MYOSIN REGULATORY LIGHT-CHAIN MODULATES THE CA2-MUSCLE FIBERS( DEPENDENCE OF THE KINETICS OF TENSION DEVELOPMENT IN SKELETAL)

To determine the role of myosin regulatory light chain (RLC) in modula ting contraction in skeletal muscle, we examined the rate of tension d evelopment in bundles of skinned skeletal muscle fibers as a function of the level of Ca2+ activation after UV flash-induced release of Ca2 from the photosensitive Ca2+ chelator DM-nitrophen. In control fiber bundles, the rate of tension development was highly dependent on the c oncentration of activator Ca2+ after the flash. There was a greater th an twofold increase in the rate of tension development when the post-f lash [Ca2+] was increased from the lowest level tested (which produced a steady tension that was 42% of maximum tension) to the highest leve l (producing 97% of maximum tension). However, when 40-70% of endogeno us myosin RLC was extracted from the fiber bundles, tension developed at the maximum rate, regardless of the post-flash concentration of Ca2 +. Thus, the Ca2+ dependence of the rate of tension development was el iminated by partial extraction of myosin RLC, an effect that was parti ally reversed by recombination of RLC back into the fiber bundles. The elimination of the Ca2+ dependence of the kinetics of tension develop ment was specific to the extraction of RLC rather than an artifact of the co-extraction of both RLC and Troponin C, because the rate of tens ion development was still Ca2+ dependent, even when nearly 50% of endo genous Troponin C was extracted from fiber bundles fully replete with RLC. Thus, myosin RLC appears to be a key component in modulating Ca2 sensitive cross-bridge transitions that limit the rate of force devel opment after photorelease of Ca2+ in skeletal muscle fibers.