MYOSIN HEAD ORIENTATION AND MOBILITY DURING ISOMETRIC CONTRACTION - EFFECTS OF OSMOTIC COMPRESSION

We have correlated the mobility and the generation of force of myosin heads by applying radial compression to isometrically contracting musc le fibers. Osmotic pressure was produced by dextran T-500, and its eff ect on the orientation and mobility of myosin heads labeled with N-(1 -oxy-2,2,5,5-tetramethyl-4-pyperidinyl)maleimide was observed by conve ntional and saturation-transfer electron paramagnetic resonance method s. A biphasic behavior in spectral changes coinciding with the tension dependence was observed as the fibers were compressed. At diameters a bove the equilibrium spacing, the large myosin head disorder character istic during contraction in the absence of compression was largely mai ntained, whereas the mobility decreased threefold, from tau(R) similar to 25 mu s to similar to 80 similar to 90 mu S. The inhibition of fas t microsecond motions was not accompanied by tension loss, implying th at these motions are not necessary for force generation. At diameters below the equilibrium spacing, both the disorder and the mobility decr eased dramatically in parallel with the tension inhibition, suggesting that slower microsecond motions and the disorder of the myosin head a re necessary for muscle function.