Myosin heavy chain composition of the human lateral pterygoid and digastric muscles in young adults and elderly

The myosin heavy chain (MyHC) content in different parts of, two jaw openin g muscle, the human lateral pterygoid and the digastric muscles of five you ng adult and five elderly subjects (mean age 22 and 73 years, respectively) was determined, using gel electrophoresis and immunohistochemical methods. The lateral pterygoid of both young and elderly contained predominantly sl ow MyHC, and fast A MyHC was the major fast isoform. In contrast, the digas tric was composed of slow, fast A and fast X MyHCs in about equal proportio ns in both age groups. About half of the lateral pterygoid fibres contained mixtures of slow and fast MyHCs, often together with alpha-cardiac MyHC. I n the digastric, co-existence of slow and fast MyHCs was rare, and alpha-ca rdiac MyHC was lacking. On the other hand, co-expression of fast A and fast X MyHCs was found more often in the digastric than in the lateral pterygoi d. In both age groups about half of the digastric IIB fibres contained sole ly fast X MyHC. In the lateral pterygoid, type IIB fibres with pure fast X MyHC was found in only one subject. The lateral pterygoid in elderly showed a significant amount of fibres with solely fast A MyHC, which were occasio nally found in young adults. In the digastric, no significant differences w ere found between young and elderly, although the muscles of elderly contai ned lower mean value of slow MyHC, as compared to that of young muscles. It is concluded that the lateral pterygoid and the digastric muscles differ n ot only in the MyHC composition but also in modifications of the MyHC pheno types during aging, suggesting that they have separate roles in jaw opening function.