Effects of amiodarone on thyroid hormone-responsive gene expression in rat liver

The hypothesis that amiodarone (AM) acts by inducing a local 'hypothyroid-like' state in thyroid hormone-responsive tissues was investigated in rat liver. Hypothyroid rats, pretreated orally for 8 consecutive days with AM (200 mg/kg) or water, were given a single i.p. injection of equimolar doses of T4, T3 or rT3 (1.00 to 1.20 mg/kg). Six hours later, the rats were killed and liver nuclear T3 receptor occupancy was determined. Simultaneously, the activity of two thyroid hormone-responsive enzymes was measured, together with the levels of their respective mRNAs by hybridization to specific cDNAs. The enzymes were phosphoenolpyruvate carboxykinase and glutamine synthetase. AM showed no effect on nuclear T3 receptor occupancy in rats injected with either vehicle, rT3, or T3, but it completely blocked the increase in receptor occupancy in rats injected with T4. With regard to postreceptor effects, T4 and T3 elicited an approximately two-fold increase in the levels of the mRNAs coding for the two enzymes, whereas rT3 had no effect. The increase of the two mRNAs was potentiated by AM, but this is probably secondary to an AM-induced state of anorexia. Remarkably, this potentiating effect of AM was not observed at the protein-level: enzyme activities were lower in rats pretreated with AM. AM-pretreatment thus results in lower enzyme activity to mRNA ratios for both enzymes, irrespective of hormonal treatment. Therefore, although no conclusions can be drawn about possible effects of AM at the transcriptional level, it is concluded that AM interferes with thyroid hormone responsive gene expression in rat liver at a post-transcriptional level. As a consequence, in the present experimental design the livers of AM-treated rats resemble the liver of hypothyroid rats with regard to specific enzyme activities, but not with regard to either nuclear T3 receptor occupancy or the levels of specific mRNAs.