Cross-bridge movement in rat slow skeletal muscle as a function of calcium concentration

A single fibre bundle from rat soleus muscle was chemically skinned with saponin and the transfer of myosin heads from the thick filaments to the thin filaments at a sarcomere length of 2.4 μm was measured as a function of Ca²⁺ concentration using an x-ray diffraction method at 4–7 °C. In the relaxed state, the 1,0 spacing was 42.08 nm. The spacing showed no significant decrease when the Ca²⁺ concentration was below the threshold (−log₁₀ Ca²⁺] or pCa 5.8). No significant transfer of the myosin heads occurred when the Ca²⁺concentration was below the threshold (pCa 5.8). When the muscle was maximally activated at pCa 4.4, the spacing decreased to 40.35 nm. During the maximum isometric contraction at pCa 4.4, 54.9 ± 6.5% (±SE of the mean) of the myosin heads were transferred to the thin filaments. The transfer of the myosin heads was approximately proportional to relative tension. These results suggest that myosin heads of both fast-twitch and slow-twitch skeletal muscles transferred on the common movement as a function of Ca²⁺ concentration. Received: 1 December 1995/Received after revision and accepted: 20 May 1996