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A novel experimental method was developed to study the contractility of single myofibrils of skeletal muscle. Single myofibrils (ca. 1 microm in diameter) prepared from glycerinated rabbit psoas muscle were suspended between rigid and flexible microneedles by the entwining method. The length changes of the preparations applied via the rigid microneedle by an actuator and the force produced were measured by photo-electrically detecting the nanometer deflections of the flexible microneedle. Single myofibril preparations maintained uniform sarcomere striations during contraction-relaxation cycles. The isometric force produced, the velocity of unloaded shortening, and the force-velocity relationship of single myofibrils were investigated at various MgATP concentrations. The contractility of single myofibrils thus obtained in the absence of ATP regenerative systems was essentially the same as that of skinned muscle fibers under comparable conditions in the presence of ATP regenerative systems. Thus, it was found that (1) the present experimental method is useful for studying the contractility of single myofibrils, and (2) in single myofibril preparations, the MgATP concentration at actomyosin sites is well equilibrated with that in bathing solutions.

Contractility of Single Myofibrils of Rabbit Skeletal Muscle Studied at Various MgATP Concentrations.