THYROID INFLUENCE ON SEQUENTIAL APPEARANCE OF MYOSIN HEAVY-CHAIN ISOFORMS AND MUSCLE GROWTH IN GROWING MALE TURKEYS (MELEAGRIS-GALLOPAVO)

Roles of thyroid hormones on the sequential transition of myosin heavy chain isoforms in growing turkeys were determined by feeding triiodot hyronine (T-3) and methimazole (MMI) at levels of 0.5 ppm and 0.1%, re spectively, from 1 day through 4 weeks of age with the crossover of tr eatments at 2 weeks of age, Then, turkeys were reared by a conventiona l feeding regimen through 16 weeks, Dietary T-3 and MMI induced hypert hyroidism and hypothyroidism, respectively, and reduced body and breas t-muscle weights, Adverse effects of earlier episodes of hyperthyroidi sm and hypothyroidism on body weights remained evident at 16 weeks. Th e transition from embryonic to neonatal and to adult isoforms in Pecto ralis muscle was delayed under both hyperthyroid and hypothyroid condi tions, The appearance of the neonatal isoform was not affected immedia tely by MMI treatment, But, 2 weeks after the treatment started, the l evel of the neonatal isoform level was depressed, The level of the adu lt isoform was depressed immediately by T-3 treatment, The interruptio n of hypothyroidism and the replacement of MMI with T-3 at 2 weeks cau sed a marked reduction in the adult isoform level, It was concluded th at the timing of isoform switch in Pectoralis muscle was altered by bo th hypothyroidism and hyperthyroidism, but by different mechanisms.