Paradoxical effects of cycloheximide on the ultra-rapid induction of two hepatic mRNA sequences by triiodothyronine (T3).

Triiodothyronine (T3) rapidly induces the accumulation of two hepatic mRNA sequences (spot 14 and spot CyT) in thyroidectomized rats as revealed by two-dimensional gel electrophoresis of in vitro translated products of isolated poly(A) containing RNA. T3 acting alone induced a 29-fold increase of spot 14 within 4 h, an increase which was completely inhibited by the concomitant administration of cycloheximide. On the other hand, CyT could be detected only after the administration of cycloheximide, and the combined action of cycloheximide and T3 resulted in a 4-fold increase in CyT 4 h after administration of both agents. These observations suggested that the early cellular action of T3 is contingent on the participation of rapidly turning over protein and mRNA.