Inhibin, a gonadal hormone capable of preferential suppression of pituitary follicle-stimulating hormone (FSH) secretion, has recently been purified. The major form of this protein is an a48 heterodimer encoded by two separate genes. In contrast to the FSH-suppressing action of the a,3 heterodimer, the 1831 homodimer stimulates FSH secretion. Earlier investigations on testicular (1, 2) and ovarian (3-6) proteins capable of differential suppression of follicle-stimulating hormone (FSH), but not luteinizing hormone (LH), secretion by the anterior pituitary gland have been followed by the recent isolation of inhibin from porcine (7-9) and bovine (10, 11) follicular fluid. Based on the amino acid sequence of these proteins, porcine (12), human (13), and bovine (14) MATERIALS AND METHODS Animals. Male and female Sprague-Dawley rats were obtained from Johnson Laboratories (Bridgeview, IL). Neonatal male rats (7-day-old) were maintained at 8 to 10 per lactating mother, whereas adult male rats were hypophysectomized at 60-70 days of age and then used at 7-10 days after operation. For theca-interstitial cell cultures, immature female rats were hypophysectomized at 21 days of age and used at 4 days after operation. In addition, theca explants were obtained from 30-day-old intact rats.

Serum steroid, gonadotropin, and alpha-subunit levels were assessed in 35 women with cycle abnormalities [11 with and 24 without polycystic ovarian disease (PCOD) according to strict clinical and biochemical criteria] and 8 regularly cycling women in the early (cycle day 3 or 4) and mid (cycle day 7 or 8) follicular phase. LH and FSH levels were estimated using two immunological techniques [RIA and immunoradiometric assay (IRMA)] and in vitro bioassays (BIO), using mouse Leydig cells and rat granulosa cells, respectively. In PCOD patients mean alpha-subunit, free androgen index [FAI; testosterone x 100/sex hormone-binding globulin (SHBG)], androstenedione, estrone, and estradiol (E2) were significantly elevated compared to levels in the early follicular phase of control cycles and non-PCOD patients. In addition, in PCOD patients mean IRMA-LH and RIA-LH levels were distinctly increased (2.8- to 3.6 fold, respectively; both comparisons, P less than 0.001) compared to control values, but in the same order of magnitude (1.3- to 1.4-fold increments) as that in non-PCOD patients. However, the median BIO-LH level in PCOD patients was 5.9-fold higher than that in non-PCOD patients and 4.0-fold higher than the BIO-LH in the early follicular phase of control women. Consequently, the median BIO/IRMA-LH ratio was 4.8-fold higher in PCOD patients compared to non-PCOD patients. In women with cycle abnormalities, individual BIO/IRMA-LH ratios correlated with BIO-LH (rs = 0.48), FAI (rs = 0.39), free estrogens (E2/SHBG ratios; rs = 0 0.47), and dehydroepiandrosterone sulfate (rs = 0.60) concentrations. Mean IRMA-, RIA-, and BIO-FSH levels and BIO/IRMA-FSH ratios were not significantly different when various groups were compared. Although RIA- and IRMA-LH levels showed good correlation (rs = 0.88), RIA-LH levels were consistently higher, resulting in distinctly higher RIA-LH/FSH ratios (mean, 4.5) compared to IRMA-LH/FSH ratios (median, 1.8) in PCOD patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibin is a gonadal glycoprotein believed to be important in the regulation of pituitary FSH secretion and/or to function as a paracrine factor within the ovary and testis. We studied serum levels of inhibin, oestradiol (E2), progesterone (P), FSH and LH during the periovulatory interval in order to determine whether there is differential control of sex steroid and inhibin secretion by the mature follicle and the emerging corpus luteum. Seven normal cyclic women were admitted 3-4 days prior to midcycle and blood samples drawn every 3 h for 5-7 days. Serum E2, P, FSH, LH and inhibin were measured by radioimmunoassay. Data were normalized around the peak LH value (0 h). Serum E2 and inhibin rose in parallel (r = 0.92, P less than 0.001) between -69 and -18 h, E2 reached a peak of 1296 +/- 154 (mean +/- SEM) pmol/l at -18 h, then fell to 1050 +/- 139 pmol/l at 0 h. Serum inhibin, on the other hand, continued to rise to a peak of 837 +/- 95 U/l at -6 h, fell to 455 +/- 48 U/l at +45 h, then rose again. On average, the peak inhibin level occurred 10.4 +/- 5.1 h after the peak E2 (P less than 0.05). Inhibin levels were positively correlated with both serum LH and FSH between -24 and +24 h (P less than 0.01). Serum E2 was negatively correlated with LH, FSH and inhibin between -24 and 0 h (P less than 0.01). Serum P levels increased from 1.8 +/- 0.3 nmol/l at -24 h to 14.3 +/- 1.0 nmol/l at +60 h. Serum inhibin was positively correlated with serum P from -24 to 0 h (P less than 0.01) and +45 to +60 h (P less than 0.01), but was inversely correlated from 0 to +45 h (P less than 0.01). We conclude that the maturing follicle secretes both E2 and inhibin in parallel until -18 h, at which time the process of luteinization is initiated by the onset of the midcycle LH surge, as evidenced by the rise in P. E2 secretion then falls while inhibin secretion rises, indicating different regulation of secretion of these two hormones by the maturing follicle. Furthermore, the close positive correlation between inhibin and gonadotrophin levels around midcycle suggests that FSH and/or LH stimulate inhibin secretion and that the presumed negative feedback effect of inhibin on FSH secretion is overcome at this time. After midcycle, inhibin secretion initially falls, then rises, while P rises progressively.(ABSTRACT TRUNCATED AT 400 WORDS)

Urinary excretion of oestrone conjugates, pregnanediol-3 alpha-glucuronide (PdG) and 20 alpha-hydroxypregn-4-en-3-one were measured from 8 weeks before oestrus to 2 weeks post partum and bioactive FSH was monitored during the periovulatory interval in a female giant panda. A biphasic urinary bioactive FSH excretory profile appeared to indicate a broad (approximately 10 day) follicular phase followed by a sharp preovulatory bioactive FSH surge coincident with an acute increase in urinary oestrone conjugates and behavioural oestrus. Weekly concentrations of urinary oestrone conjugates and PdG increased (P less than 0.001) by Week 9 of gestation with 20 alpha-hydroxypregn-4-en-3-one levels increasing 10-30-fold (P less than 0.001) between Weeks 11 and 14. These observations indicate that the monoestrous giant panda does not appear to require a prolonged period of endogenous FSH release or multiple FSH peaks for ovarian priming and follicle selection to proceed normally. Furthermore, the delayed rise in urinary steroid excretion during the second half of gestation in the giant panda supports the concept of delayed implantation while the estimation of steroid conjugates in urine offers a non-invasive approach for monitoring pregnancy status in this endangered species.