TRANSPLACENTAL PASSAGE OF A HUMAN RELAXIN ADMINISTERED TO RHESUS-MONKEYS

A synthetic version of the human relaxin encoded by the human gene 2 (hRlx-2) was administered to pregnant rhesus monkeys (Macaca mulatta) on gestational days 141-158. Monkeys (three per group) received doses of 100-mu-g/kg or 2000-mu-g/kg as a continuous i.v. infusion over 2 h into a radial vein. One monkey in the low-dose group received, along with the unlabelled hRlx-2, 25.5-mu-Ci/kg of the test material internally labelled with [S-35]cysteine. Immunoreactive hRlx-2, as measured by enzyme-linked immunosorbent assay, appeared in all fetuses within 30 min (the first sampling time) of beginning the infusions. Peak fetal plasma levels of hRlx-2 were only 0.8-1.5% of the maternal values. Only 8-15% of the fetal serum radioactivity was hRlx-2. Radioactivity from maternal urine pooled over the 4-h experiment did not elute at the volume corresponding to hRlx-2, but near the column volume.