MEASUREMENT OF CIRCULATING INHIBIN FORMS DURING THE ESTABLISHMENT OF PREGNANCY

This study investigated the forms of inhibin released into the circula tion 1) in very early pregnancy, 2) after stimulation of the corpus lu teum by exogenous hCG, and 3) in abnormal and failing human pregnancy. Samples were assayed by enzyme-linked immunosorbent assays for inhibi n A, inhibin B, and inhibin pro-alpha C-related immunoreactivity (pro- alpha C-RI). The concentration of inhibin A rose steadily during the c onception luteal phase to an initial peak 12 days after ovulation (104 +/- 23 pg/mL), then rose rapidly to a further peak 43 days after ovul ation (424 +/- 6 pg/mL). The concentration of pro-alpha C-RI exhibited a much larger peak on day 15 after ovulation (1423 +/- 361 pg/mL), bu t fell thereafter. The concentration of inhibin B was low after ovulat ion and subsequently barely detectable in pregnancy. hCG treatment res ulted in a significant rise in the concentrations of inhibin A and pro -alpha C-RI, but had no effect on the inhibin B concentration. The pro -alpha C-RI concentration was a better indicator of continuing pregnan cy viability than either hCG or inhibin A. Early trophoblast secretes proportionately more bioactive inhibin than the corpus luteum. The cor pus luteum and trophoblast do not secrete inhibin B into the circulati on. These data support the concept of different physiological roles fo r different inhibin forms.