STRETCH ACTIVATION AND ISOFORMS OF MYOSIN HEAVY-CHAIN AND TROPONIN-T OF RAT SKELETAL-MUSCLE FIBERS

Recent studies on single mammalian skeletal muscle fibres revealed a c orrelation between the kinetics of stretch-induced delayed force incre ase (stretch activation) and the isoforms of the myosin heavy chain. T his observation suggests a causal relation between stretch activation and myosin heavy chain. However, the assumption is weakened by the fac t that isoforms of other myofibrillar proteins tend to be coexpressed with myosin heavy chain isoforms. The relation between the isoforms of the tropomyosin-binding troponin subunit and myosin heavy chain is un known. For a variety of reasons, tropomyosin-binding troponin subunit is a possible candidate for being involved in stretch activation. Ther efore, we measured stretch activation of single, maximally Ca2+-activa ted skinned rat skeletal muscle fibres and characterized them by their myosin heavy chain composition,as well as by the isoform species of t ropomyosin-binding troponin subunit. Four myosin heavy chain isoforms (I, IIa, IId or IIx and IIb) and six tropomyosin-binding troponin subu nit isoforms (TnT(1s), TnT(2s), TnT(1f), TnT(2f), TnT(3f), TnT(4f)) we re distinguished. The following preferential coexpression patterns of the myosin heavy chain and tropomyosin-binding troponin subunit isofor ms were observed: MHCI-TnT(1s), MHCIIa-TnT(3f), MHCIId-TnT(1f), and MH CIIb-TnT(4f). Stretch activation kinetics was found to be correlated w ith the myosin heavy chain isoform complement also in fibres not displ aying one of the preferential MHC-TnT(f) isoform coexpression patterns . This corroborates the assumption of a causal relation between myosin heavy chain and stretch activation.