Adult human pharyngeal muscle fibers express some unusual myosin-heavy chain isoforms

Abstract

Problem: Fiber type and myosin-heavy chain (MHC) composition in the human pharyngeal constrictor (PC) and upper esophageal sphincter (UES) muscles were analyzed to better understand the muscle functions. Methods: Twelve adult human PC and UES muscles obtained from autopsies were studied using immunohistochemical and immunoblotting techniques. For comparison, masseter and biceps brachii muscles of the same subjects served as controls. Serial cross-sections were incubated with a panel of isoform-specific anti-MHC monoclonal antibodies. Results: The human PC and UES muscles were composed of 2 histochemically defined fiber layers, a slow inner layer (SIL) (78% type I) and a fast outer layer (FOL) (59% type II). The adult PC and UES fibers expressed unusual MHC isoforms (ie, slow-tonic, alpha-cardiac, embryonic, and neonatal) that were also identified in the masseter, but not in the biceps brachii. These unusual MHC isoforms demonstrated by immunocytochemistry were confirmed by electrophoretic immunoblotting. The slow-tonic and alpha-cardiac fibers were concentrated mainly in the SIL, whereas the developmental MHC-containing fibers were located primarily in the FOL. The unusual MHC isoforms were coexpressed with the major MHC isoforms (ie, slow type I and fast type II), thus forming various major/unusual (m/u) MHC hybrid fiber types. The slow-tonic and alpha-cardiac MHC isoforms coexisted mainly with type I MHC, whereas the embryonic and neonatal MHC isoforms were coexpressed primarily with type IIa MHC. At least 8 m/u MHC hybrid fiber types were identified in these muscles. Conclusion: The multiplicity of MHC isoforms in the PC and UES fibers is believed to be related to embryonic origin, innervation, and unique functional requirements. Significance: Age-related changes in the fiber type and MHC composition in the human PCs and UES may be associated with pathogeneses of dysphagia and obstructive sleep apnea that are commonly seen in the elderly. Support: None reported.