Thyroid hormone-induced regulation of polyribosomal translational efficiency in tadpole tail muscle.

A variety of mechanisms have been proposed to explain the thyroid hormone-induced involution of the anuran tadpole tail, but supportive evidence for any of these proposals is inconclusive. Our in uiuo findings support and extend the hypothesis that a decrease in the rate of protein synthesis in tadpole tail tissue is an early and progressively sustained effect induced by in uiuo administration of thyroid hormone. Quantification of tail muscle polyribosomal profiles and in viuo nascent chain labeling patterns indicate that following hormone treatment there is a progressive loss in polyribosomes and a decrease in the relative proportion of trichloroacetic acid-insoluble [Wlleucine associated with heavy and medium sized polyribosomes. In vitro studies with tail muscle heavy and medium sized polyribosomal complexes from 2and B-day hormone-treated animals demonstrate that they are less active in protein synthesis than comparable quantities of similar sized polyribosomal complexes from control tadpole tail muscle. Use of tail muscle postribosomal supernatant from control animals with polyribosomes from B-day hormone-treated animals did not enhance the in vitro translational efficiency of these polyribosomes; however, use of tail muscle postribosomal supernatant from 5-day hormone-treated animals with polyribosomes from control tail muscle decreased the translational efficiency of these polyribosomes to the level observed in polyribosomes from 5-day hormone-treated animals. The decreased in vitro translational efficiency of these polyribosomes is due to an inhibitory factor(s) present in the postribosomal supernatant fraction from tail muscle of hormone-treated tadpoles. Thus, administration of thyroid hormone to anuran tadpoles initiates, directly or indirectly, a regulatory effect on tail muscle which operates at the translational level and results in an overall decreased rate of protein synthesis.