The effects of endothelins on the secretion of prolactin, luteinizing hormone, and follicle-stimulating hormone are mediated by different guanine nucleotide-binding proteins

G proteins and autocrine signaling differentially regulate gonadotropin subunit expression in pituitary gonadotrope

Gonadotropin-releasing hormone (GnRH) acts at gonadotropes to direct the synthesis of the gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). The frequency of GnRH pulses determines the pattern of gonadotropin synthesis. Several hypotheses for how the gonadotrope decodes GnRH frequency to regulate gonadotropin subunit genes differentially have been proposed. However, key regulators and underlying mechanisms remain uncertain. We investigated the role of individual G proteins by perturbations using siRNA or bacterial toxins. In LβT2 gonadotrope cells, FSHβ gene induction depended predominantly on Gα(q/11), whereas LHβ expression depended on Gα(s). Specifically reducing Gα(s) signaling also disinhibited FSHβ expression, suggesting the presence of a Gα(s)-dependent signal that suppressed FSH biosynthesis. The presence of secreted factors influencing FSHβ expression levels was tested by studying the effects of conditioned media from Gα(s) knockdown and cholera toxin-treated cells on FSHβ expression. These studies and related Transwell culture experiments implicate Gα(s)-dependent secreted factors in regulating both FSHβ and LHβ gene expression. siRNA studies identify inhibinα as a Gα(s)-dependent GnRH-induced autocrine regulatory factor that contributes to feedback suppression of FSHβ expression. These results uncover differential regulation of the gonadotropin genes by Gα(q/11) and by Gα(s) and implicate autocrine and gonadotrope-gonadotrope paracrine regulatory loops in the differential induction of gonadotropin genes.

Coordination of calcium signals by pituitary endocrine cells in situ

The pulsatile secretion of hormones from the mammalian pituitary gland drives a wide range of homeostatic responses by dynamically altering the functional set-point of effector tissues. To accomplish this, endocrine cell populations residing within the intact pituitary display large-scale changes in coordinated calcium-spiking activity in response to various hypothalamic and peripheral inputs. Although the pituitary gland is structurally compartmentalized into specific and intermingled endocrine cell networks, providing a clear morphological basis for such coordinated activity, the mechanisms which facilitate the timely propagation of information between cells in situ remain largely unexplored. Therefore, the aim of the current review is to highlight the range of signalling modalities known to be employed by endocrine cells to coordinate intracellular calcium rises, and discuss how these mechanisms are integrated at the population level to orchestrate cell function and tissue output.

Direct stimulatory effect of ghrelin on pituitary release of LH through a nitric oxide-dependent mechanism that is modulated by estrogen

Ghrelin, a gut peptide with key actions on food intake and GH secretion, has been recently recognized as potential regulator of reproductive function. Thus, in adult female rats, ghrelin has been proven to modulate GnRH/LH secretion, with predominant inhibitory effectsin vivo. We analyze herein potential direct pituitary effects of ghrelin on basal and GnRH-stimulated gonadotropin secretion in prepubertal female rats, and its interplay with ovarian inputs, nitric oxide (NO), and hypothalamic differentiation. In the experimental setting, pituitaries from intact and ovariectomized prepubertal female rats were challenged with ghrelinin vitroand LH secretion was monitored. Our results demonstrate that 1) ghrelin consistently stimulatedin vitropituitary LH secretion under different experimental conditions; 2) the sensitivity to ghrelin, expressed either as the minimal effective dose or the amplitude of the LH response, was modulated by ovarian inputs; 3) the blockade of estrogen action significantly augmented the stimulatory effect of ghrelin; 4) the stimulatory effect of ghrelin on LH secretion required proper NO synthesis; and 5) the ability of ghrelin to elicit LH secretionin vitrowas preserved after alteration (masculinization) of brain sexual differentiation. Overall, our present data reinforce the concept that ghrelin participates in the control of LH secretion, with potential stimulatory actions at the pituitary level that require the presence of NO and are modulated by ovarian signals.

Endothelin-3 and PRL levels in the maternal and fetal circulation at delivery

OBJECTIVE

The aim of the present cross-sectional study was to test the hypothesis that endothelin-3 (ET-3) is involved in PRL secretion via systemic hormonal interaction during labor.

MATERIALS AND METHODS

Fifty healthy pregnant women with singleton pregnancies were included in the present study. At delivery, blood samples were drawn from umbilical vein and artery. At the same time, a blood sample was obtained from a peripheral vein of the mother. In all blood samples, plasma ET-3 and serum PRL concentrations were determined. The main outcome measures were the differences between maternal peripheral blood, umbilical artery and vein in terms of ET-3 and PRL levels, and the associations between ET-3 and PRL levels.

RESULTS

ET-3 values (mean+/-SEM) in umbilical artery did not differ significantly from those in umbilical vein (4.94+/-0.27 vs 5.05+/-0.32 pg/ml) but were in both vessels significantly higher than in maternal vein (1.14+/-0.56 pg/ml, p<0.001). Serum PRL values showed similar patterns. There was a significant positive correlation of the ET-3 levels between umbilical artery and vein (r=0.906, p<0.001), but not between maternal peripheral venous blood and the umbilical vessels. Similar correlations were found for PRL values. However, no significant correlations were found between ET-3 and PRL levels in all vessels studied.

CONCLUSIONS

The present study demonstrates for the first time that ET-3 levels are higher in fetal than in maternal circulation at term. The lack of correlation between ET-3 and PRL levels suggests that ET-3 does not play an important endocrine role in the control of maternal and fetal PRL secretion during labor.

Ghrelin effects on gonadotropin secretion in male and female rats

Ghrelin is a 28-amino acid peptide primarily involved in the control of food intake and growth hormone secretion. The present experiments were carried out to analyze the potential involvement of ghrelin in the control of gonadotropin secretion. Prepubertal intact and gonadectomized female and male rats, cyclic rats in diestrus, lactating rats and aged female rats were i.c.v. injected with ghrelin (3 nmol/rat) and blood samples were obtained by decapitation 15 min later. In addition, we analyzed the effects of ghrelin on in vitro basal and luteinizing hormone-releasing hormone (LHRH)-stimulated gonadotropin secretion. Our present results indicate that ghrelin inhibited luteinizing hormone (LH) secretion in vivo in prepubertal males as well as gonadectomized males and females, whereas follicle-stimulating hormone (FSH) remained unaffected. In vitro, ghrelin stimulated the secretion of both gonadotropins, and differentially modulated the response to LHRH; the LH response was inhibited, while the FSH response was enhanced. Overall, our current data open up the possibility that ghrelin may be involved in the control of LH secretion, and in the dissociation of both gonadotropins that takes place in many physiological, pathological and experimental situations.

Autocrine regulation of prolactin secretion by endothelins throughout the estrous cycle

We have previously found that the ovarian steroid background determines the efficiency of the endothelin-mediated autocrine feedback regulation of prolactin (PRL) secretion. In this study, we investigated the role of endogenous endothelins in regulating PRL secretion during the estrous cycle. Adult female rats representing different stages of the 4-d cycle were sacrificed by decapitation, and the anterior pituitary cells were enzymatically dispersed using collagenase and hyaluronidase. PRL secretion of individual lactotrophs was measured in a PRL-specific reverse hemolytic plaque assay, and the influence of endogenous endothelins on PRL secretion was assessed by applying the selective ET(A) receptor antagonist peptide, BQ123. Blocking the endothelin-mediated autocrine feedback resulted in an increase in PRL secretion when cells were obtained at proestrus, estrus, and diestrus-1, whereas PRL secretion was decreased at diestrus-2 by ET(A) receptor blockade. These observations suggest that endogenous endothelins are predominantly inhibitory during proestrus, estrus, and diestrus-1, whereas at diestrus-2 their influence on PRL secretion is stimulatory. Whereas the bell-shaped concentration-response curves with BQ123 at proestrus and diestrus-1 may indicate a transition state in which endogenous endothelins can be both stimulatory and inhibitory, at estrus the influence of endogenous endothelins is unequivocally inhibitory in nature. We propose that intensification of the endogenous endothelin- mediated negative feedback at estrus may play a role in restraining PRL secretion following the estradiol- induced proestrous PRL surge.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Endothelin and dopamine release

Endothelins and endothelin receptors are widespread in the brain. There is increasing evidence that endothelins play a role in brain mechanisms associated with behaviour and neuroendocrine regulation as well as cardiovascular control. We review the evidence for an interaction of endothelin with brain dopaminergic mechanisms. Our work has shown that particularly endothelin-1 and ET(B) receptors are present at significant levels in typical brain dopaminergic regions such as the striatum. Moreover, lesion studies showed that ET(B) receptors are present on dopaminergic neuronal terminals in striatum and studies with local administration of endothelins into the ventral striatum showed that activation of these receptors causes dopamine release, as measured both with in vivo voltammetry and behavioural methods. While several previous studies have focussed on the possible role of very high levels of endothelins in ischemic and pathological mechanisms in the brain, possibly mediated by ET(A) receptors, we propose that physiological levels of these peptides play an important role in normal brain function, at least partly by interacting with dopamine release through ET(B) receptors.

Correlation between 21 amino acid endothelin, intrafollicular steroids and follicle size in stimulated cycles

Several in-vitro studies have shown that endothelins (ET) may inhibit synthesis of progesterone and prevent luteinization of granulosa cells. In the present study, a specific radioimmunoassay was used to evaluate the correlation between concentrations of active (21 residue) ET and ovarian steroids in 47 samples of human follicular fluid (FF) following gonadotrophin stimulation for in-vitro fertilization (IVF) protocols. An isoform non-selective antibody was used in the radioimmunoassay, which recognized the C-terminal structure of the 21 residue ET, and therefore did not crossreact with their weakly active precursors - big ET. In pooled samples of follicular fluid (FF), the concentration of 21 amino acid ET correlated negatively with diameter of the follicles (r = -0.31, P < 0.05) and progesterone concentrations in FF (r = -0.56, P < 0. 001). A positive relationship (non-significant) was found between ET and testosterone concentrations. No correlation between ET and oestradiol was observed. The within-patient correlation coefficients were also evaluated in women from whom three or more samples of FF were obtained. ET were markedly inversely correlated with follicle size in all cases, and with progesterone in five of seven women. Five of seven patients also showed significant positive correlation of ET with testosterone. The results demonstrate clinical evidence that active ET play an important role in regulation of follicle development, especially in the inhibition of premature luteinization of granulosa cells.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

The effects of interleukin-2 treatment on endothelin and the activation of the hypothalamic-pituitary-adrenal axis

OBJECTIVE

Recent reports suggest that complex interactions exist between the neuroendocrine and immune systems. It has been shown for example that cytokines are able to stimulate the hypothalamo-pituitary-adrenal axis. In addition, some studies present evidence that endothelin is able to modulate the activity of several hypothalamic-pituitary axes, e.g. by inducing the ACTH production.

DESIGN

We investigated the effects of interleukin-2 on endothelin levels and the hypothalamo-pituitary-adrenal axis. We determined the interleukin-6, big-endothelin, endothelin-1, ACTH, cortisol and AVP responses to intravenously and subcutaneously administered interleukin-2 in 8 cancer patients in a randomized placebo controlled trial.

PATIENTS

8 Patients (2 female and 6 male), age 44 +/- 4.8 years, were enrolled. All patients had a World Health Organization performance status of 1 or less and a Karnofsky Index of at least 80%.

MEASUREMENTS

Blood-samples were taken before and 15, 30, 45, 60, 120, 180, 240, 300 and 360 min after interleukin-2 injection. Cytokine serum levels and the plasma levels of big-endothelin, endothelin, ACTH and AVP were analysed using radioimmuno-assays. Cortisol was assayed by an enzyme-linked immunosorbent assay.

RESULTS

Interleukin-2 treatment significantly increased plasma big-endothelin levels (P < 0.01 vs basal) and endothelin-1 levels (P < 0.05 vs basal) within two hours and this was followed by an increase in ACTH (P < 0.01 vs basal) and cortisol (P < 0.05 vs basal) within three hours. Interleukin-6 levels increased two hours after interleukin-2 administration (P < 0.01 vs basal). Interleukin-2 had no detectable effect on AVP, blood pressure or heart rate.

CONCLUSIONS

Our data demonstrate that cytokines are able to activate the human hypothalamo-pituitary-adrenal axis in vivo. On the basis of the observed time kinetics and in connection with previous findings from in vitro and animal models, we conclude that endothelin may be a link between cytokines and corticotrophin-releasing hormone, most probably functioning as a cytokine-induced neuromodulator controlling pituitary functions.

Endothelin is an autocrine regulator of prolactin secretion

The aim of this study was to establish the cellular source of ET-like peptides affecting PRL secretion. Fluorescence double label immunocytochemistry and confocal laser scanning microscopy were used to demonstrate cellular colocalization for PRL and endothelin-1 (ET1)-like immunoreactivities in the anterior lobe of the pituitary gland of rats. An ET-specific reverse hemolytic plaque assay was applied to demonstrate that lactotrophs are capable of releasing ET-like peptides. A PRL-specific reverse hemolytic plaque assay was used to assess the influence of the released endogenous ETs on PRL secretion. ET(A)-specific receptor antagonists BQ123 and BQ610, and endothelin convertase enzyme inhibitory peptide, [22Val]big ET1-(16-38), increased PRL secretion, whereas the ET(B) receptor-specific antagonist BQ788 was ineffective. The ET(A) antagonist BQ123-induced increase in PRL secretion followed a bell-shaped dose-response curve in cells obtained from female rats, whereas it followed a sigmoid curve in males. Frequency distribution of PRL plaque sizes using logarithmically binned data revealed two subpopulations of lactotrophs with differential responsiveness to endogenous ETs. These data demonstrate that a large proportion of lactotrophs is capable of expressing and secreting ET-like peptides in biologically significant quantities. As low pituitary cell density in reverse hemolytic plaque assay minimizes cell to cell communications, these findings constitute direct proof of autocrine regulation of PRL secretion by ET-like peptides.

Endothelin activates large-conductance K+ channels in rat lactotrophs: reversal by long-term exposure to dopamine agonist

Endothelin-1 (ET-1) inhibits PRL secretion from cultured rat lactotrophs. However, ET-1 stimulates PRL secretion after cultured lactotrophs have been exposed for 48 h to dopamine or D2 dopamine agonists. In the present study, we have used cell-attached and inside-out patch recordings to establish an ionic basis for these effects. Bath application of 20 nM ET-1 to untreated lactotrophs evoked a robust and persistent activation of large-conductance K+ channels in cell-attached patches. This effect of ET-1 had a long latency to onset, was maintained for as long as ET-1 was present, and required at least 10 min of washing in control saline before complete recovery was achieved. The stimulatory effect of 20 nM ET-1 on these channels was markedly attenuated in the presence of the selective ET(A) receptor antagonist BQ-610 (200 nM), or after pertussis toxin (200 ng/ml, 16 h) pretreatment. The unitary slope conductance of the ET-1 activated channels in cell attached patches was 165 and 95 pS when the recording electrodes contained 150 and 5.4 mM KCl, respectively. These channels were voltage-sensitive and their activity increased upon patch depolarization. Previously activated channels in cell-attached patches became quiescent immediately upon patch excision into Ca2+-free bath saline. Exposure of the intracellular surface to 0.1 microM Ca2+ restored the activity of these channels similar to the level seen before patch excision. In addition, preincubating the cells with the membrane-permeable Ca2+-chelator BAPTA-AM, or using Ca2+-free solution in the recording pipettes, prevented the activation of these channels by ET-1. The ET-1 activated large-conductance Ca2+-dependent K+ (BK(Ca)) channels were blocked by 20 mM tetraethylammonium but were insensitive to the K+ channel blockers apamin (1 microM), charybdotoxin (200 nM), or iberiotoxin (200 nM). Acute application of 10 microM dopamine and 20 nM ET-1 caused activation of BK(Ca) channels with indistinguishable kinetic properties, although the effect of dopamine occurred with shorter latency. After 48-h exposure to the specific D2 dopamine receptor agonist (+/-)-2-(N-phenyl-N-propyl) amino-5-hydroxytetralin hydrochloride (PPHT, 500 nM), bath application of 20 nM ET-1 resulted in inhibition of spontaneously active BK(Ca) channels. These data suggest that both the stimulatory and inhibitory effects of ET-1 on PRL secretion are mediated, at least in part, by actions on BK(Ca) channels, and that long term exposure to dopamine or D2 agonists alters the signaling pathways from the ET(A) receptor to BK(Ca) channels.

Distribution of endothelin 3-like immunoreactivity in gonadotrophs of the bullfrog (Rana catesbeiana) pituitary

Immunohistochemical and immunocytochemical techniques were employed to investigate the distribution of endothelin 3 (ET3)-like immunoreactivity in the pituitary of the bullfrog, Rana catesbeiana. ET3-immunoreactive (ET3-IR) cells were scattered all over the pars distalis of the female pituitary; however, only a few ET3-IR cells were observed in the male pituitary. ET3-IR cells were found to correspond to cells immunostained with monoclonal antibodies against the beta-subunit of bullfrog LH (fLH beta) or monoclonal antibodies against the beta-subunit of bullfrog FSH (fFSH beta) at the light microscopic level. However, we could not find ET3-IR cells which were immunoreactive for other pituitary hormones. So far, all ET3-IR cells showed both fLH beta and fFSH beta immunoreactivity. About 24% of the fLH beta-IR cells and about 33% of the fFSH beta-IR cells showed ET3-like immunoreactivity. Immunoelectron microscopic analysis using colloidal gold revealed the coexistence of ET3-like substance(s) and gonadotropins within the same granules. This study demonstrated the presence of ET3-like peptide(s) in bullfrog gonadotrophs, suggesting the possible participation of ET3 in regulating pituitary function as an autocrine and/or paracrine hormone.

Characterization of endothelin receptors in the anterior pituitary gland

To characterize endothelin (ET) receptors modulating pituitary hormone secretion, potencies of ET-like agonists were compared on prolactin (PRL), thyrotropin (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) secretion from primary cultures of female rat pituitary cells. ET-1 was more potent than ET-3 in all cases. Sarafotoxin (SRTX) S6b an ETA agonist, was also more potent than ET-3 in all cases. SRTX-c, an ETB receptor agonist, was inactive. The ET-1-to-ET-3 potency ratio was three orders of magnitude higher on PRL or TSH secretion than on LH and FSH secretion, whereas SRTX-b-to-ET-3 potency ratios were similar on all four hormones. The ETA antagonist BQ-123 caused a parallel dextral displacement of dose-response curves of ET-1 and ET-3 on all four hormones. Schild regressions for BQ-123 on ET-1-induced PRL, TSH, LH, and FSH secretion indicated that BQ-123 has a similar affinity for the receptors mediating ET-1's effects. When BQ-123 was assessed against ET-3, Schild regressions indicated greater affinity for ET-3 on lactotrophs and thyrotrophs than gonadotrophs. Thus changes in pituitary hormone secretion are mediated by ETA-like receptors. ET receptors in lactotrophs and thyrotrophs are clearly distinguishable from gonadotrophs. We suggest the existence of distinct ETA receptor subtypes (ETA1 and ETA2) on these differing pituitary cells.

The endothelin-A receptor subtype transduces the effects of the endothelins in the anterior pituitary gland

All members of the mammalian endothelin family of peptides exert significant effects on prolactin and luteinizing hormone release from dispersed anterior pituitary cells in vitro. The rank order of potency for the prolactin inhibiting effects of the endothelins is ET-1 = ET-2 much less than ET-3. This suggests an involvement of the ET-A receptor subtype. The selective ET-A receptor antagonist BQ-123 antagonized the effects of the ETs in a competitive fashion with pA2 values of 6.1 (ET-1), 5.7 (ET-2) and 6.4 (ET-3), when added simultaneously with the ETs. This suggests the involvement of the ET-A receptor subtype in the actions of the ETs within the anterior pituitary gland.

Inhibitory effects of endothelin-1 and endothelin-3 on prolactin release: possible involvement of endogenous endothelin isopeptides in the rat anterior pituitary

Spontaneous prolactin release from the isolated rat anterior pituitary was inhibited by endothelin-1 in a dose-dependent manner (10(-8)-10(-6) M). Endothelin-3 also inhibited spontaneous prolactin release with an almost identical dose-response relationship as endothelin-1. These inhibitory effects were unaffected by application of a dopamine D2-receptor antagonist, YM-09151-2 (10(-7) M). Rat anterior and posterior pituitary glands were abundant in both endothelin-1 and endothelin-3, as compared with other regions of the brain. The present results suggest that endogenous endothelin-1 and endothelin-3 in the anterior and posterior pituitary are involved in the inhibitory regulation of prolactin secretion as autocrine or paracrine factors.