Contractile properties and myosin heavy chain isoform composition in single fibre of human laryngeal muscles

In the present study we aimed to determine the functional properties and the myosin heavy chain (MHC) isoform composition of single chemically skinned fibres from the vocal muscle of four adult men (age: 55-67 years). Single fibres, dissected from the bioptic samples, were chemically skinned and isometric tension (P₀) and maximal shortening velocity (V₀) were measured at pCa 4.6. MHC and myosin light chain (MLC) composition of fibre segments and MHC distribution of the biopsy samples were analysed by SDS-poly-acrylamide gel electrophoresis (SDS - PAGE) and densitometry. Four MHC isoforms (1,2A, 2X and a fourth isoform, provisionally called L) and five MLC isoforms (MLC1s, MLC1f, MLC3f, MLC2f, MLC2s) were identified. The major findings of this study were: (1) fast MHC isoforms (in particular MHC-2A) and fast fibres were predominant, (2) one-third of the fibres were mixed or hybrid, i.e. expressed more than one MHC isoform, (3) V₀ and P₀ values were determined by the MHC isoform composition and mixed fibres showed functional properties which were intermediate between pure fibres; MHC-L was associated with V₀ values similar to those of MHC-2A, (4) compared with limb muscles, V₀ values of laryngeal fibres were similar to those of limb muscle fibres containing the same MHC isoform whereas P₀ values were lower for slow and fast 2X fibres and similar for fibres expressing MHC-2A.