Ritodrine hydrochloride is widely used in clinical obstetrics for the prevention of premature labor, but the knowledge of its effect on fetal circulation is limited. The purpose of this study was to elucidate changes in vascular resistance in the fetal placental circulation by using the dually perfused human placenta in vitro. The viability of the perfused placenta was maintained during the experiments because the production rate of hCG was constant. The transfer of ritodrine hydrochloride was observed and its concentration on the fetal side was about one third of maternal side at 120 min subsequent to the injection. The fetal perfusion pressure during the control period was 38.6 +/- 7.0 mmHg (mean +/- S.D., n=3). After the addition of ritodrine hydrochloride to the maternal circulation, the fetal perfusion pressure was decreased dose-dependently. The pressure was 95.4 +/- 1.8% at 80 ng/ml of ritodrine hydrochloride which was within the clinical concentration limits. It is concluded that ritodrine hydrochloride acts as a vasodilator on the fetal vasculature in the human placenta.
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