Serum Immunoreactive Inhibin Levels before and after Luteectomy in the Cynomolgus Monkey (Macaca fascicularis)*

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Abstract
Whereas studies in women have demonstrated that serum immunoreactive inhibin concentrations peak during the luteal phase of the menstrual cycle and that the corpus luteum (CL) encodes mRNA for the inhibin subunits, a clear link between the presence of the CL and circulating inhibin has not been established in primates. Therefore, we measured serum immunoreactive inhibin levels in monkeys before and after luteectomy as well as immunoreactive inhibin concentrations and mRNA encoding the .alpha.-inhibin subunit in luteal tissue. Monkeys were assigned to one of four groups depending on which day of the luteal phase luteectomy was performed: group A, days 4-5; group B, days 7-8; group C, days 9-10; and group D, days 11-12 (the day after the estrogen surge = day 1 of the luteal phase). Daily blood samples were obtained for 3 days before luteectomy, immediately before surgery, and for 2 days after luteectomy. Immunoreactive inhibin concentrations were measured with a double antibody RIA using an antiserum to bovine 31-kDa inhibin, bovine 31-kDa inhibin for iodination, and a human follicular fluid inhibin preparation as standard. Total RNA was isolated from luteal tissue and transferred by Northern blot onto a Zeta-probe membrane. The probe used for hybridization was the PstI/NcoI restriction enzyme fragment (381 basepairs) of .alpha.-inhibin DNA generated from a human ovarian cDNA library. Serum inhibin concentrations decreased (P < 0.05) 24 h after removal of the corpus luteum in each of the four groups studied. This reduction was especially marked in groups B (days 7-8) and C (days 9-10), in which serum inhibin concentrations were reduced (P < 0.05) by 83% and 92%, respectively. Similar reductions in serum progesterone concentrations were observed. Immunoreactive inhibin was readily measurable in luteal tissue obtained from monkeys on days 7-14 of the luteal phase and was present in similar concentrations throughout. mRNA encoding .alpha.-inhibin was detectable in CL obtained from two monkeys on day 9 of the luteal phase. In contrast, hybridization did not occur with RNA from either monkey liver or heart. We conclude that the CL is the major source of inhibin during the luteal phase of the monkey and, based on similarities of our data with those from humans, suggest that this is also true of the human luteal phase.