Cross bridge slippage induced by the ATP analogue AMP-PNP and stretch in glycerol-extracted fibrillar muscle fibres.

Glycerol-extracted insect fibillar muscle fibers in rigor exhibited both an elastic and a plastic phase in the length-tension diagram. The transition between these phases took place at a critical tension, the "yield point" or elastic limit. In the plastic phase the apparent static elastic modulus became zero, whereas the immediate elastic modulus (measured by rapid length changes completed within 4 ms) exhibited no abrupt change at the yield point. The tension value of the yield point (but not immediate stiffness) was lowered by addition of AMP-PNP and was partially restored by washing out AMP-PNP. The dependence of the critical tension at which plastic flow begins on cooperative cross bridge behaviour is discussed in terms of breaking and reforming acto-myosin linkages. Evidence is presented that addition of AMP-PNP induces slippage of cross bridges on the actin filament by affecting the interaction between myosin and actin.