Molecular heterogeneity of histochemical fibre types: a comparison of fast fibres

The relationship between histochemical fibre type and contractile protein expression was analysed in three rabbit skeletal muscles, the erector spinae, the plantaris and the diaphragm. A procedure for staining fibre bundles was developed using the same histochemical methods as those for typing fibres in cross-section. This allowed pretyped fibres to be selected and their molecular composition to be analysed by gel electrophoresis. The balance of expression of the two predominant fast troponin species, TnT1f and TnT2f, and alpha and beta tropomyosin subunits were studied in type IIA and IIB fast fibres. Type IIA fibres exhibited a restricted pattern of thin filament expression, exhibited TnT1f and both tropomyosin subunits in all three muscles. The expression in type IIB fibres, however, ranges from predominantly TnT2f and the alpha tropomyosin subunit in the erector spinae to TnT1f with both alpha and beta subunits in the diaphragm. These results indicate that there is not a simple one-to-one relationship between the fast muscle fibre subtypes and the expression of different thin filament protein isoforms.