Modulation of transcription in rat liver by benzo[a]pyrene.

Single intraperitoneal administration of benzo[a]pyrene (B[a]P) at 20 mg/kg body weight to male Wistar rats caused an early (2 h) inhibition in liver of gross transcription as measured by incorporation of [14C] orotic acid into nuclear RNA. This inhibition was reversed gradually into a partial stimulation at 2-4 days after administration of B[a]P, followed by another period of inhibition at 7 days which persisted up to 14 days. The early reversible inhibition at 2 h was probed further by studies on in vitro transcription using isolated nuclei from treated rats. It was observed that total expression of RNA polymerase activity was only marginally inhibited because, while expression of RNA polymerases II and III showed remarkable inhibition, RNA polymerase I showed stimulation in activity.