Stretch activation, unloaded shortening velocity, and myosin heavy chain isoforms of rat skeletal muscle fibres.

1. Contractile properties were investigated on single skinned-fibre preparations from rat leg muscles. Following the mechanical measurements, the myosin heavy chain (HC) composition of the same fibre was analysed by gradient gel electrophoresis. 2. Fibres were typed according to their myosin HC isoform composition (HCI, type I; HCIIA, type IIA; HCIID, type IID; HCIIB, type IIB). Many fibres showed the co-existence of two myosin HC isoforms (hybrid fibres). 3. A strong correlation was found between fibre type and time characteristics of stretch-induced delayed force increase (stretch activation) of fully Ca(2+)-activated fibres. 4. The maximal unloaded shortening velocity (Vmax), as measured with the slack test, was lowest in type I fibres. Within the type II group, a continuum of Vmax values was found, with large overlaps of the different fibre types. 5. The results suggest that the kinetics of stretch activation is determined by the myosin HCs whereas unloaded fibre shortening seems to be determined by other myofibrillar proteins in addition to the myosin HCs. Assuming that stretch activation represents certain steps of the cross-bridge turnover under isometric conditions and Vmax reflects cross-bridge detachment under unloaded conditions it can be deduced that different myofibrillar proteins are responsible for different steps within the cross-bridge turnover.