Myosin isoform expression in dog rectus muscles: patterns in global and orbital layers and among single fibers

To quantitate the distribution of myosin heavy chain (MyHC) isoforms along the global and orbital layers of dog rectus muscles and determine MyHC and myosin light chain (MLC) isoform patterns among single fibers from both layers.

Serial samples of both layers of rectus muscles were prepared for gel electrophoresis. Relative amounts of each MyHC isoform in each sample were determined with scanning densitometry. Single fibers were isolated from each layer for analyses of MyHC and MLC isoforms.

Nine MyHC isoforms were detected. Four prominent MyHC isoforms, and an additional MyHC isoform at very low levels, are expressed in the global layer. Evidence suggests that all nine MyHC isoforms are expressed in the orbital layer. There are marked gradients in the levels of some MyHC isoforms along the length of both layers. Complex patterns of coexpression of multiple MyHC isoforms exist in single fibers from both layers. Most fibers express conventional slow or fast MLC isoforms, in accordance with the type (slow or fast) of MyHC isoform(s) in a given fiber, with the exception that slow fibers in the orbital layer express the atrial/embryonic isoform of MLC1.

MyHC isoform expression patterns differ markedly between and along global and orbital layers of dog rectus muscles, with greater complexity in the orbital layer. Heterogeneity in MyHC isoform expression in rectus muscles is much greater than in limb muscles and presumably is the basis for the broad spectrum of extraocular muscle (EOM) contractile properties in driving oculomotor functions.