Expression of ovarian inhibin during pregnancy in the rat.

We have examined the expression of the rat inhibin genes in the maternal ovary during pregnancy. RNA blot analysis indicates that the inhibin-alpha chain mRNA is expressed in the ovary throughout gestation at levels comparable to those observed in cycling rats. In situ hybridization shows that the inhibin-alpha and -beta A mRNAs are produced in the granulosa cells of developing antral follicles; little or no hybridization to functional corpora lutea is observed. Early in pregnancy, a large number of follicles hybridize to both alpha- and beta A-inhibin cDNA probes. Many of these follicles undergo atresia during the first half of pregnancy, and the number of inhibin-expressing follicles reaches a nadir on day 15. This is followed by an increase in inhibin-producing follicles, which peaks just before parturition. The increase in inhibin-expressing follicles observed in late pregnancy corresponds to a small rise in serum inhibin levels, as measured using an alpha chain-specific RIA. After the first postpartum ovulation, few hybridizing follicles are observed. Ovariectomy in either early (day 6) or mid (day 15) pregnancy results in a significant fall in serum inhibin levels and a robust increase in serum FSH levels 9 h after surgery. These results suggest that inhibin is produced by the maternal ovary during pregnancy, that its synthesis is modulated during late gestation, and that inhibin may play a role in regulating FSH secretion during pregnancy.