Cross-bridge attachment in relaxed muscle.

We have measured the stiffness of relaxed, skinned rabbit psoas fibers at 5 degrees C in low ionic strength relaxing solution (mu = 0.02 M) by stretching the fibers and measuring the resulting force and sarcomere length changes. This stiffness is very dependent upon the velocity of stretch. With very slow stretches (0.5% of fiber length in greater than 30 ms), it is almost negligible but with stretches as fast as 0.5% of fiber length in 150 microseconds, the stiffness approaches 1/3 that of the rigor fiber. This stiffness is also very sensitive to ionic strength, being reduced more than 20-fold at an ionic strength of 0.17 M. This ionic strength sensitive stiffness scales with the amount of overlap between the actin and myosin filaments which strongly suggests that it is due to attached cross-bridges. The speed dependence suggests that the attached cross-bridges are not statically attached but in rapid equilibrium between attached and detached states. Experiments with adenylyl-imido-diphosphate suggest that the rates of attachment and detachment depend upon nucleotide.