The process of muscle relaxation by the combined action of MgAMPPNP and ethylene glycol.

Insect flight muscle fibres were relaxed by the combined action of MgAMPPNP and ethylene glycol, as measured by the stiffness of the fibres. Relaxation occurred over a small range of glycol concentration. Addition of Ca2+ raised the glycol required for relaxation. The speed at which the stiffness measurement was made did not influence the glycol concentration at which relaxation occurred. Glycol in excess of that needed to relax the muscle caused a slight rise in high-frequency stiffness. Removal of the glycol restored the rigor stiffness. Under glycol-relaxed conditions, much of the AMPPNP bound in muscle fibres was retained during cold-chase (elution of [3H] AMPPNP by nonradioactive AMPPNP); the intensity ratio of the inner equatorial X-ray diffraction peaks rose upon glycol relaxation to a value slightly below that characteristic of natural relaxation. The results are interpreted in terms of cooperative attachment of the crossbridges to actin.