On the possibility of interaction between neighbouring crossbridges

Demembranated insect or rabbit striated muscle fibres at equilibrium (i.e. in the absence of ATP hydrolysis) were modified either by substituting ethylene glycol for water or by adding AMPPNP. The resultant states observed by electron microscopy and X-ray diffraction appeared to contain a mixture of at least two distinct types of crossbridge, which were not randomly mixed. The crossbridges held tension for a great deal longer than they remained attached to actin in solution. In the presence of AMPPNP the muscle fibres relaxed at a critical glycol concentration. These properties indicate that the crossbridges interacted with one another.