STRETCH ACTIVATION, UNLOADED SHORTENING VELOCITY, AND MYOSIN HEAVY-CHAIN ISOFORMS OF RAT SKELETAL-MUSCLE FIBERS

1. Contractile properties were investigated on single skinned-fibre pr eparations from rat leg muscles. Following the mechanical measurements , the myosin heavy chain (HC) composition of the same fibre was analys ed by gradient gel electrophoresis. 2. Fibres were typed according to their myosin HC isoform composition (HCI, type I; HCIIA, type IIA; HCI ID, type IID; HCIIB, type IIB). Many fibres showed the co-existence of two myosin HC isoforms (hybrid fibres). 3. A strong correlation was f ound between fibre type and time characteristics of stretch-induced de layed force increase (stretch activation) of fully Ca2+-activated fibr es. 4. The maximal unloaded shortening velocity (V-max), as measured w ith the slack test, was lowest in type I fibres. Within the type II gr oup, a continuum of V-max values was found, with large overlaps of the different fibre types. 5. The results suggest that the kinetics of st retch activation is determined by the myosin HCs whereas unloaded fibr e shortening seems to be determined by other myofibrillar proteins in addition to the myosin HCs. Assuming that stretch activation represent s certain steps of the cross-bridge turnover under isometric condition s and V-max reflects cross-bridge detachment under unloaded conditions it can be deduced that different myofibrillar proteins are responsibl e for different steps within the cross-bridge turnover.