CONSTRICTIVE EFFECT OF THYROXINE ON THE DUCTUS-ARTERIOSUS IN FETAL RATS

This work was conducted to know whether thyroxine (T4) when injected i nto fetal rats would induce a constriction of the ductus arteriosus (D A). In Experiment 1, fetal rats on day 20 of gestation were given a su bcutaneous injection of 1 or 10 mug T4 through the maternal uterine wa ll, and were autopsied 3 or 6 hr later. Similarly, in Experiment 2, th e mother rats were given an injection of hydrocortisone (10 mg/kg) jus t after fetal T4 injections. In either series of experiments, uninject ed littermates served as controls. According to the whole-body freezin g and shaving method, the DA was exposed and calibrated under a dissec ting microscope. In Experiment 1, the DAs of the T4-injected fetuses, 3 hr later, were significantly smaller in caliber than those of their controls, but recovered 6 hr later. In Experiment 2 with fetal T4 inje ction followed by maternal hydrocortisone injection, the DAs of the T4 -injected fetuses were further smaller in caliber than those of their controls which were clearly shrunken as compared with those of control s in Experiment 1. These results indicate that T4 exerts a constrictiv e effect on the DA, an effect which is strengthened in the presence of hydorocortisone.