Myosin heavy chain isoforms in human laryngeal muscles: an expression study based on gel electrophoresis.

Laryngeal muscles in mammals are involved in highly specialized functions such as control of air passage, sphincter for airway protection and phonation. In this study, we aimed to assess whether such specialized functions are accompanied by specific distributions of fibre types. To this end, we studied the expression of the isoforms of the heavy subunit of myosin, myosin heavy chain (MyHC), which are considered the molecular markers of fibre types. The pattern of MyHC isoform expression was determined in 5 intrinsic laryngeal muscles: tensors as thyroarytenoid (TA), vocal (Vo), cricothyroid (CT), adductors as transverse arytenoid or interaytenoid (IA), and abductors as posterior cricoarytenoid (PCA), and 2 extrinsic muscles such as thyropharingeal (TPh) and cricopharingeal (CrPh). Muscles were sampled in 14 patients (11 males and 3 females, age 55-75 years) subjected to laryngectomy. MyHC isoforms were separated by SDS-PAGE and quantified by computer assisted densitometry. The comparison between the proportions of MyHC isoforms showed that: i) the three isoforms expressed in trunk and limb muscles (I, IIA and IIX) were present in all muscles examined, ii) the fast or type II MyHC isoforms were predominant in all muscles, representing 60-70% of the total, with no significant differences with respect to muscle types, iii) an additional isoform (indicated as MyHC L) was present in the muscles of the majority of the subjects and was more abundant in Vo and IA. MyHC L was, however, not found in any muscle of the three female patients and was never found in TA and TPh, iv) a great interindividual variability in MyHC distribution was present in all muscles.