Effectof MgATP on stiffness measured at two frequencies in Ca2+-activated muscle fibers.

The stiffness of skinned crayfish single muscle fibers was continuously monitored at two frequencies. The length of the fibers was oscillated by the sum of two sine waves (5 Hz and 100 Hz) of small amplitudes. In saline containing saturating amounts of Ca2+, the stiffness ratio (5 Hz:100Hz) was constant as the MgATP (substrate) concentration was raised from 0 to 2 mu M, then it decreased with a further increment in MgATP. The systematic decrease in the stiffness ratio in MgATP above 2 mu M indicates the presence of faster transitions in the cross-bridge cycle. This dependence of the stiffness ratio on MgATP is predictable if we use the two-state model of A. F. Huxley (1957) with a modification, in which MgATP promotes the dissociation of the attached cross-bridges.