Changes in serum concentrations of inhibin in cyclic pigs

Peripheral concentrations of inhibin A, ovarian steroids, and gonadotropins associated with follicular development throughout the estrous cycle of the sow

We investigated changes in peripheral concentrations of inhibin A, total inhibin, steroids, and gonadotropins throughout the intact estrous cycle of the sow in relation to ovarian changes determined by daily transrectal ultrasonography. All visible follicles of 3 mm or more in diameter were classified as small (≥3 and <6 mm) or large (≥6 mm). Follicular recruitment was identified in two periods of the cycle: one from the late luteal to the follicular phase, characterized by an increase in the number of small follicles followed by the appearance of large follicles; and another during the early luteal phase, consisting only of increased numbers of small follicles. Plasma concentrations of inhibin A increased (P<0.05), coinciding with the two periods of follicle emergence. Estradiol (E2) levels increased (P<0.05) during the follicular phase, but not during the early luteal phase. An inverse relationship (P<0.01) between the patterns of inhibin and FSH concentrations was noted around the two periods of follicle emergence, but there was no relationship (P≥0.1) between the patterns of plasma E2and FSH during the early luteal phase. In conclusion, measurement of plasma inhibin A levels combined with ultrasonographic examination of the ovaries revealed two periods of synchronous follicular growth during the sow's estrous cycle. The results strongly suggest that inhibin A functions as a negative feedback regulator of FSH secretion throughout the estrous cycle, whereas E2appears to influence FSH secretion only during the follicular phase.

Inhibin

Inhibin is a glycoprotein hormone, consisting of two dissimilar, disulphide-linked subunits, termed α (MW 20kD) and β (MW 3-15kD), which inhibits the production and/or secretion of pituitary gonadotrophins, preferentially follicle stimulating hormone (FSH). The most widely studied inhibin molecule has a molecular weight of 31-32kD, as purified and cloned from bovine, porcine, ovine, rat and human sources. Higher molecular weight forms have been identified in ovarian follicular fluids and in culture media of granulosa and Sertoli cells, and generally differ from the 31kD form in having larger α-subunits, designated by their molecular weights, e.g. α44 in 58kD inhibin. There are two forms of the β-subunit, named βA and βB and the corresponding inhibin dimers have been named inhibin A and inhibin B. Dimers of the β-subunit, which have been shown to have FSH stimulating activity, are termed activins and are designated activin A, B or AB depending on whether the dimer is a homodimer of βA or βB or a heterodimer of βA and βB (Figure 1). The major gonadal source of inhibin is the granulosa cell in the female and the Sertoli cell in the male. Other potential cellular sites of origin will be discussed below.

Features of follicle-stimulating hormone-stimulated follicles in a sheep model: keys to elucidate embryo failure in assisted reproductive technique cycles

OBJECTIVE

To evaluate the individual functionality of gonadotropin-stimulated preovulatory follicles, for understanding embryo failure in assisted reproductive technique cycles, in a sheep model.

DESIGN

Observational, model study.

SETTING

Public research unit.

ANIMAL(S)

Fifteen adult Manchega ewes.

INTERVENTION(S)

Synchronization of the estrous cycle with intravaginal progestagens and ovarian stimulation with FSH; evaluation of reproductive activity, plasma sampling, ovarian ultrasonography, and ovariectomies.

MAIN OUTCOME MEASURE(S)

Determination of estrus behavior, plasma and intrafollicular concentrations of E(2) and inhibin A, number and size of ovarian follicles, and developmental competence of oocytes.

RESULT(S)

These results support the usefulness of serial measurements of plasma inhibin A for assessment of follicular growth during the FSH treatment, rather than of E(2) assays commonly used. Functionality of FSH-stimulated preovulatory follicles is clearly disturbed, as confirmed by a negative correlation between follicular size and intrafollicular concentrations of inhibin A and E(2) in preovulatory follicles after individual dissection; moreover, the ability of their oocytes to resume meiosis was diminished.

CONCLUSION(S)

Functionality of follicles in controlled ovarian stimulation (COS), and developmental competence of their oocytes, is disturbed by the high doses of gonadotropin supplied and finally determined by follicular sizes at starting FSH treatment.

Effects of ACTH injections during estrus on concentrations and patterns of progesterone, estradiol, LH, and inhibin alpha and time of ovulation in the sow

This study investigated whether injections of ACTH for 48 h, from the onset of the second standing estrus after weaning, had any impact on time of ovulation and patterns of progesterone, estradiol, luteinizing hormone (LH), and inhibin alpha. The studied sows (n=15) were fitted with jugular vein catheters and randomly divided into a control (C group) and an ACTH group. From the onset of standing estrus, the sows were injected (NaCl or synthetic ACTH, 5 microg/kg) every 4h; blood samples were collected immediately before and 45 min after each injection. Ovulation was monitored using ultrasonography. The ACTH-group sows stopped displaying signs of standing estrus sooner after ovulation in their second estrus, but no impact was found on time of ovulation. There were no significant differences in the intervals between LH peak, estradiol peak, and the onset of standing estrus between the C and ACTH groups. The cortisol and progesterone concentrations were significantly elevated (p<0.001) in samples taken 45 min after ACTH injection. There were minor differences in estradiol and LH concentrations between the groups. Overall inhibin alpha concentrations were significantly higher during the treatment period in the ACTH than in the C group, but there were no significant differences between samples taken either 45 min or 4h after injection. In conclusion, injections of synthetic ACTH during estrus in the sow apparently disturb the duration of signs of standing estrus and the hormonal pattern of progesterone, and possibly of inhibin alpha, estradiol and LH.

Resumption of follicle growth in gilts after ovarian autografting

The aims of the study were to evaluate autografting of porcine ovarian tissue in terms of establishment of a blood supply, follicle survival and development, commencement of oestrous cycles and endocrine patterns in this polyovular species. Experiment 1, a preliminary study on four gilts, showed that ovarian tissue slices survived the grafting procedure and re-vascularised. In Experiment 2, a further six pre-pubertal gilts had both ovaries surgically removed and two thin cortical slices of each ovary were immediately reattached to each of the ovarian pedicles. Blood samples were taken at surgery and then weekly. Two gilts were slaughtered 2 weeks after surgery and ovarian tissue recovered. The remaining four gilts underwent daily checks for behavioural oestrus until slaughter 24 weeks after surgery. All four gilts showed standing heat at least once prior to slaughter. Plasma LH and FSH concentrations increased significantly (P<0.01) by 3 days after surgery, then fell gradually, but did not return to pre-surgery levels. Progesterone concentrations showed some evidence of cyclicity in all animals. In the grafted tissue, re-vascularisation of the tissue was apparent by 2 weeks post-grafting, although no preantral or antral follicles were observed. The tissue recovered after 24 weeks contained healthy preantral and antral follicles, luteal tissue and some large cystic follicles. It is unclear whether these cysts were the result of ovarian or hypothalamic/pituitary disturbance. In conclusion, the results of this study have shown that follicle growth and resumption of cyclicity can be achieved following ovarian autografting in pigs and indicate that this will be a useful model for investigating the mechanisms that control the early stages of follicular growth and ultimately ovulation rate in this multiovular species.

Plasma gonadotropins and ovarian hormones during the estrous cycle in high compared to low ovulation rate gilts

Mature gilts classified by low (12 to 16 corpora lutea [CL], n = 6) or high (17 to 26 CL, n = 5) ovulation rate (OR) were compared for plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), progesterone, estradiol-17beta, and inhibin during an estrous cycle. Gilts were checked for estrus at 8-h intervals beginning on d 18. Blood samples were collected at 8-h intervals beginning on d 18 of the third estrous cycle and continued for one complete estrous cycle. Analysis for FSH and LH was performed on samples collected at 8-h intervals and for ovarian hormones on samples collected at 24-h intervals. The data were standardized to the peak of LH at fourth (d 0) and fifth estrus for the follicular phase and analyzed in discrete periods during the periovulatory (-1, 0, +1 d relative to LH peak), early-luteal (d 1 to 5), mid-luteal (d 6 to 10), late-luteal (11 to 15), periluteolytic (-1, 0, +1 d relative to progesterone decline), and follicular (5 d prior to fifth estrus) phases of the estrous cycle. The number of CL during the sampling estrous cycle was greater (P < 0.005) for the high vs low OR gilts (18.8 vs 14.3) and again (P < 0.001) in the cycle subsequent to hormone measurement (20.9 vs 14.7). For high-OR gilts, FSH was greater during the ovulatory period (P = 0.002), the mid- (P < 0.05) and late-luteal phases (P = 0.01), and tended to be elevated during the early-luteal (P = 0.06), but not the luteolytic or follicular periods. LH was greater in high-OR gilts during the ovulatory period (P < 0.005), but not at other periods during the cycle. In high-OR gilts, progesterone was greater in the mid, late, and ovulatory phases (P < 0.005), but not in the follicular, ovulatory, and early-luteal phases. Concentrations of estradiol-17beta were not different between OR groups during the cycle. Inhibin was greater for the high OR group (P < 0.005) during the early, mid, late, luteolytic, and follicular phases (P < 0.001). The duration of the follicular phase (from last baseline estrogen value to the LH peak) was 6.5 +/- 0.5 d and was not affected by OR group. These results indicate that elevated concentrations of both FSH and LH are associated with increased ovulation rate during the ovulatory phase, but that only elevated FSH during much of the luteal phase is associated with increased ovulation rate. Of the ovarian hormones, both inhibin and progesterone are highly related to greater ovulation rates. These findings could aid in understanding how ovulation rate is controlled in pigs.

Effects of active immunization against GnRH on serum LH, inhibin A, sexual development and growth rate in Chinese female pigs

Surgical castration of young female pigs is common practice in Chinese pig farming today. The purpose of the present study is to investigate anti-GnRH immunization as a practical alternative to surgical castration for female pigs. Thirty-six Chinese female crossbred pigs (Chinese Yanan x Yorkshire) were selected from 12 litters, three pigs from each litter, at the age of 10-13 weeks. One pig from each litter was immunized with 62.5 microg D-Lys6-GnRH-tandem-dimer peptide conjugated to ovalbumin in Specol adjuvant at Week 0 (0 week post-vaccination, wpv), and a booster vaccination was given 8 weeks later (8 wpv). Its intact and castrate littermates (surgically castrated at the time of weaning, i.e. at 6 weeks of age) were administered the vehicle and served as controls. Antibody titers, serum LH and inhibin A were determined at the day of first vaccination, every 4 weeks thereafter and at the day of slaughter (18 wpv). At slaughter, ovaries were inspected for the presence of follicles and corpora lutea, and ovarian and uterine weights were recorded. Ten of twelve immunized pigs responded well to the immunization (immunocastrated animals), while the remaining two pigs responded poorly (nonresponders). Antibody titres in immunocastrated animals steadily increased after immunization, became maximal at 12 wpv and remained high until slaughter. Serum LH levels were reduced (P < 0.05) in immunocastrated pigs as compared to intact controls and surgical castrates. Serum inhibin A levels decreased after vaccination, and equaled surgical castrate levels from 8 wpv until the end of the experiment. Ovarian and uterine weights (1.3 +/- 0.2 and 43.9 +/- 11.4 g, respectively; mean +/- S.E.M.) were significantly lower (P < 0.05) in immunocastrates than in intact controls (9.4 +/- 1.1 and 390.9 +/- 67.2 g, respectively). Antibody titers were significantly lower (P < 0.05) in nonresponders than in immunocastrated pigs from 12 wpv to slaughter. Ovarian and uterine weights were similar in nonresponders and in intact controls. Macroscopically, no follicular structures were found in ovaries of immunocastrated pigs, while large follicles or corpora lutea were observed in the ovaries of both nonresponders and intact controls. Although not significant, immunocastrates had a numerically higher average daily gain than surgical castrates and intact controls (0.74 +/- 0.04 versus 0.66 +/- 0.04 versus 0.66 +/- 0.03 kg per day, respectively; mean +/- S.E.M., P = 0.09). Results obtained in the present study demonstrate that anti-GnRH immunization can be an attractive alternative to surgical castration for Chinese crossbred female pigs. Our results also question the beneficial effect of surgical castration on growth as compared to intact controls.

Mechanisms responsible for increase in circulating inhibin levels at the time of ovulation in mares

In female mammals, inhibin is secreted by the granulosa cells and selectively inhibits secretion of FSH. Although circulating immunoreactive (ir)-inhibin levels decrease after ovulation as a result of the disappearance of its main source, they abruptly increase at the time of ovulation in mares. To investigate the mechanisms responsible for this increase, 50 ml of equine follicular fluid (eFF) was administered into the abdominal cavity of mares during the luteal phase (eFF, n = 4). One hour after treatment, plasma levels of ir-inhibin and inhibin pro-alphaC (but not estradiol-17beta) were significantly higher in eFF-treated mares than in control mares (n = 4). The hormone profiles in eFF-treated mares were similar to those in mares with the spontaneous or hCG induced ovulations. The present study demonstrates that the release of follicular fluid into the abdominal cavity when the follicle ruptures is responsible for the ovulatory inhibin surge in the mare. These findings also suggest that circulating inhibin pro-alphaC may be useful for determining the time of ovulation in the mare.

A selective increase in circulating inhibin and inhibin pro-alphaC at the time of ovulation in the mare

The relationship between a selective increase in circulating immunoreactive (ir)-inhibin and the time of ovulation was investigated in mares. Concentrations of plasma ir-inhibin were measured every 4 h during the periovulatory period. Inhibin pro-alphaC, a precursor protein of the inhibin alpha-subunit, was also measured. The changes in ir-inhibin and inhibin pro-alphaC in circulation were parallel. Concentrations of both ir-inhibin and inhibin pro-alphaC in the plasma increased at the same time when ovulatory follicles ruptured, and the peak levels of circulating ir-inhibin and inhibin pro-alphaC were maintained for 4-8 h. There was no selective increase in plasma concentrations of estradiol-17beta during the process of ovulation. These results suggest that the selective increase in ir-inhibin and inhibin pro-alphaC was caused by the absorption of follicular fluid after the rupture of ovulatory follicles. These results also suggest that the measuring of plasma concentrations of ir-inhibin or inhibin pro-alphaC in mares might be a useful method for detecting the time of ovulation.

Concentrations of gonadotropins, estradiol and progesterone in sows selected on an index of ovulation rate and embryo survival

The objective of this study was to determine concentrations of follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone (P4) and 17beta-estradiol (E2) in sows from a line selected on an index which emphasized ovulation rate (Select) and from a control line. A further classification of the sows in each line was made according to the estimated number of ovulations during an estrous cycle. Sows in the Select line were ranked into a high (HI) or low group (LI) when their estimated number of ovulations were 25 or more and 14 to 15, respectively. Sows of the control line were classified into groups as high (HC) or low (LC) when the estimated values for ovulation rate were 14-15 and 8-9 ovulations, respectively. Blood samples were collected every 12 h during a complete estrous cycle and samples were analyzed for concentrations of FSH and LH. Samples collected every 24 h were assayed for P4 and E2. Mean concentrations of FSH, LH, P4 and E2 did not differ (P > 0.10) between lines or between HI and LI or HC and LC groups. Selection of pigs for ovulation rate and embryonal survival did not affect concentrations of FSH, LH, P4 and E2 in sows during the estrous cycle.

Inhibin secretion in the mare: localization of inhibin alpha, betaA, and betaB subunits in the ovary

To determine the source of circulating inhibin and estradiol-17beta during the estrous cycle in mares, the cellular localization of the inhibin alpha, betaA, and betaB subunits and aromatase in the ovary was determined by immunohistochemistry. Concentrations of immunoreactive (ir-) inhibin, estradiol-17beta, progesterone, LH, and FSH in peripheral blood were also measured during the estrous cycle in mares. Immunohistochemically, inhibin alpha subunits were localized in the granulosa cells of small and large follicles and in the theca interna cells of large follicles, whereas inhibin betaA and betaB subunits were localized in the granulosa cells and in the theca interna cells of large follicles. On the other hand, aromatase was restricted to only the granulosa cells of large follicles. Plasma ir-inhibin concentrations began to increase 9 days before ovulation; they remained high until 2 days before ovulation, after which they decreased when the LH surge was initiated. Thereafter, a further sharp rise in circulating ir-inhibin concentrations occurred during the process of ovulation, followed by a second abrupt decline. After the decline, plasma concentrations of ir-inhibin remained low during the luteal phase. Plasma estradiol-17beta concentrations followed a profile similar to that of ir-inhibin, except during ovulation, and these two hormones were positively correlated throughout the estrous cycle. Plasma FSH concentrations were inversely related to ir-inhibin and estradiol-17beta. These findings suggest that the dimeric inhibin is mainly secreted by the granulosa cells and the theca cells of large follicles; granulosa cells of small follicles may secrete inhibin alpha subunit, and estradiol-17beta is secreted by the granulosa cells of only large follicles in mares.

Effects of passive immunization using antibody against an alpha-inhibin peptide on follicle-stimulating hormone concentrations and litter size in sows

The objective of the experiment was to determine whether passive immunization against inhibin at weaning would increase FSH secretion and thereby influence postweaning reproductive performance in sows. Commercial Yorkshire sows (n = 173) were assigned within parity to 5 alpha-inhibin fragment antibody (alpha-IF-Ab) dosage groups: 0 (control), 3.25, 6.5, 13 and 26 RP-2 kU/kg alpha-IF-Ab. Antibody had been semipurified from ovine antisera raised against alpha-IF, a peptide that mimicked the N-terminal region of inhibin's alpha-subunit. A RP-2 U refers to a laboratory reference preparation. Sows were administered a single intramuscular injection of control solution or alpha-IF-Ab just before 21-d-old piglets were weaned. Blood samples were taken immediately before immunization and 24 h later. Sows were bred upon expression of estrus. Serum alpha-IF-Ab titers in sows 24 h following passive immunization increased (P < 0.001) with dosage. In control sows serum FSH concentrations decreased 24% by 24 h postweaning (P < 0.001). The decrease was diminished or prevented by alpha-IF-Ab treatment in a dose-responsive manner (P < 0.001). Most (167/173) sows were bred within 10 d postweaning, and wean-to-service intervals tended (P < 0.1) to be shorter in the 13 and 26 RP-2 kU/kg alpha-IF-Ab dosage groups. Farrowing rate was 72% (124/173) and was similar among sows in the alpha-IF-Ab dosage groups. Litter size, expressed as total or live piglets born per sow or per sow farrowed, was unchanged by alpha-IF-Ab treatment. Results demonstrate that 1) inhibin plays a key role in regulating FSH secretion at weaning, and 2) blocking the acute postweaning drop in FSH secretion has little if any effect on subsequent reproductive performance.

Peripheral inhibin levels in relation to climatic variations and stage of estrous cycle in buffalo ()

The present study investigated the peripheral plasma inhibin levels in relation to 1) the stage of estrous cycle and the effect of climatic variations. Blood samples were collected from cyclic buffalo (n=5) once daily for 32 consecutive days during the tropical hot humid (summer) and cold (winter) seasons. Estrus was recorded by parading a vasectomized bull as well as by plasma progesterone determination. In the winter season, peripheral inhibin concentrations which were lowest (0.35 +/- 0.02 ng/ml) during the mid-luteal phase of estrous cycle (Day 6 to Day 14, Day 0 = day of estrus) increased significantly (P < 0.02) to 0.47 +/- 0.04 ng/ml during the late luteal phase (Day -4 to Day -2) and then further to 0.52 +/- 0.03 ng/ml (P< 0.02) during the periestrus phase (Day -1 to Day 1). Inhibin concentrations then decreased significantly (P < 0.02) to 0.40 +/- 0.03 ng/ml during the early luteal phase (Day 2 to Day 5). In the summer season the differences in peripheral inhibin concentrations among different phases of estrous cycle were found to be nonsignificant. A comparison of the circulating inhibin concentrations between the two seasons indicated that inhibin concentrations were significantly higher in the late luteal phase (P < 0.01) and periestrus phase (P < 0.05) during the winter season compared with corresponding periods during the summer season. The present study suggests that peripheral inhibin concentrations change in the estrous cycle during cooler breeding season and that environmental heat stress can cause a reduction in peripheral inhibin concentrations.

Measurement of dimeric inhibin in porcine serum: evidence for low concentrations and existence of binding proteins

An immunoradiometric assay and serum extraction procedure were developed to measure dimeric inhibin in porcine serum with minimal interference by putative inhibin-binding proteins. Assay sensitivity was 50 pg/tube, and it incorporated antibodies against the N-terminal region of inhibin's alpha-subunit, alpha-(1-25)-Ab, and against the C-terminal region of inhibin's beta A-subunit. To determine whether inhibin-binding proteins were present in porcine serum, serum was incubated with [125I]-recombinant human (rh)-inhibin and then chromatographed by gel filtration. Radioiodinated rh-inhibin was associated with protein(s) > 600 kDa. Radioiodinated rh-inhibin also was incubated with alpha 2-macroglobulin, an inhibin-binding protein in human and rat serum. Elution profiles were similar for serum and alpha 2-macroglobulin. Serum- and alpha 2-macroglobulin-[125I]rh-inhibin complexes dissociated upon exposure to 8 M urea. Porcine serum was treated with urea, after which inhibin was isolated and concentrated. The recovery of rh-inhibin added to starting serum was 28%. Concentrations of endogenous dimeric inhibin were < 28 pg/ml in serum collected from sows at random stages of the estrous cycle and were < 21 pg/ml in serum collected from sows 2 d postweaning. Results demonstrate that 1) concentrations of dimeric inhibin are low in porcine serum, and 2) an inhibin-binding protein(s), consistent with alpha 2-macroglobulin, is present in porcine serum.

Peripheral inhibin levels during estrous cycle in buffalo ()

A sensitive, specific RIA was validated and used for measurement of peripheral plasma immunoreactive inhibin (irinhibin) levels during the estrous cycle in Murrah buffalo. The RIA employed an 125-I iodinated inhibin as tracer and an antiserum against dimeric inhibin. The procedure had a sensitivity of 16 pg/tube, and the nonspecific effects of buffalo plasma were compensated for by including 200 ul bullock plasma in the standards. Separation of free and bound inhibin was affected by the use of a second antibody and precipitation with polyethylene glycol. Blood samples were collected once daily for 30 d from Murrah buffalo (n = 6) during the hot month of July. Cyclic activity and estrus were confirmed by plasma progesterone determination. Peripheral plasma concentrations of ir-inhibin fluctuated between 0.40 +/- 0.07 and 0.67 +/- 0.13 ng/ml during the estrous cycle in buffalo. During the same period, plasma progesterone levels increased from 0.21 +/- 0.01 ng/ml at Day 0 to a peak of 3.30 +/- 0.72 ng/ml on Day 13, declining sharply by Day -5. Ir-inhibin levels exhibited an increase during the follicular phase, with the maximum concentration of 0.65 +/- 0.01 ng/ml occuring on the day of estrus, a decline thereafter, and no pattern during the luteal phase. The differences, however, were not statistically significant throughout the estrous cycle.

Sertoli cell function declines earlier than Leydig cell function in aging Japanese men

In order to evaluate the age-related changes in Leydig cell and Sertoli cell function, we measured serum levels of total testosterone (TT), free testosterone (FT), inhibin, LH and FSH in 116 healthy Japanese men, aged 24-92 years. Serum TT remained constant up to the age of 80 years and decreased thereafter. Serum FT declined linearly with aging and was significantly lower in men in their forties than in younger men (24-39 years old). Serum inhibin levels also declined with aging, with serum concentrations significantly lower in men older than 40 and markedly lower in men over 80 years old. LH and FSH were elevated in men over 60 and 40 years old, respectively. We also examined relationships between gonadotropins and gonadal hormones in these men divided into three age groups, young (24-39 years old), middle aged (40-59 years old) and aged (60-92 years old) men. Although there was a significant inverse correlation between LH and TT or FT for the entire population, subset analysis demonstrated that this inverse correlation was confined to men over 60 years old. In fact, in young men, TT and FT were positively correlated with LH. Overall, there was also an inverse correlation between FSH and inhibin. In subset analysis this relationship was present in both middle aged and aged men. These findings suggest that in Japanese men testicular endocrine functions decline after the fourth decade of life, and that Sertoli cell function declines earlier than Leydig cell function.

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)

Changes in follicular estradiol-17 beta, progesterone and inhibin immunoactivity in healthy and atretic follicles during preovulatory maturation in the pig

Follicular hormones, growth and granulosa cell gonadotropin sensitive adenylate cyclase activity were determined in healthy and atretic follicles during preovulatory maturation in pigs. Ovaries were recovered at slaughter which was 1, 3, 5 or 7 d after the last administration of a progesterone agonist (altrenogest). Plasma FSH decreased (P < .05) by 64% between days 1 and 3 and remained low through day 5. The number of large (> 5 mm) follicles increased from 2.7 on day 1 to 14.8 on day 3 and did not differ significantly among days 3, 5 and 7. The number of small (1-2 mm) and medium (3-5 mm) follicles decreased (P < or = .05) by 82% between days 3 and 5. Follicles first became estrogen-active (EA) (> or = 100 ng of estradiol-17 beta/ml of follicular fluid) on day 3, with 14.3% of medium and 73.8% of large follicles being EA. About 30% of small and 13% of medium follicles were morphologically atretic on days 1 and 3. However, by day 5, the proportion of atretic small and medium follicles had increased (P < or = .05) to 100 and 59%, respectively. Follicular fluid inhibin immunoactivity and estradiol-17 beta were lower (P < or = .05) and progesterone was greater (P < or = .05) in atretic than healthy follicles. Granulosa cells from large follicles produced (P < or = .05) more cAMP than cells from healthy or atretic small/medium follicles. Compared to control or pFSH treatment, pLH increased cAMP production by granulosa cells from large follicles on all days and from small/medium follicles on days 1 and 5; pLH had no effect on granulosa cells from atretic follicles. Compared to control, pFSH increased cAMP production in granulosa cells from healthy small/medium follicles only on day 1; no effect was detected in granulosa cells from large or atretic follicles on any day. We conclude that decreased secretion of FSH increased loss and atresia among non-ovulatory follicles. Atretic follicles were marked by loss of granulosa cell gonadotropin-sensitive adenylate cyclase activity and by low concentrations of estradiol-17 beta.

Activin-A as an intraovarian modulator: actions, localization, and regulation of the intact dimer in human ovarian cells

The actions, localization, and regulation of activin in the human ovary are unknown. Therefore, the aims of this study were (a) to define the effects of recombinant activin-A and its structural homologue, inhibin-A, on mitogenesis and steroidogenesis (progesterone secretion and aromatase activity) in human preovulatory follicular cells; (b) to localize the activin-A dimer in the human ovary by immunohistochemistry; and (c) to examine regulation of intracellular activin-A production in cultured human follicular cells. In addition to stimulating mitogenic activity, activin-A causes a dose- and time-dependent inhibition of basal and gonadotropin-stimulated progesterone secretion and aromatase activity in human luteinizing follicular cells on day 2 and day 4 of culture. Inhibin-A exerts no effects on mitogenesis, basal or gonadotropin-stimulated progesterone secretion and aromatase activity, and does not alter effects observed with activin-A alone. Immunostaining for dimeric activin-A occurs in granulosa and cumulus cells of human ovarian follicles and in granulosa-lutein cells of the human corpus luteum. cAMP, and to a lesser degree human chorionic gonadotropin and follicle-stimulating hormone, but not inhibin-A, activin-A, or phorbol 12-myristate 13-acetate, increased the immunostaining for activin-A in cultured granulosa cells. These results indicate that activin-A may function as an autocrine or paracrine regulator of follicular function in the human ovary.

Control of FSH, follicular development and estrus synchronization in the mare with steroid-free follicular fluid

Twenty-two pony mares were used in a project designed to determine the effectiveness of different treatments in controlling FSH, follicular development and synchronization of estrus and ovulation. Mares in Group 1 (n=8) received daily oral altrenogest (0.044 mg/kg); those in Group 2 (n=7) received daily altrenogest (0.044 g/kg) and, during the last 4 days of treatment they received steroid-free follicular fluid, (15 cc) intravenously (I.V.) two times a day; Mares in Group 3 (n=7) received daily intramuscular (I.M.) injections of progesterone (80 mg) and estradiol valerate (7 mg). All treatments lasted for 10 days, at the end of which prostaglandin (PgF(2)alpha, 10 mg) was administered. Sexual behavior, follicular development and FSH concentrations were monitor daily. Concentrations of FSH in Group 2 mares, were not significantly different (P>0.05) from those of Group 1 until the mares in Group 2 were treated with follicular fluid (P<0.05). Concentrations of FSH in Group 3 mares, were significantly lower than those of Groups 1 and 2 (P<0.05) until the mares in Group 2 were treated with steroid-free follicular fluid. At this point there was no significant difference between groups 2 and 3 (P>0.05). Steroid-free follicular fluid appears to induce atresia in larger follicles (>11 mm), and the initiation of new follicular wave. The combination of progesterone and estradiol valerate appears to delay follicular growth and not to induce atresia, since larger follicles (>11 mm) continued to grow after treatment. Both treatments (groups 2 and 3) resulted in ovulations within 5 days period. The treatment in Group 1 did not have any effect on FSH or follicular development and ovulations were dispersed through a 9-day period. We concluded that steroid-free follicular fluid offers a new possibility to synchronize ovulation in the mare by controlling FSH and follicular development.

Immunoreactive inhibin in follicular fluid is related to meiotic stage of the oocyte during final maturation of the porcine follicle

The concentration and content of inhibin was determined in individual porcine follicles from gilts ovariectomized at various times after the onset of estrus. In one experiment, gilts (n = 5) were ovariectomized at 0, 10, or 20 hr after the onset of estrus and the follicular fluids from all large follicles individually aspirated. In a second experiment, gilts (n = 6) were ovariectomized at 21, 24, 27, 30, or 34 hr after the onset of estrus; follicular fluids were aspirated; and each oocyte was stained and evaluated for cytogenetic stage of meiotic maturation. Inhibin was determined in diluted follicular fluids with a radioimmunoassay based on a synthetic peptide replica of part of the alpha subunit of porcine inhibin. Inhibin values were expressed in terms of thousands of units (kU) of a World Health Organization inhibin standard (86/690). Concentration of inhibin did not vary among hours (overall mean 248 kU/ml). Total follicular content of inhibin also was not different among hours (overall mean 57 kU/follicle). When follicles were classified on the basis of the maturation of the oocyte, significant differences were found. Concentration of inhibin in follicles with a germinal vesicle-stage oocyte was 138 kU/ml, whereas follicles with more mature oocytes had concentrations of between 204 and 254 kU/ml. Follicular content of inhibin showed a similar pattern with 34.9 kU/follicle at germinal vesicle stage, increasing to 42.5-56.1 kU/follicle at later stages. Quantities of inhibin were also negatively skewed and were positively correlated to follicular content of estradiol and dermatan sulfate.

Isolation of bovine ovarian inhibin, its immunoneutralization in vitro and immunolocalization in bovine ovary

A purification scheme involving gel permeation chromatography, anion exchange chromatography and reversed-phase high performance liquid chromatography (RP-HPLC) was used to isolate from bovine follicular fluid (FF) biologically-active inhibin of molecular weight 32 kDa. Chromatographic fractions were monitored for inhibin-like biological activity (ILA) using a simplified bioassay procedure in which a suppression of total basal FSH production by rat pituitary cells in monolayer culture indicates the presence of ILA. Approximately 3 mg protein having an ILA potency (ED50 value in in vitro bioassay) of 1.7 ng/ml was obtained from 4 1 crude bovine FF (260 g protein; ILA potency 3750 ng/ml) reflecting an approximate 2200-fold purification factor with an overall recovery of about 3%. The isolated material appeared as a single major UV absorbance peak on RP-HPLC and as a single band (32 kDa) when subjected to SDS-PAGE (15% gel) under non-reducing conditions. Under reducing conditions the molecule dissociated into 2 subunits of apparent molecular weight 22 and 14 kDa confirming that it is probably identical to the 31/32 kDa form of bovine ovarian inhibin previously reported by two other independent research groups. An antiserum raised in a chicken against the isolated material completely neutralized the suppressive effects of both 32 kDa inhibin and bovine FF on basal production of FSH by rat pituitary cells in vitro but only partially reversed the suppressive effects of both porcine and human FF. Immunohistochemical staining of sections of bovine ovary and of isolated preparations of bovine granulosa cells using this antiserum confirmed that granulosa cells are a major source of inhibin. The observation that specific immunostaining was not confined to these cells, however, suggests that they may not be the exclusive source of immunoreactive inhibin in the bovine ovary.

Inhibin and related proteins: localization, regulation, and effects

Inhibin has originally been defined as a gonadal hormone that exerts a specific negative feedback action on the secretion of FSH from the gonadotropic cells of the pituitary gland. The existence of inhibin was postulated by Mottram and Cramer (15) as early as 1923. However, only after reliable and sensitive bioassay systems had been developed for detection and estimation of inhibin and an ample source of inhibin was found in the form of ovarian follicular fluid, was progress made on the isolation and characterization of the hormone. It is apparent now that inhibin, which itself consists of a dimer of two different subunits, alpha and beta, is a member of a much larger family of (glyco)protein hormones and growth factors that includes Müllerian inhibiting substance, transforming growth factor-beta, activin/erythroid differentiation factor, bone morphogenetic proteins, and an insect and a Xenopus protein. All play important roles in cell differentiation. Gonadal inhibin is produced in the Sertoli cells in the testis and in the granulosa cells in the ovary. The production of inhibin is stimulated by FSH, but controversy exists about other factors that might play a role in the regulation of the production of inhibin. It appears likely that inhibin plays an important role in the feedback regulation of peripheral concentrations of FSH during the period in which Sertoli cells and granulosa cells--the target cells for FSH--divide, i.e., during puberty in male animals and during the development of ovarian follicles in female animals. In this way, inhibin may be an important regulator of the number of developing Sertoli cells and of the length of the seminiferous tubuli in the testis and of the number of developing follicles in the ovary. Apart from its function in the pituitary-gonadal axis, inhibin and activin may be produced and act in a number of other organs such as the placenta, hypothalamus, adrenal, and bone marrow. Investigation of the role of the members of the inhibin family in these systems has only begun, but will certainly be a field of major interest in the near future.

Changes in plasma follicle-stimulating hormone, luteinizing hormone, estrogen and progesterone during growth of ovulatory follicles in the pig

This experiment was conducted to determine the changes in secretion of LH, FSH, estrogen and progesterone during follicle maturation. Ovaries were recovered from 11 non-treated (control) gilts, three on day 13, four on day 16, and four on day 19 of the estrous cycle, and from four altrenogest-treated gilts on day 19. Altrenogest, a progesterone agonist, was fed at a dose of 20 mg once daily from days 13 to 18 to block spontaneous follicle maturation. Gilts were bled daily from day 12 until slaughter. For control gilts, the number of follicles/gilt 1-6 mm in diameter decreased (P less than .05) from 93.5 on day 13 to 21.5 on day 19, and the number of large (greater than 6 mm) follicles increased (P less than .05) from 5.3 to 13.2. Altrenogest treatment blocked loss of small follicles and growth of large follicles between days 13 and 19. Plasma progesterone decreased (P less than .001) between days 12 and 16 in both control and altrenogest-treated gilts. Plasma FSH decreased (P less than .05) between days 12 and 16 only in control gilts. Plasma LH was not significantly affected by day or altrenogest treatment. Plasma estrogen increased (P less than .05) between days 15 and 19 only in control gilts. These results indicate that 1) no increased LH secretion was detected in conjunction with emergence of ovulatory follicles, and 2) atresia of nonovulatory follicles was associated with decreased secretion of FSH. Both atresia and decreasing FSH secretion began before estrogen concentration increased in the systemic circulation.

Correlation of serum inhibin concentrations with results in an ovarian hyperstimulation program

In order to appraise follicular development, human serum inhibin was measured daily by radioimmunoassay in 18 women undergoing ovarian hyperstimulation. Serum inhibin in the early follicular phase correlated with the number of follicles greater than or equal to 17 mm in diameter (r = 0.57, n = 18, P less than 0.05), follicles greater than or equal to 14 mm in diameter (r = 0.67, n = 18, P less than 0.01) on the day of human chorionic gonadotropin (hCG) administration and the number of oocytes retrieved (r = 0.67, n = 15, P less than 0.01). The number of oocytes retrieved showed a significant correlation with serum inhibin level on the day of hCG administration (r = 0.84, n = 15, P less than 0.001). These data suggest that: (1) In the early follicular phase, serum inhibin may be a valid index to predict ensuing follicular growth; (2) In the preovulatory phase, serum inhibin may be one of applicable indexes of follicular development during the hyperstimulation cycle.