INCREASED CONCENTRATIONS OF IMMUNOREACTIVE INHIBIN DURING CONCEPTION CYCLES IN THE MARMOSET MONKEY - SUPPRESSION WITH AN LHRH ANTAGONIST AND CLOPROSTENOL

Peripheral concentrations of immunoreactive (ir) inhibin have been measured during the ovarian cycle and early pregnancy in the marmoset monkey. Blood samples were taken (three per week) during conception (n = 6) and non-conception (n = 5) cycles. Ir-inhibin was measured by radioimmunoassay using an antiserum raised against a synthetic peptide fragment of the alpha-subunit of human inhibin. Monomeric bovine alpha-subunit and 32 kDa bovine inhibin were used as tracer and standard respectively. In all animals low concentrations of ir-inhibin were recorded during the follicular phase (40-60-mu-g/l) of the cycle. After ovulation, ir-inhibin concentrations increased but the peak concentrations attained differed between conception and non-conception cycles. In non-pregnant animals ir-inhibin concentrations reached a maximum of 242 +/- 16-mu-g/l on days 12/13 after ovulation. In pregnant animals ir-inhibin concentrations were significantly (P < 0.05) higher (1.8-fold) than in non-pregnant animals on days 8/9 after ovulation, and reached a maximum value of 636 +/- 141-mu-g/l on days 20/21 after ovulation. Administration of an LHRH antagonist during the luteal phase on days 6-8 after ovulation resulted in a significant (P < 0.05) decrease in progesterone and ir-inhibin concentrations within 4 and 8 h respectively. This was prevented by co-administration with human chorionic gonadotrophin. Administration of cloprostenol to pregnant animals between days 17 and 20 after ovulation halved the initial concentrations of both inhibin and progesterone within 1.5 h. The increase in plasma ir-inhibin concentrations in the luteal phase and the apparent similarity in control of ir-inhibin and progesterone supports a luteal source of ir-inhibin in both conception and non-conception cycles. The higher levels of ir-inhibin from days 8/9 after ovulation in conception cycles were not related to any detectable increase in peripheral concentrations of chorionic gonadotrophin and occurred at least 4 days before the expected time of implantation. This suggests a role for the conceptus in inhibin secretion which may involve the release of an embryo message before implantation.