Why are endocrine pituitary cells excitable?

Differential Regulation of Gonadotropins as Revealed by Transcriptomes of Distinct LH and FSH Cells of Fish Pituitary

From mammals to fish, reproduction is driven by luteinizing hormone (LH) and follicle-stimulating hormone (FSH) temporally secreted from the pituitary gland. Teleost fish are an excellent model for addressing the unique regulation and function of each gonadotropin cell since, unlike mammals, they synthesize and secrete LH and FSH from distinct cells. Only very distant vertebrate classes (such as fish and birds) demonstrate the mono-hormonal strategy, suggesting a potential convergent evolution. Cell-specific transcriptome analysis of double-labeled transgenic tilapia expressing GFP and RFP in LH or FSH cells, respectively, yielded genes specifically enriched in each cell type, revealing differences in hormone regulation, receptor expression, cell signaling, and electrical properties. Each cell type expresses a unique GPCR signature that reveals the direct regulation of metabolic and homeostatic hormones. Comparing these novel transcriptomes to that of rat gonadotrophs revealed conserved genes that might specifically contribute to each gonadotropin activity in mammals, suggesting conserved mechanisms controlling the differential regulation of gonadotropins in vertebrates.

Common and diverse elements of ion channels and receptors underlying electrical activity in endocrine pituitary cells

The pituitary gland contains six types of endocrine cells defined by hormones they secrete: corticotrophs, melanotrophs, gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs. All these cell types are electrically excitable, and voltage-gated calcium influx is the major trigger for their hormone secretion. Along with hormone intracellular content, G-protein-coupled receptor and ion channel expression can also be considered as defining cell type identity. While many aspects of the developmental and activity dependent regulation of hormone and G-protein-coupled receptor expression have been elucidated, much less is known about the regulation of the ion channels needed for excitation-secretion coupling in these cells. We compare the spontaneous and receptor-controlled patterns of electrical signaling among endocrine pituitary cell types, including insights gained from mathematical modeling. We argue that a common set of ionic currents unites these cells, while differential expression of another subset of ionic currents could underlie cell type-specific patterns. We demonstrate these ideas using a generic mathematical model, showing that it reproduces many observed features of pituitary electrical signaling. Mapping these observations to the developmental lineage suggests possible modes of regulation that may give rise to mature pituitary cell types.

Control of anterior pituitary cell excitability by calcium-activated potassium channels

In anterior pituitary endocrine cells, large (BK), small (SK) and intermediate (IK) conductance calcium activated potassium channels are key determinants in shaping cellular excitability in a cell type- and context-specific manner. Indeed, these channels are targeted by multiple signaling pathways that stimulate or inhibit cellular excitability. BK channels can, paradoxically, both promote electrical bursting as well as terminate bursting and spiking dependent upon intrinsic BK channel properties and proximity to voltage gated calcium channels in somatotrophs, lactotrophs and corticotrophs. In contrast, SK channels are predominantly activated by calcium released from intracellular IP3-sensitive calcium stores and mediate membrane hyperpolarization in cells including gonadotrophs and corticotrophs. IK channels are predominantly expressed in corticotrophs where they limit membrane excitability. A major challenge for the future is to determine the cell-type specific molecular composition of calcium-activated potassium channels and how they control anterior pituitary hormone secretion as well as other calcium-dependent processes.

Is bursting more effective than spiking in evoking pituitary hormone secretion? A spatiotemporal simulation study of calcium and granule dynamics

Endocrine cells of the pituitary gland secrete a number of hormones, and the amount of hormone released by a cell is controlled in large part by the cell's electrical activity and subsequent Ca(2+) influx. Typical electrical behaviors of pituitary cells include continuous spiking and so-called pseudo-plateau bursting. It has been shown that the amplitude of Ca(2+) fluctuations is greater in bursting cells, leading to the hypothesis that bursting cells release more hormone than spiking cells. In this work, we apply computer simulations to test this hypothesis. We use experimental recordings of electrical activity as input to mathematical models of Ca(2+) channel activity, buffered Ca(2+) diffusion, and Ca(2+)-driven exocytosis. To compare the efficacy of spiking and bursting on the same cell, we pharmacologically block the large-conductance potassium (BK) current from a bursting cell or add a BK current to a spiking cell via dynamic clamp. We find that bursting is generally at least as effective as spiking at evoking hormone release and is often considerably more effective, even when normalizing to Ca(2+) influx. Our hybrid experimental/modeling approach confirms that adding a BK-type K(+) current, which is typically associated with decreased cell activity and reduced secretion, can actually produce an increase in hormone secretion, as suggested earlier.

Roles of connexins and pannexins in (neuro)endocrine physiology

To ensure appropriate secretion in response to demand, (neuro)endocrine tissues liberate massive quantities of hormones, which act to coordinate and synchronize biological signals in distant secretory and nonsecretory cell populations. Intercellular communication plays a central role in this control. With regard to molecular identity, junctional cell-cell communication is supported by connexin-based gap junctions. In addition, connexin hemichannels, the structural precursors of gap junctions, as well as pannexin channels have recently emerged as possible modulators of the secretory process. This review focuses on the expression of connexins and pannexins in various (neuro)endocrine tissues, including the adrenal cortex and medulla, the anterior pituitary, the endocrine hypothalamus and the pineal, thyroid and parathyroid glands. Upon a physiological or pathological stimulus, junctional intercellular coupling can be acutely modulated or persistently remodeled, thus offering multiple regulatory possibilities. The functional roles of gap junction-mediated intercellular communication in endocrine physiology as well as the involvement of connexin/pannexin-related hemichannels are also discussed.

Dependence of spontaneous electrical activity and basal prolactin release on nonselective cation channels in pituitary lactotrophs

All secretory anterior pituitary cells fire action potentials spontaneously and exhibit a high resting cation conductance, but the channels involved in the background permeability have not been identified. In cultured lactotrophs and immortalized GH(3) cells, replacement of extracellular Na(+) with large organic cations, but not blockade of voltage-gated Na(+) influx, led to an instantaneous hyperpolarization of cell membranes that was associated with a cessation of spontaneous firing. When cells were clamped at -50 mV, which was close to the resting membrane potential in these cells, replacement of bath Na(+) with organic cations resulted in an outward-like current, reflecting an inhibition of the inward holding membrane current and indicating loss of a background-depolarizing conductance. Quantitative RT-PCR analysis revealed the high expression of mRNA transcripts for TRPC1 and much lower expression of TRPC6 in both lactotrophs and GH(3) cells. Very low expression of TRPC3, TRPC4, and TRPC5 mRNA transcripts were also present in pituitary but not GH(3) cells. 2-APB and SKF-96365, relatively selective blockers of TRPC channels, inhibited electrical activity, Ca(2+) influx and prolactin release in a concentration-dependent manner. Gd(3+), a common Ca(2+) channel blocker, and flufenamic acid, an inhibitor of non-selective cation channels, also inhibited electrical activity, Ca(2+) influx and prolactin release. These results indicate that nonselective cation channels, presumably belonging to the TRPC family, contribute to the background depolarizing conductance and firing of action potentials with consequent contribution to Ca(2+) influx and hormone release in lactotrophs and GH(3) cells.

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.

Endocrine cells and blood vessels work in tandem to generate hormone pulses

Hormones are dynamically collected by fenestrated capillaries to generate pulses, which are then decoded by target tissues to mount a biological response. To generate hormone pulses, endocrine systems have evolved mechanisms to tightly regulate blood perfusion and oxygenation, coordinate endocrine cell responses to secretory stimuli, and regulate hormone uptake from the perivascular space into the bloodstream. Based on recent findings, we review here the mechanisms that exist in endocrine systems to regulate blood flow, and facilitate coordinated cell activity and output under both normal physiological and pathological conditions in the pituitary gland and pancreas.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.

Electrical activity in endocrine pituitary cells in situ: a support for a multiple-function coding

The anterior pituitary is an endocrine gland that controls basic body functions. Pituitary cell functioning depends on membrane excitability, which induces cytosolic calcium rises. Here, we reported the first identification of small-amplitude voltage fluctuations that controlled spike firing in endocrine cells recorded in situ. Three patterns of voltage fluctuations were distinguishable by their durations (1-100 s). These patterns could be ordered on top of each other, namely in response to secretagogues. Thus, pituitary endocrine cells express in situ a cell code in which small-amplitude voltage fluctuations lead to a multimodal arrangement of spike firing, which may finely tune calcium-dependent functions.

Dampening of cytosolic Ca2+ oscillations on propagation to nucleus

Ca(2+) signals may regulate gene expression. The increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) promotes activation and/or nuclear import of some transcription factors, but others require the increase of the nuclear Ca(2+) concentration ([Ca(2+)](N)) for activation. Whether the nuclear envelope may act as a diffusion barrier for propagation of [Ca(2+)](c) signals remains controversial. We have studied the spreading of Ca(2+) from the cytosol to the nucleus by comparing the cytosolic and the nuclear Ca(2+) signals reported by targeted aequorins in adrenal chromaffin, PC12, and GH(3) pituitary cells. Strong stimulation of either Ca(2+) entry (by depolarization with high K(+) or acethylcholine) or Ca(2+) release from the intracellular Ca(2+) stores (by stimulation with caffeine, UTP, bradykinin, or thyrotropin-releasing hormone (TRH)) produced similar Ca(2+) signals in cytosol and nucleus. In contrast, both spontaneous and TRH-stimulated oscillations of cytosolic Ca(2+) in single GH(3) cells were considerably dampened during propagation to the nucleus. These results are consistent with the existence of a kinetic barrier that filters high frequency physiological [Ca(2+)](c) oscillations without disturbing sustained [Ca(2+)](c) increases. Thus, encoding of the Ca(2+) signal may allow differential control of Ca(2+)-dependent mechanisms located at either the cytosol or the nucleus.

Spontaneous calcium oscillations control c-fos transcription via the serum response element in neuroendocrine cells

In excitable cells the localization of Ca2+ signals plays a central role in the cellular response, especially in the control of gene transcription. To study the effect of localized Ca2+ signals on the transcriptional activation of the c-fos oncogene, we stably expressed various c-fos beta-lactamase reporter constructs in pituitary AtT20 cells. A significant, but heterogenous expression of c-fos beta-lactamase was observed in unstimulated cells, and a further increase was observed using KCl depolarization, epidermal growth factor (EGF), pituitary adenylate cyclase-activating polypeptide (PACAP), and serum. The KCl response was almost abolished by a nuclear Ca2+ clamp, indicating that a rise in nuclear Ca2+ is required. In contrast, the basal expression was not affected by the nuclear Ca2+ clamp, but it was strongly reduced by nifedipine, a specific antagonist of l-type Ca2+ channels. Spontaneous Ca2+ oscillations, blocked by nifedipine, were observed in the cytosol but did not propagate to the nucleus, suggesting that a rise in cytosolic Ca2+ is sufficient for basal c-fos expression. Inactivation of the c-fos promoter cAMP/Ca2+ response element (CRE) had no effect on basal or stimulated expression, whereas inactivation of the serum response element (SRE) had the same marked inhibitory effect as nifedipine. These experiments suggest that in AtT20 cells spontaneous Ca2+ oscillations maintain a basal c-fos transcription through the serum response element. Further induction of c-fos expression by depolarization requires a nuclear Ca2+ increase.

An extracellular sulfhydryl group modulates background Na(+) conductance and cytosolic Ca(2+) in pituitary cells

Treatment of GH(3) pituitary cells with p-chloromercurybenzenesulfonate (PCMBS) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)). This effect was reversed by dithiothreitol and blocked by L-type Ca(2+) channel antagonists or Na(+) removal. PCMBS increased membrane conductance and depolarized the plasma membrane. Apart from minor effects on K(+) and Ca(2+) channels, PCMBS increased (6 times at -80 mV) an inward Na(+) current whose properties were similar to those of a background Na(+) conductance (BNC) described previously, necessary for generation of spontaneous electrical activity. In rat lactotropes and somatotropes in primary culture, PCMBS also produced a Na(+)-dependent [Ca(2+)](i) increase, whereas little or no effect was observed in thyrotropes, corticotropes, and gonadotropes. The Na(+) conductance elicited by PCMBS in somatotropes seemed to be the same as that stimulated by the hypothalamic growth hormone (GH)-releasing hormone, which regulates membrane excitability and GH secretion. The BNC studied here could play a physiological role, regulating excitability and spontaneous activity, and explains satisfactorily the [Ca(2+)](i)-increasing actions of the mercurials reported previously in several excitable tissues.

Rac and Rho mediate opposing hormonal regulation of the ether-a-go-go-related potassium channel

BACKGROUND

Previous studies of ion channel regulation by G proteins have focused on the larger, heterotrimeric GTPases, which are activated by heptahelical membrane receptors. In contrast, studies of the Rho family of smaller, monomeric, Ras-related GTPases, which are activated by cytoplasmic guanine nucleotide exchange factors, have focused on their role in cytoskeletal regulation.

RESULTS

Here we demonstrate novel functions for the Rho family GTPases Rac and Rho in the opposing hormonal regulation of voltage-activated, ether-a-go-go-related potassium channels (ERG) in a rat pituitary cell line, GH(4)C(1). The hypothalamic neuropeptide, thyrotropin-releasing hormone (TRH) inhibits ERG channel activity through a PKC-independent process that is blocked by RhoA(19N) and the Clostridium botulinum C3 toxin, which inhibit Rho signaling. The constitutively active, GTPase-deficient mutant of RhoA(63L) rapidly inhibits the channels when the protein is dialysed directly into the cell through the patch pipette, and inhibition persists when the protein is overexpressed. In contrast, GTPase-deficient Rac1(61L) stimulates ERG channel activity. The thyroid hormone triiodothyronine (T3), which antagonizes TRH action in the pituitary, also stimulates ERG channel activity through a rapid process that is blocked by Rac1(17N) and wortmannin but not by RhoA(19N).

CONCLUSIONS

Rho stimulation by G(13)-coupled receptors and Rac stimulation by nuclear hormones through PI3-kinase may be general mechanisms for regulating ion channel activity in many cell types. Disruption of these novel signaling cascades is predicted to contribute to several specific human neurological diseases, including epilepsy and deafness.

Mitochondrial [Ca(2+)] oscillations driven by local high [Ca(2+)] domains generated by spontaneous electric activity

Mitochondria take up calcium during cell activation thus shaping Ca(2+) signaling and exocytosis. In turn, Ca(2+) uptake by mitochondria increases respiration and ATP synthesis. Targeted aequorins are excellent Ca(2+) probes for subcellular analysis, but single-cell imaging has proven difficult. Here we combine virus-based expression of targeted aequorins with photon-counting imaging to resolve dynamics of the cytosolic, mitochondrial, and nuclear Ca(2+) signals at the single-cell level in anterior pituitary cells. These cells exhibit spontaneous electric activity and cytosolic Ca(2+) oscillations that are responsible for basal secretion of pituitary hormones and are modulated by hypophysiotrophic factors. Aequorin reported spontaneous [Ca(2+)] oscillations in all the three compartments, bulk cytosol, nucleus, and mitochondria. Interestingly, a fraction of mitochondria underwent much larger [Ca(2+)] oscillations, which were driven by local high [Ca(2+)] domains generated by the spontaneous electric activity. These oscillations were large enough to stimulate respiration, providing the basis for local tune-up of mitochondrial function by the Ca(2+) signal.

Spontaneous calcium oscillatory patterns in mammotropes display non-random dynamics

We previously showed that primary rat mammotropes exhibited four distinct patterns of 'spontaneous' free intracellular calcium ([Ca2+]i) oscillatory behavior: a quiescent state A and three oscillatory states B,C&D, which differed in frequency/amplitude characteristics. When [Ca2+]i was monitored in 10 min windows separated by several hours, these phenotypes were frequently found to interconvert, raising the question about whether these transitions were random or ordered events. We reasoned that if such activity were random, then neither episode duration nor transitional probabilities should differ among phenotypes. We tested this logic in the current study by making long-term, continuous measurements of [Ca2+]i in mammotropes microinjected with Fura-2-dextran and identified by their ability to express a prolactin promoter-driven reporter plasmid. We found that transitions occurred in ~25% of cells (n = 36 from 9 independent experiments) once every 1-5 h and demarcated phenotype episodes of different duration (A, 1.04 +/- 0.2 h; B, 1.64 +/- 0.3 h; C, 2.45 +/- 0.62 h; D, 0.90 +/- 0.2 h, mean +/- SEM). Moreover, some transitions occurred more frequently than others and linked specific phenotypes into a common pattern: C to B to A. Our results demonstrate that the seemingly spontaneous nature of [Ca2+]i phenotype transitions are, in fact, ordered and support the view that they comprise a structured 'code' like that proposed to underlie calcium-dependent regulation of exocytosis and gene expression.

Rhythmic bursts of calcium transients in acute anterior pituitary slices

Endocrine cells isolated from the anterior pituitary fire intracellular Ca2+ ([Ca2+]i) transients due to voltage-gated Ca2+ entry. However, the patterns of [Ca2+]i transients within the glandular parenchyma of the anterior pituitary are unknown. Here we describe, using real-time confocal laser microscopy, several spontaneous patterns of calcium signaling in acute pituitary slices prepared from male as well as cycling and lactating female rats. Forty percent of the cells demonstrated a spontaneous bursting mode, consisting of an active period of [Ca2+]i transients firing at a constant frequency, followed by a rest period during which cells were either silent or randomly active. The remaining recordings from endocrine cells either demonstrated random [Ca2+]i transients or were silent. These rhythmic bursts of [Ca2+]i transients, which required extracellular calcium, were detected in lactotrophs, somatotrophs, and corticotrophs within the acute slices. Of significance was the finding that the bursting mode could be adjusted by hypothalamic factors. In slices prepared from lactating rats, TRH recruited more bursting cells and finely adjusted the average duty cycle of [Ca2+]i bursts such that cells fired patterned bursts for approximately 70% of the recording period. Eighty-six percent of these cells were lactotrophs. Thus, the rhythmic [Ca2+]i bursts and their tuning by secretagogues may provide timing information that could encode for one or more cellular functions (e.g. exocytosis and/or gene expression) critical for the release of hormones by endocrine cells in the intact gland.

Calcium influx and intracellular stores in angiotensin II stimulation of normal and hyperplastic pituitary cells

In rat pituitary cells from estrogen-induced hyperplasia, angiotensin II (ANG II) does not evoke a clear spike elevation of intracellular Ca2+ concentration ([Ca2+]i) but induces a plateau increase. The present work was undertaken to establish whether this difference was related to a differential participation of intracellular and/or plasma membrane Ca2+ channels. We first tested the effect of 10 nM ANG II on [Ca2+]i in the absence of extracellular Ca2+ in cells depolarized with 25 mM K+ or in the presence of blockers of L-type voltage-sensitive Ca2+ channels (VSCC). These treatments did not alter spike elevation in [Ca2+]i in controls but reduced plateau levels in hyperplastic cells. Intracellular Ca2+ stores were similar in both groups, as assessed by thapsigargin treatment, but this drug abolished spike increase in controls and scarcely modified plateau levels in hyperplastic cells. Finally, inositol trisphosphate (InsP3) production in response to ANG II was significantly higher in control cells. We conclude that the observed plateau rise in hyperplastic cells results mainly from Ca2+ influx through VSCC. In contrast, in control cells, the ANG II-induced spike increase in [Ca2+]i results from mobilization of Ca2+ from thapsigargin-sensitive internal channels, activated by higher inositol 1,4,5-trisphosphate generation.

Expression of calcium-sensing receptor and characterization of intracellular signaling in human pituitary adenomas

Extracellular Ca(2+)-sensing receptor (CaSR) has been recently identified in rat and mouse pituitary and in AtT-20 cells. The aim of the study was to investigate the presence of CaSR in the human pituitary and its signaling pathway. Normal parathyroid biopsies, autoptic normal pituitaries, and seven nonfunctioning and six GH-secreting adenomas were studied. Southern blot analysis of the RT-PCR products from pituitary adenomas indicated that the PCR fragments obtained were products of specific amplification of CaSR messenger ribonucleic acid. Sequence analysis showed nucleotide identity of these products with the available human parathyroid CaSR. By immunoblotting analysis CaSR, was detected in normal and adenomatous pituitary tissues. In all tumors studied, extracellular Ca2+ (2.5 mmol/L) induced a significant increase in intracellular Ca2+, mainly due to Ca2+ mobilization (from 82.7+/-11 to 148+/-36 nmol/L; P < 0.001). Similar results were obtained with the CaSR activators gadolinium and neomycin. Moreover, CaSR activators significantly increased cAMP levels; this effect was not mimicked by other agents able to increase intracellular Ca2+, such as TRH. CaSR agonists did not increase resting GH secretion in any GH-secreting adenomas, but amplified the GH response to GHRH. In this study we first demonstrate CaSR expression in the human pituitary and provides evidence for an additional mechanism by which calcium might regulate pituitary cell function.

Calcium signalling and gene expression

A wide variety of compounds acting as extracellular signals cause changes in the free cytosolic Ca2+ concentration. These factors include hormones, growth factors, neurotransmitters, but also nutrient and metabolic activators. Ca2+ signalling is caused by mobilization of Ca2+ from internal stores and by well controlled and timed Ca2+ influx from the extracellular space. Ca2+ signals address Ca2+ dependent enzymes, most importantly Ca2+ sensitive protein kinases and phosphatases. The profound influence of Ca2+ signalling on gene expression has been recognized a long time ago. As Ca2+ signals are short-lived when compared to alterations in differentiated gene expression, it is generally considered that genes coding for short-lived transcription factors (i.e. fos, jun) are the immediate target of Ca2+ signalling. Transcription of these immediate early genes (IEG) can be activated without the need for protein synthesis. Ca2+ signalling affects differentiated gene expression via changes in the absolute and relative abundance of IEG products, which in turn control the expression of differentiated genes. Ca2+ signals can stimulate both transcriptional initiation as well as transcriptional elongation. Initiation of transcription is stimulated by the Ca2+ dependent phosphorylation of binding proteins addressing two response elements in the promoter of IEGs: the cAMP response element, CRE, and the serum response element, SRE. Distinct protein kinases are involved in either case. We study the elongation of transcripts of the IEG c-fos beyond the first intron which is favoured by Ca2+ signals, involving mechanisms which still are poorly understood. We can show that intron sequences contribute to the control of elongation by Ca2+, and that there is a strong interrelation between the transcription control by the promoter and by the intron.

Effects of cellular interactions on calcium dynamics in prolactin-secreting cells

Signals derived from other pituitary cells can have a dramatic effect on PRL gene expression and secretion by mammotropes. However, the intracellular mechanisms by which these effects are manifested on the target cell remain unexplored. Inasmuch as calcium is a key modulator of both gene expression and hormone export in mammotropes, we evaluated the effects of cell to cell contact vs. specific cellular interactions on calcium dynamics within these cells. This was accomplished by digital-imaging fluorescence microscopy of fura-2 in pituitary cells that were isolated in culture (singles) or adjoining one other cell (doublets). After calcium imaging, we then subjected cells to immunocytochemistry for PRL. Doublets were further categorized into mammotropes attached to another mammotrope (M-M) or to a nonmammotrope (M-nonM). We then calculated and compared Mean [Ca2+]i values as well as Oscillation Indices (which reflect the oscillatory behavior of cells) in singles and doublets and found that they were not different (P > 0.05). However, the phenotype of the adjoining cell had a profound influence on both of these calcium parameters, such that the presence of one mammotrope could consistently decrease (P < 0.05) the Mean [Ca2+]i value (39.17 +/- 3.83 vs. 56.24 +/- 5.56 in M-nonM) and Oscillation Index (10.19 +/- 1.76 vs. 21.21 +/- 3.73 in M-nonM) of its neighboring counterpart. A more detailed analysis of oscillatory patterns in these cells revealed that nonoscillators were more abundant in M-M (23%) than in M-nonM (12%) doublets. Taken together, our results indicate that PRL-secreting cells convey a signal that dampens the oscillatory behavior of neighboring mammotropes. Thus, it appears that it is the phenotype rather than the physical presence of a neighbor that controls intercellular regulation of calcium dynamics among mammotropes.

Calcium signaling and secretion in pituitary cells

An important trigger of hormone secretion from pituitary cells is a rise in cytosolic Ca(2+) ([Ca(2+)](i)). Pituitary cells may modulate [Ca(2+)](i) by an increased membrane flux from the extracellular space and/or by a release from intracellular stores. Both mechanisms can support exocytosis, although in different pituitary cell types one or the other mechanism may predominate. Molecular events transducing a rise in [Ca(2+)](i) into hormone secretion are still poorly understood. Here, the exocytotic machinery in pituitary cells is briefly reviewed in terms of the spatial organization of [Ca(2+)](i) elevation relative to the Ca(2+) sensor(s).

GnRH-induced calcium and current oscillations in gonadotrophs

The rat pituitary gonadotroph is a well-studied cell model for investigation of the oscillatory nature of calcium signaling in agonist-stimulated excitable cells. Cytosolic calcium levels ([Ca(2+)](i)) in gonadotrophs are controlled by two distinct oscillators, a plasma membrane oscillator that generates extracellular calcium-dependent low-amplitude [Ca(2+)](i) spiking in unstimulated cells and an endoplasmic reticulum oscillator that is activated by calcium-mobilizing receptors for GnRH, endothelin, and pituitary adenylate cyclase-activating polypeptide. In this review, the characteristics of the spontaneous and agonist-induced calcium oscillations in gonadotrophs and the coordinate actions of the two oscillators during GnRH action discussed.

Pituitary adenylate cyclase-activating polypeptide regulates [Ca(2+)](i) and electrical activity in pituitary cells through cell type-specific mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified hypothalamic factor that acts on a variety of anterior pituitary cell types. It is clear, however, that its actions are not mediated by the same intracellular signaling mechanisms in each cell type. The signaling pathways by which PACAP regulates changes in [Ca(2+)], and electrical activity in rat somatotrophs and gonadotrophs is described in the present article. Finally, the possibility that the differences in PACAP-regulated signaling in anterior pituitary cells is due to the differential expression and coupling of PACAP receptor subtypes is discussed.