Expression of Messenger Ribonucleic Acid for Inhibin Subunits and Ovarian Secretion of Inhibin and Estradiol at Various Stages of the Sheep Estrous Cycle1
The relationship between expression of inhibin mRNA and ovarian secretion of estradiol (E2) and immunoactive inhibin was investigated at midluteal phase and throughout the follicular phase of the sheep estrous cycle. At laparotomy, timed samples of ovarian blood were collected and ovaries were removed from 39 Scottish Blackface ewes (ovulation rate 1.3 +/- 0.1) on Day 10 of the luteal phase or 24, 48, 60, 72, or 84 h after injection of cloprostenol (PG; 100 micrograms) on Days 10-12. Ovaries were removed and fixed for in situ hybridization using 35S-labeled antisense riboprobes transcribed from inhibin alpha, beta A, and beta B cDNAs. LH, E2, and inhibin concentrations were determined by RIA. On the basis of peripheral LH levels and the presence of estrogen-active follicles (E-A; > or = 3 mm in diameter secreting > 1 ng/min E2) or recent ovulations, animals were grouped as follows: presurge (24 or 48 h post-PG; LH < 5 ng/ml; n = 7), midsurge (with E-A; LH > 5 ng/ml; n = 6), late surge (large follicle not E-A; LH > 5 ng/ml; n = 4), postsurge (large follicle not E-A; LH < 5 ng/ml; n = 7), and postovulation (n = 10). As expected, E2 secretion by the "active" ovary (containing preovulatory follicle) tended to increase with follicular development such that secretion was maximal at midsurge and then declined. E2 secretion by the "inactive" ovary was low at all stages. Immunoactive inhibin, in contrast, was secreted in substantial quantities by both ovaries, although secretion from active ovaries was higher at all stages (p < 0.05). Effects of stage on secretion were not significant, but immunoactive inhibin secretion from active ovaries was high in postsurge animals when E2 secretion was very low. Hybridization for inhibin mRNA was specific for granulosa cells of antral follicles. While most sheep in the luteal (4 of 5), presurge (2 of 3), and midsurge groups (5 of 5) had at least one inhibin-positive large follicle (expressing both alpha- and beta-subunit mRNA), none were present between the LH surge and ovulation (late and postsurge groups). Inhibin mRNA was undetectable in midcycle CL, but 4 of 10 recent ovulations hybridized weakly with the alpha probe and one very weakly with the beta A probe. The mean number of inhibin-positive large follicles per animal (in those having at least one) was 1.3 +/- 0.15 (n = 15 ewes).(ABSTRACT TRUNCATED AT 400 WORDS)