Regulation of tension development by MgADP and Pi without Ca²⁺. Role in spontaneous tension oscillation of skeletal muscle

The length of sarcomeres in isolated myofibrils fixed at both ends spontaneously oscillates when MgADP and Pi coexist with MgATP in the absence of Ca²⁺ (Okamura, N., and S. Ishiwata, 1988. J. Muscle Res. Cell. Motil. 9:111-119). Here, we report that MgADP and Pi function as an activator and an inhibitor, respectively, of tension development of single skeletal muscle fibers in the absence of Ca²⁺ and the coexistence of MgADP and Pi with MgATP induces spontaneous tension oscillation. First, the isometric tension sharply increased when the concentration of MgADP became higher than ~3x that of MgATP and saturated at ~9% of the tension obtained under full Ca²⁺ activation; in parallel with this sigmoidal increase of tension, MgATPase activity appeared. The inhibition of contraction by the regulatory system seems to be desuppressed by the allosteric effect of actomyosin-ADP complex, similarly to so-called rigor complex. The ADP-induced tension was decreased along a reversed sigmoidal curve by the addition of Pi; actomyosin-ADP-Pi complex, which has no desuppression function, may be formed by exogenous Pi; accompanying the decline of tension, spontaneous oscillations of tension and sarcomere length appeared. It is suggested that the length oscillation of each (half) sarcomere would occur through the transition of cross-bridges between force-generating (on) and non-force-generating (off) states, which may be regulated by the mechanical states (strain) of cross-bridges and/or thin filaments.