Distribution and Inactivation of Labeled Parathyroid Hormone in Rat Fetus

Abstract

Labeled parathyroid hormone (¹²⁵I-PTH) injected IV into rat fetuses is removed from the circulation with a half-life of 31 min. In pregnant female rats the value obtained for the half-life is 21 min. The absence of ¹²⁵I-PTH in maternal or in fetal plasma after injection of ¹²⁵I-PTH to the fetus or to the mother respectively, provides evidence for the absence of placental transfer of PTH. A very high accumulation of trichloracetic acid precipitable radioactivity is observed in the liver after injection of labeled PTH into the fetus. Low values are found in kidneys, gastrointestinal tract and placenta. In the pregnant rat the in vitro ¹²⁵I-PTH inactivation is much higher in the kidney than in the liver. On the contrary, in the fetus, the kidney and the liver have the same activity; the placenta has a low activity inferior to that of the fetal kidney. The most active fetal tissue for inactivating ¹²⁵I-PTH is the gastrointestinal tract.

Key words

Fetus - Labeled Parathyroid Hormone (¹²⁵I-PTH) - ¹²⁵I-PTH Inactivation