Skeletal muscule fiber types in C57BL6J mice

RESUMO

Rat skeletal muscle contains up to four myosin isoforms. Analysis of single muscle fibers has identified type I, IIA, IID and IIB fibers containing type I, IIa, IId and IIb myosin isoforms, respectively. Hybrid fibers have at least two myosin isoforms, such as myosin heavy chain (MHC) I+MHCIIa, MHCIIa+ MHCIId or MHCIId+ MHCIIb. These fiber types are identified as IC, IIC, IIAD, IIDA, IIDB or IIBD. The muscles of C57BL6J mice have not been extensively characterized, especially with regard to the presence and composition of hybrid fibers. The aim of this study was to examine the fiber composition of the soleus (SOL), extensor digitorum longus (EDL), tibialis anterior (TA) and gastrocnemius (GAS) muscle of C57BL6J mice using histochemical and biochemical methods. The SOL muscle contained type I fiber poncentage, median +- semi amplitude, (37, 42 +- 8, 20 per cent), IIa (38, 62 +- 6, 81 per cent), IIAD (18, 74 +- 6, 95 per cent) and IIAD (5, 69 +- 3, 09 per cent) fibers. The EDL had type I (0, 44 +- 1, 27 per cent), IC/IIC (3, 05 +- 3, 49 per cent), IIA (0, 46 +- 0, 68 per cent), IIAD (7, 56 +- 4, 51 per cent), IID (0, 46 +- 1, 34 per cent), IIDB (21, 48 +- 7, 33 per cent) and IIB (66, 01 +- 8, 51 per cent) fibers. The Ta muscle also showed a predominance of type IIB (59 ,68 +- 9, 95 per cent) and IIDB (33, 83 +- 15, 85 per cent) fibers followed by type IIAD (3, 42 +- 4, 87 per cent), IID (2, 25 +- 1, 64 per cent), IIA (1, 12 +- 2, 17 per cent), I (0, 17 +- 1, 25 per cent) and IC/IIC (0, 008 +- 1, 04 per cent) fibers. The GAS muscle had type IIB (54, 42 +- 8, 11 per cent), IIDB (19, 37 +- 2, 98 per cent), IID (2, 26 +- 2, 24 per cent), IIAD (12, 40 +- 2 ,34 per cent), IIA (5, 73 +- 3, 24 per cent) and I (5, 74 +- 2, 55 per cent) fibers. Thus, the SOL muscles of C57BL6J mice had predominantly type IIA fibers, unlike rats in which almost all of the fibers are type I. The EDL, TA and GAS muscles had prenominantly IIB and IIDB fibers as in rats.