Myosin heavy chain 2B isoform is expressed in specialized eye muscles but not in trunk and limb muscles of cattle

Myosin heavy chain isoforms (MHC) of adult skeletal muscles are codified by four genes named: slow, or type 1, and fast types 2A, 2X and 2B. The slow, 2A and 2X isoforms have been found expressed in all mammalian species studied so far whereas there is a large inter-species variability in the expression of MHC-2B. In this study histochemistry (m-ATPase), immunohistochemistry with the use of specific monoclonal antibodies and RT-PCR were combined together to assess whether the MHC-2B gene is expressed in bovine muscles. ATPase staining and RT-PCR experiments showed that three MHC isoforms (1, 2A, 2X) were expressed in trunk and limb muscles. Slow or type 1 expression was confirmed using a specific antibody (BA-F8) whereas the detection of fast MHC isoforms were validate by means of BF-35 antibody although not by the SC-71 antibody. MHC-2B was absent in limb and trunk muscles, but was present in specialized eye muscles (rectus lateralis and retractor bulbi) as consistently showed by RT-PCR and reactivity with a specific antibody (BF-F3). Interestingly, a cardiac isoform, MHC-a-cardiac was found to be expressed not only in extraocular muscles but also in masticatory muscles as masseter.