Distribution of Myosin Heavy Chain Isoform in Human Extraocular Muscles

Abstract

Purpose: To provide quantitative data on the distribution of MyHCeom and compare the proportion of myosin heavy chain (MyHC) isoforms between the central and peripheral regions of human extraocular muscles (EOMs). Methods: Medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique muscle samples were taken from three men with brain death. To examine the longitudinal distribution of myosin isoforms, the muscles were divided into central and peripheral portions of equal length. Electrophoresis and densitometry were used to quantify the distribution of MyHC isoforms. Results: Electrophoresis of whole-muscle extracts of sampled EOMs revealed four MyHC bands that were identified as MyHCI, MyHCeom, MyHCIIa, and MyHCIIx. The proportion of MyHCeom was higher in the central region, whereas the proportion of MyHCIIa was higher in the peripheral region. The relative proportions of MyHCI, MyHCeom, and MyHCIIa were not significantly different among the EOMs. There was a tendency for higher levels of MyHCIIx in the inferior rectus muscle. Conclusions: The proportion of MyHCeom was higher in the central region of human EOMs. Further studies are needed to investigate the consequences of this distributional difference on the function of EOMs.