Two attached non-rigor crossbridge forms.

It is possible to produce a graded progression from rigor toward relaxation using MgAMPPNP and substituting ethylene glycol for part of the solvent water. Fibers have been brought through this progression to various stages while measuring isometric force and stiffness, then fixed for thin-section electron microscopy. Distinct state-dependent crossbridge forms were observed in thin cross and longitudinal sections. When MgAMPPNP was added to rigor fibers at 23 degrees C, the tension dropped to about one-third of its original value, but crossbridge angle remained at 45 degrees. Distinct changes were seen in crossbridge shape and angle close to the thick filament, presumably in the S2 region of myosin. Adding 30% glycol in the presence of AMPPNP reduced tension to nearly zero while stiffness remained high, provided either calcium was present or the muscle was kept cold. Under these conditions, the crossbridges were oriented at approximately 90 degrees to the filaments, and in cross-section appeared straight and joined the thick filament at separate azimuths. Raising the glycol concentration to 40% or the temperature to 23 degrees C in the absence of calcium lowered the stiffness to a value slightly above that of MgATP relaxed muscle. The 90 degrees crossbridge forms seen in stiff versus relaxed fibers were closely similar but the distribution of bridges and the optical transforms suggested more bridge attachments when stiffness was high. The 90 degrees crossbridges appear structurally distinct from the rigor form and may in the stiff fibers represent a stable but weak binding state of the actomyosin contact.