Pituitary self-priming actions of gonadotropin-releasing hormone. Kinetics of estradiol's potentiating effects on gonadotropin-releasing hormone-facilitated luteinizing hormone and follicle-stimulating hormone release in healthy postmenopausal women

Continuous Kisspeptin Administration in Postmenopausal Women: Impact of Estradiol on Luteinizing Hormone Secretion

CONTEXT

Kisspeptin stimulates the reproductive endocrine cascade in both men and women. Circulating sex steroids are thought to modulate the ability of kisspeptin to stimulate gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release.

OBJECTIVE

To probe the effects of sex steroids on kisspeptin-stimulated GnRH-induced LH pulses.

PARTICIPANTS

Eight healthy postmenopausal women.

INTERVENTION

Subjects underwent every-10-minute blood sampling to measure GnRH-induced LH secretion at baseline and in response to a continuous kisspeptin infusion (12.5 µg/kg/h) over 24 hours. A subset of the participants also received kisspeptin (0.313 µg/kg) and GnRH (75 ng/kg) intravenous boluses.

RESULTS

Postmenopausal women are resistant to the stimulatory effect of continuous kisspeptin on LH secretion. Postmenopausal women receiving estradiol replacement therapy are also resistant to kisspeptin initially, but they demonstrate a significant increase in LH pulse amplitude in direct proportion to the circulating estradiol concentration and duration of kisspeptin administration.

CONCLUSIONS

Kisspeptin administration has complex effects on GnRH, and by extension, on LH secretion. The ability of kisspeptin to affect LH secretion can be modulated by the ambient sex-steroid milieu in a time- and dose-dependent manner.

Joint basal and pulsatile hypersecretory mechanisms drive the monotropic follicle-stimulating hormone (FSH) elevation in healthy older men: concurrent preservation of the orderliness of the FSH release process: a general clinical research center study

To appraise the neuroendocrine mechanisms that underlie a selective (monotropic) elevation of serum FSH concentrations in healthy older men, we sampled blood in 11 young (ages 21-34) and 8 older men (ages 62-72) men every 2.5 min overnight. Serum FSH concentrations were quantitated in an automated, high-sensitivity, chemiluminescence-based assay. Rates of basal and pulsatile FSH secretion were estimated by deconvolution analysis, and the orderliness of the FSH release process via quantitated the approximate entropy statistic. Statistical analysis revealed that healthy older men manifest dual neuroendocrine hypersecretory mechanisims; specifically, a 2-fold increase in the basal (nonpulsatile) FSH secretion rate, and a concurrent 50% amplification of FSH secretory burst mass (and amplitude). The regularity or orderliness of ad seriatim FSH release is preserved in older individuals. We postulate that higher basal FSH secretion in older men is a consequence of reduced testosterone negative feedback, whereas amplified FSH secretory burst mass reflects net enhanced stimulation of gonadotrope cells by endogenous FSH secretagogues (e.g. GnRH and/or activin). The foregoing specific mechanisms driving heightened FSH secretion in older men contrast with the lower-amplitude pulsatility and more disorderly patterns of LH release in the same individuals. Thus, the present data illuminate an age-dependent disparity in the disruption of FSH neuroregulation in the aging male.

Neuroendocrine mechanisms mediating awakening of the human gonadotropic axis in puberty

The hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator presides over the pulsatile and feedback-regulated activities of the pituitary-gonadal axis. Awakening of synchronous activity of the GnRH neuronal ensemble in the earliest stages of puberty heralds the onset of full activation of the reproductive axis in girls and boys. Progression from prepuberty to adulthood in boys is directed by marked (30-fold) amplitude enhancement of pulsatile luteinizing hormone (LH) secretion, as assessed by an ultrasensitive immunofluorometric assay and deconvolution analysis. There is a much less apparent rise in LH secretory burst frequency (approximately 1.3-fold increase). Consequently, human puberty is an amplitude-driven neuroendocrine maturational process. However, less is known about pulsatile follicle-stimulating hormone (FSH) release in puberty. Multiple pathophysiologies that result in hypogonadotropic hypogonadism can converge on a final common mechanism of attenuated hypothalamic GnRH pulse generator output and hence reduced LH (and FSH) secretion. Disturbances may take the form of reduced GnRH pulse frequency and/or attenuated GnRH secretory burst mass. When the pathophysiology of hypogonadism originates exclusively in a failed GnRH pulse generator, then either treatment of the primary disease process where possible (e.g., by refeeding in starvation, improved metabolic control in diabetes mellitus, dopamine agonist treatment in hyperprolactinemia, etc) and/or treatment with pulsatile GnRH (e.g., in Kallmann's syndrome, isolated hypothalamic lesions, etc.) can provide relevant therapeutic options in children and adults.

Influence of the ovary on parameters of LH secretion during the recovery from buserelin-induced desensitization

This study examined the effect of the ovary on LH pulsatility and on the secretory performance of gonadotrophs during the phase of recovery after treatment with buserelin, a GnRH analogue. We included 12 patients, who received buserelin (1.2 mg/day, intranasally for 3 months) as a reductive therapy for uterine leiomyomatosis prior to hysterectomy. Six patients were oophorectomized and the other 6 patients had their ovaries preserved. LH was measured in samples taken basally up to 36 days after suppression of buserelin. LH pulsatility was studied on day 9 along a 24-h cycle, and the response of the hormone to a double-stimulus GnRH test on days 0, 9, 20, and 34. The concentration of LH reached normal premenopausal levels after an average of 2 weeks in women with ovaries but increased until 4-5 weeks in oophorectomized patients. The pulsatility of LH on day 9 was similar for both groups, but parameters related to LH amplitude or to baseline secretory activity of gonadotrophs were higher in the oophorectomized women. The response of LH to the GnRH tests was also significantly higher in the oophorectomized group from day 9. The conclusions are as follows. (1) At the early stage of recovery from desensitization, as represented by day 9, LH pulsatility was not substantially influenced by the presence or absence of the ovary. (2) There was an increase in parameters related to the amplitude of the LH bursts in the oophorectomized women. Although a higher amplitude of the endogenous GnRH pulses cannot be discarded, most probably that difference is due to a higher sensitivity at a pituitary level, as reflected by the GnRH stimulation tests.

Effects of oestrogen treatment on serum gonadotrophin bioactivity, immunoreactivity and isohormone distribution, and on immunoreactive inhibin levels, in prostatic cancer patients

OBJECTIVE AND DESIGN

No data are available on effects of long-term exposure to oestrogen on bioactivity of gonadotrophins in men. We studied the effects of a 6-month oestrogen therapy on serum FSH and LH bioactivity (B), immunoreactivity (I) and isohormone distribution, and on serum I-inhibin levels, in patients with prostatic carcinoma.

PATIENTS

Eleven men with advanced prostatic cancer were studied, each receiving 160 mg of polyoestradiol phosphate (Estradurin) once a month intramuscularly for 6 months.

MEASUREMENTS

Serum samples were collected before, and after 2 and 6 months of oestrogen treatment. Serum B- and I-FSH levels were measured by immature rat granulosa cell bioassay and immunofluorometric (IFMA, Delfia) assay, respectively, and those of B- and I-LH by mouse interstitial cell bioassay and IFMA, respectively. Serum oestradiol (E2) concentrations were measured by IFMA assay, and serum testosterone (T) and inhibin levels by radioimmunoassay. Isoelectric focusing was used for fractionation of the FSH and LH isoforms.

RESULTS

The pretreatment levels of B-FSH and I-FSH were 84.7 +/- 21.6 and 11.4 +/- 3.2 IU/l (mean +/- SEM), respectively, and the B/I ratio of FSH was 8.3 +/- 1.0. The pretreatment levels of B-LH and I-LH were 23.5 +/- 3.2 and 10.1 +/- 2.3 IU/l, respectively, and the B/I ratio was 3.0 +/- 0.4. After 6 months of oestrogen therapy, B-FSH and I-FSH decreased to 37.5 +/- 8.1 (P < 0.05) and 1.3 +/- 0.3 IU/l (P < 0.01), respectively, but the B/I ratio of FSH increased to 28.5 +/- 4.2 (P < 0.05). B- and I-LH levels decreased in 6 months to 7.4 +/- 0.9 and 2.3 +/- 0.5 IU/l (P < 0.01), respectively, but no change was found in the B/I ratio of LH. Serum T levels decreased from 19.0 +/- 2.6 to 2.7 +/- 0.9 nmol/l (P < 0.01) during the 6-month treatment, and the respective E2 levels increased from 0.2 +/- 0.01 to 4.4 +/- 0.5 nmol/l (P < 0.01). Serum I-inhibin levels were analysed from eight patients. The levels at 0, 2 and 6 months were 0.81 +/- 0.09, 0.50 +/- 0.03 and 0.54 +/- 0.01 microgram/l, respectively. Gonadotrophins in the pretreatment and 6-month samples of four patients were analysed by isoelectric focusing. In FSH of all subjects, and in LH of three subjects, a shift from acidic to more basic isoforms occurred after oestrogen therapy. This is in keeping with the increase of the B/I ratio of FSH. With LH, the isoform shift occurred between fractions with similar B/I ratios, and hence there was no shift in the overall B/I ratio.

CONCLUSIONS

Oestrogen therapy of men suppressed bioactive and immunoreactive levels of gonadotrophins. The B/I ratio of FSH increased, and this increase was associated with a shift in the isohormone profile to more basic forms. In contrast, no change occurred in the B/I ratio of LH, even though changes in the isohormone profile were observed. Hence, not all changes in the isohormone distribution of gonadotrophins result in changes of the intrinsic in-vitro bioactivity.

Suppressive actions of a gonadotropin-releasing hormone antagonist on luteinizing hormone, follicle-stimulating hormone, and prolactin release in estrogen-deficient postmenopausal women

We investigated time- and dose-dependent actions of a gonadotropin-releasing hormone antagonist, the "Nal-Glu" peptide [Ac-D2Nal1, 4CIDPhe2, D3Pal3, Arg5, DGlu6(AA), DAla10], in nine healthy estrogen-withdrawn postmenopausal women. Gonadotropin-releasing hormone antagonist was administered subcutaneously at doses of 10, 30, 100, and 300 micrograms/kg. Suppression of immunoactive luteinizing hormone concentrations was achieved with a 30 micrograms/kg dose of antagonist. Suppression of immunoactive follicle-stimulating hormone levels was less (40%) even at the highest antagonist dose (300 micrograms/kg). Bioactive luteinizing hormone concentrations also significantly decreased (greater than 60%) at the two antagonist doses tested (30 and 300 micrograms/kg). However, the lower antagonist dose showed an "escape" of bioactive luteinizing hormone values after 18 hours. No suppressive effects of the antagonist on prolactin secretion occurred at any dose tested. We conclude that this gonadotropin-releasing hormone antagonist can achieve effective, potent, and long-lasting suppression of pituitary secretion of biologically active luteinizing hormone at higher doses, but secretion of biologically active luteinizing hormone may "escape" at lower doses.

Influence of oestradiol and progesterone on pulsatile LH secretion in postmenopausal women

Pulsatile LH secretion was studied in six healthy postmenopausal women. Blood samples were obtained every 10 min during an 8-h saline infusion performed before and during the administration of transdermal oestradiol alone (E2; 50 micrograms/day) and in combination with vaginal progesterone (P; 100 mg twice daily). Plasma E2 and P levels reached values similar to those found in the early follicular phase and in the luteal phase of the menstrual cycle, respectively. The mean plasma LH levels significantly decreased (P less than 0.01) during transdermal E2 with and without vaginal P. A significant increase in the frequency (P less than 0.025) and the amplitude (P less than 0.05) of LH pulses was observed during transdermal E2. The administration of vaginal P to oestrongenized women significantly blunted the frequency (P less than 0.05) and enhanced the amplitude (P less than 0.05) of LH pulses. In all experimental conditions, the mean plasma LH levels showed a positive linear correlation with the amplitude of LH pulses. The present results show that peripheral levels of E2, similar to those of the early follicular phase of the menstrual cycle, can influence the pulsatile pattern of LH secretion, enhancing the frequency and the amplitude of LH pulses. In oestrogenized patients, the increase of peripheral P plasma levels to postovulatory values restored a pulsatile pattern of LH secretion similar to that of the early luteal phase of menstrual cycle.

Pathophysiological features of the pulsatile secretion of biologically active luteinizing hormone in man

The development of an in vitro bioassay of high specificity, sensitivity and precision for the measurement of low circulating concentrations of biologically active glycoprotein hormones has offered exciting new insights into the in vivo secretion and metabolic clearance of luteinizing hormone (LH) in various pathophysiological states. Moreover, the most recent combined application of the rat interstitial cell testosterone (RICT) bioassay and a novel multiple-parameter deonvolution model has allowed investigators to dissect plasma concentration profiles of bioactive LH into defined secretory bursts, which have numerically explicit amplitudes, locations in time, and durations, and are acted upon by determinable subject- and study-specific endogenous metabolic clearance rates. Here, we have: (i) reviewed the ability of the endogenous GnRH pulse signal to regulate the in vivo secretion of biologically active LH molecules as assessed in the RICT and by deconvolution mechanics; (ii) demonstrated that low-dose exogenous GnRH pulses effectively mimic spontaneous bioactive LH pulsatility; (iii) investigated the role of endogenous androgen and estrogen in modulating bioactive gonadotropin secretion in men and women; and (iv) described significant alterations in endogenous LH bioactivity in puberty and healthy aging.

Nature of alpha subunit secretion in men: circadian rhythms, pulsatile release and secretory profiles

Alpha subunit complements LH as a marker of the activity of the hypothalamic GnRH pulse generator. To characterize episodes of alpha subunit release and to determine if a circadian pattern of alpha subunit secretion is present in man, spontaneous alpha subunit pulsatility was analyzed in six healthy young men by blood sampling every 5 min for 24 h. The resulting alpha subunit concentration time series were analyzed by two statistically based independent peak detection methods, and subjected to Fourier transformation to assess underlying circadian rhythms. Cross-correlation analyses and multiple parameter deconvolution were used to estimate the concordance of spontaneous and exogenous GnRH-stimulated LH and alpha subunit secretion. These analyses revealed that two independent discrete peak detection algorithms yielded similar estimates of spontaneous alpha subunit pulse frequency, namely, 21 +/- 1.1 (Cluster) and 21 +/- 1.5 (Detect) alpha subunit peaks/24 h. Sampling intensity markedly influenced the estimate of endogenous alpha subunit pulse frequency, inasmuch as estimates from 5-min sampling were significantly greater than those of 10-min or 20-min sampling. Fourier transformation unmasked a significant circadian alpha subunit rhythm in all six men, with maximal concentrations at 0836 h and an average amplitude of 28% of the 24-hr mean hormone concentration. Cross-correlation analysis of spontaneous glycoprotein release revealed that serum LH and alpha subunit concentrations were highly cross-correlated when considered simultaneously, but not at various lags. Finally, deconvolution analysis of exogenous GnRH-stimulated glycoprotein release disclosed distinct half-times of alpha subunit and LH clearance with virtually simultaneous underlying secretory bursts. These data indicate that human alpha subunit is secreted in both a circadian and a discrete pulsatile fashion at a pulse frequency that is significantly underestimated at conventional sampling rates. The approximately hourly alpha subunit interpulse interval (68 +/- 4.6 min) is similar to that reported earlier for LH in peripheral blood and for testosterone in gonadal vein blood in healthy men. Moreover, cross-correlation analysis of endogenous GnRH-driven alpha subunit and deconvolution analysis of exogenous GnRH-stimulated alpha subunit and LH secretion suggest that these glycoproteins are secreted virtually simultaneously, but have significantly different endogenous clearance properties. The remarkably similar in vivo pulse frequencies for alpha subunit, LH, and testosterone in man suggest that the release of these three hormones is coordinately regulated.

Alterations in the pulsatile properties of gonadotropin secretion in alcoholic men

The functional characteristics of the hypothalamic-pituitary-testicular axis were examined quantitatively in 10 chronic alcoholic men without hepatic dysfunction or clinical nutritional deficiencies. Spontaneous gonadotropin pulsatility was analyzed in blood sampled every 20 minutes over a 24-hour period 3 to 16 days after abstinence from alcohol and again 29 to 39 days later. The numbers of LH and FSH pulses per 24 hours were normal in these alcoholic men compared with controls. However, we found increased mean 24-hour concentrations of immunoactive LH (P = 0.012) and FSH (P = 0.018), increased peak heights for LH (P = 0.035) and FSH (P = 0.004), decreased fractional LH (P = 0.002) and FSH (P = 0.044) pulse amplitudes and increased interpulse valley mean LH (P = 0.010) and FSH (P = 0.018) concentrations. Serum levels of total testosterone, total estradiol and estrone were normal, whereas concentrations of free testosterone and free estradiol were increased. Pituitary release of LH and FSH was normal in response to low (5-micrograms) and high (95-micrograms) doses of GnRH given intravenously. The present observations indicate that in chronically alcoholic men, acute abstinence from ethanol is associated with elevated circulating concentrations of immunoactive gonadotropins in the presence of intact spontaneous gonadotropin pulsatility, preserved pituitary responsiveness to exogenous GnRH, and increased concentrations of free testosterone and free estradiol. Such findings are consistent with alterations in the endogenous feedback actions of sex steroid hormones in this setting.

Modulating actions of estradiol on gonadotropin-releasing hormone-stimulated prolactin secretion in postmenopausal individuals

The temporal aspects of estrogen treatment on serum prolactin concentrations basally and in response to a 10 micrograms intravenous injection of gonadotropin-releasing hormone were assessed in eight postmenopausal women. The response of prolactin to gonadotropin-releasing hormone stimulation was compared with that of follicle-stimulating hormone and luteinizing hormone obtained simultaneously. Basal serum prolactin increased significantly (p less than 0.001) in response to estrogen treatment, and a positive correlation was found between the serum concentrations of estradiol and prolactin (r2 = 0.266; p = 0.0011). Gonadotropin-releasing hormone induced a significant increase in serum prolactin concentrations, which was greater after 5 to 10 days of estrogen treatment compared with that in the estrogen-depleted state (p = 0.031). No correlation was found between gonadotropin-releasing hormone-stimulated prolactin and luteinizing hormone or follicle-stimulating hormone release. These data demonstrate that estrogen treatment of previously hypoestrogenemic postmenopausal women potentiates gonadotropin-releasing hormone-stimulated prolactin release. Furthermore, our data suggest that the previously described paracrine interaction between the gonadotropins and lactotropes exerted by gonadotropin-releasing hormone does not appear to be mediated via increased gonadotropin release.

Appraising the instantaneous secretory rates of luteinizing hormone and testosterone in response to selective mu opiate receptor blockade in late pubertal boys

The pulsatile properties of gonadotropin and testosterone release were examined before and after chronic mu opiate receptor blockade with naltrexone, 50 mg every other day, in four normal boys in late puberty (ages 14 8/12 to 15 1/12 years). The nature of spontaneous secretory events was appraised for immunoactive LH and testosterone in blood withdrawn every 20 minutes for 24 hours, using a novel, discrete deconvolution algorithm to estimate apparent instantaneous secretory rates. The application of this methodology revealed that the frequency of discrete LH instantaneous secretory rates increased after mu opiate receptor blockade (P = 0.011). More strikingly, all parameters of testosterone secretory events responded significantly to mu opiate receptor blockade, including increases in mean estimated secretory rate (+47%, P = 0.02), testosterone pulse frequency (+ 64%, P less than 0.001) and amplitude (+ 20%, P = 0.027). Correspondingly, decreases in testosterone interpulse secretory intervals (-35%, P = 0.001), secretory pulse duration (-19%, P = 0.042) and interpulse valley duration (-35%, P = 0.006) also were noted. There was a prominent diurnal rhythm in testosterone secretion with maximal values in the morning and late evening, and marked reductions in the afternoon, sometimes to prepubertal levels. This variation in the testosterone secretory profile paralleled that of LH. In response to naltrexone, the FSH concentration series showed a significant increase in the mean FSH concentration (+ 18%) P = 0.003) and mean peak amplitude (+ 15%, P = 0.002). These data provide indirect evidence of functional coupling of the opiate system with the hypothalamic GnRH pulse generator.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of estradiol on discrete attributes of the luteinizing hormone pulse signal in man. Studies in postmenopausal women treated with pure estradiol

We assessed the time-dependent impact of estradiol on properties of the luteinizing hormone (LH) pulse signal in 12 hypoestrogenemic postmenopausal volunteers studied basally and after 1, 5, 10, and 30 d of estradiol delivery via an intravaginal Silastic ring. Computerized analysis of the plasma LH time series revealed a significant decrease in LH pulse frequency within 24 h of estrogen treatment, followed by a secondary increase (days 5 and 10), and then a sustained decline (day 30) in LH pulsatility. Estradiol also significantly suppressed incremental and maximal (but not fractional) LH pulse amplitudes in a biphasic manner. In contrast, LH peak duration was invariant until day 30 of estradiol replacement. These observations indicate that the well recognized biphasic actions of estradiol on mean serum LH concentrations can be modeled in relation to specific and time-dependent alterations in LH pulse frequency and amplitude.