ISOMYOSIN PATTERNS OF SINGLE TYPE-IIB, TYPE-IID AND TYPE-IIA FIBERS FROM RABBIT SKELETAL-MUSCLE

The present study demonstrates for the first time isomyosin patterns o f the three fast-twitch fibre types IIB, IID/X, and IIA. Single muscle fibres were dissected from freeze-dried fibre bundles of rabbit adduc tor magnus, extensor digitorum longus, and psoas muscles. Pure fibre t ypes, expressing only one myosin heavy chain isoform (MHCIIb, MHCIId/x , MHCIIa), were delineated by electrophoresis of fibre fragments under denaturing conditions. Pieces of the same fibres were then subjected to electrophoresis under non-denaturing conditions. A three-band patte rn of fast isomyosins, representing the LC3f homodimer (FM1), the LC1f /LC3F heterodimer (FM2), and the LC1f homodimer (FM3), was detected in each of the three pure fibre types. Therefore, three isomyosins, diff erent in their light chain complement, coexist in each pure fibre. The relative mobilities of the three bands, migrating in the order FM1 > FM2 > FM3, were the same in the three fibre types. The absolute electr ophoretic mobilities of the MHCIlb-, MHCIId- and MHCIIa-based isomyosi n triplets differed in the order MHCIIb triplets > MHCIId triplets > M HCIIa triplets. The proportions of FM1, FM2, and FM3 varied in type II B, IID, and IIA fibres. FM2 was the dominant isomyosin in all three fi bre types, but fibre type-related differences existed in the FM1 to FM 3 ratio. This ratio was lowest in IIA fibres and highest in IIB fibres which agrees with our previous observations that the LC3f/(LC1f + LC3 f) fraction is lowest in type IIA and highest in type IIB fibres.