Disposition of rhIGF-I. (2). Transfer of 125I-rhIGF-I into fetus and milk in pregnant or lactating rats.

The transfer of 125I-rhIGF-I to the fetus or into the milk after a single subcutaneous administration was investigated in pregnant rats on 12th and 18th day of gestation and in lactating rats. The level of radioactivity in the fetus on the 18th day of gestation was higher than that on the 12th day of gestation, but was at least six times lower than that in maternal plasma. The level of radioactivity in fetal tissues disappeared parallelling the maternal plasma except that in thyroid gland, which might be due to free 125I. Most of radioactive substances of high molecular weight, which were found in the milk of lactating rats, might be on endogenous proteins which had in corporated a free iodine, otherwise the permeability of rhIGF-I into milk was thought to be low.