Differential Synthesis of Human Placental Lactogen and Human Chorionic Gonadotropinin Vitro*

Abstract

The dynamics of the synthesis and secretion of human placental lactogen (hPL) and human chorionic gonadotropin (hCG) were investigated by culturing explants from normal term placentae in minimal essential medium with 15% fetal calf serum for periods up to 4 days. The total amounts of hPL and hCG in the media and tissues at the end of each day were determined by radioimmunoassay, and new hormone synthesis was measured by incorporation of [3H]leucine into hPL and hCG at several intervals during the first 2 days after a 1 h pulse with 25 .mu.Ci of [3H]leucine. The secretion of hPL was maximal on the 1st day, decreased by 83.5 .+-. 9.3% on the 2nd day, and then remained constant over the next 2 days. The secretion of hCG also decreased by 78.0 .+-. 3.7% from the 1st to the 2nd day of culture. In contrast to hPL, the secretion of hCG increased by an average of 780% from the 2nd to the 4th days. Hormone released into the medium during the 1st day in culture was almost all preformed since the addition of cycloheximide to explants in doses sufficient to inhibit hPL and hCG synthesis by greater than 95% resulted in less than 5% decrease in total hPL and hCG secretion. When cycloheximide was asded to explants during the 3rd day, hPL secretion decreased by 23.1% and hCG secretion decreased by 96%. hPL synthesis remained relatively constant over the 4 day period, whereas the synthesis of hCG increased by approximately 800%. It is unlikely that the increase in hCG synthesis can be attributed to an increase in the mass of trophoblast since no apparent increase in either the syncytiotrophoblast or cytotrophoblast was noted. More likely, the synthesis of hCG by explants from the term placenta is subject to an inhibitory influence which declines with time in culture.