Distribution and diaplacental passage of infused 3H-fenoterol hydrobromide (partusisten) in the gravid rabbit.

Tritium-labelled fenoterol hydrobromide was infused into an ear-vein of pregnant rabbits over a period of 4 hours. Two doses were used. The tritum concentrations in the blood of the dams and the amniotic fluid were determined from samples obtained during infusion and up to 11 hours after completion of infusion. The tritium concentrations were also determined in selected organ samples from dams and fetuses, taken immediately and 5 and 11 hours after infusion. Within 1.5 hours after commencement of infusion, the tritium activity in the whole blood of the dams reached a plateau on a level corresponding to the dose administered. Immediately after completion of infusion, the level of radioactivity fell rapidly. The tritium concentration in the amniotic fluid began to rise far more slowly after a time lag, so that approximately 2 hours after completion of infusion, the concentrations in maternal whole blood and amniotic fluid were equal. As the amniotic fluid level fell at a slower rate than the maternal whole blood level, the former somewhat exceeded the latter at this advanced stage of the study. The tritium distribution patterns of the dams indicate rapid elimination via kidneys and intestine. The tritium concentration in all the tissue fell rapidly after completion of infusion. The lung and uterus have a slightly greater affinity. The tritium concentrations in the fetal organs remained below those in the organs of the dams for the whole duration of the study. The highest radioactive concentration in the fetuses was found in the urine. Further observation of the rapid conversion of fenoterol hydrobromide into conjugates leads us to expect only minimal amounts of active substance to be found in the fetuses after infusion.