FORCE RESPONSES FOLLOWING STEPWISE LENGTH CHANGES OF RAT SKELETAL-MUSCLE FIBER TYPES

1. Force responses following stepwise length changes of Ca2+-activated skinned leg muscle fibres (6 degrees C) of the rat were correlated wi th their myosin heavy chain (HC) isoforms (myosin HC I, fibre type I; myosin HC IIA, type IIB; myosin HC IID (HC IIX), type IID (type IIX); myosin HCIIB, 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 v arious muscle fibres, and a conspicuous correlation with their myosin HC complement was noticed (order of velocity: IIB > IID > IIA much gre ater than I). No differences were found in the relationship between th e applied length step and the resulting force (T-1, T-2 curves). 3. Ou r results suggest that the heads of various myosin heavy chain isoform s exhibit different kinetic properties. The differences concern the ki netics of the myosin head movements and the duration of cyclic interac tions between myosin heads and thin filaments. The extent of force-gen erating movements and the mean elongation of attached heads in the iso metric state seem to be independent of the isoform.