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' hearts 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.