LOCALIZATION OF INHIBIN ACTIVIN SUBUNIT MESSENGER-RNAS DURING THE LUTEAL-PHASE IN THE PRIMATE OVARY

Localization of inhibin/activin subunit mRNAs within the macaque ovary from the immediate preovulatory period of the menstrual cycle, when serum immunoreactive inhibin begins to rise, to day 9 of the luteal phase, when serum inhibin concentrations are maximal, was investigated using in-situ hybridization. Ovaries were studied on the day of the LH surge (day 0) and on days 2, 5, and 9 of the luteal phase by hybridizing frozen tissue sections with radio-labelled riboprobes specific to the inhibin/activin alpha-, beta(A)-and beta(B)-subunits. After autoradiographic exposure for 10 and 21 days, grain concentrations were quantified by image analysis. Moderate expression of alpha-, beta(A)- and beta(B)-subunit mRNA was present within the granulosa cells of the pre-ovulatory follicle (day 0). The granulosa-lutein cells of the corpora lutea expressed high levels of alpha-subunit at days 2, 5 and 9. mRNAs for beta(A) and beta(B) were detected at low but significant levels in all of the corpora lutea. All healthy antral follicles exhibited a high level of expression of beta(B)-subunit mRNA in the granulosa cells. On day 2 after ovulation these follicles also expressed high alpha- and moderate beta(A)-subunit mRNA. On day 9 the beta(B)-inhibin mRNA in antral follicles was found in association with low expression of the other subunits. Small follicles in ovaries on day 2 expressed moderate alpha- and low levels of beta(B)-subunit mRNA, while mRNA for beta(A) was absent. Alpha-subunit mRNA expression was present on day 5 while neither beta(A)- nor beta(B)-subunit mRNA was detected. On day 9 a proportion of small follicles expressed alpha- and beta(A)-subunit mRNA. These results demonstrate that marked differences are present in the levels of expression of the three inhibin/activin subunit genes between follicles and the corpus luteum. The predominance of the beta(B)-subunit mRNA within antral follicles would be consistent with the synthesis of activin. The predominance of the alpha-subunit combined with the low expression of the beta-subunits in the corpus luteum suggests that both biologically active inhibin and free alpha-subunit are produced by the primate corpus luteum.