Analysis of actions of 5‐hydroxytryptamine in pregnancy

1. When 5‐hydroxytryptamine creatinine sulphate is injected subcutaneously in a dose of 2 mg into mice during the second half of pregnancy, the foetuses die within 1 hr. The mode of action of 5‐hydroxytryptamine (5‐HT) in producing this effect has been studied. 2. It has been demonstrated that this is not a direct toxic effect of 5‐HT. Very little passes through the placenta into the foetus, and after the injection of much larger amounts directly into the foetus no lethal effect was observed. 3. It was shown that 5‐HT had little effect on uterine motility in vivo , and that the much larger contractions produced by oxytocin did not produce any effect on the foetus. 4. No constriction of the umbilical vessels was produced by 5‐HT injected into the mother, nor did the local application of 5‐HT to the cord produce any effect. 5. The administration of 5‐HT to the mother markedly reduced the passage of 22 Na from the maternal circulation into the placenta and foetus. This was accompanied by a reduction in the blood supply to the placenta from a normal value of 87 μl./min/g to 4 μg/min/g. 6. It is suggested that the effect of 5‐HT in producing foetal death is a consequence of the reduction in the transfer function of the placenta.