Thyroidal influence on nuclear accumulation and DNA replication in skeletal muscles of young chickens.

Regulation of DNA accumulation may be a particularly important aspect of the regulation of skeletal muscle growth. Since thyroid hormones influence the growth of chicken skeletal muscle, including DNA accumulation, we ascertained whether triiodothyronine (T3) affects the accumulation of nuclei within muscle fibers and 3H-thymidine uptake into nuclei within the basal lamina and muscle total DNA. The number of nuclei and the diameter of pectoralis muscle fiber segments freed of adhering connective tissue were reduced markedly in cockerels treated with propylthiouracil (PTU) during the first month after hatching. T3 replacement for 1 week increased the number of nuclei per unit length of isolated fiber section by as much as 54%, compared to the value for birds receiving PTU alone, without affecting fiber diameter. There was also a 24% increase in the number of nuclei beneath the basal lamina of fiber cross sections near the origin of anterior latissimus dorsi (ALD) muscles within 48 hr after initiating T3 replacement in 25-day-old, PTU-treated cockerels. The incorporation of 3H-thymidine into nuclei beneath the basal lamina of ALD fiber cross sections was increased approximately two-fold at 24 hr after administering T3 to 8-day-old thyroidectomized (Tx) cockerels. Thymidine (-3H) incorporation into total DNA of ALD muscles incubated in vitro for 1 hr was also markedly enhanced as early as 6-12 hour after T3 administration to Tx chicks during the first week after hatching. Data from this study suggest that T3 enhances DNA replication and the accumulation of nuclei within the basal lamina of muscle fibers of thyroid-deprived chicks.(ABSTRACT TRUNCATED AT 250 WORDS)