[Effects of a beta 2-stimulant, ritodrine hydrochloride, on the fetuses and dams in the late stage of pregnancy: an experimental study using rabbits].

Ritodrine hydrochloride (ritodrine) is a beta 2-adrenoceptor agonist and has been effectively prescribed for the prevention of premature labor. The present study was carried out to investigate the effect of ritodrine on uterine motility, cardiovascular system, fetal heart rate and placental blood flow in late pregnant rabbits, in comparison with the effects of isoproterenol and isoxsuprine. Furthermore, we investigated the effect of ritodrine on the damage to utero-placental circulation evoked by drugs. Ritodrine (10-1,000 micrograms/kg) suppressed the spontaneous motility of pregnant rabbit uterus (27-29th days of gestation) in a dose-dependent manner. Maternal blood pressure and heart rate were little affected by ritodrine. Isoxsuprine induced hypotension and isoproterenol increased the heart rate. Ritodrine and isoproterenol had little effect on the fetal heart rate. On the other hand, isoxsuprine induced marked fetal bradycardia. Ritodrine, isoxsuprine and isoproterenol showed a tendency to decrease the placental blood flow. Isoxsuprine was more potent than the other two drugs and needed a long time for restoration. Ritodrine restored the changes in placental blood flow or fetal heart rate evoked by drugs. These results suggest that ritodrine effectively suppresses uterine motility and has little effect on the maternal land fetal cardiovascular system.