Myosin heavy chain composition of rat middle ear muscles

Objective - To quantitatively analyze myosin heavy chain (MHC) mRNA composition in two rat middle ear muscles (the tensor tympani and stapedius) using competitive polymerase chain reaction (PCR). Material and Methods - An exogenous template, including oligonucleotide sequences specific for the seven rat MHCs (2A, 2B, 2X, 2L/EOM, embryonic, neonatal and β-cardiac) as well as β-actin, was constructed and used as the competitor. Results - The tensor tympani and stapedius contained all MHC isoforms except 2L. The tensor tympani contained approximately equal proportions of 2X (40.4%±6.5%) and 2A (34.0%±1.3%) MHCs, with a smaller percentage of 2B (16.6%±1.5%) and neonatal (7.5%±0.6%) MHCs, while β-cardiac and embryonic MHCs were minimally expressed. The stapedius contained predominantly 2X (58.0%±4.2%) and 2A (32.3%±6.7%) MHCs, with a smaller percentage of 2B (7.4%±0.2%) and β-cardiac (1.9%±0.1%) MHCs. Neonatal and embryonic MHCs were detected at very low levels. Conclusion - These results suggest that two middle ear muscles, which are mainly composed of two fast-twitching myosins (2X and 2A MHCs), contract fast and are fatigue-resistant.