relaxation effect’ was found to develop after the end of tetanic responses manifested as a reduction of muscle tension followed by its recovery to the initial level. Possible mechanisms of these events are discussed. It is hypothesized that transformation of elementary contractile responses in skeletal muscles can be fulfilled due to the existence of specialized microdomains in muscle fibers which regulate accumulation and extrusion of Ca²⁺ ions during tetanic activity. The possibility that the basic, depolarization-induced, Ca²⁺ release (DICR) is complemented by an additional, Ca²⁺-induced, Ca²⁺release (CIRC) is analyzed.