PARALLEL INHIBITION OF ACTIVE FORCE AND RELAXED FIBER STIFFNESS BY CALDESMON FRAGMENTS AT PHYSIOLOGICAL IONIC-STRENGTH AND TEMPERATURE CONDITIONS - ADDITIONAL EVIDENCE THAT WEAK CROSS-BRIDGE BINDING TO ACTIN IS AN ESSENTIAL INTERMEDIATE FOR FORCE GENERATION

Previously we showed that stiffness of relaxed fibers and active force generated in single skinned fibers of rabbit psoas muscle are inhibit ed in parallel by actin-binding fragments of caldesmon, an actin-assoc iated protein of smooth muscle, under conditions in which a large frac tion of cross-bridges is weakly attached to actin (ionic strength of 5 0 mM and temperature of 5 degrees C). These results suggested that wea k cross-bridge attachment to actin is essential for force generation. The present study provides evidence that this is also true for physiol ogical ionic strength (170 mM) at temperatures up to 30 degrees C, sug gesting that weak cross-bridge binding to actin is generally required for force generation. In addition, we show that the inhibition of acti ve force is not a result of changes in cross-bridge cycling kinetics b ut apparently results from selective inhibition of weak cross-bridge b inding to actin. Together with our previous biochemical, mechanical, a nd structural studies, these findings support the proposal that weak c ross-bridge attachment to actin is an essential intermediate on the pa th to force generation and are consistent with the concept that isomet ric force mainly results from an increase in strain of the attached cr oss-bridge as a result of a structural change associated with the tran sition from a weakly bound to a strongly bound actomyosin complex. Thi s mechanism is different from the processes responsible for quick tens ion recovery that were proposed by Huxley and Simmons (Proposed mechan ism of force generation in striated muscle. Nature. 233:533-538.) to r epresent the elementary mechanism of force generation.