Changes in Serum Immunoreactive Inhibin and Follicle-Stimulating Hormone during Gonadal Development in Male and Female Rats1

We have measured serum and ovarian immunoreactive inhibin alpha (irIα) and serum FSH in fetal and neonatal rats from 20 days of gestation until 40 days of age. For animals aged 10 days or older, serum measurements were made on intact and gonadectomized animals. Serum irIα was detectable in intact male and female rats at all ages studied. In females, irIα levels were low until Day 5 and then increased steadily to peak at Day 25. Thereafter they declined until Day 35 to reach levels typical of adult females. There was a significant decrease in irIα levels 24 h after ovariectomy at all ages. Serum FSH levels in females were low until Day 7, then increased rapidly to plateau from Days 10–15. The levels then declined until Day 25 and were generally unchanged after that time. There was a significant increase in FSH 24 h after ovariectomy in rats aged 20 days and older, and in younger rats by 48 h after ovariectomy. In male rats, serum irIα levels were significantly higher than females until Day 7. The levels increased at Day 7 and then remained relatively constant until Day 20, after which they declined to reach typical adult male levels. Serum irIα levels were significantly lower in males than females from Days 25–40. There was a significant decrease in serum irIα 24 h after castration at all ages studied. Serum FSH levels in males were low until Day 20, increased at Day 25, and thereafter remained relatively unchanged. Serum FSH was significantly lower in males than in females until Day 20 and significantly greater than in females from Day 25 onwards. Serum FSH increased significantly 24 h after castration at all ages. irIα was detectable in ovarian extracts of all ages studied (Day 3 onwards). The content and concentration of irIα were very low until Day 5, after which they increased rapidly with age until Day 25. The content and concentration then decreased slightly on Days 30 and 35 and increased again on Day 40. These results show that irIα is present in male and female rats before birth and that the source of inhibin is primarily gonadal. They also show that there are male/female differences in irla levels and that there is a reciprocal relationship between inhibin and FSH at all ages in the male rat and in the older female rats.