Dynamic characteristics of luteinizing hormone release from perifused sheep anterior pituitary cells stimulated by combined pulsatile and continuous gonadotropin-releasing hormone

We aim to quantify and relate the dominant dynamic factors of GnRH signals and of the resultant patterns of LH release from pituitary cells. Using perifused sheep cells we have already shown that rising edges of GnRH pulses are major effectors of LH release and that a longer absence of signal between pulses improves response. This study reports the effects on LH release dynamics of continuous levels of GnRH with superimposed pulses and of slowing the important rising edge of the GnRH pulses. Low baseline GnRH perifusions at physiological levels (5-60 pM) reduced the response to hourly pulses of 850 pM GnRH. Continuous GnRH (420 pM), which initially yielded maximal LH release followed by desensitization, prevented extra stimulation by pulses of equal concentration, but 10-fold higher pulses gave additional LH output. After desensitization an hour's respite from stimulation resensitized cells to 420 pM pulses. Whereas continuous stimulation of cells with GnRH even at the very low level of 5-10 pM [ED50 = 58 +/- 6 (SE) pM] produced desensitization in 10-15 min, slowly rising GnRH (0.56-14 pM/min) caused increasing LH output with time. However, in comparison with square wave pulses, stimulatory signals consisting of slowly rising concentrations of GnRH produced peaks characterized by less total LH output and a changed shape. This was consistent with desensitization at low concentrations of GnRH reducing response to later increases in the level of stimulation. The mechanism for detecting GnRH signals and/or the mechanisms controlling release of LH were desensitized to constant GnRH at any concentration but retained a reduced sensitivity, or developed an additional release capacity, to increased levels of GnRH. Properties of four distinct types of LH release dynamics were described quantitatively and were shown to be controlled by different time constants in the GnRH pulse stimulation patterns.     

Feedback of follicle-stimulating hormone to inhibit luteinizing hormone and stimulate follicle-stimulating hormone release in ovariectomized rats

Injection of ovine follicle-stimulating hormone (o-FSH) into the third cerebral ventricle significantly suppressed plasma luteinizing hormone (LH) release in conscious, unrestrained ovariectomized rats. Intraventricular injection of o-FSH (0.5 microgram) significantly lowered the plasma levels of LH by 70 min and decreased pulse frequency, whereas the pulsatile release of FSH was unchanged during the 180-min duration of the experiment. In a 12-hour experiment, the suppression of LH release reached the maximum within 3 h (p less than 0.001), followed by a return of plasma LH level to initial values 6-8 h after intraventricular injection of o-FSH at doses of 0.5, 4 and 8 micrograms. Plasma levels of FSH were not significantly changed by the low dose of o-FSH (0.5 microgram), but there was a delayed elevation of plasma FSH during the 8-12 h after intraventricular injection of o-FSH at higher doses (4 and 8 micrograms). On the other hand, plasma prolactin was unchanged after intraventricular injection of o-FSH. Since the pituitary responsiveness to synthetic LH-releasing hormone (LHRH) was unimpaired at the end of the sampling and incubation with o-FSH suppressed the release of LHRH from median eminence terminals, these data indicate that FSH acts intrahypothalamically to suppress LHRH release. The late elevation of plasma FSH may be the result of enhanced release of FSH-releasing factor or of a delayed, direct effect of the hormone on the gonadotropes to increase release of FSH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of in vivo treatment with estrogen on luteinizing hormone synthesis and release by rat pituitaries in vitro.

The influence of estrogen on uptake of [3H]glucosamine and [14C]alanine and their incorporation into LH and total protein was investigated. Ovariectomized rats were sacrificed 22 h after injection with either oil or estradiol benzoate (EB, 50 microng/rat). Quartered anterior pituitary glands were incubated for 4 h with radioactive precursors in the presence or absence of 3.6 X 10-8M synthetic gonadotropin-releasing hormone (GnRH). Labeled LH was isolated by immunoprecipitation with specific anti-LH-beta serum. Both EB and GnRH significantly elevated the amount of [3H]glucosamine-LH appearing in the medium, the tissue, and the total system (medium + tissue), but they increased the amount of [14C]alanine-LH only in the medium. There was a significant positive interaction between EB and GnRH on the amounts of [3H]glucosamine-LH and [14C]alanine-LH in the medium and of [3H]glucosamine-LH in the tissue and total system. EB enhanced [3H]glucosamine uptake and incorporation into total protein, but GnRH had little or no effect on these parameters. In time course studies rats were injected with either oil or EB at 22, 11, or 5.5 h prior to sacrifice. At all times EB significantly increased synthesis and release of [3H]-glucosamine-LH and release of total immunoreactive LH (IR-LH) by pituitaries incubated with GnRH. The amounts of labeled and IR-LH released into the medium increased linearly with time after EB injection, but the amount of labeled LH in the total system plateaued at 5.5 h after EB injection. In another study, estradiol (E2, 5 microng/rat) dissolved in 1% ethanol-saline was injected at 0.5, 1.0, 2.0, or 4 h prior to sacrifice. Incorporation of [3H]glucosamine into tissue protein and release of [3H]glucosamine-LH was stimulated within 2 h after E2 injection. However, incorporation of [3H]glucosamine into LH was not stimulated until 4 h after E2 injection. These results suggest that estrogen and GnRH regulate LH synthesis at different sites, and that the effect of estrogen is non-specific compared to that of GnRH. The synthesis of the carbohydrate moiety of LH appears to be subjected to hormonal regulation more readily than the synthesis of the polypeptide moiety.     

Age-related differences in the release of luteinizing hormone and gonadotropin-releasing hormone in ovariectomized rats

The effect of aging on the release of gonadotropin-releasing hormone (GnRH) in vitro and of luteinizing hormone (LH) both in vivo and in vitro in ovariectomized (Ovx) rats was studied. Old (21-24 months) and young (3-4 months) rats were Ovx before use. They were injected subcutaneously with estradiol benzoate (25 micrograms/kg) or sesame oil for 3 days and then challenged with GnRH (0.5, 2 or 10 micrograms/kg) via a jugular catheter. Blood samples were collected immediately before and at 5, 10, 20, 40 and 60 min following GnRH injection. For in vitro study, Ovx rats were decapitated. The anterior pituitary glands (APs) were incubated with GnRH (0.1 or 10 nM) and estradiol (0, 0.1, 1 or 10 nM) at 37 degrees C for 30 min. The mediobasal hypothalamus was superfused with Locke's solution at 37 degrees C for 210 min, and stimulated with 60 mM KCl at 90 and 150 min. The medium samples were collected at 10-min intervals. Concentrations of GnRH and LH in plasma and medium samples were measured by radioimmunoassay. In all rats, the basal and GnRH-stimulated levels of plasma LH were lower in old than in young rats. The spontaneous release of LH in vitro from APs of Ovx rats was increased by aging, whereas GnRH-stimulated release of LH in vitro was lower in old than in young animals. The potassium-stimulated, but not spontaneous, release of GnRH was lower in old than in young Ovx rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of glucocorticoids on responsiveness of luteinizing hormone and follicle-stimulating hormone to gonadotropin-releasing hormone by male rat pituitary cells in vitro

To determine if the inhibitory effects of glucocorticoids on GnRH-stimulated secretion of LH observed in male rats in vivo are exerted directly on the pituitary, dispersed pituitary cells from adult male rats were treated with 60 or 600 ng/ml cortisol (F) or corticosterone (B) during one or two 48-h incubations. Control cells received no glucocorticoids. During the second 48 h, some cells from each group were treated with GnRH (2.4 X 10(-11)-6.2 X 10(-8) M). Concentrations of LH and FSH in media and cells were measured by RIA. Treatment with steroids had no effect on basal secretion or maximal GnRH-stimulated secretion of LH, or on maximal secretion of FSH. Treatment with 600 ng/ml B for 96 h increased basal secretion of FSH relative to controls. All treatments with glucocorticoids increased the slopes of the GnRH dose-response curves for both LH and FSH, cell content of LH, total (cells + medium) LH, and total FSH. Incubation with 6 micrograms/ml F or B or 60 ng/ml dexamethasone gave similar results. Decreasing the time period of the second incubation to 6 h results in no significant differences between control cells and cells treated with B or F. These results show that glucocorticoids have different effects in vivo and in vitro, suggesting that inhibitory effects of glucocorticoids on secretion of LH in vivo may not be exerted directly on the pituitary but are exerted elsewhere, perhaps by altered hypothalamic secretion of GnRH. Also, these results show that male and female pituitaries in vitro respond differently to glucocorticoids.     

Effect of human growth hormone-releasing hormone on the release of dynorphin-like immunoreactivity, luteinizing hormone, and follicle-stimulating hormone from rat adenohypophysis in vitro

The effect of GH-releasing hormone (GHRH) on the release of the endogenous opioid dynorphin from rat adenohypophysis was investigated in vitro. Rat anterior pituitary quarters were incubated in vitro, and hormone release into the incubation medium was measured by RIAs. Human pancreatic GHRH [hpGHRH-(1-44)] as well as human Leu27,Gly45-GHRH [GHRH-(1-45)] enhanced the secretion of dynorphin A1-13-like immunoreactivity (Dyn A1-13-IR) in a concentration-dependent manner. The concentrations of hpGHRH-(1-44) that stimulated the release of Dyn A1-13-IR were about 100-fold higher than those that enhanced GH secretion. GH release induced by hpGHRH-(1-44) was blocked by somatostatin (IC50, approximately 10 nM) without affecting hpGHRH-(1-44)-induced release of Dyn A1-13-IR. GH release was elicited by prostaglandin E2, while Dyn A1-13-IR secretion remained unchanged. At concentrations that enhanced Dyn A1-13-IR release, hpGHRH-(1-44) also elicited LH and FSH secretion. The LHRH antagonist D-pGlu1, D-Phe2,D-Trp3,6-LHRH blocked the secretion of Dyn A1-13-IR, LH, and FSH induced by hpGHRH-(1-44), whereas the LHRH antagonist did not influence the simultaneous GH release elicited by hpGHRH-(1-44). A possible direct effect of GHRH on the LHRH receptor was examined in radioligand binding studies using iodinated D-Ala6, des-Gly10-LHRH ethylamide (LHRH-A). The binding of [125I]iodo-LHRH-A to rat anterior pituitary membranes was completely displaced by hpGHRH-(1-44) and GHRH-(1-45). The deduced apparent dissociation constants were about 3 orders of magnitude higher than that of LHRH-A, but were close to those concentrations that enhanced Dyn A1-13-IR release. We conclude that GHRH-induced release of Dyn A1-13-IR is unrelated to GH release. High concentrations of GHRH may interact directly with LHRH receptors on gonadotrophs and thereby enhance the release of LH, FSH, and Dyn A1-13-IR.     

Regulation of hypothalamic gonadotropin-releasing hormone and neuropeptide Y concentrations by progesterone and corticosteroids in immature rats: correlation with luteinizing hormone and follicle-stimulating hormone release.

In a previous study, we demonstrated that progesterone (P4) and the synthetic glucocorticoid triamcinolone acetonide (TA), but not cortisol, could induce LH and FSH release in estrogen-primed ovariectomized immature rats. Therefore, the purpose of this study was to determine if the stimulatory effect of P4 and TA on LH and FSH release were associated with changes in GnRH or NPY concentrations in the medial basal hypothalamus (MBH) or preoptic area (POA). Ovariectomized immature rats primed with estradiol at 27 and 28 days received either vehicle, P4, TA or cortisol (1 mg/kg BW) at 9.00 h on day 29. Animals were killed at 9.30, 10.00, 12.00 and 13.00 h on day 29 for serum LH and FSH measurements, and the MBH and POA were dissected and analyzed for GnRH and NPY concentrations via RIAs. P4- and TA-treated animals showed significantly elevated serum LH and FSH levels from 13.00 h to 15.00 h. Cortisol was without effect. P4 significantly increased MBH GnRH and NPY concentrations at 12.00 h followed by a significant fall at 13.00 h. P4 modulated POA GnRH and NPY concentrations in a fashion similar to that seen in the MBH, except POA NPY concentrations did not fall at 13.00 h after the elevation at 12.00 h. TA had no significant effect on MBH GnRH and NPY levels at 12.00 h compared to the values at 9.30 h and 10.00 h but, as with P4, there was a significant fall in MBH GnRH and NPY levels at 13.00 h. TA had no significant effect on POA GnRH and NPY concentrations at any time point studied.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Pubertal acceleration of pulsatile gonadotropin-releasing hormone release in male rats as revealed by microdialysis

A microdialysis technique was used in male rats to directly assess the postulate that pubertal maturation is associated with accelerated GnRH pulsatility. Juvenile male rats, postnatal d 43 or 45 (n = 4) were stereotaxically fitted with guide cannulas directed toward the lateral median eminence, and repeated microdialysis experiments were conducted over 4-6 d. In each session, samples were collected continuously over 12 h (0900-2100 h) at 5-min intervals Results from individual peripubertal animals were pooled into two time bins for postnatal d 45-47 and 48-50, respectively, and GnRH characteristics were compared between the two epochs. The GnRH pulse frequency and mean GnRH concentration were significantly elevated at 48-50 d compared with 45-47 d. The GnRH pulsatility characteristics for 45-47 d vs. 48-50 d were as follows: pulse frequency, 0.74 +/- 0.16 vs. 1.79 +/- 0.19 pulses/h (P < 0.05); pulse amplitude, 254.1 +/- 22.3 vs. 347.2 +/- 15.8 deltapg/ml (difference in value from trough to peak); and mean release, 0.55 +/- 0.03 vs. 2.04 +/- 0.04 pg/5 min (P < 0.05). An additional two rats were dialyzed only once on postnatal d 50 to assess the effects of repeated sampling; the GnRH pulse characteristics in these animals were similar to those in rats sampled for a third or fourth time on postnatal d 48-50. To further assess the possible effects of repeated sampling on GnRH release profiles, a group of adult male rats (postnatal d 95-105; n = 3) was also dialyzed on four consecutive days. In these rats no significant alteration in GnRH pulse generator activity was observed over the four sessions. Moreover, the increase in GnRH pulse frequency observed in the peripubertal rats was found to be sustained in adult animals. To better understand the temporal relationship of GnRH pulse generator activity to reproductive maturation, groups of male rats were killed from postnatal d 45-56 along with an adult group at 95-105 d (n = 5/group) and examined for physiological signs of reproductive development. Gradual increases in serum levels of LH and testosterone and decreases in FSH and inhibin B were seen from postnatal d 45-56 to adulthood. Mature spermatozoa were found in the vas deferens by postnatal d 53. Our results demonstrate that in the late juvenile stage of male rat development, GnRH pulse generator activity is gradually accelerated over the course of consecutive days. This acceleration occurs over a period during which serum LH and testosterone are rising to adult levels, and it precedes the presence of mature spermatozoa in the vas deferens by 3 d. Our observations provide direct support for the hypothesis that an acceleration of GnRH pulsatility is the critical neural stimulus for the initiation of pubertal maturation in males. The peripheral and central cues that prompt the pubertal activation of the GnRH pulse generator remain to be characterized.     

Pathophysiology of pulsatile and copulsatile release of thyroid-stimulating hormone, luteinizing hormone, follicle-stimulating hormone, and alpha-subunit.

Under physiological conditions, TSH, LH, FSH, and alpha-subunit are released in discrete pulses. To further characterize their neuroregulation and to investigate possible copulsatile secretion of these glycoprotein hormones, we studied the 24-h pulse profiles of all four hormones in each of four subject groups: young men, young women, postmenopausal women, and subjects with untreated primary hypothyroidism. Gonadotropin pulse properties in euthyroid men and women were similar to those previously reported, and hypothyroid subjects had normal gonadotropin pulse patterns. TSH release was pulsatile in all groups; hypothyroid subjects had increased pulse amplitude, but loss of the usual nocturnal increases in pulse amplitude. alpha-Subunit concentrations were pulsatile in all groups, with minimal circadian variation; postmenopausal and hypothyroid subjects had increased alpha-subunit pulse amplitude. We then tested pulse concordance among the four simultaneous hormone series. alpha-Subunit and the gonadotropins were significantly coreleased (triple coincidence), suggesting that all three hormones are closely linked to processes that regulate GnRH secretion. alpha-Subunit bursts were also significantly coincident with those of TSH in men, postmenopausal women, and hypothyroid subjects. Interestingly, TSH pulses were significantly concordant with those of LH and FSH, and all four hormones were significantly concordant in men, postmenopausal women, and hypothyroid subjects. In conclusion, the present findings imply that an underlying unified signal coordinates pulsatile hormone secretion from both gonadotrophs and thyrotrophs.     

Regulation of luteinizing hormone release by pulsatile and continuous administration of gonadotropin-releasing hormone to superfused rat and hamster pituitary cells

Regulation of LH release by GnRH was studied in superfused anterior pituitary cells from 30-day-old female rats or hamsters. Dispersed cells were cultured 4-6 days on Cytodex beads, then loaded into water-jacketed columns, and perfused with medium (0.5 ml/min) at 37 C. Three-minute fractions of effluent were assayed for LH by RIA. LH release was dose related between 10(-10) and 10(-7) M GnRH. Rat and hamster cells released LH at peak rates of 11.3 and 12.5 ng/(min X 10(6) cells), respectively, when first exposed to 10(-8) M GnRH. Short pulses (6 min) of 10(-8) M GnRH given at 30-min intervals had little effect on the rate of LH release by rat pituitary cells; however, if the interpulse interval was reduced to 12 min, release declined 72% by the fifth pulse. In contrast, pulses of 10(-6) M GnRH at 30-min intervals desensitized rat cells. Hamster cells were desensitized by 10(-8) M GnRH after a single pulse regardless of whether a second pulse was given 30 min or 2.5 h later. Similar desensitization also occurred at other doses (10(-9) and 10(-6) M). After five pulses at 30-min intervals, the LH release rate in hamster cells was depressed 65%. Release was depressed 80% by pulses at 12-min intervals. Thus, in rats, desensitization is both frequency and dose dependent, whereas in hamsters, it is independent of frequency and dose. Stimulation with 10(-6) M GnRH pulses completely overcame desensitization in both species. Continuous exposure of anterior pituitary cells to 10(-8) M GnRH caused an initial rapid LH release, followed by a steady decline in the rate of release from peak rates to baseline levels by 2.5 h in both species. A 6-min, 10(-6) M GnRH pulse given immediately after a 3-h 10(-8) M GnRH exposure rapidly stimulated the cells to release LH at rates up to 192% of initial rates. When these pulses were continued at 30-min intervals, additional desensitization occurred. This overcoming of desensitization shows that desensitized anterior pituitary cells are not refractory to GnRH and suggests that the GnRH regulation of LH release may involve more than one GnRH receptor-mediated phenomenon.     

Further studies on norepinephrine-induced suppression of pulsatile luteinizing hormone release in ovariectomized rats

The present study examined three aspects of the inhibitory effects of continuous intraventricular infusion of norepinephrine (NE) on pulsatile luteinizing hormone (LH) release in ovariectomized, nonsteroid-primed rats: whether the inhibitory effects of NE infusion were exerted on LH pulse frequency and/or amplitude; whether central nervous system desensitization occurred in response to the inhibitory effects of continuous NE infusion on pulsatile LH secretion, and whether dopamine of serotonin were involved as possible interneuronal transmitter mediators of NE-induced suppression of pulsatile LH release. Unanesthetized rats with external jugular cannulae were bled continuously at a rate of 50 microliters whole blood/7 min for 2 h prior to infusion and for 2-3 h during continuous intraventricular infusion of artificial cerebrospinal fluid or NE. Infusion of cerebrospinal fluid had no effect on pulsatile LH release, while continuous infusion of 0.3 or 1.8 micrograms NE/h for 2-3 h produced suppression of pulsatile LH secretion. Although desensitization to the stimulatory effects of NE on LH release in ovariectomized, steroid-primed rats had been observed to occur rapidly within 90 min after the onset of infusion, desensitization to the inhibitory effect of NE on pulsatile LH release did not occur even after continuous infusion of NE for periods up to 20 h. Mean blood LH levels were as low in rats bled 17-20 h after the onset of NE infusion as in those bled at 0-3 h. The suppressive effect of NE on pulsatile LH release was not prevented by prior blockade of dopamine or serotonin receptors with pimozide or metergoline, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Abrogation by a potent gonadotropin-releasing hormone antagonist of the estrogen/progesterone-stimulated surge-like release of luteinizing hormone and follicle-stimulating hormone in postmenopausal women.

In both the rodent and primate, administration of progesterone elicits an acute surge-like release of LH in the setting of prior estrogen treatment. Whether these facilitative effects of estrogen and progesterone on gonadotropin secretion reside at pituitary or hypothalamic loci is not known. To further investigate the mechanisms by which estrogen combined with progesterone amplifies gonadotropin secretion, we studied the responses of seven estrogen-primed postmenopausal women to progesterone administration with or without cotreatment with a potent GnRH antagonist, [Ac-D2Nal1,D4ClPhe2,D3Pal3,Arg5,DGlu6(AA), DAla10]GnRH. Repetitive blood sampling for the later measurement of serum concentrations of LH, FSH, and PRL was begun 4 h before the administration of progesterone and continued for 36 h. We observed that progesterone administration after 72 h of priming with ethinyl estradiol resulted in a surge-like release of LH and FSH in all subjects. Concomitant administration of the GnRH antagonist abolished the surge-like release of both gonadotropins in all subjects. In contrast, administration of the antagonist had no effect on PRL release. These results indicate that endogenous GnRH action is an obligatory component of the progesterone-induced surge-like release of both gonadotropic hormones in the estrogen-primed human.     

Interleukin 1 alpha inhibits prostaglandin E2 release to suppress pulsatile release of luteinizing hormone but not follicle-stimulating hormone.

Interleukin 1 alpha (IL-1 alpha), a powerful endogenous pyrogen released from monocytes and macrophages by bacterial endotoxin, stimulates corticotropin, prolactin, and somatotropin release and inhibits thyrotropin release by hypothalamic action. We injected recombinant human IL-1 alpha into the third cerebral ventricle, to study its effect on the pulsatile release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in conscious, freely moving, ovariectomized rats. Intraventricular injection of 0.25 pmol of IL-1 alpha caused an almost immediate reduction of plasma LH concentration; this decrease was statistically significant 20 min after injection and occurred through a highly significant reduction in the number of LH pulses, with no effect on pulse amplitude. In contrast, there was no change in pulse frequency but a small significant elevation in amplitude of FSH pulses. Intraventricular injection of the diluent had no effect on gonadotropin release. The results provide further evidence for separate hypothalamic control mechanisms for FSH and LH release. To determine the mechanism of the suppression of LH release, mediobasal hypothalamic fragments were incubated in vitro with IL-1 alpha (10 pM) and the release of LH-releasing hormone (LHRH) and prostaglandin E2 into the medium was measured by RIA in the presence or absence of norepinephrine (50 microM). IL-1 alpha reduced basal LHRH release and blocked LHRH release induced by norepinephrine. It had no effect on the basal release of prostaglandin E2; however, it completely inhibited the release of PGE2 evoked by norepinephrine. To evaluate the possibility that IL-1 alpha might also interfere with the epoxygenase pathway of arachidonic acid metabolism, epoxyeicosatrienoic acids were also measured. IL-1 alpha had no effect on the content of epoxyeicosatrienoic acids in the hypothalamic fragments as measured by gas chromatography and mass spectrometry. In conclusion, IL-1 alpha suppresses LH but not FSH release by an almost complete cessation of pulsatile release of LH in the castrated rat. The mechanism of this effect appears to be by inhibition of prostaglandin E2-mediated release of LHRH.     

Simultaneous measurement of gonadotropin-releasing hormone, luteinizing hormone, and follicle-stimulating hormone in the orchidectomized rat

In the present study two recently developed techniques have been combined to enable the simultaneous in vivo determination of pulsatile release of GnRH, LH, and FSH in the orchidectomized rat. The first of these techniques involves the implantation of two vascular catheters and collecting serial blood samples through one while simultaneously infusing a replacement blood mixture through the other; consequently, blood samples can be collected for an extended period of time, and detailed plasma LH and FSH release profiles can be established for individual animals. The second technique involves push-pull perfusion of the pituitary gland to determine changes in GnRH concentration as might be perceived by the gonadotropes. For each animal (n = 6), blood (150 microliters) and push-pull perfusate (200 microliters) samples were collected at 5- and 10-min intervals, respectively, for approximately 6 h, and the hormone release profiles were determined by RIA. All of the rats showed a clear pulsatile release pattern for GnRH, LH, and FSH. Moreover, the interpulse interval was remarkably similar for each of these hormones (36.9, 41.5, and 43.5 min, respectively, as determined by PULSAR). The percentage of GnRH pulses associated with a gonadotropin pulse was 72% for LH and 76% for FSH; only 14% of the pulses were silent for both gonadotropins. These results demonstrate that in the orchidectomized rat the pulsatile pattern of GnRH release is reflected in the pulsatile pattern of not only LH but also FSH. They may, therefore, be construed to support the concept that the pulsatile secretion of both gonadotropins is primarily orchestrated by a single hypothalamic releasing hormone. Alternatively, if two separate hypothalamic releasing hormones do indeed exist (LHRH and FSH-releasing hormone), it would appear that in the orchidectomized rat their episodic release is tightly coupled to the same hypothalamic pulse generator.