THE CONTINUUM OF PURE AND HYBRID MYOSIN HEAVY CHAIN-BASED FIBER TYPESIN RAT SKELETAL-MUSCLE

The myosin heavy chain (MHC)-based fibre composition of adult rat addu ctor magnus (AM) and tibialis anterior (TA) muscles was investigated u sing single. fibre analysis. Microelectrophoresis performed on single fibre fragments demonstrated a predominance of pure fast MHC-based fib re types (expressing only one fast MHC). Most of the fibres analysed f rom both the AM (72%) and TA (50%) were pure type IIB (expressing only MHCIIb). Pure type IID fibres (expressing only MHCIId) were also abun dant in AM (20%) and TA (18%). In addition, hybrid fibres coexpressing MHCIIb and MHCIId in varying proportions (fibre types IIBD and IIDB) were found, as well as fibres coexpressing MHCIId and MHCIIa with a pr edominance of MHCIId (type IIDA) and some C fibres (coexpressing MHCI and MHCIIa in varying proportions). Considered altogether, these data reflect the dynamic nature of adult skeletal muscle fibres and indicat e a continuum of MHC-based fibre types in normal rat muscle with trans itions in the order IIB<-> IIBD<->IIDB<->IID<->IIDA<->IIAD<->IIA <->II C<->IC<->I.