Single-fiber myosin heavy chain polymorphism during postnatal development:modulation by hypothyroidism

The primary objective of this study was to follow the developmental time co urse of myosin heavy chain (MHC) isoform transitions in single fibers of th e rodent plantaris muscle. Hypothyroidism was used in conjunction with sing le-fiber analyses to better describe a possible Linkage between the neonata l and fast type IIB MHC isoforms during development. In contrast to the gen eral concept that developmental MHC isoform transitions give rise to muscle fibers that express only a single MHC isoform, the single-fiber analyses r evealed a very high degree of MHC polymorphism throughout postnatal develop ment. In the adult state, MHC polymorphism was so pervasive that the rodent plantaris muscles contained similar to 12-15 different pools of fibers (i. e., fiber types). The degree of polymorphism observed at the single-fiber l evel made it difficult to determine specific developmental schemes analogou s to those observed previously for the rodent soleus muscle. However, hypot hyroidism was useful in that it confirmed a possible link between the devel opmental regulation of the neonatal and fast type IIB MHC isoforms.