[Different sensitivity of cross-bridges in the muscle fiber sarcomere in Ca2+-activation].

The investigation of transient kinetics of force development by glycerinated rabbit m. psoas fibres at consecutive pCa stepwise drops in the presence of 0.1 mM Na3VO4 was carried out. Stationary content of trapped vanadate in the fibres at low levels of Ca2+-activation was deduced to be attained slowly as compared to the initial rate of force development and vanadate trapping. This result is considered an indication of the fact, that not all cross-bridges can be activated by turns at low levels of Ca2+-activation.