The physiology of testicular inhibin and related proteins

Inhibin-A and inhibin-B in cyclic and pregnant mares, and mares with granulosa-theca cell tumors: Physiological and diagnostic implications

Studies in mares have examined serum inhibin concentrations using immuno-assays unable to distinguish dimeric inhibin-A from inhibin-B isoforms. Inhibin-A and inhibin-B immuno-assays were used to investigate concentrations in cyclic mares, young and old (6 vs 19 years old, respectively) mares following hemi-ovariectomy, mares during pregnancy and in mares with confirmed granulosa cell tumors (GCTs). Mares with inter-ovulatory intervals of 26 days had ovulatory peaks of inhibin-A averaging 80 pg/mL with a mid-cycle nadir of 5 pg/mL. Inhibin-A and inhibin-B concentrations were highly correlated (r = + 0.79, P < 0.01) though peak and nadir concentrations of inhibin-B were not significantly different. However, the ratio of inhibin-A to inhibin-B (A/B) changed significantly through the cycle, highest at ovulation and <1 (more inhibin-B than -A) at mid-cycle. Two mares with grossly extended inter-ovulatory intervals demonstrated mid-cycle inhibin-A (and inhibin-B) excursions suggestive of follicular waves. Follicle-stimulating hormone was negatively correlated with inhibin-A and -B concentrations in all 6 mares. Hemi-ovariectomy in young mares resulted in a significant decrease in inhibin-A and inhibin-B concentrations one day later (P < 0.05) but older mares did not, suggesting a possible extra-ovarian source(s) of these hormones. Both inhibin isoforms dropped to very low levels during pregnancy (P < 0.0001), inhibin-A (P < 0.0001) more rapidly than -B (P < 0.05), so that inhibin-B became the predominant measured form throughout most of gestation (P < 0.05). Mares with confirmed GCTs had elevated inhibin-B concentrations more reliably than inhibin-A but neither inhibin-A or -B was correlated with anti-Müllerian hormone concentrations. Collectively, concentrations of inhibin-A and -B were aligned with physiological events in healthy mares, though more pronounced cyclic changes were seen with inhibin-A. Inhibin-B concentrations were significantly associated with GCTs (P < 0.01), inhibin-A concentrations were not. While both inhibin-A and -B concentrations track physiological events such as cyclic follicular activity, only inhibin-B concentrations effectively signal ovarian neoplasia in mares.

Insights into the Development of the Adult Leydig Cell Lineage from Stem Leydig Cells

Adult Leydig cells (ALCs) are the steroidogenic cells in the testes that produce testosterone. ALCs develop postnatally from a pool of stem cells, referred to as stem Leydig cells (SLCs). SLCs are spindle-shaped cells that lack steroidogenic cell markers, including luteinizing hormone (LH) receptor and 3β-hydroxysteroid dehydrogenase. The commitment of SLCs into the progenitor Leydig cells (PLCs), the first stage in the lineage, requires growth factors, including Dessert Hedgehog (DHH) and platelet-derived growth factor-AA. PLCs are still spindle-shaped, but become steroidogenic and produce mainly androsterone. The next transition in the lineage is from PLC to the immature Leydig cell (ILC). This transition requires LH, DHH, and androgen. ILCs are ovoid cells that are competent for producing a different form of androgen, androstanediol. The final stage in the developmental lineage is ALC. The transition to ALC involves the reduced expression of 5α-reductase 1, a step that is necessary to make the cells to produce testosterone as the final product. The transitions along the Leydig cell lineage are associated with the progressive down-regulation of the proliferative activity, and the up-regulation of steroidogenic capacity, with each step requiring unique regulatory signaling.

Inhibin A and B in peri- and postmenopause

OBJECTIVE

The aim of the study was to investigate the origin of inhibin A and B during the last years of the reproductive age and after menopause by measuring their levels in the ovarian and peripheral venous blood.

METHODS

The study population consisted of 43 women, aged 42-69 years (mean 50), who underwent hysterectomy with ovarian removal for a benign disease. A total of 24 of them were in follicular phase, 11 in luteal phase, and eight were postmenopausal. Peripheral and ovarian venous blood was collected for measurement of inhibin A and B. In addition, sex steroid hormone and gonadotropin levels were measured.

RESULTS

Ovarian venous inhibin B correlated significantly with ovarian estradiol secretion (r = 0.5, P = 0.001). The levels of inhibin B were significantly higher in the ovarian vein than in the peripheral vein (P = 0.006). The highest inhibin B concentrations were detected in the mid-proliferative (mid-follicular) phase (median 31.6 pg/ml range 25.9-47.9). In postmenopausal women, inhibin B was not detectable. No correlation between FSH and ovarian inhibin B was found. Inhibin A rose rapidly in late proliferative (late follicular) phase (median 28.5, range < 2-51.8) and dominated in the circulation throughout the luteal phase (median 20.9, range 8.8-60). For inhibin A, no concentration gradient existed between the ovarian and peripheral vein. Unlike inhibin B, inhibin A was detectable in ovarian and peripheral blood in postmenopausal women. A significant negative correlation between ovarian and peripheral inhibin A and FSH was found (r = -0.386, P = 0.015; r = -0.345, P = 0.034, respectively).

CONCLUSION

Inhibin B correlates with ovarian estradiol secretion and seems to reflect follicular function. Inhibin A dominates in circulation during the luteal phase but is detectable at low concentrations both in follicular phase and even in postmenopause. Our findings suggest that inhibin A may play a role in FSH suppression in the female reproduction. In addition to the ovary, there may be extragondal source(s) of inhibin A.

Diagnostic and clinical features in azoospermia

BACKGROUND AND OBJECTIVE

The recent advances in assisted fertilization and gamete micromanipulation techniques have enabled fertilization in some forms of azoospermia; for example, epididymal sperm aspiration in obstructive azoospermia. Therefore knowledge of the specific degree of testicular damage is of primary importance, since other clinical parameters, such as FSH plasma levels and testicular volume, do not discriminate between the different testiculopathies. In order to further characterize the specific testicular conditions present in azoospermia, we have examined a large group of azoospermic subjects on the basis of testicular cytological analysis obtained by fine needle aspiration.

DESIGN AND PATIENTS

One hundred and twenty-two infertile, azoospermic men were studied by physical examination, FSH radioimmunoassay, testicular ultrasound examination and fine needle aspiration of the testes. Thirty-five infertile normozoospermic subjects were studied as controls.

RESULTS

The cytological analysis identified five different sub-types in azoospermic subjects: I, Sertoli cell-only syndrome; II, hypospermatogenesis; III, spermatogonial and/or spermatocytic arrest; IV, spermatidic arrest; and V, normal germ line. The testicular volume was reduced in groups I and II, while the FSH plasma levels were increased in groups I, II and III, suggesting a primary role of spermatids in the control of FSH secretion.

CONCLUSIONS

In azoospermic subjects, testicular cytological analysis permits the identification of different sub-types and this classification may be very important in determining therapy, particularly the choice between surgical treatment and the hypothetical use of assisted fertilization techniques by retrieval of epididymal or intratesticular spermatozoa or spermatids.

Inhibin activity in the mare and stallion

An overnight double antibody RIA, employing a rabbit antiserum raised to bovine 31 kDa inhibin (rAs-#1989, NICHD) and purified bovine 31 kDa inhibin (bINH-I-90/1, NICHD) as trace and standard, was validated to measure immunoreactive inhibin (iINH) concentrations in equine peripheral plasma, follicular fluid (FF), ovarian vein (OV) plasma, testicular tissue extracts (TTE) and testicular vein (TV) plasma. The dynamic relationship of iINH and follicle stimulating hormone (FSH) was investigated during the estrous cycle of the mare and the annual reproductive cycle of the stallion. In the RIA, parallel dose-response curves were observed between the bovine inhibin standard and serial dilutions of equine FF, OV, TTE, TV and plasma. The average recovery of a known amount of purified bovine inhibin added to gelding plasma was approximately 100%. In the inhibin bioassay, serial dilution of equine FF and TTE were observed to be parallel to the bovine inhibin standard. A five-fold difference (p < 0.05) between jugular and gonadal vein plasma iINH concentrations was observed in the mare and an eight-fold difference (p < 0.05) was observed in the stallion. Plasma levels of iINH in ovariectomized mares or geldings were undetectable in the RIA. Concentrations of FSH, estradiol and iINH changed significantly in the mare during the estrous cycle (p < 0.05). Immunoreactive inhibin levels were highest (0.54 +/- 0.06 ng/ml) on the day of ovulation, declined rapidly following ovulation and reached a nadir (0.21 +/- 0.03 ng/ml) on day 7 post-ovulation. Plasma iINH and estradiol concentrations followed a similar profile and were found to be positively correlated (r = 0.7064; p < 0.01), whereas iINH and FSH levels demonstrated an inverse relationship (r = -0.7359, p < 0.01) throughout the estrous cycle. Concentrations of FSH were also inversely related (-0.8498, p < 0.01) with estradiol during the cycle. In the stallion, plasma iINH and FSH levels changed significantly during the year (p < 0.05). The iINH profile reflected seasonal changes in testicular activity, with highest concentrations in late spring (3.37 +/- 0.44 ng/ml) and lowest concentrations in the fall (2.21 +/- 0.33 ng/ml). Plasma concentrations of iINH were positively correlated (r = 0.7691, p < 0.01) with FSH concentrations throughout the year. In conclusion, a specific and sensitive RIA for iINH has been validated for plasma and biological fluids in the horse. Furthermore, the gonads appear to be the source of bioactive and immunoreactive inhibin as observed in other species.(ABSTRACT TRUNCATED AT 400 WORDS)