Ontogenesis of triiodothyronine nuclear receptors in three skeletal muscles in male and female chicks.

The ontogenesis of nuclear triiodothyronine receptors was determined in the pectoralis (alpha W fibers: fast contracting, glycolytic metabolism), adductor brevis (alpha R fibers: fast contracting, oxido-glycolytic metabolism) and adductor magnus (beta R fibers: slow contracting, oxidative metabolism), muscles of male and female chickens at 18 days in ovo and 0, 6, and 30 days ex ovo. In the fast muscles (adductor brevis and pectoralis major), the T3 receptor number decreases from the 18th day of incubation to hatching or the 6th day after hatching, respectively, and then increases. In the slow muscle (adductor magnus), the T3 receptor number increases from 18 days in ovo to hatching and then decreases. At 30 days after hatching, the number of T3 receptors was higher in the fast muscles than in the slow one. A comparison of the two fast muscles reveals that the number of T3 receptors was more elevated in the pectoralis (glycolytic) fibers than in the adductor brevis (oxido-glycolytic) fibers. An overall test of significance showed a higher number of T3 nuclear receptors in muscles from females than from males.