INHIBIN-A AND INHIBIN-B ARE INVERSELY CORRELATED TO FOLLICLE-STIMULATING-HORMONE, YET ARE DISCORDANT DURING THE FOLLICULAR PHASE OF THE RATESTROUS-CYCLE, AND INHIBIN A IS EXPRESSED IN A SEXUALLY DIMORPHIC MANNER

Inhibin A and inhibin B are related dimeric protein hormones and endoc rine regulators of the reproductive axis. Specifically, inhibin inhibi ts FSH secretion from the anterior pituitary. The inhibins are synthes ized by the gonads and are themselves modulated by FSH, Although the a ctivity of these ligands has been well characterized, the circulating concentrations of dimeric inhibin A and dimeric inhibin B have not pre viously been reported for the rat. Our group examined the serum concen tration of inhibin A and inhibin B in normally cycling female rats, ma le rats, and in gonadectomized animals. Both inhibin isoforms are dete cted in intact female rat serum. Interestingly, inhibin B, but not inh ibin A, is detected in intact male rat serum. Neither inhibin isoform is detected in long-term castrate female or male rats. In normally cyc ling female rats, inhibin A was low on the morning of metestrus and ro se steadily to a peak on proestrus. In contrast, inhibin B was elevate d on the mornings of metestrus, diestrus, and proestrus. Both Ligands persisted in the serum until proestrus evening. Serum inhibins then de clined beginning at 2100 h (inhibin A) or 1800 h (inhibin B) on proest rus, and the concentrations reached a nadir on the morning of estrus ( 0600 h). The nadir coincided with the peak of the secondary FSH surge. Both inhibins rebounded later on the morning of estrus. The results o f this study demonstrate that dimeric, ovarian-derived inhibin A and i nhibin B circulate in the female rat. The inverse relationship bf the inhibins during the secondary FSH surge is consistent with the hypothe sis that they Participate in the regulation of B during the period of follicular development on metestrus and diestrus suggest different fol licle sources or regulation of these molecules during this period. We further demonstrate that inhibin B is the dominant form of FSH regulat ing protein in the male rat.