Abstract
Contractile properties of skinned muscle fibres from the masseter muscle and strips of heart atrium muscle from rabbits, both containing the alpha-cardiac myosin heavy chain isoform (alpha-cardiac MHC), were investigated and compared with those of other skeletal muscle fibre types. The stretch-induced delayed force increase (stretch activation) was investigated on maximally Ca(2+)-activated skinned preparations as an index of the kinetic properties of the myosin heads of various MHC isoforms. Skeletal muscle fibres containing exclusively alpha-cardiac MHC (type alpha) and muscle strips of heart atrium showed specific kinetics of stretch activation intermediate between those of types IIA and I fibres. In agreement with available data the unloaded shortening velocity V(u) of type alpha fibres was also intermediate between that of types IIA and I fibres. Compared with skeletal muscle type alpha fibres, muscle strips of heart atrium exhibited significantly (P<0.001) faster kinetics of both stretch activation and V(u). We conclude that type alpha fibres have characteristic kinetic properties that fill the gap between the fast type II and the slow type I fibres, and the following order of velocity can be established: IIB>IID(X)>IIA>alpha>I.
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