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In excitable cells, voltage-gated calcium influx provides an effective mechanism for the activation of exocytosis. In this study, we demonstrate that although rat anterior pituitary lactotrophs, somatotrophs, and gonadotrophs exhibited spontaneous and extracellular calcium-dependent electrical activity, voltage-gated calcium influx triggered secretion only in lactotrophs and somatotrophs. The lack of action potential-driven secretion in gonadotrophs was not due to the proportion of spontaneously firing cells or spike frequency. Gonadotrophs exhibited calcium signals during prolonged depolarization comparable with signals observed in somatotrophs and lactotrophs. The secretory vesicles in all three cell types also had a similar sensitivity to voltage-gated calcium influx. However, the pattern of action potential calcium influx differed among three cell types. Spontaneous activity in gonadotrophs was characterized by high amplitude, sharp spikes that had a limited capacity to promote calcium influx, whereas lactotrophs and somatotrophs fired plateau-bursting action potentials that generated high amplitude calcium signals. Furthermore, a shift in the pattern of firing from sharp spikes to plateau-like spikes in gonadotrophs triggered luteinizing hormone secretion. These results indicate that the cell type-specific action potential secretion coupling in pituitary cells is determined by the capacity of their plasma membrane oscillator to generate threshold calcium signals.

Pituitary cells express purinergic receptor-channels (P2XR), the activation of which by ATP is associated with the facilitation of Ca2+ influx. Pharmacological, RT-PCR, and nucleotide sequence analyses confirm the presence of a wild type P2X2aR and a spliced isoform P2X2bR, which lacks a portion of carboxyl terminal amino acids. Wild type and spliced isoform receptors have a similar EC50 for ATP and time-course for activation, but the spliced isoform exhibits rapid and complete desensitization, whereas the wild type channel desensitizes slowly and incompletely. Deletion and insertion studies have revealed that a 6 residue sequence located in carboxyl tail (Arg371-Pro376) is required for sustained Ca2+ influx through wild type receptors. When co-expressed, the wild type and spliced channels form functional heteropolymeric channels. The patterns of Ca2+ signaling in the majority of pituitary cells expressing ATP-gated receptor-channels are highly comparable to those observed in cells co-transfected with P2X2aR and P2X2bR. ATP-induced [Ca2+]i response in pituitary cells is partially inhibited by nifedipine, a blocker of voltage-gated L-type Ca2+ channels, suggesting that P2X2R not only drive Ca2+ into the cell, but also activate voltage-gated Ca2+ entry. Our results indicate that ATP represents a paracrine and (or) autocrine factor in the regulation of Ca2+ signaling, and that its actions are mediated in part by heteropolymeric P2X2R.

Gonadotropin-releasing hormone (GnRH) receptors are expressed in hypothalamic tissues from adult rats, cultured fetal hypothalamic cells, and immortalized GnRH-secreting neurons (GT1 cells). Their activation by GnRH agonists leads to an overall increase in the extracellular Ca2+-dependent pulsatile release of GnRH. Electrophysiological studies showed that GT1 cells exhibit spontaneous, extracellular Ca2+-dependent action potentials, and that their inward currents include Na+, T-type and L-type Ca2+ components. Several types of potassium channels, including apamin-sensitive Ca2+-controlled potassium (SK) channels, are also expressed in GT1 cells. Activation of GnRH receptors leads to biphasic changes in intracellular Ca2+ concentration ([Ca2+]i), with an early and extracellular Ca2+-independent peak and a sustained and extracellular Ca2+-dependent plateau phase. During the peak [Ca2+]i response, electrical activity is abolished due to transient hyperpolarization that is mediated by SK channels. This is followed by sustained depolarization and resumption of firing with increased spike frequency and duration. The agonist-induced depolarization and increased firing are independent of [Ca2+]i and are not mediated by inhibition of K+ currents, but by facilitation of a voltage-insensitive and store depletion-activated Ca2+-conducting inward current. The dual control of pacemaker activity by SK and store depletion-activated Ca2+ channels facilitates voltage-gated Ca2+ influx at elevated [Ca2+]i levels, but also protects cells from Ca2+ overload. This process accounts for the autoregulatory action of GnRH on its release from hypothalamic neurons.

Polychlorinated biphenyls (PCBs) are complex mixtures of congeners that exhibit carcinogenic and toxicant activities in a variety of mammalian tissues. Here, we studied the acute in vivo and in vitro effects of a commercially used PCB product, Aroclor 1248 (A1248), a mixture of tri-, tetra-, and pentachloro congeners. Single intraperitoneal (i.p.) or bilateral intratesticular (i.t.) injections of A1248 decreased serum androgen levels in both groups 24 h after injection. Chorionic gonadotropin-stimulated androgen production by acute testicular cultures from both groups was also reduced, and progesterone production was attenuated in cultures from i.t.-treated animals. The capacity of the postmitochondrial fractions from testes of i.t.-treated animals to convert pregnenolone to progesterone and progesterone to testosterone was reduced as well. In vitro studies revealed that a 10- to 15-min exposure of postmitochondrial testicular fractions and intact interstitial cells from normal animals to A1248 in a subnanomolar concentration range was sufficient to attenuate the conversion of pregnenolone to progesterone and progesterone to testosterone. At micromolar concentrations, A1248 added in vitro also inhibited the conversion of Delta(4)-androstendione to testosterone without affecting the viability of interstitial cells. These results indicate that A1248 down-regulates the testicular androgenesis by an acute inhibition of 3beta-hydroxysteroid dehydrogenase, 17alpha-hydroxylase/lyase, and 17beta-hydroxysteroid dehydrogenase activities.

The role of ATP as a positive feedback element in Ca2+ signaling and secretion was examined in female rat pituitary gonadotrophs. ATP and ADP, but not AMP or adenosine, induced a dose- and extracellular Ca2+-dependent rise in [Ca2+]i in identified gonadotrophs in a Mg2+- and suramin-sensitive manner. ATP, adenosine-5'-O-(3-thiotriphosphate), adenosine-5'-O-(1-thiotriphosphate), 2-methylthio-ATP, and 3'-O-(4-benzoyl)benzoyl-ATP were roughly equipotent in rising [Ca2+]i in gonadotrophs, while ADP was effective only at submillimolar concentration range, and none of these compounds permeabilized the cells. On the other hand, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, and UTP were unable to induce any rise in [Ca2+]i. This pharmacological profile is consistent with expression of P2X2 and/or P2X5 purinergic receptor channels. Patch-clamp experiments showed that ATP induced an inward depolarizing current in gonadotrophs clamped at -90 mV, associated with an increase in [Ca2+]i. The ATP-induced [Ca2+]i response was partially inhibited by nifedipine, a blocker of voltage-sensitive Ca2+ channels (VSCC), but was not affected by tetrodotoxin, a blocker of voltage-sensitive Na+ channels. Thus, the P2-depolarizing current itself drives Ca2+ into the cell, but also activates Ca2+ entry through VSCC. In accord with this, low [ATP] induced plasma membrane-dependent [Ca2+]i oscillations in quiescent cells, and increased the frequency of spiking in spontaneously active cells. ATP-induced Ca2+ influx also affected agonist-induced and InsP3-dependent [Ca2+]i oscillations by increasing the frequency, base line, and duration of Ca2+ spiking. In addition, ATP stimulated gonadotropin secretion and enhanced agonist-induced gonadotropin release. ATP was found to be secreted by pituitary cells during agonist stimulation and was promptly degraded by ectonucleotidase to adenosine. These observations indicate that ATP represents a paracrine/autocrine factor in the regulation of Ca2+ signaling and secretion in gonadotrophs, and that these actions are mediated by P2 receptor channels.

To examine the role of calcium signaling in the joint modulation of gonadotropin-releasing hormone (GnRH) secretion by gamma-aminobutyric acid (GABA) and glutamate, cytoplasmic calcium ([Ca2+]i) responses to the two transmitters were analyzed in monolayer networks of the GT1-7 line of immortalized GnRH neurons. [Ca2+]i was increased by GABA and the GABAA receptor agonist, muscimol, and these responses were inhibited by the GABAA receptor antagonist, bicuculline. In contrast, the GABAB receptor agonist, baclofen, and the GABAB receptor antagonist, phaclofen, had no effect on basal and GABA- and glutamate-induced Ca2+ levels in GT1-7 neurons. The GABA- and muscimol-induced responses consisted of a spike increase in [Ca2+]i followed by a decrease to a plateau; both the increase and the subsequent decrease in [Ca2+]i depended on agonist concentration. Glutamate, N-methyl-D-aspartate (NMDA), and kainate also increased [Ca2+]i, but were less effective than GABA. GABA-, glutamate-, NMDA-, and kainate-induced [Ca2+]i responses were almost abolished in Ca(2+)-free medium and were markedly attenuated by nifedipine. The [Ca2+]i response to GABA was unaffected by prior application of glutamate, and vice versa. This additive effect of glutamate on the GABA-induced [Ca2+]i response was mimicked by prior or simultaneous application of low (10 mM) KCl. The [Ca2+]i response to simultaneous application of GABA and glutamate was also equal to the sum of the individual responses, whereas the GnRH secretory response was larger. However, the secretory responses to GABA and glutamate applied individually or together, were markedly attenuated by nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanisms by which the generation and frequency of cytoplasmic Ca2+ oscillations are controlled were investigated in pituitary gonadotrophs. In these cells, two Ca(2+)-mobilizing receptors, the gonadotropin-releasing hormone and endothelin receptors, induce frequency-modulated Ca2+ spiking at the rate of up to 30 min-1. The cytoplasmic oscillator is also activated by discharge of luminal Ca2+ (initiated by ionomycin, thapsigargin, or thimerosal) but not by increased voltage-sensitive Ca2+ influx or treatment with caffeine. The basic difference between these two types of Ca2+ oscillations is related to their requirement for inositol-1,4,5-triphosphate (InsP3). Thapsigargin-, thimerosal-, and ionomycin-induced spiking occurs without the rise in InsP3 production that is essential for the generation of receptor-controlled oscillatory responses. The differential requirement for InsP3 in the two types of Ca2+ spiking is indicated by two lines of evidence. First, agonist-induced Ca2+ spiking of frequency similar to that of non-receptor-mediated oscillations was accompanied by a significant increase in InsP3, whereas none of the non-receptor-mediated oscillations was associated with measurable changes in inositol phosphate production. Second, agonist-induced InsP3 formation and Ca2+ spiking were abolished by treatment with the phospholipase C inhibitors U73122 and neomycin sulfate, whereas non-receptor-mediated Ca2+ spiking was not affected by these agents. When the oscillator was activated by agents that do not increase InsP3 formation, it operated only at the basal rate of approximately 5 min-1 and spiking frequency did not rise with increasing drug concentrations, in contrast to the situation in agonist-stimulated gonadotrophs. However, both types of oscillations were affected by depletion of luminal Ca2+ and by changes in the intracellular Ca2+ concentration ([Ca2+]i) but were not inhibited by ryanodine. These findings are consistent with the operation of a single-pool Ca2+ oscillator that is responsible for generation of both types of Ca2+ oscillations. The oscillator is controlled by the coagonist actions of InsP3 and Ca2+ on the InsP3 receptor channels and by the activation of Ca(2+)-ATPase by rising [Ca2+]i. It can be induced to operate at low frequency without an increase in InsP3 production by agents that reduce intraluminal [Ca2+]i, and it exhibits a dose-dependent increase in spiking frequency during agonist stimulation.

Glutamate and its receptors are present in the hypothalamus and have been proposed to participate in neuroendocrine regulation, including the control of GnRH secretion. To address the mechanism of glutamate action, we measured [Ca2+]i, inositol phosphate, and secretory responses to glutamate receptor subtype agonists and antagonists in the immortalized GT1-7 cell line of GnRH-secreting hypothalamic neurons. Glutamate, N-methyl-D-aspartate (NMDA), kainate, and trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid increased GnRH secretion. In monolayer cultures of GT1-7 cells, L- but not D-glutamate induced a moderate, concentration-dependent rise in [Ca2+]i. The action of glutamate on [Ca2+]i was mimicked by NMDA, alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA), and kainate. Responses to NMDA were potentiated by the coagonist, glycine, and were inhibited by an antagonist of the glycine site on the NMDA receptor, 5,7-dichlorokynurenic acid (DCKA). NMDA-induced [Ca2+]i responses were also inhibited by Mg2+ and by the NMDA receptor antagonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK-801), but not by the AMPA/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, responses to AMPA and kainate were inhibited by CNQX but not by Mg2+, DCKA, or MK-801. Responses to glutamate were more inhibited by MK-801 plus CNQX than by either antagonist alone. All [Ca2+]i responses were nearly abolished in Ca(2+)-free solution. None of the agonists stimulated inositol phosphate formation.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of membrane potential (Vm) on cytoplasmic calcium ([Ca2+]i) oscillations during the sustained extracellular Ca(2+)-dependent phase of the Ca2+ signaling response to gonadotropin-releasing hormone (GnRH) was analyzed in cultured pituitary gonadotrophs. In agonist- and inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3)-stimulated cells, sustained [Ca2+]i oscillations were extinguished by hyperpolarization after 3-15 min despite the availability of Ca2+ in the extracellular medium. Single depolarizing pulses transiently restored the amplitude of the sustained spiking in a dihydropyridine- and extracellular Ca(2+)-sensitive manner. The responses to depolarization showed a marked dependence on Vm that was correlated with the steady-state inward Ca2+ current. In addition, repetitive application of brief depolarizing pulses modulated the frequency of agonist- and Ins(1,4,5)P3-controlled spiking; depolarization pulses at frequencies lower than the intrinsic rate of episodic Ca2+ release triggered large transients between the autonomous spikes, whereas higher frequencies of depolarizing pulses overcame the original Ca2+ spiking frequency. These extrinsically driven and extracellular Ca(2+)-dependent oscillations were sensitive to the Ca(2+)-ATPase blocker, thapsigargin, but not to ryanodine. On the other hand, spontaneous firing and application of depolarizing pulses to nonstimulated cells failed to induce thapsigargin-sensitive oscillations. These findings demonstrate that the pattern of Ca2+ signaling in gonadotrophs does not depend exclusively on the Ins(1,4,5)P3 concentration, but also on the excitable status of the cell. Such modulation of the Ins(1,4,5)P3-controlled Ca2+ signaling system by changes in Vm could provide a mechanism for the integration of multiple inputs that utilize diverse signal transduction pathways.

It has been demonstrated that endothelins (ETs) induce LH secretory responses in cultured rat pituitary cells. Because estradiol and progesterone are known to be potent modulators of GnRH-induced gonadotropin secretion, we examined whether these steroids also influence the secretory responses of gonadotrophs to ETs. Cultured female rat pituitary cells were treated for 48 h with vehicle (0.2% ethanol), 1 nM estradiol alone, or a combination of 1 nM estradiol and 100 nM progesterone or for 48 h with 1 nM estradiol and a further 4 h with 100 nM progesterone and subsequently stimulated with 10 pM-100 nM ET-1 or ET-3. Forty-eight-hour estradiol treatment led to enhanced LH secretory responses to both ETs. This action was facilitated by short term progesterone treatment (3-fold vs. vehicle), while long term progesterone treatment was inhibitory. Perifusion experiments were performed to study the kinetics of individual and pulsatile LH secretory responses after steroid exposure of pituitary cells. ET-1 induced immediate biphasic LH responses that were augmented by long term estradiol treatment. Two-hour progesterone exposure led to marked increases in LH secretion, whereas 48-h progesterone treatment was inhibitory. Estradiol and progesterone were able to modulate both the initial spike and the secondary plateau phase of the secretory profile in response to ET-1, although these actions did not always reach statistical significance. The steroid treatment paradigms employed also induced inhibitory and stimulatory effects in cells that were stimulated with ET-3 in a pulsatile fashion. In these experiments it could be demonstrated that the facilitatory action of progesterone was present after 50 min of treatment and was maximum after 150 min (5-fold enhancement). The present data support the hypothesis that ETs are involved in the physiological regulation of gonadotropin secretion and demonstrate that ovarian steroids can act as potent modulators of ET-induced LH secretion.