Human growth hormone does not cross the placenta of the pregnant rat

The purpose of this study was to investigate if human growth hormone (hGH) crosses the placenta to the fetus of the pregnant rat. Pregnant rats were injected i.v. with 125I-hGH alone or co-injected with unlabelled hormone on gestational day 20 or 21. The rats were sacrificed 10 min after injection, and the distribution of the radioactivity was determined by direct measurement in brain, thymus, liver, kidney, rectus femoris muscle, placenta and fetus. Free iodine was determined in samples from maternal liver, placental and fetal tissue homogenates after precipitation of proteins by trichloroacetic acid. Two animals were injected with a higher dose of radioactivity. One of them was co-injected with a high dose of unlabelled hormone. They were sacrificed after 10 min, frozen, and sections of 20 microns were cut sagittally for autoradiography. A small amount of radioactivity was detected in the fetal tissue. However, this activity was shown to be free iodine, and no inhibition of that uptake was found by unlabelled hGH. No radioactivity was detected in the fetuses of either rat exposed to autoradiography. Therefore, we conclude that there is no transfer of hGH from the mother to the fetus of the pregnant rat. Our result is in agreement with some early observations in humans and rabbits but disagrees to some extent with recent results found in rats, where a small passage of 125I-hGH from mother to fetus was reported.