Freeze-fracture studies on the cross-bridge angle distribution at various states and the thin filament stiffness in single skinned frog muscle fibers.

To give information about the changes of cross-bridge (myosin head) orientation during muscle contraction, mechanically skinned frog muscle fibers were rapidly frozen at various states, and the cross-bridges were observed on freeze-etch replicas. The number of cross-bridges per unit length of thick filament in relaxed state was less than one-third of that in contracting and rigor states. The interval between adjacent cross-bridges was maximum around 35 nm, a value close to the crossover repeat of actin helix. The cross-bridge angle distribution, as measured with a digital image processor, showed a peak around 90 degrees in all the states examined. The proportion of cross-bridges with angles around 90 degrees decreased either after stretch or after release of fibers in rigor state. The axial spacing of actin monomers in the thin filament was found to increase with increasing rigor force, to give the thin filament stiffness (times unit length) of about 1.8 x 10(4) pN. These results are discussed in connection with the mechanical properties of cross-bridges.