Thyroid hormone induces synthesis and accumulation of tropomyosin and myosin heavy chain in limb buds of premetamorphic tadpoles

Myosin heavy chain (MHC) and tropomyosin (Tm) have been isolated from limb muscles of the North American bullfrog, Rana catesbeiana, and injected into rabbits to raise monospecific antibodies. These antibodies were used to study the localization and synthesis of myosin heavy chain and tropomyosin in the limb buds of premetamorphic (stage VI–VII) tadpoles treated with triiodothyronine (T3) to induce metamorphosis. Indirect immunofluorescence localization detects the accumulation of both MHC and Tm in the developing thigh region within 24 h of T3 treatment. During the subsequent 48 h, the accumulation of these proteins is enhanced in the thigh and progresses from the thigh to the distal regions of the limb. Quantitative immunochemical determinations indicate that within 24 h of T3 treatment, synthesis of Tm and MHC are increased 23-fold and 6-fold, respectively. Following 5 days of T3 treatment, the synthetic rates of Tm and MHC are 266 and 70 times the control values, respectively. Both methods suggest that Tm is synthesized and accumulated at a greater rate than myosin heavy chain. These observations suggest that T3 promotes the differentiation of muscle in the limb buds of premetamorphic tadpoles and that limb development promoted by T3 in tadpoles is similar to that described during the embryonic development of higher vertebrates.