Mouse and rat skeletal muscles contain slow (type 1) fibres, which express MyHC-β/slow, and three fast fibre types, 2A, 2X and 2B fibres, defined by the presence of MyHC-2A, -2X and -2B, respectively (Schiaffino & Reggiani, 1996). In contrast, human skeletal muscle contains only two fast fibre types that have been traditionally classified as type 2A and 2B based on histochemical ATPase staining. However, the finding that the human type 2B fibres contain MyHC-2X transcripts (Smerdu et al. 1994; Ennion et al. 1995) has raised the possibility that these fibres may in fact correspond to the rodent 2X fibres. The human genome contains a MyHC gene that appears to be the homologue of the murine MyHC-2B gene both with respect to sequence identity in the 5Ì and 3Ì untranslated regions and to its position in the synthenic regions of chromosomes 17 and 11 (Weiss et al. 1999). A recent paper has implied an expression of the MyHC 2B transcript in human masseter muscle (Horton et al. 2001). To clarify the nature of the human type 2B fibres we have therefore investigated the pattern of expression of this gene in human skeletal muscles. After ethical approval we examined a total of more than 100 human muscles originating from 69 different individuals (aged from 3 to 94 years), as well as fetal muscle. In short, various different muscles from the extremities including muscles originating from well trained, sedentary, and paralysed individuals and in addition an extensive number of masseter muscles, eye muscles and laryngeal muscles were analysed. The mRNA expression pattern of the human MyHC 2B was assessed by RT-PCR, Northern blotting, in situ hybridisation and dot blot with probes specific for all known human MyHCs, including 2B probes. Furthermore, SDS-PAGE, Western blotting and immunohistochemistry were used to investigate the nature of the MyHC 2B expression at the protein level, as well. We found no substantial evidence of MyHC 2B mRNA in any of the examined muscles from the extremities. Some very weak expression was detected in a single masseter muscle. Clear expression was seen in several laryngeal muscles and in all examined eye muscles. In the masseter muscles the 2B transcript was only found to be expressed in very faint amounts in some muscles. In the larynx muscle varying amounts of the 2B transcript were found, ranging from very small amounts, to being the dominant MyHC isoform transcript in the transverse arytheniodeus muscle. In the eye muscles the 2B transcript was found in all examined muscles ranging from very minor expression to being the most dominant MyHC isoform transcript. SDS-PAGE of the muscles, which showed a high 2B mRNA expression, revealed a protein band corresponding to the rat 2B band. This band was confirmed as a MyHC type 2 band by Western blotting with an antibody reacting with all rat and human type 2 MyHC.

In conclusion, we have substantial data to document that the MyHC 2B gene is expressed both at the mRNA and protein level in human skeletal muscle, but that the occurrence of the 2B MyHC is apparently restricted to very specialised muscles of the eye and the larynx.