Modulation of myosin heavy chains in rat laryngeal muscle

Objectives: To test the hypothesis that myosin heavy chain (MHC) compositio n is a biological marker indicative of appropriate and functional reinnerva tion. Study Design: Age-matched adult rats were randomized for prospective study under three experimental conditions. Methods: In adult rats, three ex perimental conditions were surgically created, including transient recurren t laryngeal nerve (RLN) crush injury, RLN transection and repair, and crico arytenoid joint fixation with intact RLN. Animals were survived for 30, 90 and 180 days. At each interval, vocal fold mobility was assessed by rigid m icrolaryngoscopy, Laryngeal electromyography (EMG) was performed before eut hanasia. The thyroarytenoid and posterior cricoarytenoid muscles were then excised, each muscle was processed far sodium dodecyl sulfate-polyacrylamid e gel electrophoresis, and MHC composition was determined. Results: Thirty days after nerve crush injury, three of six animals regained vocal fold mob ility and normal MHC composition. Impaired vocal fold motion in three of si x animals was associated with MHC composition characteristic of denervation . At 90 and 180 days, normal vocal fold motion and normal MHC composition w ere observed in all animals. Following nerve transection and repair, impair ed vocal fold motion and MHC composition characteristic of denervation were observed in all animals, despite evidence of reinnervation on EMG. Followi ng joint fixation, alteration in MHC composition consistent with denervatio n was observed only at 30 days, as was evident in the nerve crush model. Co nclusion: Temporary injury and vocal fold immobilization result in transien t shifts in MHC composition. Nerve transection and repair result in persist ent alteration of MHC composition and vocal fold dysfunction. The expressio n of normal MHC composition is dependent on the condition of appropriate ne ural contact and functional reinnervation.