Differential regulation of luteinizing hormone, follicle-stimulating hormone, and free alpha-subunit secretion from the gonadotrope by gonadotropin-releasing hormone (GnRH): evidence from the use of two GnRH antagonists

To examine the differential regulation of glycoprotein hormone secretion from the gonadotrope by GnRH, the Nal-Glu GnRH antagonist was administered to euthyroid women in the early follicular phase (days 1-5) of the menstrual cycle, and the results compared to previous studies with the Nal-Arg GnRH antagonist. After a 4-h period of baseline sampling at a frequency of every 10 min, a single sc dose of the GnRH antagonist was administered to each subject. Frequent sampling continued for 8 h, followed by hourly sampling for a further 16 h. LH, FSH, and free alpha-subunit were measured serially in assays with high specificity. There was a 90% concordance of LH and free alpha-subunit pulses during the baseline sampling period. Pulsatile secretion of LH and free alpha-subunit was immediately abolished at the highest dose of the Nal-Glu antagonist for at least 8 h. The maximum percent suppression of LH after administration of the Nal-Glu GnRH antagonist was 70 +/- 4%, 80 +/- 4%, and 83 +/- 1% at doses of 15, 50, and 150 micrograms/kg, respectively, compared to 51 +/- 10%, 70 +/- 5%, and 69 +/- 5% at doses of 50, 150, and 500 micrograms/kg Nal-Arg antagonist. Decreases in FSH were 28 +/- 2%, 32 +/- 7%, and 39 +/- 2%, with increasing doses of the Nal-Glu antagonist compared with 25 +/- 6%, 17 +/- 6%, and 28 +/- 4% reductions at increasing doses of the Nal-Arg antagonist. Free alpha-subunit decreased 22 +/- 4%, 23 +/- 4%, and 28 +/- 3% at increasing doses of the Nal-Glu antagonist and 12 +/- 4%, 27 +/- 4%, and 30 +/- 7% with increasing doses of the Nal-Arg antagonist. For the Nal-Glu antagonist, suppression of LH was greater than that of FSH and free alpha-subunit at all doses (P less than 0.001), while FSH suppression was greater than that of free alpha-subunit at the highest dose only (P less than 0.05). For the Nal-Arg antagonist, LH suppression was greater than that of FSH or free alpha-subunit at all doses (P greater than 0.01), and FSH suppression exceeded that of free alpha-subunit at the 50 micrograms/kg dose. Suppression of LH was greater with the Nal-Glu antagonist than with the Nal-Arg antagonist at doses of 50 and 150 micrograms/kg (P less than 0.05), and FSH suppression was greater with the Nal-Glu antagonist at 150 micrograms/kg (P less than 0.01), while the degrees of maximum suppression were similar for the two different GnRH antagonists for free alpha-subunit.(ABSTRACT TRUNCATED AT 400 WORDS)     

Gonadotrophin-releasing hormone (GnRH) overcomes GnRH antagonist-induced suppression of LH secretion in primates

If the suppressive effects of gonadotrophin-releasing hormone (GnRH) antagonists on gonadotrophin secretion are mediated through GnRH-receptor occupancy alone, it should be possible to restore serum gonadotrophin levels by displacing the antagonist with exogenous GnRH. To test this hypothesis, eight adult crab-eating macaques (Macaca fascicularis), weight 4.7-7.6 kg, were subjected to the following treatment regimens. A GnRH-stimulation test was performed before and 4, 12 and 24 h after a single s.c. injection of the GnRH antagonist (N-Ac-D-p-Cl-Phe1,2, D-Trp3, D-Arg6, D-Ala10)-GnRH (ORG 30276). The stimulation tests were performed with 0.5, 5.0 or 50 micrograms GnRH given as a single i.v. bolus. Blood was taken before and 15, 30 and 60 min after each bolus for analysis of bioactive LH and testosterone. The GnRH-challenging doses were given as follows: 0.5 microgram GnRH was injected at 0 and 4 h, followed by 5.0 micrograms after 12 h and 50 micrograms after 24 h. One week later, 5.0 micrograms GnRH were given at 0 and 4 h, followed by 50 micrograms after 12 h and 0.5 microgram after 24 h. Finally, after another week, the GnRH challenges began with 50 micrograms at 0 and 4 h, followed by 0.5 microgram at 12 h and 5.0 micrograms at 24 h. This design permitted comparison of the LH and testosterone responses with respect to the dose of GnRH and the time after administration of GnRH antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)     

Graded inhibition of pulsatile luteinizing hormone secretion by a selective gonadotropin-releasing hormone (GnRH)-receptor antagonist in healthy men: evidence that age attenuates hypothalamic GnRH outflow

Healthy older men manifest concomitant hypoandrogenemia and attenuation of LH pulse size. Because exogenous GnRH remains effective, a plausible intuition is that aging reduces hypothalamic GnRH secretion, thus mediating relative hypogonadotropic hypogonadism. To assess the impact of age on central GnRH outflow indirectly, we quantitated graded suppression of pulsatile LH secretion by saline and escalating doses of a potent and selective GnRH-receptor antagonist, ganirelix, in 18 healthy men ages 23-72 yr. The rationale is that ganirelix should reduce the amplitude of LH pulses in proportion to both drug concentration and endogenous GnRH feedforward. To this end, blood was sampled every 10 min for 2 h before and 16 h after sc administration of saline or ganirelix and for 3 additional hours after iv injection of a fixed dose of GnRH (100 ng/kg); concentrations of LH and ganirelix were measured by immunochemiluminometry and RIA, respectively; and pulsatile LH secretion was quantitated by a deconvolution procedure. Log-linear regression analysis was used to estimate the sensitivity of pulsatile LH secretion to inhibition by a unit increase in serum ganirelix concentrations in each subject. Statistical analyses revealed that increasing age markedly attenuated the capability of ganirelix to decrease LH pulse size (viz., r = -0.648; P = 0.004). In contrast, age did not modify the competitive interaction between injected GnRH and ganirelix. These joint outcomes support the clinical hypothesis that age diminishes hypothalamic GnRH outflow without impairing GnRH action in healthy men.     

Exogenous gonadotrophin-releasing hormone (GnRH) stimulates LH secretion in male monkeys (Macaca fascicularis) treated chronically with high doses of a GnRH antagonist.

We reported previously that after a single injection of a gonadotrophin-releasing hormone (GnRH) antagonist to male monkeys, exogenous GnRH stimulated LH secretion in a time- and dose-dependent manner, indicating that GnRH antagonist-induced blockade of LH secretion resulted from pituitary GnRH receptor occupancy. The present study was performed to investigate whether GnRH can also restore a blockade of LH and testosterone secretion during chronic GnRH antagonist administration. Four adult male cynomolgus monkeys (Macaca fascicularis) received daily s.c. injections of the GnRH antagonist [N-Ac-D-pCl-Phe1,2,D-TRP3,D-Arg6-D-Ala10]-GnRH (ORG 30276) at a dose of 1400-1600 micrograms/kg for 8 weeks. Before the GnRH antagonist was given and during weeks 3 and 8 of treatment, pituitary stimulation tests were performed with 0.5, 5, 50 and 500 micrograms synthetic GnRH, administered in increasing order at intervals of 24 h. At 8 weeks, a dose of 1000 micrograms GnRH was also given. All doses of GnRH significantly (P less than 0.05) stimulated serum concentrations of bioactive LH (3- to 8-fold) and testosterone (2.6- to 3.8-fold) before the initiation of GnRH antagonist treatment. After 3 weeks of GnRH antagonist treatment, only 50 and 500 micrograms GnRH doses were able to increase LH and testosterone secretion. Release of LH was significantly (P less than 0.05) more elevated with 500 micrograms compared with 50 micrograms GnRH. After 8 weeks, only the highest dose of 1000 micrograms elicited a significant (P less than 0.05) rise in LH secretion. Basal hormone levels just before the bolus injection of GnRH were similar (P greater than 0.10-0.80).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of follicle-stimulating hormone secretion from gonadotroph adenomas by repetitive administration of a gonadotropin-releasing hormone antagonist

As a preliminary step in searching for a pharmacological treatment for gonadotroph adenomas, we administered the GnRH antagonist analog Nal-Glu GnRH to five patients, four men and a woman, with FSH-secreting gonadotroph adenomas in order to determine its effect on FSH secretion. Administration of a single 10-mg dose of Nal-Glu GnRH to four of the patients produced a significant decrease in the serum FSH concentration in two patients and returned the FSH level to normal in only one. Administration of 5 mg Nal-Glu every 12 h for 7 days, however, produced a significant (P less than 0.001) decrease, and to within the normal range, in four of the five patients (mean +/- SEM, 32.7 +/- 5.6 IU/L during the 3 days before treatment and 9.8 +/- 1.4 IU/L during the last 3 days of treatment). Also, in response to the 7-day treatment, LH fell significantly in all five patients, alpha-subunit fell in three, and testosterone fell in all four men. Administration for 6 weeks of the GnRH agonist analog leuprolide did not decrease the serum FSH concentration of one of the patients whose serum FSH did decrease in response to Nal-Glu GnRH. We conclude that repetitive administration of Nal-Glu GnRH may often inhibit FSH secretion by gonadotroph adenomas and that FSH secretion by gonadotroph adenomas may be dependent on endogenous GnRH secretion.     

Dexamethasone suppresses gonadotropin-releasing hormone (GnRH) secretion and has direct pituitary effects in male rats: differential regulation of GnRH receptor and gonadotropin responses to GnRH

Endogenous or exogenous glucocorticoid excess leads to the development of hypogonadotropic hypogonadism, but the site(s) and mechanisms of glucocorticoid action are uncertain. We studied the effects of various doses of dexamethasone (Dex) on the hypothalamic-pituitary-gonadal axis in intact and castrate testosterone-replaced (cast + T) male rats and attempted to determine possible sites of Dex effects. A dose-dependent suppression of basal gonadotropin secretion was induced by 5 days of Dex treatment (20, 100, 500, or 2,500 micrograms/kg.day), and the highest dose completely abolished the postcastration rise in pituitary GnRH receptor number (GnRH-R) and serum gonadotropin levels. Administration of exogenous GnRH (0.02-200 micrograms/day over 2 days) resulted in a dose-dependent induction in GnRH-R in both intact and cast + T rats, but the effect was significantly (P less than 0.01) augmented in Dex-treated animals. In contrast, acute LH and FSH responses to GnRH (10, 25, 50, 100, or 250 ng, iv) were significantly blunted in Dex-treated rats. The data suggest that 1) Dex suppresses hypothalamic GnRH secretion, thereby preventing the postcastration rises in GnRH-R and gonadotropins; 2) at the pituitary level, Dex dissociates GnRH-R and gonadotropin responses to GnRH, augmenting GnRH-R induction by GnRH and suppressing gonadotropin responses to GnRH at a postreceptor site; and 3) the model of Dex-treated rats may be useful to study differential GnRH regulation of GnRH-R and gonadotropin secretion.     

LH down-regulates gonadotropin-releasing hormone (GnRH) receptor, but not GnRH, mRNA levels in the rat testis.

The demonstration of an inhibitory effect of gonadotropin-releasing hormone (GnRH) agonists upon steroidogenesis in hypophysectomized rats and the presence of mRNA coding for GnRH and GnRH receptors (GnRH-R) in rat gonads suggests that GnRH can act locally in the gonads. To assess this hypothesis, we investigated the effects of GnRH analogs, gonadotropins and testosterone on the levels of both GnRH and GnRH-R mRNA in the rat testis. Using dot blot hybridization, we measured the mRNA levels 2 to 120 h after the administration of the GnRH agonist, triptorelin. We observed an acute reduction of both GnRH and GnRH-R mRNAs 24 h after the injection (about 38% of control). However, the kinetics for testis GnRH-R mRNA were different from those previously found for pituitary GnRH-R mRNA under the same conditions. Initially, the concentrations of serum LH and FSH peaked, then declined, probably due to the desensitization of the gonadotrope cells. In contrast, the GnRH antagonist, antarelix, after 8 h induced a 2.5-fold increase in GnRH-R mRNA, but not in GnRH mRNA, while gonadotropins levels were reduced. Human recombinant FSH had no significant effect on either GnRH or GnRH-R mRNA levels. Inversely, GnRH-R mRNA levels markedly decreased by 21% of that of control 24 h after hCG injection. Finally, 24 h after testosterone injection, a significant increase in GnRH-R mRNA levels (2.3 fold vs control) was found, but a reduction in the concentration of serum LH, probably by negative feedback on the pituitary, was observed. In contrast, GnRH mRNA levels were not significantly altered following testosterone treatment. Since LH receptors, GnRH-R and testosterone synthesis are colocalized in Leydig cells, our data suggest that LH could inhibit the GnRH-R gene expression or decrease the GnRH-R mRNA stability in the testis. However, this does not exclude the possibility that GnRH analogs could also affect the GnRH-R mRNA levels via direct binding to testicular GnRH-R. In contrast, the regulation of GnRH mRNA levels appeared to be independent of gonadotropins. Taken together, our results suggest a regulation of GnRH and GnRH-R mRNA specific for the testis.     

Differential suppression of follicle-stimulating hormone and luteinizing hormone secretion in vivo by a gonadotropin-releasing hormone antagonist

Because of some indication that FSH secretion is less dependent than LH secretion on GnRH in vivo, we performed experiments to examine the effects of a GnRH antagonist (antag) on LH and FSH secretion. We first showed that pituitary cells superfused with GnRH showed a similar pattern of suppressed secretion of both LH and FSH in response to addition of antag. In contrast, antag administration to ovariectomized rats had differing effects on LH and FSH secretion. Serum LH was suppressed in a dose-dependent fashion by 2 h (20-50% of control values). Recovery from the lower doses of antag was seen by 12 h, but the two highest doses maintained serum LH levels at 10% of control values for 72 h. In contrast, the effect on serum FSH was not manifested until 12 h. FSH was maximally decreased only to 40-60% of control values. The two highest doses maintained this effect for 72 h. These results reinforce previous suggestions that FSH secretion in vivo may occur independently of acute changes in GnRH secretion, and may have an GnRH-independent component.     

Effects of gonadotropin-releasing hormone (GnRH) on gastric secretion and gastrin release in the dog

The effects of GnRH on gastric secretion and gastrin release from dogs provided with gastric fistulae and Heidenhain pouches have been investigated. A transient yet significant inhibition of pentagastrin-stimulated secretion from gastric fistulae was observed, while secretion from Heidenhain pouches was unchanged. The maximal inhibitory effect of GnRH on both acid and pepsin secretion stimulated by 2-deoxy-D-glucose was obtained from gastric fistulae. On the contrary, GnRH failed to affect either acid secretion stimulated by bethanechol or acid secretion and gastrin release induced by bombesin. The present results indicate that GnRH possesses an inhibitory action on gastric secretion from the vagally innervated stomach of the dog. The most likely inhibitory mechanism seems to be represented by a decrease of the vagal activity.     

Dynamics of gonadotropin-releasing hormone (GnRH) secretion during the GnRH surge: insights into the mechanism of GnRH surge induction.

Recent studies demonstrate unequivocally that a preovulatory surge of GnRH is secreted into pituitary portal blood during the estrous cycle of the ewe and that this surge is induced by the follicular phase rise in estradiol. These data, obtained at 10-min intervals, suggested the surge results from a continuous elevation of GnRH rather than from a sequence of discrete pulses. This study examines the dynamics of GnRH secretion in more detail to determine if the surge results from strictly episodic release of the decapeptide. Our approach was to monitor GnRH secretion into pituitary portal blood at very frequent intervals during several "windows" of the GnRH surge induced using a physiological model for the estrous cycle. Samples of portal blood were obtained at either 2-min intervals (6 ewes), or 30-sec intervals (12 ewes) at several times during the surge; at other times portal blood was sampled less often to monitor progression of the GnRH surge. All ewes had an unambiguous GnRH surge; amplitude ranged from 100- to 500-fold over pressure levels. Regardless of sampling interval, our results provide no convincing evidence to indicate the enhanced secretion of GnRH is strictly episodic; values remained continuously elevated in portal blood. Our findings are consistent with the hypothesis that the GnRH surge is not composed entirely of discrete synchronous secretory events, and they raise the possibility that one action of estradiol in inducing the GnRH surge may be to switch the pattern of GnRH secretion into portal blood from episodic to continuous.     

Evidence for a physiological role of gonadotropin-releasing hormone (GnRH) or GnRH-like material in the ovary

The possibility that GnRH or a GnRH-like material of ovarian origin may play a physiological role in follicular development was explored in immature hypophysectomized rats by testing whether a potent synthetic antagonist of GnRH action [( N-acetyl-dehydro-Pro1,D-p-chloro-Phe2,D-Trp3,6]GnRH), would potentiate FSH-induced maturation of ovarian follicles to an ovulable stage. Rats were hypophysectomized on day 25 of their life and implanted with a Silastic capsule containing diethylstilbestrol. On day 30, they were started on injections of 10 micrograms NIH FSH-S12 twice daily alone (control) or in combination with 10 micrograms of either native GnRH or GnRH antagonist. On day 35, all rats received 30 IU hCG to trigger ovulation and luteinization of mature follicles. Rats were killed 25.5-28 h later and inspected for number of ova in Fallopian tubes, ovarian weight, number of corpora lutea (CL) on ovarian surface, and appearance of hematoxylin-eosin-stained ovarian slices. In control animals (n = 6), we found some ovulations (mean +/- SEM, 3.2 +/- 1.1/rat), many more CL (16.5 +/- 4.5/rat), and ovarian weights of 37.7 +/- 1.1 mg/rat. In GnRH-treated rats (n = 5), there were no CL formed, no ova were found, and ovarian weights were 16.0 +/- 1.5 mg/rat. In contrast, in GnRH antagonist-treated rats (n = 5), 16.4 +/- 1.6 ova/rat were recovered from the Fallopian tubes, and ovaries contained 20.8 +/- 2.5 CL/rat and weighed 52.7 +/- 3.2 mg/rat. All changes were statistically significant. We conclude that an antagonist of GnRH action is able to potentiate the action of FSH on ovarian follicle development and suggest that it does so by inhibiting the action of an endogenous GnRH or GnRH-like substance that may play a role as a physiological atretic signal.     

Differential gonadotropin secretion: blockade of periovulatory LH but not FSH secretion by a potent LHRH antagonist

The dependence of periovulatory gonadotropin secretion on LHRH was assessed with the use of a potent LHRH antagonist [ ALHRH ; (Nac-L- Ala1 ,p-Cl-D-Phe2,D-Trp3,6)LHRH]. Blood samples were collected hourly from 14.00 h proestrus (P) through 09.00 h estrus (E) from intact cycling female rats. ALHRH was administered at 09.00 or 13.00 h P before the proestrous increases in gonadotropins had commenced or at 23.00 h P after the LH and primary FSH surges had occurred but preceding the secondary FSH surge. Antagonist given at 09.00 or 13.00 h P completely blocked the LH release with levels remaining undetectable in most animals (less than 30 ng/ml) throughout the sampling period. However, administration of antagonist at these times failed to block completely the primary FSH surge although peak values were reduced when compared with controls, which displayed normal gonadotropin surges. In addition, ALHRH administered at 23.00 h failed to alter the magnitude or other characteristics of the secondary FSH surge when compared with controls. The present study demonstrates that the estrous surge of FSH in the rat is independent of acute hypothalamic release of LHRH. Furthermore, although the proestrous release of FSH is to a large extent LHRH dependent, our data suggest that some other mechanism may also contribute to this primary FSH surge.     

Pattern of gonadotropin-releasing hormone (GnRH) secretion leading up to ovulation in the ewe: existence of a preovulatory GnRH surge

We have previously shown LH surges induced by physiological estradiol levels are invariably accompanied by robust and sustained GnRH surges in the ewe. Such an increase, however, has not been observed consistently during the preovulatory LH surge. In the present study, we examined GnRH secretion in Suffolk and Ile de France ewes during the preovulatory period using a method for pituitary portal blood collection which allows simultaneous portal and jugular blood samples to be taken at frequent intervals for up to 48 h. Ewes were sampled either during the mid-late luteal phase (n = 8) or follicular phase (n = 20). During the follicular phase, a robust increase in GnRH secretion occurred at the onset of the LH surge in 11 of 12 ewes sampled during the LH surge. The GnRH increase in most ewes was a massive surge, reaching values averaging 40-fold greater than baseline and extending well beyond the end of the preovulatory LH surge. In the single ewe not exhibiting a GnRH surge during the LH surge, postmortem inspection indicated blood was probably not sampled from the pituitary portal vessels. In the early follicular phase, GnRH-pulse frequency was greater than that observed in the luteal phase and, within the follicular phase, GnRH-pulse frequency increased further and amplitude decreased as the surge approached. These data demonstrate GnRH secretion leading up to ovulation in the ewe is dynamic, beginning with slow pulses during the luteal phase, progressing to higher frequency pulses during the follicular phase and invariably culminating in a robust surge of GnRH. The LH surge, however, ends despite continued elevation of GnRH.     

FSH and LH response to testosterone and gonadotropin-releasing hormone in aging male rats

Gonadotropins were measured in young (3 months) and old (24 months) male Sprague-Dawley rats. Basal levels of LH were lower in old animals, whereas the basal levels of FSH were unchanged. The levels of gonadotropins increased significantly following castration in both age groups, However the old animals secreted less LH than the young. When 50 mg/100 g BW of testosterone propionate (TP) was given, the postcastration levels of FSH and LH were suppressed only in the old rats. When higher doses of TP were given, the levels of FSH and LH were reduced in both groups. However these levels remained suppressed longer in the old rat than in the young. The levels of FSH and LH were elevated after an iv injection of 10 or 100 ng gonadotropin releasing hormone (GnRH/100 g BW. FSH levels were similar in both groups of animals. LH values were higher in the young rats only after the injection of 100 ng GnRH/100 g BW. These data suggest that age differentially influences the control mechanisms for FSH and LH secretion.     

Early prepubertal ontogeny of pulsatile gonadotropin-releasing hormone (GnRH) secretion: I. Inhibitory autofeedback control through prolyl endopeptidase degradation of GnRH.

GnRH[1-5], a subproduct resulting from degradation of GnRH by prolyl endopeptidase (PEP) and endopeptidase 24.15 (EP24.15) was known to account for an inhibitory autofeedback of GnRH secretion through an effect at the N-methyl-D-aspartate (NMDA) receptors. This study aimed at determining the possible role of such a mechanism in the early developmental changes in frequency of pulsatile GnRH secretion. Using retrochiasmatic explants from fetal male rats (day 20-21 of gestation), no GnRH pulses could be observed in vitro, whereas pulses occurred at a mean interval of 86 min from the day of birth onwards. This interval decreased steadily until day 25 (39 min), during the period preceding the onset of puberty. Based on GnRH[1-10] or GnRH[1-9] degradation and GnRH[1-5] generation after incubation with hypothalamic extracts, EP24.15 activity did not change with age, whereas PEP activity was maximal at days 5-10 and decreased subsequently until day 50. These changes were consistent with the ontogenetic variations in PEP messenger RNAs (mRNAs) quantitated using RT-PCR. Using fetal explants, the NMDA-evoked release of GnRH was potentiated in a dose-dependent manner by bacitracin, a competitive PEP inhibitor and the desensitization to the NMDA effect was prevented using 2 mM of bacitracin. At day 5, a higher bacitracin concentration of 20 mM was required for a similar effect. Pulsatile GnRH secretion from fetal explants was not caused to occur using bacitracin or Fmoc-Prolyl-Pyrrolidine-2-nitrile (Fmoc-Pro-PyrrCN), a noncompetitive PEP inhibitor. At postnatal days 5 and 15, a significant acceleration of pulsatility was obtained using 1 microM of Fmoc-Pro-PyrrCN or 2 mM of bacitracin. At 25 and 50 days, a lower bacitracin concentration of 20 microM was effective as well in increasing the frequency of GnRH pulsatility. We conclude that the GnRH inhibitory autofeedback resulting from degradation of the peptide is operational in the fetal hypothalamus but does not explain the absence of pulsatile GnRH secretion at that early age. After birth, PEP activity is high and may account for the low frequency of pulsatility. The potency of that effect decreases before the onset of puberty and may contribute to the acceleration of GnRH pulsatility.     

FSH and LH response to testosterone and gonadotropin-releasing hormone in aging male rats

Abstract

Gonadotropins were measured in young (3 months) and old (24 months) male Sprague-Dawley rats. Basal levels of LH were lower in old animals, whereas the basal levels of FSH were unchanged. The levels of gonadotropins increased significantly following castration in both age groups. However the old animals secreted less LH than the young. When 50 mg/100 g BW of testosterone propionate (TP) was given, the postcastration levels of FSH and LH were suppressed only in the old rats. When higher doses of TP were given, the levels of FSH and LH were reduced in both groups. However these levels remained suppressed longer in the old rat than in the young. The levels of FSH and LH were elevated after an iv injection of 10 or 100 ng gonadotropin releasing hormone (GnRH)/100 g BW. FSH levels were similar in both groups of animals. LH values were higher in the young rats only after the injection of 100 ng GnRH/100 g BW. These data suggest that age differentially influences the control mechanisms for FSH and LH secretion.     

Differential suppression of FSH and LH secretion by follicular fluid in the presence or absence of GnRH

The differential role of porcine follicular fluid (pFF) in regulating follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release in vivo in situations of different gonadotropin releasing hormone (GnRH) backgrounds was studied. In experiment 1, 2-week ovariectomized rats injected intravenously with 4, 16 or 64 mg of protein from pFF, showed a dose-dependent suppression of FSH over time, with a maximal suppression to 40% of control values by 10 h. LH levels were slightly, but significantly, elevated by the two lower doses, but not by the highest dose of pFF. In experiment 2, 64 mg pFF was superimposed (i.v. injection) in ovariectomized rats injected subcutaneously with a high dose of GnRH antagonist (500 micrograms) 24 h earlier. The pFF suppressed FSH 35% below the level achieved in the absence of GnRH stimulation, with no effect on LH. In experiment 3, the rise in FSH secretion in acutely ovariectomized rats was shown to be inhibited by 8 or 32 mg pFF administered intravenously 3.5 h after surgery. Injection of GnRH (250 or 1,000 ng) 4.5 h after pFF could not overcome the inhibitory action of pFF on FSH, although non-pFF-treated controls responded in a dose-dependent fashion to GnRH stimulation. The expected LH response to GnRH was not affected by pFF, except in the group receiving 1,000 ng GnRH and 8 mg pFF. In these rats, LH was enhanced in one trial, but suppressed in a replicate trial, illustrating the inconsistent effects of pFF on LH under conditions of high GnRH stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)