Force responses following stepwise length changes of Ca(2+)-activated skinned leg muscle fibres (6 degrees C) of the rat were correlated with their myosin heavy chain (HC) isoforms (myosin HC I, fibre type I; myosin HC IIA, type IIA; myosin HC IID (HC IIX), type IID (type IIX); myosin HC IIB, type IIB) in order to study the mechanical properties of these molecules. 2. Marked differences in the time behaviour of force transients following quick releases of fibre length existed between various muscle fibres, and a conspicuous correlation with their myosin HC complement was noticed (order of velocity: IIB > IID > IIA > > I). No differences were found in the relationship between the applied length step and the resulting force (T1, T2 curves). 3. Our results suggest that the heads of various myosin heavy chain isoforms exhibit different kinetic properties. The differences concern the kinetics of the myosin head movements and the duration of cyclic interactions between myosin heads and thin filaments. The extent of force-generating movements and the mean elongation of attached heads in the isometric state seem to be independent of the isoform.
