Fibre types and myosin heavy chain expression in the ocular medial rectus muscle of the adult rat

Myosin heavy chain (MHC) expression was determined immunohistochemically in individual muscle fibre types characterised by activities of ATPase and th e key oxidative and glycolytic enzymes in rat ocular medial rectus (MR) mus cles. In the global layer (GL), glycolytic activity of muscle fibres was hi gher and oxidative activity lower, than in the orbital layer (OL). Muscle f ibres in the former displayed rosette-like organisation with a slow fibre s urrounded by several fast fibres, which expressed either MHCIIa or MHCIIb, but many co-expressed both isoforms. In the OL some slow fibres co-expresse d MHCIIa. Extraocular MHC isoform (MHCeom) could not be determined immunohi stochemically and no pure MHCIIx/d containing fibres were found, suggesting that these isoforms, demonstrated electrophoretically, are co-expressed wi th others. Slow muscle fibres in both layers co-expressed MHC beta slow, MH C alpha cardiac and MHC-slow tonic. Neonatal isoform (MHCneo) was co-expres sed in several fast and slow muscle fibres in the orbital, but not global l ayer. Slow fibres in the GL displayed very low oxidative activity. Electrop horetic analysis of ocular MR muscle homogenates revealed that about 50% of total MHC was MHCIIb, MHCeom was quite prominent (25%), and MHCIIa, MHCIIx /d and MHCI contributed each about 8%. MHCneo, MHCslow tonic and MHC alpha cardiac could not be identified as separate bands.