Interrelationship between estrogen and thyroxine on the release of luteinizing hormone and gonadotropin-releasing hormone in vitro

The present experiments were designed to study the interaction between estradiol benzoate (EB) and thyroxine (T4) given in vivo on the responsiveness of pituitary luteinizing hormone (LH) to gonadotropin-releasing hormone (GnRH) and the release of GnRH in vitro. Ovariectomized-thyroidectomized (Ovx-Tx) rats were injected s.c. with saline or T4 (2 micrograms/100 g b.wt), and oil or EB (0.1 microgram) once daily for 40 days following a 2 x 2 factorial design. All animals were then decapitated and blood samples were collected. Anterior pituitaries (APs) were incubated in vitro with and without 0.1 ng GnRH at 37 degrees C for 4 h. Mediobasal hypothalami (MBHs) were excised and then incubated with and without APs from Ovx donor rats. Concentrations of LH and GnRH in the medium and that of LH in the serum were measured by radioimmunoassay. The LH level in media containing MBHs and donor APs was used as the index of bioactive GnRH release. In Ovx-Tx rats, T4 injections reduced the serum LH concentration, the pituitary LH response to GnRH, and the bioactive as well as the immunoreactive GnRH release. The serum LH levels and the spontaneous as well as the GnRH-stimulated release of LH in vitro were suppressed in Ovx-Tx rats following administration of EB. By contrast, the serum LH concentration, as well as pituitary LH response to GnRH and GnRH release in vitro, were higher in the group treated with both T4 and EB than in that treated with saline and EB. These results suggest that the differential changes in the LH secretion after thyroidectomy of Ovx versus non-Ovx rats are due to an antagonistic effect between T4 and estrogen on the response of pituitary LH to GnRH, and the release of GnRH.     

Effects of thyroidectomy and thyroxine replacement on the release of luteinizing hormone and gonadotropin-releasing hormone in vitro

The effects of thyroidectomy and thyroxine (T4) replacement on the release of luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) in ovariectomized (Ovx) rats were studied. Immediately after ovariectomy, rats were thyroidectomized (Tx) or sham-Tx. The Ovx-Tx rats were injected subcutaneously with either saline or T4 (2 micrograms/100 g body weight) daily for 30 days before sacrifice. Sham-Tx rats were treated with saline only. Twenty hours after the last injection, the blood sample was obtained by decapitation. The excised anterior pituitary gland (AP) was bisected and incubated in vitro with or without 0.1, 0.5, 2.5, and 50 ng GnRH at 37 degrees C for 4 h. The mediobasal hypothalamus (MBH) was bisected and incubated with or without the AP of Ovx donor rat in vitro. Concentrations of LH and GnRH in the medium and that of LH in the serum were measured by radioimmunoassay. LH in the serum of Tx rats was higher than that in the serum of sham-Tx and Tx-T4 rats. Thyroidectomy resulted in an increase of LH release by Ovx rat AP, stimulated with or without 0.1 and 50 ng GnRH, as well as in an increase of immunoreactive GnRH release from MBH of Ovx rats in vitro. After a 4-hour incubation with donor APs, the LH in the medium containing MBH obtained from Tx rats was significantly higher than that obtained from sham-Tx and Tx-T4 rats. LH concentrations, in both sera and media, as well as GnRH concentration in the media of euthyroid and T4-replaced Tx groups were nonsignificantly different. These results suggest that T4 is inhibitory to the basal and GnRH-stimulated LH release as well as to the release of GnRH in the absence of ovarian hormones.     

Differential changes in the mechanisms controlling luteinizing hormone and prolactin secretion in middle-aged female rats

Serum luteinizing hormone (LH) and prolactin (PRL) concentrations were measured in young (3-4 month old) and middle-aged (10-12 month old) intact female rats on proestrus, in ovariectomized rats after two estrogen injections (estradiol benzoate; EB, 10 micrograms/100 g body weight, s.c.) or after preoptic stimulation in EB-primed ovariectomized rats. Only animals showing regular 4-day estrous cycles were selected for the experiment. The magnitude of proestrous LH surge was significantly smaller in middle-aged than in young rats. Two BE injections, at noon on Days 0 and 3, in ovariectomized middle-aged rats failed to induce surges in LH secretion on Day 4 whereas the same treatment produced LH surges in ovariectomized young rats. The preoptic electrochemical stimulation (50 microA for 60 sec) produced a prompt rise in serum LH levels in ovariectomized EB-primed young but not in middle aged rats. The preoptic stimulation with a larger current (200 microA) induced LH secretin in middle-aged rats. In none of these situations serum PRL concentrations were different between young and middle-age rats. These results suggest differential aging rates in the preoptic mechanisms governing LH and PRL secretion in the rat. The function of the preoptic ovulatory center in responding to the estrogen positive feedback action and inducing LH secretion may become impaired and independent of the PRL control mechanism, even before the regular estrous cycle terminates.     

The effect of testosterone on serum gonadotropins of castrated rats kept under different lighting conditions.

Testosterone feedback sensitivity was measured as the ability of testosterone propionate to decrease serum LH and FSH of long-term castrated (4 wk) rats under four different lighting conditions: periodic light (12L:12D), constant light (LL), constant darkness (DD), and dim night illumination (1 lx) with a 12L:12D photoperiod. Rats were exposed to the different lighting conditions for 1 wk, during which they received daily testosterone propionate (125 micrograms or 250 micrograms s.c.). At the end of the experiment the rats were decapitated at 1100 h, and serum gonadotropin levels were measured by RIA. Serum LH of the rats kept under LL was reduced to the level of the intact rats with the smaller testosterone dose (125 micrograms/day). Under all other lighting conditions only the large dose (250 micrograms/day) was able to restore the serum LH concentration to the level of the intact rats. Serum FSH was restored only partially, and the effect was the same with both doses and similar under all lighting conditions. We conclude that the increase in testosterone negative feedback sensitivity was not caused by the lack of periodicity of illumination alone, but that sufficient intensity of lighting throughout the 24 h was needed as well.     

Alterations in the proestrous pattern of median eminence LHRH, serum LH, FSH, estradiol and progesterone concentrations in middle-aged rats

The purpose of the following study was to assess the changes in the proestrous hormone profile in middle-aged cycling rats to better understand the inter-relationship and possible interaction of these hormones during the transition to estrous acyclicity. Median eminence LHRH concentrations and serum LH, FSH, estradiol and progesterone concentrations were measured in young (3-4 months old) and middle-aged (8-10 months old) proestrous rats at 0900, 1200, 1500 and 1800h. The data demonstrate that (1) baseline hormone concentrations prior to the surge at 0900h are the same in middle-aged and young rats; (2) the proestrous gonadotropin surge is temporally delayed in middle-aged rats; (3) this delay is preceded by lower median eminence LHRH concentrations and serum estradiol concentrations at 1200h; (4) serum progesterone concentrations are lower in middle-aged rats during the preovulatory gonadotropin surge (at 1500 and 1800h) probably as a consequence of the delayed LH surge.     

Demonstration of potent, gonadotropin-like biological activity in the serum of rats during midpregnancy

The gonadotropin-like activity (GnLa) of serum from pregnant rats was measured using the rat interstitial cell testosterone (RICT) bioassay. Serum GnLA was elevated on day 9 of pregnancy, peaked at 7.2 μg rat LH-RPl equivalents/ml on day 11 and declined to undetectable levels by day 15. Serum LH, measured by homologous RIA, was consistently low (<20 ng/ml) during pregnancy, except near term.Rat placental lactogen (rPL), which was measured in the same serum samples by rat radioreceptor assay (RRA), reached maximal concentrations on days 12 and 13 of pregnancy.These data suggest the presence in pregnancy serum of a potent-gonadotropin-like hormone, different from pituitary LH, whose origin is unknown. Furthermore, there are discrepancies between the times of appearance of this GnLA and rPL.     

Effect of adrenalectomy and 5-hydroxytryptophan on phasic release of luteinizing hormone

The effect of 5-hydroxytryptophan (5-HTP) on serum progesterone and the possible role of adrenal progesterone in mediating stimulation by 5-HTP of phasic release of luteinizing. hormone (LH) were investigated in estradiol benzoate (EB)-treated ovariectomized rats. LH surges were induced in long-term (at least two weeks) ovariectomized rats by two injections of EB (20 micrograms/rat, s.c.) with an interval of 72 hrs. Administration of 5-HTP (50 mg/kg, i.p.) at 1000 hr in EB-treated ovariectomized rats resulted in a four-fold increase in serum progesterone within 30 mins, and significantly stimulated the LH surge at 1600 hr. This facilitative effect of 5-HTP on serum LH, but not progesterone, was further potentiated in rats pretreated with P-chlorophenylalanine (PCPA) 72 hrs earlier. Adrenalectomy shortly before 5-HTP administration attenuated the LH surge in saline treated controls, and completely blocked the facilitative effect of 5-HTP on the afternoon surge of LH in rats pretreated with PCPA 72 hrs earlier. On the other hand, chronic adrenalectomy (for 6 days) followed by hydrocortisone (0.2 mg/rat/day) replacement not only had no effect on the LH surge in saline treated controls, but also failed to prevent 5-HTP from facilitating the LH surge in PCPA pretreated rats. On the first day of bleeding, the basal LH value at 1000 hr in sham operated controls was significantly suppressed by PCPA pretreatment 48 hrs earlier. The second dose of 5-HTP administered on the next day failed to potentiate LH surges in either sham operated or adrenalectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the antiandrogen hydroxyflutamide on progesterone secretion by preovulatory rat follicles in vivo and in vitro.

Serum and ovarian progesterone levels and in vitro production of progesterone by preovulatory follicles were measured on proestrus in pregnant mare's serum gonadotropin (PMSG) primed immature rats in which the luteinizing hormone (LH) surge and ovulation were blocked by administration of the antiandrogen hydroxyflutamide. Serum progesterone levels observed at 12:00 on proestrus were significantly elevated, twofold above those observed in vehicle-treated controls, by in vivo administration of 5 mg hydroxyflutamide 4 h earlier. In control rats, proestrous progesterone did not increase until 16:00, in parallel with rising LH levels of the LH surge. No LH surge occurred in the hydroxyflutamide-treated rats, ovulation was blocked, and serum progesterone declined throughout the afternoon of proestrus, from the elevated levels present at 12:00. Administration of human chorionic gonadotropin (hCG) at 11:00 advanced the elevation of serum progesterone by 2 h in vehicle-treated controls and prevented the decline in progesterone levels in hydroxyflutamide-treated rats. The patterns of change in ovarian tissue concentrations with time and treatment were essentially similar to those observed for serum progesterone. In in vitro experiments, progesterone secretion during 24 h culture of preovulatory follicles obtained on PMSG-induced proestrus was significantly increased, sixfold, by addition to the culture media of 370 microM but not of 37 microM hydroxyflutamide. Testosterone (50 nM) and hCG (20 mIU/mL) caused 26- and 14-fold increases, respectively, in progesterone secretion by cultured follicles. Hydroxyflutamide significantly reduced the stimulatory effect of testosterone but not of hCG on progesterone secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Periovulatory changes in serum concentration and ovarian content of 5 alpha-androstane-3 alpha, 17 beta-diol in the adult rat

The high amounts of 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-diol) present in immature female rats decline towards first ovulation, but on the day of first proestrus a peak is seen. This raises the possibility that during adulthood similar proestrous peaks may occur. Therefore, serum concentrations and ovarian content of 3 alpha-diol were estimated every two hours between 0900 and 2100 h in adult cyclic rats on the day of proestrus. In the same rats, serum concentrations of estradiol (E2), progesterone (P) and luteinizing hormone (LH) were measured, as were ovarian contents of E2 and P. A significant elevation in ovarian 3 alpha-diol was found between 0900 and 1700 h proestrus, whereas serum concentrations of 3 alpha-diol were elevated from 1300 to 2100 h. The high morning values of ovarian 3 alpha-diol correlated with those for ovarian E2 (p less than 0.005); the elevated serum concentrations of 3 alpha-diol during the afternoon correlated with serum P (p less than 0.005) and with serum LH concentrations (p less than 0.005). Serum and ovarian values were positively correlated for P and E2, but not for 3 alpha-diol. The rise in serum 3 alpha-diol could be prevented by blocking the LH surge with sodium pentobarbitone (Nembutal; 35 mg/kg b.w.) administered at 1300 h. In Nembutal-treated rats, the concentration of 3 alpha-diol at 1700 h (886 pg/ml) was significantly lower than in saline-treated control rats (1135 pg/ml; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)     

Neonatal release of gonadotropins is essential for development of ovarian follicle-stimulating hormone receptors

In the rat, ovarian follicle-stimulating hormone (FSH) receptors increase markedly during the first two postnatal weeks, when serum gonadotropin levels are most elevated. This study was conducted to evaluate the hypothesis that these high gonadotropin levels, and in particular FSH, are involved in the acquisition of FSH receptors by the developing ovary. Gonadotropin release was suppressed by administration of several non-aromatizable androgens, among which dihydrotestosterone propionate (DHTP) was the most effective. In one series of experiments the steroids were administered from Days 5 to 11, and serum FSH and luteinizing hormone (LH) were measured on Day 12. Surprisingly, FSH receptor content was greater in rats with suppressed serum gonadotropins than in controls. The greatest increase in available receptors was observed in DHTP-treated rats in which serum FSH was reduced to 20% of control values and LH suppressed to undetectable values. DHTP failed to directly increase available FSH receptors in hypophysectomized immature rats. Magnesium chloride (MgCl2) treatment of ovarian membranes removed bound 125I-hFSH by 87% without affecting receptor viability. Exposure of control 12-day-old ovaries to MgCl2 increased available FSH receptors to a level similar to that of ovaries from DHTP-treated rats not exposed to MgCl2, suggesting that more receptors were available in DHTP-treated rats because serum FSH was suppressed. Earlier initation of DHTP treatment (postnatal Day 1) suppressed serum FSH and LH to undetectable values by Day 5 and decreased FSH receptor content below control values by Day 12. MgCl2 treatment only slightly increased available receptors in these DHTP-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Puberty and ovulatory release of gonadotropins in spontaneously hypertensive rats

The age at vaginal opening, estrous cyclicity, serum levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) on the day of proestrus, and number of ova and ovarian weight as measured on the day of estrus in spontaneously hypertensive (SH) and genetically matched normotensive Wistar Kyoto (WKY) female rats were compared. In SH rats, there was a significant delay in the vaginal opening, but the regular 4-day estrous cycle followed afterwards. No significant changes were observed in the afternoon increase in serum LH, FSH and PRL on the day of proestrus in SH and WKY rats, although the basal levels of LH and PRL in the morning (11:00 h) were lower in SH rats than in WKY rats. The mean number of ova in SH rats was also less than in WKY rats, whereas the ovarian relative weight was similar in both species of rats. It can be said that SH rats undergo certain, but not critical, endocrine and/or neuroendocrine changes related to reproduction.     

Development of time-resolved immunofluorometric assays for rat follicle-stimulating hormone and luteinizing hormone and application on sera of cycling rats

The aim of this study was to develop time-resolved immunofluorometric assays (TR-IFMA) for measuring rat (r)FSH and rLH. The advantages of these IFMAs are higher sensitivity due to lower background values, higher specificity as only intact molecules of FSH and LH can be measured, and a very long shelf life of the nonradioactive biotin antigens compared with radiolabeled iodine antigens. For rFSH, IFMAs are lacking, while for rLH, if present, the resources for antibodies are scarce or the mouse monoclonal antibodies (mMAbs) against LHalpha are inactive with FSH. Thus specific antibodies need to be obtained. With the final TR-IFMAs, rFSH and rLH levels were assessed during the estrous cycle and compared with those obtained with the more classical RIAs and fluoroimmunoassays (FIAs). Two IFMAs for rFSH were developed with mMAbs against the recombinant human (rec h)FSHbeta subunit (FSH56A) attached to the wall and two different rabbit polyclonal antibodies (PAbs) against the alpha subunit of rec hFSH (R93-2705) or recombinant rat (rec r)LH (R95-2715) conjugated with biotin as signal antibody. With both IFMAs, rFSH holo-molecules can be measured. Rat FSH standards could be assessed between 0.02 and 10 ng/ml with a detection limit of 0.05 and 0.24 ng/ml in buffer and serum, respectively. These detection limits in four IFMAs were 8- to 16-fold lower than those in RIAs and FIAs. This detection level allowed the measurement of FSH levels in serum of hypophysectomized (HYPEX) rats at 0.18 ng/ml. In serum of cycling rats, the FSH levels of the IFMA were 2-fold lower than those of the FIA, while in ovariectomized (OVX) rats the IFMA levels were comparable. A peak level of FSH was found during proestrus of Day 2 and gestation with both RIA and FIA, but with IFMAs at gestation only. An IFMA for rLH was set up with mMAb (hCG77A) reacting with rLHbeta as capture and rabbit PAb to rec rLHalpha (R95-2712) as signal antibody. Rat LH standard could be assessed between 0.001 and 10 ng/ml with a detection limit of 0.012 and 0.1 ng/ml in buffer and serum, respectively, which was 8-fold lower than that in RIA/FIA. In serum of HYPEX rats, LH was undetectable (< 0.04 ng/ml), whereas a high background level of 2.5 ng/ml was measured in the FIA. In serum of cycling rats, only a very low LH level of 0.14 ng/ml was measured, which strongly deviated from the level of 3.46 ng/ml with an FIA. The load of LH in serum of OVX rats was 2.91 ng/ml, which was 12-fold lower than that for the FIA. The peak level of LH was detected on proestrus Day 2 with RIA, FIA, and IFMA. In conclusion, two IFMAs for rFSH and one for rLH have been developed with high sensitivity and specificity for intact gonadotropins. The LH pattern during the estrous cycle was comparable between IFMA, RIA, and FIA, although the overall level in the IFMA was much lower, as were HYPEX levels. The FSH pattern differed only on proestrus Day 2 in the IFMA from that of RIA/FIA, showing a peak level with RIA/FIA and a basal level with the IFMA. This implies that in RIA/FIA measurements, proteins other than intact FSH and LH interfere with the analysis at proestrus Day 2 for FSH and in HYPEX, cycling, and OVX rats for LH.     

Circulating radioimmunoassayable inhibin during periods of transient follicle-stimulating hormone rise: secondary surge and unilateral ovariectomy

We have measured changes in circulating immunoreactive (ir-) inhibin in male and female rats using an RIA with an antiserum raised against porcine inhibin alpha (1-26)-Gly-Tyr. The same synthetic peptide was used for standards and for the preparation of tracer. Serum ir-inhibin levels were significantly higher in intact female than in intact male rats (p less than 0.001). Immunoreactive inhibin was significantly reduced in both sexes 24 h after bilateral gonadectomy (p less than 0.0001). Unilateral ovariectomy (ULO) of female rats on metestrus caused a transient decrease in serum inhibin 8 h after surgery, but levels were not significantly different from those of sham-operated controls at later times after surgery. Increases in serum FSH and LH were observed for 8-18 h after ULO. Serum ir-inhibin levels were also measured on the early morning of estrus during the secondary FSH surge. At this time, ir-inhibin levels were low, while FSH levels were high and LH levels were low. These results show that serum ir-inhibin levels in rats are decreased at times when serum FSH levels are high.     

Reduced ability of naloxone to stimulate LH and testosterone release in aging male rats; possible relation to increase in hypothalamic met5-enkephalin

Blood luteinizing hormone (LH) and testosterone levels are lower in old than in young male rats. The specific opiate antagonist, naloxone, previously shown to increase serum LH in mature male rats, exhibited relatively little ability to raise serum LH and testosterone levels in old (18–20 mo) as compared to young (4–5 mo) male rats. The brain opiate, met⁵-enkephalin, which depresses LH, was found to be significantly higher in the hypothalamus of old than of young male rats. These observations suggest that hypothalamic opiates may be partially responsible for the lower serum LH and testosterone levels in old male rats, and for reduced release of these hormones in response to naloxone administration.     

Effect of Immobilization Stress on Gonadotropins and Sex Steroids

The effects of the acute immobilization stress on follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol, and testosterone levels in rats were evaluated. The male and female rats were grouped in control and stress groups (7 animals in each group). Vaginal smears were taken daily from female rats and the rats in the proestrus were involved in the experiments. After a 1-h immobilization stress, blood samples from the heart were taken and the serum FSH, LH, estradiol and testosterone levels were measured by radioimmunoassay (RIA). The results were statistically analyzed using the Mann–Whitney U-test. A statistically significant increase of the levels of FSH and estradiol, but not of LH, was found in the female rats after the stress. A statistically significant increase of the FSH concentration was also revealed in the male rats, but there were no significant changes of the LH and testosterone levels. The scientific and applied (medical) aspects of the data obtained are discussed.     

Naloxone stimulation of luteinizing hormone release in prepubertal female rats; role of serotonergic system

The purpose of this study was to determine whether naloxone stimulated LH release via a serotonergic mechanism. Injection of naloxone hydrochloride (2 mg/kg B.W.) into 25-day old female prepubertal rats resulted in a significant elevation in serum LH 30 min later. Injection of this dose of naloxone together with morphine sulfate (2 or 5 mg/kg B.E.) resulted in inhibition of naloxone-induced LH release. When rats were first injected with 5-hydroxytryptophan (5-HTP) to increase hypothalamic serotonin content, naloxone failed to increase serum LH levels. On the other hand, when parachlorophenylalanine (PCPA) was given first to reduce hypothalamic serotonin content, naloxone-induced LH release was potentiated. Morphine failed to inhibit the naloxone-induced rise in serum LH when PCPA was first administered. Neither 5-HTP nor PCPA, when injected alone, altered serum LH values. These results suggest that naloxone promotes LH release by reducing hypothalamic serotonergic activity, and morphine inhibits LH release by increasing hypothalamic serotonergic activity. This does not exclude possible involvement of other neurotransmitters.     

Crystalloid formation in Leydig cells of rats (Rattus fuscipes)

Summary The ultrastructure of Leydig cells in a seasonally breeding rodent, Rattus fuscipes, was studied in the breeding and non-breeding season and compared with Leydig cell morphology after suppression of gonadotrophin secretion induced by hypophysectomy or chronic administration of testosterone. Serum luteinizing hormone (LH) and testosterone (T) were measured and in-vitro T production by testes was assessed by stimulation with human chorionic gonadotrophin (hCG). In non-breeding wild-trapped rats and rats with experimental suppression of gonadotrophins, the Leydig cells were atrophied and exhibited variable amounts of cytoplasmic lipid and crystalloid inclusions, the latter commonly dominating the cytoplasmic area. Compared with fertile rats, serum LH and hCG-stimulated T production of experimentally regressed rats was significantly reduced, confirming structural features indicative of Leydig cell inactivity. Atrophy of Leydig cell nuclei was accompanied by the formation of unusual intranuclear vesicles sometimes containing small crystalloids. Ultrastructural analysis suggested transfer of the vesicles to the cytoplasm where their unification gave rise to much larger crystalloid bodies. Crystalloids occurred when serum LH was depressed and with either full (T treatment) or arrested spermatogenesis (hypophysectomy) suggesting that their formation is governed by pituitary function and is not dependent upon the degree of spermatogenic activity.     

Delayed suppression of serum luteinizing hormone after naloxone treatment in neonatal female rats

In female neonatal rats, opiate receptor blockade markedly raises serum luteinizing hormone (LH) levels. The LH effect of acute treatment with opiate antagonists is apparently brief in older rats; however, age-related differences in antagonist pharmacokinetics may result in different LH response patterns. The duration of LH response to naloxone (NAL) and naltrexone (NTX) was examined in 5 day-old (d.o.) female rats and compared to the duration of analgesia blockade. The rise in serum LH following opiate receptor blockade in 5 d.o. rats was of similar duration to that previously observed in older animals and much briefer than blockade of analgesia. Furthermore, neonatal rats exhibited a delayed suppression of LH 6 hr following NAL, but not NTX, treatment. Stimulation and later suppression of LH were still observed after five repetitive NAL treatments at 6 hr intervals.     

Chronic administration of corticosterone impairs LH signal transduction and steroidogenesis in rat Leydig cells

The mechanism involved in the inhibitory actions of chronic corticosterone treatment on Leydig cell steroidogenesis was studied in adult Wistar rats. Rats were treated with corticosterone-21-acetate (2 mg/100 g body weight, i.m., twice daily) for 15 days and another set of rats was treated with corticosterone plus ovine luteinizing hormone (oLH) (100 microg/kg body weight, s.c., daily) for 15 days. Chronic treatment with corticosterone increased serum corticosterone but decreased serum LH, testosterone, estradiol and testicular interstitial fluid (TIF) testosterone and estradiol concentrations. Administration of LH with corticosterone partially prevented the decrease in serum and TIF testosterone and estradiol. Leydig cell LH receptor number, basal and LH-stimulated cAMP production were diminished by corticosterone treatment which remained at control level in the corticosterone plus LH treated rats. Activities of steroidogenic enzymes, 3beta- and 17beta-hydroxysteroid dehydrogenase (3beta-HSD and 17beta-HSD) were significantly decreased in corticosterone treated rats. LH plus corticosterone treatment did not affect 3beta-HSD activity but decreased 17beta-HSD activity, indicating a direct inhibitory effect of excess corticosterone on Leydig cell testosterone synthesis. The indirect effect of corticosterone, thus, assume to be mediated through lower LH which regulates the activity of 3beta-HSD. Basal, LH and cAMP-stimulated testosterone production by Leydig cells of corticosterone and corticosterone plus LH treated rats were decreased compared to control suggesting the deleterious effect of excess corticosterone on LH signal transduction and thus steroidogenesis.     

Role of luteinizing hormone-releasing factor (LH-RF) and LH on maintenance of early gestation in rats.

The role of luteinizing hormone (LH) and LH-releasing hormone (LH-RH) in the maintenance of early pregnancy in rats was studied. Serum levels of progesterone (P) and LH were measured daily in untreated pregnant rats from Day 4 through parturition. Serum levels of P and LH were determined on Days 11 and 15 of pregnancy in animals treated with antisera to LH (LH-A/S) and to LH-RH (LH-RH-A/S) on Days 8-10. Serum levels of P peaked on Days 7 and 16 in untreated animals, after which they declined sharply just before delivery. Serum LH fluctuated between 30-160 ng/ml during pregnancy but did not exhibit any distinctive peaks. Treatment with .2 ml LH-A/S on Days 8-10 reduced serum P to virtually undetectable levels on Day 11, and only a slight recovery was evident on Day 15. Lower doses of LH-A/S had no effect. Administration of 1.3 ml LH-RH-A/S had no effect on serum levels of P or LH, and did not impede fetal development. The results indicate that LH is essential to the luteotropic complex of early pregnancy in the rat, and also that LH-RH-A/S can maintain to some extent basal levels of P and LH during early pregnancy.