PLASMA CLEARANCE AND DISTRIBUTION OF ¹²⁵I‐OVINE PLACENTAL LACTOGEN IN SHEEP

The plasma clearance of ¹²⁵I‐ovine placental lactogen (¹²⁵I‐oPL) in pregnant, non‐pregnant and foetal sheep followed a biphasic exponential curve. The initial half‐life (t₁(½)) of the hormone ranged from 5·7–12·7 min. The molecular identity of ¹²⁵I‐oPL in plasma samples was examined by gel filtration chromatography. At the end of blood sampling, the high concentrations of radioactivity in the thyroid, kidney and urine were largely non‐protein‐bound. A similar pattern was obtained for the low concentration of radioactivity in amniotic fluid. The maternal‐foetal compartments showed negligible bi‐directional transfer of radioactivity.

The plasma t₁(½) of ¹²⁵I‐oPL determined here provides evidence that the hormone has a short biological half‐life in sheep. The virtual tack of placental passage of the hormone implies discrete mechanisms for unidirectional secretion of oPL into the two circulatory systems. Our data also suggest the importance of the kidneys in the metabolism of the hormone.