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 wi th saponin and the transfer of myosin heads from the thick filaments t o the thin filaments at a sarcomere length of 2.4 mu m was measured as a function of Ca2+ concentration using an x-ray diffraction method at 4-7 degrees C. In the relaxed state, the 1,0 spacing was 42.08 nm. Th e spacing showed no significant decrease when the Ca2+ concentration w as below the threshold (-log(10) [Ca2+] or pCa 5.8). No significant tr ansfer of the myosin heads occurred when the Ca2+ concentration was be low the threshold (pCa 5.8). When the muscle was maximally activated a t pca 4.4, the spacing decreased to 40.35 nm. During the maximum isome tric 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. T hese results suggest that myosin heads of both fast-twitch and slow-tw itch skeletal muscles transferred on the common movement as a function of Ca2+ concentration.