EXPRESSION OF ALPHA-CARDIAC MYOSIN HEAVY-CHAIN IN NORMAL AND DENERVATED RAT MUSCLE-SPINDLES

Whether any fibers in rat hindlimb muscles express alpha-cardiac myosi n heavy chain (MHC) is uncertain. Expression of alpha-cardiac MHC mRNA and the polypeptide for which it codes were examined in control and d enervated rat muscle spindles using in situ hybridization (ISH) and im munocytochemistry (ICC). Both nuclear bag(2) and bag(1) intrafusal fib ers in the extensor digitorum longus (EDL) muscles expressed alpha-car diac MHC and its precursor mRNA. Furthermore, denervation of the hindl imb down-regulated alpha-cardiac MHC mRNA expression in rat nuclear ba g intrafusal fibers, even though they continued to display a strong af finity fro anti-alpha-cardiac MHC monoclonal antibody. These data show that (1) intrafusal fibers express the alpha-cardiac MHC gene; (2) in nervation regulates alpha-cardiac MHC gene expression at a pre-transla tional level; and (3) ISH is more sensitive than ICC to changes in alp ha-cardiac MHC gene expression in adult rat muscle spindles.