In vivo and in vitro effects of thyrotropin on ribosomal pattern of dog thyroid

Abstract

The proportion of ribosomes bound to mRNA ( i.e. polysomes) was expressed as the P/P+M ratio, where P and M were the polysomes and monosomes area, respectively, measured on a ribosomal density gradient centrifugation pattern. Thyrotropin both in vivo and in vitro enhanced this P/P+M ratio. The average increase of P/P+M by thyrotropin was 20 % ( P < 0.001). Thyrotropin also increased the H/H+L ratio, where H represents the heavy polysome population ( > 6) and L the light polysome population (14 %; P < 0.02). These effects are specific for thyrotropin, they are observed within 30 min of adding thyrotropin and they are obtained for concentrations of 1 munit/ml or higher. Cyclic AMP and dibutyryl cyclic AMP reproduced the stimulation. There is a close parallelism between these effects of thyrotropin and the activation of protein synthesis by this hormone. The rapid response to thyrotropin, as well as labeling experiments with [ 3H]uridine, support the hypothesis that these effects of thyrotropin bear on translation and not on transcription. It is suggested that thyrotropin rapidly activates protein synthesis in dog thyroid slices by a process mediated by cyclic AMP which bears on translation either by unmasking of heavy mRNA or by increasing the rate of initiation of protein synthesis.