Placental transfer of radioactive salts in the pregnant rabbit

The risks of radionuclidic contamination due to the easy transfer of water-soluble ions from the circulation of the pregnant woman to her fetus, encouraged us to study whether the ease of penetration of such ions is size-dependent. Three radiochemicals--22NaCl, 99mTcO4Na and 201TlCl were injected i.v. into pregnant rabbits on the 16th day of their pregnancy, and the rabbits were killed 15, 30, 60 or 120 min later. From each rabbit the blood, heart, kidney, liver and muscle were sampled and counted as well as placenta, amniotic fluid and some fetuses. At 15 and 60 min hearts and livers were excised from selected fetuses and blood clearance, organ-to-muscle and fetus-to-placenta ratios were calculated. The results indicate that the transplacental transfer of the small radionuclide 22Na+ is faster than that of 99mTcO4- and 201Tl+, reaching equilibrium about 3 h after its injection to the pregnant rabbit. 201Tl+ demonstrated a high localization in the pregnant rabbits' and fetuses' heart and kidneys, with a similar myocardial retention in both groups. Due to the concentration of 201Tl+, in spite of its large diameter, into the fetuses' heart muscle, careful consideration should be taken when injecting 201TlCl into pregnant women for myocardial imaging.