Transfer of terbutaline across the human placenta in late pregnancy.

Placental transfer of terbutaline was studied in 22 women in late pregnancy who were delivered by elective Caesarian section. A single i.v. dose of terbutaline (0.25 or 0.5 mg) was given at various times (13-295 min) before delivery. Immediately after delivery, one blood sample was drawn from the placental side of the umbilical vein and one from the mother's antecubital vein. By use of gas chromatography plus mass spectrometry terbutaline was assayed in maternal plasma and in plasma and whole blood from the umbilical vein. Plasma concentrations in the mothers (Cmv) were initially 7 micrograms/L, while the highest umbilical venous level ( Cuv ) recorded was 3.5 micrograms/L. The ratio Cuv /Cmv increased continuously during the time interval studied and approached unity after 2-3 h. The blood:plasma concentration ratio in venous umbilical blood was initially low. It reached unity after about 60 min, but increased steadily to about 1.5 during the time of study. Thus there was a continuous uptake of terbutaline from plasma into the erythrocytes. The slow in vivo equilibration of terbutaline between plasma and erythrocytes is probably due to the low lipophilicity of the drug. However, the latter characteristic did not seem to impede its diffusion across the placenta to any great degree.