Characterization of actomyosin bond properties in intact skeletal muscle by force spectroscopy

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Abstract
Force generation and motion in skeletal muscle result from interaction between actin and myosin myofilaments through the cyclical formation and rupture of the actomyosin bonds, the cross-bridges, in the overlap region of the sarcomeres. Actomyosin bond properties were investigated here in single intact muscle fibers by using dynamic force spectroscopy. The force needed to forcibly detach the cross-bridge ensemble in the half-sarcomere (hs) was measured in a range of stretching velocity between 3.4 × 10 3 nm·hs −1 ·s −1 or 3.3 fiber length per second ( l 0 s −1 ) and 6.1 × 10 4 nm·hs −1 ·s −1 or 50 l 0 ·s −1 during tetanic force development. The rupture force of the actomyosin bond increased linearly with the logarithm of the loading rate, in agreement with previous experiments on noncovalent single bond and with Bell theory [Bell GI (1978) Science 200:618–627]. The analysis permitted calculation of the actomyosin interaction length, x β and the dissociation rate constant for zero external load, k 0 . Mean x β was 1.25 nm, a value similar to that reported for single actomyosin bond under rigor condition. Mean k 0 was 20 s −1 , a value about twice as great as that reported in the literature for isometric force relaxation in the same type of muscle fibers. These experiments show, for the first time, that force spectroscopy can be used to reveal the properties of the individual cross-bridge in intact skeletal muscle fibers.