Comparison of Functional Characteristics of Myosin in Fast and Slow Skeletal Muscles.

Myosins of fast and slow skeletal muscles differ by the isoform composition of the heavy and light chains. We compared functional characteristics of myosin from the fast (m. psoas) and slow (m. soleus) muscles of rabbits. The parameters of single actin-myosin interaction were measured in an optical trap, and the characteristics of the Ca regulation of actin-myosin interaction were studied using an in vitro motility assay. The duration of interaction of myosin from the fast muscle with actin was shorter and the filament sliding velocity over this myosin was higher than the corresponding parameters for myosin from the slow muscle. The dependence pCa-velocity for myosin from the fast muscle was less sensitive to Ca than that of slow muscle myosin. Thus, functional properties of myosin determine not only mechanical and kinetic characteristics of muscle contraction, but also the peculiarities of its Ca regulation.