Differences in the transient response of fast and slow skeletal muscle fibers. Correlations between complex modulus and myosin light chains.

Sinusoidal analysis of the mechanochemical properties of skinned muscle fibers under conditions of maximal activation was applied to fibers from several rabbit skeletal muscles (psoas, tibialis anterior, extensor digitorum longus, diaphragm, soleus, semitendinosus). This investigation distinguished between two general classes of fibers, which on the basis of their myosin light chain complements could be classified as fast and slow. In fast fibers (e.g., psoas) we identified the presence of at least three exponential processes (A), (B), (C) of comparable magnitudes. In slow fibers (e.g., soleus) we identified the presence of at least four exponential processes (A)-(D) of very different magnitudes; magnitudes of processes (A) and (B) are very small compared with those of (C) and (D). The apparent rate constants are 8-29-fold slower in slow fibers. Because our sinusoidal characterization takes less than or equal to 22 s and does not involve chemical denaturation or other means of disruption of the myofilament lattice, it allows the different physiological classes of fibers to be characterized and then studied further by other techniques. The perfect correlation between physiological and molecular properties as assayed by gel electrophoresis after sinusoidal analysis demonstrates this and justifies its use in distinguishing between fiber types.