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P.J. Reiser, S. Bicer; Myosin Heavy Chain and Light Chain Isoforms in Slow Orbital and Global Fibers of Feline Rectus Muscles . Invest. Ophthalmol. Vis. Sci. 2004;45(13):56.
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Purpose: Myosin isoforms play a pivotal role in determining contractile properties of vertebrate skeletal, cardiac and smooth muscles. We recently reported (Bicer and Reiser, Invest. Ophthalmol. Vis. Sci., 2004) that slow muscle fibers in canine global and orbital layers differ with respect to myosin light chain (MLC) 1 isoform expression, with slow orbital fibers expressing atrial MLC1 (identical to embryonic MLC1 in skeletal muscle). We chose to examine feline extraocular muscles in this study to determine whether myosin isoform expression differs between slow orbital fibers and slow global fibers in another mammalian species. Methods: Single muscle fibers were isolated from global and orbital layers of feline rectus muscles and were analyzed with denaturing gel electrophoresis to determine patterns of myosin heavy chain (MHC) and MLC isoform compositions among individual fibers. Results: Slow and fast fibers from the global layer of feline rectus muscles appear to express the same MLC isoforms as slow and fast limb muscle fibers, as is the case for canine global layer fibers (Bicer and Reiser, IOVS, 2004). Slow fibers from the orbital layer of feline rectus muscles, on the other hand, express the slow isoform of MLC2 but do not express slow MLC1. Instead, slow orbital fibers appear to express predominantly the fast isoform of MLC1. These fibers also express two MHC isoforms – MHC–I (the same MHC as in all other feline slow fibers) and an isoform that co–migrates with MHC–alpha, the predominant MHC isoform in cardiac atria which is not normally expressed in mammalian limb muscles. Conclusions: Slow fibers in feline orbital layers differ from slow fibers in global layers and limb muscles with respect to MHC and MLC isoform composition. Therefore, feline and canine slow orbital fibers express isoforms of myosin subunits that distinguish them from other rectus muscle fibers. Unusual patterns of myosin isoform expression may be a common pattern among slow fibers in orbital layers of mammalian rectus muscles and may contribute to unique contractile properties in these fibers.
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