Inhibin Suppresses Human Chorionic Gonadotropin Secretion in Term, but not First Trimester, Placenta

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Abstract
Inhibin is produced by the placenta, with serum concentrations rising throughout pregnancy. In contrast, hCG serum concentrations peak in the first trimester and are 80-90% lower at term. This study was designed to determine the effect of inhibin on hCG secretion both early and late in gestation. Villus tissue from 3 term and 3 first trimester (8-10 week) placentas was maintained in an in vitro explant culture model for 5 days. Tissue from each placenta was incubated with control medium in 24 replicate wells for the first 72 h. During the final 48 h, 12 wells received control medium, and 12 wells received medium containing 1% rabbit antiserum raised against the .alpha.-subunit (residues 1-32) of the human inhibin peptide. The antiserum demonstrated biological activity by increasing serum FSH concentrations in an immature female rat bioassay. The relative increase in hCG secretion at the conclusion of days 4 and 5 in control and antiserum-treated groups for each first trimester and term placenta were compared to pretreatment hCG concentrations on day 3. The relative increases in hCG secretion of first trimester control groups on day 4 (mean .+-. SD, 34 .+-. 11%) and day 5 (63 .+-. 23%) were compared to those in antiserum-treated groups on day 4 (39 .+-. 13%) and day 5 (54 .+-. 5%) and showed no significant difference between groups on either day. The same comparison in term cultures showed the relative increases in hCG secretion of control groups on day 4 (31 .+-. 10%) and day 5 (64 .+-. 50%) to be significantly lower than those in antiserum-treated groups on day 4 (100 .+-. 41%) and day 5 (150 .+-. 108%; P < 0.001). These findings suggest that inhibin suppresses hCG secretion in term, but not first trimester, placentas.