Gonadotropin-releasing hormone-induced Ca2+ transients in single identified gonadotropes require both intracellular Ca2+ mobilization and Ca2+ influx

Gonadotropin-releasing hormone inhibits ether-à-go-go-related gene K+ currents in mouse gonadotropes

Secretion of LH from gonadotropes is initiated by a GnRH-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) is the result of Ca(2+) release from intracellular stores and Ca(2+) influx through voltage-dependent Ca(2+) channels. Here we describe an ether-à-go-go-related gene (erg) K(+) current in primary mouse gonadotropes and its possible function in the control of Ca(2+) influx. To detect gonadotropes, we used a knock-in mouse strain, in which GnRH receptor-expressing cells are fluorescently labeled. Erg K(+) currents were recorded in 80-90% of gonadotropes. Blockage of erg currents by E-4031 depolarized the resting potential by 5-8 mV and led to an increase in [Ca(2+)](i), which was abolished by nifedipine. GnRH inhibited erg currents by a reduction of the maximal erg current and in some cells additionally by a shift of the activation curve to more positive potentials. In conclusion, the erg current contributes to the maintenance of the resting potential in gonadotropes, thereby securing a low [Ca(2+)](i) by restricting Ca(2+) influx. In addition, the erg channels are modulated by GnRH by an as-yet unknown signal cascade.

Changes in expression of hypothalamic releasing hormone receptors in individual rat anterior pituitary cells during maturation, puberty and senescence

Anterior pituitary (AP) is formed by five different cell types, each one producing a different AP hormone whose secretion is regulated by a specific hypothalamic-releasing hormone (HRH). On the other hand, a significant number of AP cells express multiple HRH receptors (multiresponsive cells). Plastic changes in expression of HRH receptors in individual AP cells are involved in critical endocrine events. Here we have characterized the changes in functional responses to CRH, LHRH, TRH, and GHRH in individual AP cells throughout the whole life span of the rat. To this end, calcium responses to the HRHs were followed by single-cell imaging in freshly dispersed AP cells prepared from rats of different ages (0-540 postnatal days). Three different cell pools were identified: 1) monoresponsive cells, holding a single class of HRH receptor; 2) multiresponsive cells; and 3) nonresponsive cells. The relative abundance of each pool changed with age. Nonresponsive cells were abundant at birth, multiresponsive cells were abundant at puberty, and monoresponsive cells dominated at senescence. The relative abundance of each HRH receptor changed largely with age but not gender. In addition, the contribution of monoresponsive and multiresponsive cells to responses to each HRH changed very much with age. Thus, the anterior pituitary shows large changes in cell populations typed by functional responses to HRHs during maturation, puberty, and senescence.

Phenotypic characterization of multi-functional somatotropes, mammotropes and gonadotropes of the mouse anterior pituitary

The existence of bihormonal anterior pituitary (AP) cells co-storing growth hormone and either prolactin (mammosomatotrope) or gonadotropins (somatogonadotrope) has been described. These cells have been proposed to be involved in "paradoxical" secretion [secretion of an AP hormone induced by a non-related hypothalamic releasing factor (HRH) and transdifferentiation (a phenotypic switch between different cell types without cell division]. Here we combine calcium imaging (to assess HRH responsiveness) and multiple sequential immunoassay of the six AP hormones to perform a single-cell phenotypic study of multifunctional somatotropes, mammotropes and gonadotropes in the normal male and female mouse pituitaries. AP cell phenotypes differed from the classic view, showing multiple HRH-receptor expression and/or hormone storage. Mammosomatotropes represented only 5-6% of somatotropes and were poorly responsive to HRHs, suggesting that their contribution to paradoxical secretion should be very limited. Somatogonadotropes were present only in females and contained adrenocorticotropic hormone. They responded to growth hormone-releasing hormone but failed to respond to gonadotropin-releasing hormone (LHRH). Other polyhormonal cells identified include (1) gonadocorticotropes, restricted to females, where they make up more than 50% of all the gonadotropes and contain other AP hormones; (2) gonadomammotropes, which are present preferentially in female cells and respond to LHRH; and (3) gonadothyrotropes, which are present similarly in male and female pituitaries.

Anterior pituitary thyrotropes are multifunctional cells

Anterior pituitary (AP) contains some unorthodox multifunctional cells that store and secrete two different AP hormones (polyhormonal cells) and/or respond to several hypothalamic-releasing hormones (HRHs; multiresponsive cells). Multifunctional cells may be involved in paradoxical secretion (secretion of a given AP hormone evoked by a noncorresponding HRH) and transdifferentiation (phenotypic switch between different mature cell types without cell division). Here we combine calcium imaging (to assess responses to the four HRHs) and multiple sequential immunoassay of the six AP hormones to perform a single-cell phenotypic study of thyrotropes in normal male and female mice. Surprisingly, most of the thyrotropes were polyhormonal, containing, in addition to thyrotropin (TSH), luteinizing hormone (40-42%) and prolactin (19-21%). Thyrotropes costoring growth hormone and/or ACTH were found only in females (24% of each type). These results suggest that costorage of the different hormones does not happen at random and that gender favors certain hormone combinations. Our results indicate that thyrotropes are a mosaic of cell phenotypes rather than a single cell type. The striking promiscuity of TSH storage should originate considerable mix-up of AP hormone secretions on stimulation of thyrotropes. However, response to thyrotropin-releasing hormone was much weaker in the polyhormonal thyrotropes than in the monohormonal ones. This would limit the appearance of paradoxical secretion under physiological conditions and suggests that timing of hormone and HRH receptor expression during the transdifferentiation process is finely and differentially regulated.

Recovery of Ins(1,4,5)-trisphosphate-dependent calcium signaling in neonatal gonadotrophs

Pituitary gonadotrophs express non-desensitizing gonadotropin-releasing hormone (GnRH) receptors and their activations leads to inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ mobilization. When added in physiological concentration range GnRH induces baseline Ca2+ oscillations, whereas in higher concentrations it induces a prolonged spike response accompanied with non-oscillatory or oscillatory plateau response. Here, we studied the recovery of calcium signaling during repetitive stimulation with short (10-30 s) GnRH pulses and variable interpulse intervals in neonatal gonadotrophs perfused with Ca2+/Na+ -containing, Ca2+ -deficient/Na+ -containing, and Ca2+ -containing/Na+ -deficient media. In Ca2+/Na+ -containing medium, baseline Ca2+ oscillations recovered without refractory period and with a time constant of approximately 20 s, whereas the recovery of spike response occurred after 25-35 s refractory period and with a time constant of approximately 30 s. During repetitive GnRH stimulation, removal of Ca2+ had only a minor effect on baseline oscillations but abolished spike response, whereas removal of Na+ slightly extended duration of baseline oscillations and considerably prolonged spike response. These results indicate that two calcium handling mechanisms are operative in gonadotrophs: redistribution of calcium within InsP3-sensitive and -insensitive pools and a sodium-dependent calcium efflux followed by calcium influx. Redistribution of Ca2+ within the cell leads to rapid recovery of InsP3-dependent pool, whereas the Na+ -dependent Ca2+ efflux pathway is activated by spike response and limits the time of exposure to elevated cytosolic Ca2+ concentrations.

Multifunctional cells of mouse anterior pituitary reveal a striking sexual dimorphism

The existence of cells storing and secreting two different anterior pituitary (AP) hormones (polyhormonal cells) or responding to several hypothalamic releasing hormones (HRHs) (multiresponsive cells) has been reported previously. These multifunctional cells could be involved in paradoxical secretion (AP hormone secretion evoked by a non-corresponding HRH) and transdifferentiation (phenotypic switch between mature cell types without cell division). Despite their putative physiological relevance, a comprehensive characterization of multifunctional AP cells is lacking. Here we combine calcium imaging (to assess responses to the four HRHs) and multiple sequential immunoassay of the six AP hormones in the same individual cells to perform a complete phenotypic characterization of mouse AP cells. Polyhormonal and multiresponsive cells were identified within all five AP cell types. They were scarce in the more abundant cell types, somatotropes and lactotropes, but quite frequent in corticotropes and gonadotropes. Cells with mixed phenotypes were the rule rather than the exception in thyrotropes, where 56-83 % of the cells stored two to five different hormones. Multifunctional AP cells were much more abundant in females than in males, indicating that the hormonal changes associated with the sexual cycle may promote transdifferentiation. As the phenotypic analysis was performed here after stimulation with HRHs, the fraction of polyhormonal cells might have been underestimated. With this limitation, the polyhormonal cells detected here responded to the HRHs less than the monohormonal ones, suggesting that they might contribute less than expected a priori to paradoxical secretion. Overall, our results reveal a striking sexual dimorphism, the female pituitary being much more plastic than the male pituitary.

The regulation of gonadotropin-releasing hormone-induced calcium signals in male rat gonadotrophs by testosterone is mediated by dihydrotestosterone

The biological effects of testosterone (T) may be mediated directly by T or indirectly by its metabolites, dihydrotestosterone (DHT) and estradiol. The present study examined whether the metabolism of T is involved in the regulation of GnRH-induced Ca2+ signaling at the pituitary. In gonadotrophs from castrated rats, a significantly greater percentage of gonadotrophs demonstrated oscillatory Ca2+ responses to 100 nM GnRH than cells from intact rats (72% vs. 24%; P < 0.05). This increase was prevented by the administration of T propionate (0.1 mg/kg x day), DHT benzoate (2 mg/kg x day,), estradiol benzoate (EB; 5 microg/kg x day), or the combination of the above doses of DHT benzoate and EB. In all cases the proportion of gonadotrophs from the steroid-treated rats having oscillatory Ca2+ responses to 100 nM GnRH was between 21-25% (P > 0.05, compared with intact rats). To assess the importance of T metabolism, intact male rats were treated with the aromatase inhibitor letrozole (1 mg/kg x day), the 5alpha-reductase inhibitor finasteride (50 mg/kg x day), or their respective vehicles for 7 days. Letrozole had no effect on GnRH-induced Ca2+ signals, serum LH concentrations, or ventral prostate or testes weight. Finasteride treatment, however, mimicked the effects of castration, with significantly more gonadotrophs exhibiting Ca2+ oscillations in response to 100 nM GnRH than gonadotrophs from the vehicle-treated group (71% vs. 20% respectively; P < 0.05). Finasteride also caused a significant (P < 0.05) decrease in prostatic weight and DHT concentration, but had no significant effect on either prostatic T or serum LH concentrations. These findings suggest that in the intact male rat, the effects of T on GnRH-induced Ca2+ signaling are preferentially mediated via DHT. The results of this study also show that in the absence of androgens, estradiol may regulate GnRH-induced Ca2+ signaling in the male rat pituitary.

Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C

Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.

Multi-responsiveness of single anterior pituitary cells to hypothalamic-releasing hormones: a cellular basis for paradoxical secretion

The classic view for hypothalamic regulation of anterior pituitary (AP) hormone secretion holds that release of each AP hormone is controlled specifically by a corresponding hypothalamic-releasing hormone (HRH). In this scenario, binding of a given HRH (thyrotropin-, growth hormone-, corticotropin-, and luteinizing hormone-releasing hormones) to specific receptors in its target cell increases the concentration of cytosolic Ca2+ ([Ca2+]i), thereby selectively stimulating the release of the appropriate hormone. However, "paradoxical" responses of AP cells to the four well-established HRHs have been observed repeatedly with both in vivo and in vitro systems, raising the possibility of functional overlap between the different AP cell types. To explore this possibility, we evaluated the effects of HRHs on [Ca2+]i in single AP cells identified immunocytochemically by the hormone they stored. We found that each of the five major AP cell types contained discrete subpopulations that were able to respond to several HRHs. The relative abundance of these multi-responsive cells was 59% for lactotropes, 33% for thyrotropes, and in the range of 47-55% for gonadotropes, corticotropes, and somatotropes. Analysis of prolactin release from single living cells revealed that each of the four HRHs tested were able to induce hormone release from a discrete lactotrope subpopulation, the size of which corresponded closely to that in which [Ca2+]i changes were induced by the same secretagogues. When viewed as a whole, our diverse functional measurements of multi-responsiveness suggest that hypothalamic control of pituitary function is more complicated than previously envisioned. Moreover, they provide a cellular basis for the so-called "paradoxical" behavior of pituitary cells to hypothalamic hypophysiotropic agents.

Sensing and refilling calcium stores in an excitable cell

Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ mobilization leads to depletion of the endoplasmic reticulum (ER) and an increase in Ca2+ entry. We show here for the gonadotroph, an excitable endocrine cell, that sensing of ER Ca2+ content can occur without the Ca2+ release-activated Ca2+ current (Icrac), but rather through the coupling of IP3-induced Ca2+ oscillations to plasma membrane voltage spikes that gate Ca2+ entry. Thus we demonstrate that capacitative Ca2+ entry is accomplished through Ca(2+)-controlled Ca2+ entry. We develop a comprehensive model, with parameter values constrained by available experimental data, to simulate the spatiotemporal behavior of agonist-induced Ca2+ signals in both the cytosol and ER lumen of gonadotrophs. The model combines two previously developed models, one for ER-mediated Ca2+ oscillations and another for plasma membrane potential-driven Ca2+ oscillations. Simulations show agreement with existing experimental records of store content, cytosolic Ca2+ concentration ([Ca2+]i), and electrical activity, and make a variety of new, experimentally testable predictions. In particular, computations with the model suggest that [Ca2+]i in the vicinity of the plasma membrane acts as a messenger for ER content via Ca(2+)-activated K+ channels and Ca2+ pumps in the plasma membrane. We conclude that, in excitable cells that do not express Icrac, [Ca2+]i profiles provide a sensitive mechanism for regulating net calcium flux through the plasma membrane during both store depletion and refilling.

Modulation of the kinetics of inositol 1,4,5-trisphosphate-induced [Ca2+]i oscillations by calcium entry in pituitary gonadotrophs

Inositol 1,4,5-trisphosphate (InsP3) binds to its receptor channels and causes liberation of Ca2+ from intracellular stores, frequently in an oscillatory manner. In addition to InsP3, the activation and inactivation properties of these intracellular channels are controlled by Ca2+. We studied the influence of Ca2+ entry on the kinetics of InsP3-triggered oscillations in cytosolic calcium ([Ca2+]i) in gonadotrophs stimulated with gonadotropin-releasing hormone, an agonist that activates InsP3 production. The natural expression of voltage-gated Ca2+ channels (VGCC) in these cells was employed to manipulate Ca2+ entry by voltage clamping the cells at different membrane potentials (Vm). Under physiological conditions, the frequency of the GnRH-induced oscillations increased with time, while the amplitude decreased, until both reached stable values. However, in cells with Vm held at -50 mV or lower, both parameters progressively decreased until the signal was abolished. These effects were reverted by a depolarization of the membrane positive to -45 mV in both agonist- and InsP3-stimulated gonadotrophs. Depolarization also led to an increase in the fraction of time during which the [Ca2+]i remained elevated; this effect originated from both an increase in the mean duration of spikes and a decrease in the interval between spikes. The frequency and amplitude of spiking depended on the activity of VGCC, but displayed different temporal courses and voltage relationships. The depolarization-driven recovery of the frequency was instantaneous, whereas the recovery of the amplitude of spiking was more gradual. The midpoints of the Vm sensitivity curve for amplitude and duration of spiking (-15 mV) were close to the value observed for L-type Ca2+ current and for depolarization-induced increase in [Ca2+]i, whereas this parameter was much lower (-35 mV) for interval between spikes and frequency of oscillations. These observations are compatible with at least two distinct effects of Ca2+ entry on the sustained [Ca2+]i oscillations. Calcium influx facilitates its liberation from intracellular stores by a direct and instantaneous action on the release mechanism. It also magnifies the Ca2+ signal and decreases the frequency because of its gradual effect on the reloading of intracellular stores.

Spontaneous and agonist-induced calcium oscillations in single human nonfunctioning adenoma cells

The effects of gonadotropin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) on cytosolic free calcium concentration ([Ca(2+)](i)) were investigated in 20 human nonfunctioning pituitary adenomas. We divided these tumors into three classes according to their response pattern to hypothalamic peptides. In type I adenomas (8 out of 20 adenomas), GnRH and GAP mobilized intracellular calcium ions stored in a thapsigargin (TG)-sensitive store. For the same concentration of agonist, two distinct patterns of GnRH-GAP-induced Ca(2+) mobilization were observed (1) sinusoidal oscillations, and (2) monophasic transient. The latter is followed by a protein kinase C (PKC)-dependent increase in calcium influx through L-type channels. In type II adenomas (7 out of 20 adenomas), GnRH and GAP only stimulate calcium influx through dihydropyridine-sensitive Ca(2+) channels by a PKC-dependent mechanism. TG (1 μM) did not affect [Ca(2+)](i) in these cells, suggesting that they do not possess TG-sensitive Ca(2+) pools. All the effects of GnRH and GAP were blocked by an inhibitor of phospholipase C (PLC), suggesting that they were owing to the activation of the phosphoinositide turnover. Type I and type II adenoma cells showed spontaneous Ca(2+) oscillations that were blocked by dihydropyridines and inhibition of PKC activity. GnRH and GAP had no effect on the [Ca(2+)](i) of type III adenoma cells that were also characterized by a low resting [Ca(2+)](i) and by the absence of spontaneous Ca(2+) fluctuations. K(+)-induced depolarization provoked a reduced Ca(2+) influx, whereas TG had no effect on the [Ca(2+)](i) of type III adenoma cells. The variety of [Ca(2+)](i) response patterns makes these cells a good cell model for studying calcium homeostasis in pituitary cells.

GnRH-induced cytosolic calcium oscillations in pituitary gonadotrophs: phase resetting by membrane depolarization

Cultured rat pituitary gonadotrophs under whole-cell voltage clamp conditions respond to the hypothalamic hormone GnRH with synchronized oscillatory changes in both cytosolic Ca2+ concentration ([Ca2+]i) and [Ca2+]i-activated, apamin-sensitive K+ current (IK(Ca)). We found, and report here for the first time, that in GnRH-stimulated cells a brief depolarizing pulse can elicit a transient [Ca2+]i rise similar to the endogenous cycle. Furthermore, Ca2+ entry during a single depolarizing pulse was found to shift the phase of subsequent endogenous [Ca2+]i oscillations, which thereafter continue to occur at their previous frequency before the pulse. Application of two consecutive depolarizing pulses showed that the size of the [Ca2+]i rise evoked by the second pulse depended on the time lapsed between two consecutive pulses, indicating that each endogenous or evoked [Ca2+]i rise cycle leaves the Ca2+ release mechanism of the gonadotroph in a refractory state. Recovery from this condition can be described by an exponential function of the time lapsed between the pulses (time constant of ca. 1 s). We propose that the underlying mechanism in both refractoriness after endogenous cycles and phase resetting by a brief pulse of Ca2+ entry involves the InsP3 receptor-channel molecule presumed to be located on the cytosolic aspect of the endoplasmic reticulum membrane.

Calcium homeostasis in identified rat gonadotrophs

1. Whole-cell voltage clamp was used in conjunction with the fluorescent Ca2+ indicator indo-1 to measure extracellular Ca2+ entry and intracellular Ca2+ concentrations ([Ca2+]i) in rat gonadotrophs identified with the reverse haemolytic plaque assay. 2. Depolarizations to potentials more positive than -40 mV elicited inward Ca2+ current (ICa) and transient elevations of [Ca2+]i. 3. The relationship between [Ca2+]i elevations and Ca2+ entry with different Ca2+ buffer concentrations in the pipette showed that endogenous Ca2+ buffers normally bind approximately 99% of the Ca2+ entering the cell. 4. With [Ca2+]i elevations less than 500 nM, decay of [Ca2+]i could be approximated by an exponential whose time constant increased with the concentration of exogenous Ca2+ buffers. 5. Inhibitors of intracellular Ca(2+)-ATPases, thapsigargin, cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ), caused [Ca2+]i to rise. Application of BHQ during [Ca2+]i oscillations induced by gonadotrophin-releasing hormone (GnRH) terminated the oscillation in a slowly decaying elevation. BHQ slowed the decay of depolarization-induced [Ca2+]i elevations about 3-fold. 6. Taking into account the Ca2+ buffering properties of the cytoplasm permitted estimation of the fluxes and rate constants for Ca2+ movements in gonadotrophs. The intracellular store is a major determinant of Ca2+ homeostasis in gonadotrophs.

Ca2+ oscillations in neutrophils triggered by immune complexes result from Ca2+ influx

Although the mechanisms of Ca2+ signalling in neutrophils by certain chemotactic agents have been well characterized, the signalling by immune complexes is poorly understood. Here we demonstrate that immune complex stimulation, acting via Fc receptors, leads to repetitive Ca2+ spiking in neutrophils. Although the initial Ca2+ rise was the result of release of Ca2+ from intracellular stores, subsequent repetitive Ca2+ spikes resulted from transmembrane influx, as they were prevented by removal of extracellular Ca2+ and were accompanied by Mn2+ influx. The transmembrane Ca2+ spikes induced dramatic neutrophil cell shape changes. The Ca2+ spiking phase was inhibited by a phospholipase C (PLC) inhibitor, U73122, and removal of immune complex, but not by cytochalasin B. It was concluded that Ca2+ spiking was dependent upon the initial release of Ca2+ from an intracellular Ca2+ store, and driven by continued binding of immune complex, which triggered pulsatile changes in transmembrane influx.

Calcium oscillations in pituitary gonadotrophs: comparison of experiment and theory

We have developed a mathematical model that describes several aspects of agonist-induced Ca2+ signaling in single pituitary gonadotrophs. Our model is based on fast activation of the inositol 1,4,5-trisphosphate (InsP3) receptor Ca2+ channels at low free cytosolic Ca2+ concentration ([Ca2+]i) and slow inactivation at high [Ca2+]i. Previous work has shown that these gating properties, when combined with a Ca(2+)-ATPase, are sufficient to generate simulated Ca2+ oscillations. The Hodgkin-Huxley-like description we formulate here incorporates these different gating properties explicitly and renders their effects transparent and easy to modulate. We introduce regulatory mechanisms of channel opening which enable the model, both in the absence and in the presence of Ca2+ entry, to give responses to a wide range of agonist doses that are in good agreement with experimental findings, including subthreshold responses, superthreshold oscillations with frequency determined by [InsP3], and nonoscillatory "biphasic" responses followed occasionally by small-amplitude oscillations. A particular added feature of our model, enhanced channel opening by reduced concentration of Ca2+ in the lumen of the endoplasmic reticulum, allows oscillations to continue during pool depletion. The model predicts that ionomycin and thapsigargin can induce oscillations with basal [InsP3] and zero Ca2+ entry, while Ca2+ injection cannot. Responses to specific pairings of sub- or superthreshold stimuli of agonist, ionomycin, and thapsigargin are also correctly predicted. Since this model encompasses a wide range of observed dynamic behaviors within a single framework, based on well-established mechanisms, its relevance should not be restricted to gonadotrophs.

Intracellular calcium and the signaling mechanism of luteinizing hormone-releasing hormone in rat granulosa cells

OBJECTIVE

The purpose of these studies was to determine the source(s) of the increase in intracellular free calcium in response to luteinizing hormone-releasing hormone in ovarian granulosa cells.

STUDY DESIGN

Rat granulosa cells were cultured and loaded with fura-2-acetoxy-methyl ester, a fluorescent calcium indicator dye, and intracellular free calcium measured by microspectrofluorometry. The source of the luteinizing hormone-releasing hormone induced increase in intracellular Ca++ was investigated with various calcium channel blockers (verapamil, diltiazem, and nifedipine), high K+ buffer, and perifusion with media lacking Ca++.

RESULTS

All three voltage-sensitive calcium channel blockers (10(-5) mol/L) tested were ineffective in blocking the luteinizing hormone-releasing hormone induced intracellular Ca++ increase. Treatment with high K+ buffer also had no effect. Perifusion with media lacking calcium resulted in a gradual loss of the luteinizing hormone-releasing hormone response, an effect that was accelerated by repeated stimulation with hormone. Transient replacement of extracellular Ca++ failed to restore the response but continued perifusion with Ca(++)-replete media allowed a luteinizing hormone-releasing hormone response 10 minutes later.

CONCLUSIONS

The luteinizing hormone-releasing hormone induced intracellular Ca++ increase does not appear to result from the opening of voltage-sensitive or K(+)-dependent Ca++ channels. To the contrary, this response likely results from the release of Ca++, primarily from intracellular stores.

Integration of cytoplasmic calcium and membrane potential oscillations maintains calcium signaling in pituitary gonadotrophs

Pituitary gonadotrophs exhibit spontaneous low-amplitude fluctuations in cytoplasmic calcium concentration ([Ca2+]i) due to intermittent firing of nifedipine-sensitive action potentials. The hypothalamic neuropeptide, gonadotropin-releasing hormone, terminates such spontaneous [Ca2+]i transients and plasma-membrane electrical activity and initiates high-amplitude [Ca2+]i oscillations and concomitant oscillations in membrane potential (Vm). The onset of agonist-induced [Ca2+]i oscillations is not dependent on Vm or extracellular Ca2+ but is associated with plasma-membrane hyperpolarization interrupted by regular waves of depolarization with firing of action potentials at the peak of each wave. The Vm and Ca2+ oscillations are interdependent during continued gonadotropin-releasing hormone action (greater than 3-5 min), when sustained Ca2+ entry is necessary for the maintenance of [Ca2+]i spiking. The initial and sustained agonist-induced Ca2+ transients and Vm oscillations are abolished by blockade of endoplasmic reticulum Ca(2+)-ATPase, consistent with the role of Ca2+ re-uptake by internal stores in the oscillatory response during both phases. Such a pattern of synchronization of electrical activity and Ca2+ spiking in cells regulated by Ca(2+)-mobilizing receptors shows that the operation of the cytoplasmic oscillator can be integrated with a plasma-membrane oscillator to provide a long-lasting signal during sustained agonist stimulation.

Apamin-sensitive potassium channels mediate agonist-induced oscillations of membrane potential in pituitary gonadotrophs

In cultured rat pituitary gonadotrophs, gonadotropin-releasing hormone (GnRH) induces rapid hyperpolarization of the cell membrane and causes cessation of the spontaneous electrical activity present in non-stimulated cells. This initial response to GnRH is followed by slow oscillations of membrane potential (Vm) which often exhibit brief bursts of action potentials (AP) fired from the peak of the oscillations. The hyperpolarization waves are synchronous with GnRH-induced elevations of cytoplasmic Ca2+ concentration ([Ca2+]i), such that Vm maxima alternate with the peak values of [Ca2+]i. The Vm oscillations result from repetitive activation of apamin-sensitive K+ channels by cytoplasmic Ca2+. Thus, GnRH activation of Ca2+ mobilization can generate a bursting pattern of membrane potential through the activation of K+ channels against a background of spontaneous electrical activity.

GnRH-induced Ca2+ oscillations and rhythmic hyperpolarizations of pituitary gonadotropes

Secretion of gonadotropic hormones from pituitary gonadotropes in response to gonadotropin-releasing hormone (GnRH) is essential for regulation of reproductive potential. Gonadotropes from male rats exhibited an unusual form of cellular excitation that resulted from periodic membrane hyperpolarization. GnRH induced an oscillatory release of intracellular Ca2+ via a guanosine triphosphate (GTP) binding protein-coupled phosphoinositide pathway and hyperpolarized the gonadotrope periodically by opening apamin-sensitive Ca(2+)-activated K+ (SK) channels. Each hyperpolarization was terminated by firing of a few action potentials that may result from removal of inactivation from voltage-gated Na+ and Ca2+ channels.

Calcium-activated K+ channels: metabolic regulation

Calcium-activated potassium (KCa) channels are highly modulated by a large spectrum of metabolites. Neurotransmitters, hormones, lipids, and nucleotides are capable of activating and/or inhibiting KCa channels. Studies from the last few years have shown that metabolites modulate the activity of KCa channels via: (1) a change in the affinity of the channel for Ca2+ (K 1/2 is modified), (2) a parallel shift in the voltage axis of the activation curves, or (3) a change in the slope (effective valence) of the voltage dependence curve. The shift of the voltage dependence curve can be a direct consequence of the change in the affinity for Ca2+. Recently, the mechanistic steps involved in the modulation of KCa channels are being uncovered. Some interactions may be direct on KCa channels and others may be mediated via G-proteins, second messengers, or phosphorylation. The information given in this review highlights the possibility that KCa channels can be activated or inhibited by metabolites without a change in the intracellular Ca2+ concentration.

Calcium-dependent control of volume regulation in renal proximal tubule cells: II. Roles of dihydropyridine-sensitive and -insensitive Ca2+ entry pathways

The Ca2+ entry pathways in the basolateral plasma membrane of the isolated, nonperfused proximal straight tubule (PST) of rabbit kidney were investigated using fura-2 fluorescence microscopy. Under isotonic conditions, reduction of bath [Ca2+] from 1 mM to 1 microM caused intracellular free calcium concentration ([Ca2+]i) to fall close to zero. Treatment with 10 microM verapamil, a calcium channel blocker, had a similar effect. Treatment with verapamil or low Ca2+ also induced fluctuations in cell volume. However, isotonic treatment with 10 microM nifedipine, a dihydropyridine (DHP)-type calcium channel blocker, did not affect [Ca2+]i or cell volume, indicating that the endogenous Ca2+ entry pathway is verapamil-sensitive but DHP-insensitive. When cells were exposed to hypotonic solutions in the presence of 1 mM Ca2+, they swelled and underwent normal RVD while [Ca2+]i increased transiently to a peak before decreasing to a late phase plateau level above the baseline level (see McCarty, N.A., O'Neil, R.G. 1991. J. Membrane Biol. 123:149-160). When cells were swollen in the presence of verapamil or low bath [Ca2+], RVD was abolished and [Ca2+]i fell well below the baseline during the late phase response. In contrast, when cells were swollen in the presence of nifedipine, RVD and the late phase rise in [Ca2+]i were abolished, but [Ca2+]i did not fall below the baseline level in the late phase, indicating that nifedipine inhibited the swelling-induced Ca2+ entry but that Ca2+ entry by another pathway was undisturbed. It was concluded that PST cells are characterized by two Ca2+ permeability pathways in the basolateral membrane. Under both isotonic and hypotonic conditions, Ca2+ entry occurs at a slow rate via a verapamil-sensitive, DHP-insensitive "baseline" Ca2+ entry pathway. Cell swelling activates a separate DHP-sensitive, verapamil-sensitive Ca2+ entry pathway, which is responsible for the supply of Ca ions to the Ca(2+)-dependent mechanism by which cell volume regulation is achieved.

Dependence of hormone secretion on activation-inactivation kinetics of voltage-sensitive Ca2+ channels in pituitary gonadotrophs

The relationships between the activation status of voltage-sensitive Ca2+ channels and secretory responses were analyzed in perfused rat gonadotrophs during stimulation by high extracellular K+ concentration ([K+]e) or the physiological agonist, gonadotropin-releasing hormone (GnRH). Increase of [K+]e to 50 mM evokes an on-off secretory response, with a rapid rise in luteinizing hormone (LH) secretion to a peak at 35 sec (on response) followed by an exponential decrease to the steady-state level. Cessation of K+ stimulation elicits a transient (off) response followed by an exponential decrease to the basal level. The LH response to high [K+]e is nifedipine-sensitive and its amplitude depends on membrane potential. There is a close relationship between the LH secretory response to high [K+]e and the amplitude of the inward Ca2+ current measured at 100 msec in whole-cell patch clamp experiments. In addition, the profile of the LH secretory response is similar to that of the response of intracellular Ca2+ concentration ([Ca2+]i) in K(+)-stimulated cells. In Ca2(+)-deficient medium, the effect of high [K+]e is abolished; subsequent elevation of [Ca2+]e during the K+ pulse is followed by restoration of the on response, but with reduced magnitude. Agonist stimulation during the steady-state phase of the [K+]e pulse or after repetitive stimulation by high [K+]e elicited biphasic [Ca2+]i and secretory responses with a significantly reduced plateau phase; conversely, K(+)-induced LH release was reduced in cells treated with desensitizing doses of GnRH. These findings indicate that depolarization-induced changes in the status of voltage-sensitive Ca2+ channels determine the profiles of [Ca2+]i and LH responses to stimulation by high [K+]e; the initial activation of dihydropyridine-sensitive Ca2+ channels is clearly dependent on membrane potential, whereas their subsequent inactivation depends on increased [Ca2+]i. Such inactivation of voltage-sensitive Ca2+ channels also occurs during GnRH action and may represent an additional regulatory mechanism to limit the entry of extracellular Ca2+ during prolonged or frequent agonist stimulation.

Calcium stimulates luteinizing-hormone (lutropin) exocytosis by a mechanism independent of protein kinase C

Using permeabilized gonadotropes, we examined whether Ca2(+)-stimulated luteinizing-hormone (LH) exocytosis is mediated by the Ca2(+)-activated phospholipid-dependent protein kinase (protein kinase C). In the presence of high [Ca2+]free (pCa 5), alpha-toxin-permeabilized sheep gonadotropes secrete a burst of LH and then become refractory to maintained high [Ca2+]free. The protein kinase C activator phorbol myristate acetate (PMA) is able to stimulate further LH release from cells made refractory to high [Ca2+]free, suggesting that Ca2+ does not stimulate LH release by activating protein kinase C. Staurosporine, a protein kinase C inhibitor, inhibited PMA-stimulated (50% inhibition at 20 nM), but not Ca2(+)-stimulated, LH exocytosis. In cells desensitized to PMA by prolonged exposure to a high PMA concentration, Ca2(+)-stimulated LH exocytosis (when corrected for depletion of total cellular LH) was not inhibited. Ba2+ was able to stimulate LH exocytosis to a maximal extent similar to Ca2+, although higher Ba2+ concentrations were necessary. Ba2+ and Ca2+ stimulated LH exocytosis with a similar time course, and both were inhibitory at high concentrations. Furthermore, cells made refractory to Ca2+ were also refractory to Ba2+. These data strongly suggest that Ba2+ and Ca2+ act through the same mechanism. Since Ba2+ is a poor activator of protein kinase C, these findings are additional evidence against a major role for protein kinase C in mediating Ca2(+)-stimulated LH exocytosis.

Minimal model for signal-induced Ca2+ oscillations and for their frequency encoding through protein phosphorylation

In a variety of cells, hormonal or neurotransmitter signals elicit a train of intracellular Ca2+ spikes. The analysis of a minimal model based on Ca2(+)-induced Ca2+ release from intracellular stores shows how sustained oscillations of cytosolic Ca2+ may develop as a result of a rise in inositol 1,4,5-trisphosphate (InsP3) triggered by external stimulation. This rise elicits the release of a certain amount of Ca2+ from an InsP3-sensitive intracellular store. The subsequent rise in cytosolic Ca2+ in turn triggers the release of Ca2+ from a second store insensitive to InsP3. In contrast to the model proposed by Meyer and Stryer [Meyer, T. & Stryer, L. (1988) Proc. Natl. Acad. Sci. USA 85, 5051-5055], the present model, which contains only two variables, predicts the occurrence of periodic Ca2+ spikes in the absence of InsP3 oscillations. Such results indicate that repetitive Ca2+ spikes evoked by external stimuli do not necessarily require the concomitant, periodic variation of InsP3. The model is closely related to that proposed by Kuba and Takeshita [Kuba, K. & Takeshita, S. (1981) J. Theor. Biol. 93, 1009-1031] for Ca2+ oscillations in sympathetic neurones, based on Ca2(+)-induced Ca2+ release. We extend their results by showing the minimal conditions in which the latter process gives rise to periodic behavior and take into account the role of the rise in InsP3 caused by external stimulation. The analysis further shows how signal-induced Ca2+ oscillations might be effectively encoded in terms of their frequency through the phosphorylation of a cellular substrate by a protein kinase activated by cytosolic Ca2+.

Angiotensin II effects on the cytosolic free Ca2+ concentration in N1E-115 neuroblastoma cells: kinetic properties of the Ca2+ transient measured in single fura-2-loaded cells

The effect of angiotensin II on the cytosolic free Ca2+ concentration was measured in single mouse neuroblastoma N1E-115 cells loaded with fura-2. Angiotensin II induced a transient concentration-dependent increase in Ca2+ and also increased the production of inositol polyphosphates. The Ca2+ increase did not require extracellular Ca2+ and was unaffected by pretreatment with pertussis toxin. These data suggest that angiotensin II increased Ca2+ by an inositol trisphosphate-mediated release of intracellular Ca2+ following activation of phospholipase C via a pertussis toxin-insensitive guanine nucleotide binding protein. Similar results were obtained with bradykinin. The angiotensin II- or bradykinin-induced increase in Ca2+ occurred after a concentration-dependent latent period. Low concentrations of agonist elicited a small increase in Ca2+ following a variable lag that sometimes exceeded 1 min, whereas at maximally effective angiotensin II concentrations a larger, more rapid increase in Ca2+ occurred without a measurable delay. In some cells, oscillatory increases in Ca2+ were induced by angiotensin II and bradykinin. Possible mechanisms to explain the concentration dependency of the latent period and the oscillatory nature of the increases of Ca2+ are discussed. These results indicate that the mouse neuroblastoma N1E-115 cell represents a useful model for studying the signal response transduction mechanisms regulating the effects of angiotensin II in neuronal cells.

Mechanisms of luteinizing-hormone exocytosis in Staphylococcus aureus-alpha-toxin-permeabilized sheep gonadotropes

We have used primary gonadotropes permeabilized with the pore-forming protein Staphylococcus aureus alpha-toxin to investigate luteinizing hormone (lutropin, LH) exocytosis. The diameter of the alpha-toxin pores (2-3 nm) allows the exchange of small molecules, whereas larger cytosolic proteins are retained. Because of the slow exchange of small molecules through the pores, we have developed a protocol which combines prolonged pre-equilibration of the permeabilized cells at 0 degrees C before stimulation with strong Ca2+ buffering. Under these conditions, increasing the free Ca2+ concentration from 0.1 microM to 10 microM [EC50 (concentration effecting half-maximal response) 2-3 microM] resulted in a 15-20-fold increase in LH exocytosis. LH exocytosis was maximal in the first 3 min and completed by 12 min. When permeabilized cells were equilibrated for prolonged periods in the absence of MgATP, Ca2(+)-stimulated LH secretion gradually declined (greater than 90% decrease by 60 min). Addition of MgATP (5 mM) rapidly restored full Ca2(+)-stimulated LH secretion. MgATP supported Ca2(+)-stimulated LH secretion at a half-maximal concentration of 1.5 mM. UTP and adenosine 5'-[gamma-thio]triphosphate were 40 and 31% as effective as MgATP, whereas other nucleotide triphosphates were ineffective. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA; 50 nM) stimulated LH exocytosis at free Ca2+ concentrations as low as 1 nM and was additive with Ca2+ at higher free Ca2+ concentrations. PMA-stimulated exocytosis required MgATP at concentrations similar to those required for Ca2(+)-stimulated LH exocytosis. These results demonstrate that LH exocytosis can be triggered both by micromolar Ca2+ concentrations or, in the virtual absence of Ca2+, by PKC activation. Both mechanisms of stimulated exocytosis have an absolute requirement for millimolar ATP. Because they retain cytosolic proteins, alpha-toxin-permeabilized cells may have advantages over alternative permeabilization methods provided that conditions are used that compensate for slow diffusion through alpha-toxin pores.

The role of exogenous calcium for gonadotropin-stimulated progesterone production by human granulosa-luteal cells

Previous studies of cells from various species have indicated that exogenous calcium is necessary for gonadotropic stimulation of steroidogenesis. To determine whether this requirement for exogenous calcium is a universal attribute of steroidogenic cells, we studied baseline and stimulated progesterone (P) production by cultured human granulosaluteal cells obtained at the time of oocyte retrieval for in vitro fertilization (IVF). During 4 hours in culture, both cholera toxin (1.25 micrograms/mL) and human chorionic gonadotropin (hCG, 1 IU/mL) stimulated a significant (P less than 0.05) 2- to 4-times increase in P production. Both baseline and stimulated (cholera toxin or hCG) increases in P were unaffected when cellular uptake of exogenous calcium was inhibited by the calcium channel blocker nitrendipine (10 microM), or by culturing the cells in calcium-free medium or in calcium-free medium with [ethylenebis(oxyethylenenitrilo)]-tetra-acetic acid (EGTA, to chelate any possible free extracellular calcium). At later time points (24 and 48 hours), lack of available exogenous calcium began to have an inhibitory effect on P production, and the hCG effect was more sensitive to the lack of exogenous calcium than was the cholera toxin effect. We speculate that this apparent independence from exogenous calcium over a short culture period is due to the prior stimulation of these cells by exogenous gonadotropins employed in IVF cycles.

Characteristics of voltage-gated Ca2+ currents in ovine gonadotrophs

1. Voltage-clamp recordings were obtained from gonadotrophs of the ovine pars tuberalis in dissociated cell culture, utilizing the whole-cell recording mode of the patch-clamp technique. 2. The amplitudes of Ca2+ and Ba2+ currents were dependent on the extracellular concentration of divalent cation. 3. Ba2+ tail currents were observed on termination of depolarizing voltage steps. The extrapolated amplitudes of 'instantaneous' tail currents increased with membrane depolarization and showed saturation beyond +15 mV. 4. True inactivation of currents occurred in the presence of both external Ca2+ and Ba2+, judged from decrease in tail current amplitudes with progressive increases in duration of the activating voltage pulse. The inactivation process was fitted by a single-exponential function at membrane potentials below -25 mV, while at more depolarized potentials the inactivation was better described by a double-exponential function. The inactivation time constants decreased with positive shifts in membrane potential favouring a voltage-dependent inactivation. 5. The half-value of steady-state inactivation was observed at -40 mV using a two-pulse protocol. 6. Power spectral analysis of Ba2+ current noise from the steady-state portion of inward current showed a double Lorentzian fit of the power spectrum. 7. Two types of voltage-activated Ca2+ currents were identified based on their kinetics, voltage dependence, dependence on activation frequency, differential sensitivity to intracellular ATP and cyclic AMP, and to extracellular application of nifedipine. The channels with faster kinetics had a lower activation threshold (-50 mV) and the amplitude of the current was sensitive to clamping frequency. 8. From ensemble noise analysis of mean maximal inward current, single-channel amplitude of about 1 pA was estimated in 50 mM-Ba2+.