Stretch activation and isoforms of myosin heavy chain and troponin-T of rat skeletal muscle fibres

Recent studies on single mammalian skeletal muscle fibres revealed a correlation between the kinetics of stretch-induced delayed force increase (stretch activation) and the isoforms of the myosin heavy chain. This observation suggests a causal relation between stretch activation and myosin heavy chain. However, the assumption is weakened by the fact 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 unknown. For a variety of reasons, tropomyosin-binding troponin subunit is a possible candidate for being involved in stretch activation. Therefore, we measured stretch activation of single, maximally Ca2+-activated skinned rat skeletal muscle fibres and characterized them by their myosin heavy chain composition,as well as by the isoform species of tropomyosin-binding troponin subunit. Four myosin heavy chain isoforms (I, IIa, IId or IIx and IIb) and six tropomyosin-binding troponin subunit isoforms (TnT(1s), TnT(2s), TnT(1f), TnT(2f), TnT(3f), TnT(4f)) were distinguished. The following preferential coexpression patterns of the myosin heavy chain and tropomyosin-binding troponin subunit isoforms were observed: MHCI-TnT(1s), MHCIIa-TnT(3f), MHCIId-TnT(1f), and MHCIIb-TnT(4f). Stretch activation kinetics was found to be correlated with the myosin heavy chain isoform complement also in fibres not displaying 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.