Accumulation of newly synthesized myosin heavy chain during thyroxine-induced myofiber type transition.

Slow-tonic myofiber to fast-twitch myofiber conversion was induced in chickens by feeding thyroxine. Incorporation of newly synthesized myosin heavy chain (MHC) into myofibers and myofibrils was followed by immunofluorescence with antibodies specific for fast-twitch MHC and slow-tonic MHC. Presence of more than one type of myosin heavy chain was detected in thyroxine-induced transitional myofibers of chicken pectoralis. Myofibers undergoing a transition were histochemically identical to immunologically cross-reacting, transitional myofibers of normal anterior latissimus dorsi. Newly synthesized MHC appeared to be incorporated uniformly across the cross sectional area of transitional myofibers and incorporated homogeneously into each sarcomere of transitional myofibrils. These observations are consistent with a theory of continuous protein exchange between myofibrillar protein and a non-myofibrillar protein pool, such that every sarcomere of a myofibril, and every myofibril of a myofiber, would be turned over simultaneously.