Does thin filament compliance diminish the cross-bridge kinetics? A study in rabbit psoas fibers

The effect of thin filament compliance on our ability to detect the cross-bridge kinetics was examined. Our experiment is based on the facts that in rabbit psoas the thin filament (1.12 mu m) is longer than half the thick filament length (0.82 mu m) and that the thick filament has a central bare zone (0.16 mu m). Consequently. when sarcomere length is increased from 2.1 to 2.4 mu m, the same number of cross-bridges is involved in force generation but extra series compliance is introduced in the I-band. Three apparent rate constants (2 pi a, 2 pi b, and 2 pi c) were characterized by sinusoidal analysis at pCa 4.66. Our results demonstrate that 2 pi a and 2 pi b increased 13-16% when sarcomere length was increased from 2.0 to 2.5 mu m, and 2 pi c decreased slightly (9%). This slight decrease can be explained by compression of the lattice spacing. These observations are at variance with the expectation based on increased series compliance, which predicts that the rate constants will decrease. We also determined compliance of the I-band during rigor. I-band compliance during rigor induction was 35% of sarcomere compliance at sarcomere length 2.4 mu m, and 24% at sarcomere length 2.1 mu m. We conclude that the presence of thin filament compliance does not seriously interfere with our ability to detect cross-bridge kinetics using sinusoidal analysis.