Single-fiber myosin heavy-chain isoform composition of rodent laryngeal muscle - Modulation by thyroid hormone

Background: Studies have shown that canine laryngeal muscle contains a larg e number of muscle fibers that coexpress varying combinations of myosin hea vy-chain (MyHC) isoforms. Currently, it is not clear whether this phenomeno n is unique to canine laryngeal muscle or occurs in all mammals. Objectives: To examine the single-fiber MyHC isoform composition of rodent laryngeal muscle and to examine the plasticity of single-fiber MyHC isoform composition via manipulation of thyroid state. Results: (1) Findings of single-fiber electrophoretic analyses clearly demo nstrate that most fibers in both the posterior cricoarytenoid and thyroaryt enoid muscles exhibit MyHC polymorphism. However, the proportions and patte rns of polymorphism appear to be muscle specific. (2) Although the fast typ e IIL isoform was observed in fibers from both muscles, it was always coexp ressed in combination with other MyHC isoforms tie, no pure type IIL fibers were found), and always represented a minor proportion of the total MyHC p ool. (3) Altering the thyroid state proved a useful tool for exploring the scope of MyHC isoform expression in these muscles. While the posterior cric oarytenoid muscle seemed more sensitive to the thyroid state, transitions i n both muscles were primarily confined to the fast type II); and IIB MyHC i soforms. Conclusion: The findings of this study support the concept that single-fibe r MyHC polymorphism occurs commonly in mammalian laryngeal muscle.