Redistribution of myosin heavy chain mRNA in the midregion of stretched muscle fibers.

Myosin mRNA distribution was compared to the distribution of striations, nuclei, and cytoskeletal components in normal fibers and in fibers undergoing growth and repair processes in response to stretch. Plantarflexion of rabbit lower hindlimb for 4 or 6 days resulted in a 35% increase in weight of the tibialis anterior muscle. Slow myosin expression in stretched fibers increased such that the proportion of fibers shifted from the fast type towards an intermediate type. Semi-quantitative in situ hybridization revealed a large increase in concentration of slow myosin mRNA in stretched fibers. Polysomes translating myosin heavy chain were excluded from the intact myofibrillar lattice. Significant increases of myosin mRNA concentration occurred only in the outer 8 microns subsarcolemmal annulus of these stretched fibers (P less than 0.001) where myofibril formation also was evident. In some fibers, stretch caused myofibrillar disorder where nuclei became centrally located, and focal concentrations of myosin mRNA also occurred. We discuss mechanisms for mRNA accumulation and favor free diffusion to loosely packed cytoplasmic regions where myosin is needed for myofibrillar growth and repair.