The effects of thyroxine and methimazole treatment on the synthesis of prostacyclin (PGI2) in the rat

The effects of thyroxine (T4) and methimazole administration on plasma prostacyclin (PGI₂) levels $\text{in vivo}$ and on PGI₂ release by aortic rings incubated $\text{in vitro}$ were investigated in rats. Male rats were given single injection of T4 (200 μg/100 g body wt) ip every 24 h for either 3, 7 or 14 days for hyperthyroid rats. For hypothyroid rats, a group of rats were given methimazole (0.01 % in drinking water) for 14 days. PGI₂ concentrations were determined in plasma and also in the medium in which aortic rings were incubated. PGI₂ was measured as 6-keto-PGF1α by RIA. Plasma PGI₂ levels in T₄-treated groups were found to be significantly higher than those of control animals. Aortic rings obtained from rats given single injection of T₄ for 7 and 14 days showed significant increases in release of PGI₂ into the incubation medium. In contrast, rats given methimazole for 14 days showed a significant decrease in the production of PGI₂ by aortic rings without any significant changes in plasma levels. Direct addition of T₄ into the incubation medium did not cause any significant changes in PGI₂ release by aortic rings obtained from control rats.These results suggest the regulatory role of thyroid hormone in PGI₂ synthesis $\text{in vivo}$.