Biphasic activation of cytosolic free calcium and LH responses by gonadotropin-releasing hormone

Gonadotropin inhibitory hormone (GnIH) as a regulator of gonadotropes

Gonadotropin inhibitory hormone (GnIH) has emerged as a negative regulator of gonadotrope function in a range of species. In rodents, such as rats and mice, GnIH exerts influence upon GnRH cells within the brain. In other species, however, the peptide is secreted into hypophysial portal blood to act on pituitary gonadotropes. In particular, a series of studies in sheep have demonstrated potent actions at the level of the pituitary gland to counteract the function of GnRH in terms of the synthesis and secretion of gonadotropins. This review focuses on the action of GnIH at the level of the gonadotrope.

Mechanisms of melatonin action in the pituitary and SCN

We have compared melatonin effects in two different cell types in order to determine general intracellular mechanisms of its action. In neonatal rat pituitary, melatonin acts via the specific membrane receptors to inhibit GnRH-induced LH release. The melatonin effect disappears in adulthood due to the disappearance of the receptors. The mechanism of the melatonin action involves inhibition of the GnRH induced increase of intracellular calcium ([Ca2+])i. Our observations indicate that melatonin has dual inhibitory effect on GnRH-induced [Ca2+]i: it inhibits mobilisation of Ca2+ from endoplasmic reticulum as well as Ca2+ influx through voltage sensitive channels. Besides, melatonin also decreases basal and GnRH- or forskolin-induced increase of cAMP concentration in the pituitary. Although cAMP is not of primary importance for regulation of LH release, the cAMP decrease may participate in the mechanism of inhibitory melatonin action on LH release. Rat suprachiasmatic nuclei (SCN) have a high density of the melatonin receptors throughout the postnatal life. Cultures of dispersed SCN cells show circadian rhythm of vasopressin (AVP) release, with several fold increase in the middle of the day and decrease during night. Melatonin inhibits the spontaneous AVP release. Melatonin also inhibits the AVP release induced by vasoactive intestinal peptide (VIP). Intracellular mechanisms of the melatonin effect may involve cAMP, because melatonin inhibits the VIP-induced increase of cAMP and increase of cAMP formation by forskolin stimulates AVP release from the cultures. On the other hand, involvement of intracellular calcium in the regulation of AVP release may not be excluded. VIP induces [Ca2+]i increase in 14% of the SCN cells and AVP release is stimulated by Ca2+ ionophore ionomycin. Our observations indicate that some of the mechanisms of melatonin action are similar in the pituitary and SCN.

Cellular mechanisms of melatonin action

The pineal hormone melatonin is involved in photic regulations of various kinds, including adaptation to light intensity, daily changes of light and darkness, and seasonal changes of photoperiod lengths. The melatonin effects are mediated by the specific high-affinity receptors localized on plasma membrane and coupled to GTP-binding protein. Two different G proteins coupled to the melatonin receptors have been described, one sensitive to pertussis toxin and the other sensitive to cholera toxin. On the basis of the molecular structure, three subtypes of the melatonin receptors have been described: Mel1A, Mel1B, and Mel1C. The first two subtypes are found in mammals and may be distinguished pharmacologically using selective antagonists. Melatonin receptor regulates several second messengers: cAMP, cGMP, diacylglycerol, inositol trisphosphate, arachidonic acid, and intracellular Ca2+ concentration ([Ca2+]i). In many cases, its effect is inhibitory and requires previous activation of the cell by a stimulatory agent. Melatonin inhibits cAMP accumulation in most of the cells examined, but the indole effects on other messengers have been often observed only in one type of the cells or tissue, until now. Melatonin also regulates the transcription factors, namely, phosphorylation of cAMP-responsive element binding protein and expression of c-Fos. Molecular mechanisms of the melatonin effects are not clear but may involve at least two parallel transduction pathways, one inhibiting adenylyl cyclase and the other regulating phospholipide metabolism and [Ca2+]i.

Signal transduction of the gonadotropin releasing hormone (GnRH) receptor: cross-talk of calcium, protein kinase C (PKC), and arachidonic acid

1. The decapeptide neurohormone gonadotropin releasing hormone (GnRH) is the first key hormone of the reproductive system. Produced in the hypothalamus, GnRH is released in a pulsatile manner into the hypophysial portal system to reach the anterior pituitary and stimulates the release and synthesis of the gonadotropin hormones LH and FSH. GnRH, a Ca2+ mobilizing ligand, binds to its respective binding protein, which is a member of the seven transmembrane domain receptor family and activates a G-protein (Gq). 2. The alpha subunit of Gq triggers enhanced phosphoinositide turnover and the elevation of multiple second messengers required for gonadotropin release and biosynthesis. 3. The messenger molecules IP3, diacylglycerol, Ca2+, protein kinase C, arachidonic acid and leukotriene C4 cross-talk in a complex networks of signaling, culminating in gonadotropin release and gene expression.

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.

Simultaneous measurements of exocytosis and intracellular calcium concentration with fluorescent indicators in single pituitary gonadotropes

Previously, we established a method for the estimation of exocytosis in single gonadotropes using an impermeable fluorescent membrane probe, TMA-DPH. In this study, we have developed a method for the simultaneous measurement of exocytosis and intracellular free Ca2+ concentration ([Ca2+]i) by double-labeling with TMA-DPH and the intracellular Ca2+ probe, Fura-2/AM, using a fluorescence microscope with a 3-wavelength excitation and 2-wavelength emission system. We, therefore, clarified the relationship between spontaneous [Ca2+]i oscillation or gonadotropin releasing hormone (GnRH)-induced intracellular Ca2+ mobilization and exocytosis in gonadotropes. Under resting conditions, some gonadotropes showed various types of spontaneous [Ca2+]i oscillations, while others did not, but all showed basal exocytosis. Each [Ca2+]i peak oscillation did not cause Ca(2+)-regulated exocytosis, and even complete blockage of the [Ca2+]i increase by the intracellular Ca2+ chelator BAPTA/AM had no effect on basal exocytosis. Both GnRH-induced intracellular Ca2+ mobilization and regulated exocytosis showed a similar pattern of peaks and plateaus. Blockage of the [Ca2+]i increase by BAPTA/AM almost completely inhibited the GnRH-stimulated exocytosis. These results show that spontaneous [Ca2+]i oscillations under resting conditions are not linked to regulated or basal exocytosis, and that intracellular Ca2+ mobilization is essential for GnRH-stimulated exocytosis.

The differential effects of protein kinase C activators and inhibitors on rat anterior pituitary hormone release

We investigated the possibility that various protein kinase C (PKC) activators and inhibitors may differentially affect luteinizing hormone (LH) and growth hormone (GH) release from rat anterior pituitary tissue, incubated in vitro. Activators of PKC induced LH release with the following order of potency: mezerein > phorbol 12,13-dibutyrate (PDBu). Mezerein and PDBu were equipotent on GH release. A range of PKC inhibitors (including compounds highly selective for PKC) potently and completely inhibited PKC activator-induced LH and GH release. Chelerythrine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7) were less potent inhibitors of PDBu-induced GH release than of LH release. A component of PDBu- and mezerein-induced LH release was inhibited by H7 with high potency, but a second H7-insensitive component was detected. Mezerein- and PDBu-induced GH release consisted of an H7-resistant component only. When the regulatory domain of PKCs from different sources was investigated by displacement of [3H]PDBu binding, the affinity for mezerein was 3-5-fold greater than that for PDBu at PKCs from cerebral cortex, lung and alpha and beta isoforms extensively purified from brain. Anterior pituitary PKCs were unusual in showing closely matched affinity for mezerein and PDBu, reminiscent of their equivalent potency on GH release. In order to investigate the potency of the catalytic domain inhibitor H7 on PKCs from different sources, enzyme activity assays were carried out on partially purified cytosolic PKCs from midbrain and anterior pituitary and on extensively purified PKC alpha and PKC beta. The Ca(2+)-independent component of PDBu-induced (phosphatidylserine-dependent) activity from anterior pituitary alone showed unusually low potency of inhibition by H7 but was potently inhibited by staurosporine and Ro 31-8220. In contrast, the Ca(2+)-dependent PKC activity in anterior pituitary was inhibited by H7, staurosporine and Ro-31-8220 with high potency as in all other preparations. These results are consistent with the presence and active role in secretion of pharmacologically distinct forms of PKC (or PKC-like kinases) in rat anterior pituitary cells.

Transient but not oscillating component of the calcium mobilizing response to gonadotropin-releasing hormone depends on calcium influx in pituitary gonadotrophs

Gonadotropin-releasing hormone (GnRH)-stimulated changes in the cytosolic free Ca2+ concentration ([Ca2+]i) were studied in gonadotrophs cultured from 3-week ovariectomized rat pituitaries. One animal was used per cell preparation. [Ca2+]i was monitored in individual gonadotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short stimulation with GnRH evoked a complex concentration-dependent Ca2+ response in individual gonadotrophs. 0.1-1 nM GnRH triggered a series of sinusoidal-like [Ca2+]i oscillations superimposed upon a modest slow [Ca2+]i rise--the oscillating response mode--while 10-100 nM GnRH caused a biphasic increase in [Ca2+]i consisting of a monophasic transient and oscillations--the transient/oscillating response mode. Despite the consistency of Ca2+ responses, an inter-preparation heterogeneity of [Ca2+]i oscillations frequency was noticed. Moreover, we observed that, within a given cell preparation, the frequency of [Ca2+]i oscillations was independent of GnRH concentration whereas both peak [Ca2+]i and area under the [Ca2+]i versus time curve were concentration-dependent. Thus, in gonadotrophs, the presence of the GnRH signal would lead to [Ca2+]i oscillations, while the amplitude of the [Ca2+]i responses would code for the concentration of agonist. Both transient and oscillating components of GnRH responses depended on releasing activity of Ca(2+)-sequestering pools in as much as GnRH responses were unaffected by brief removal of external Ca2+, but suppressed by chelating intracellular free Ca2+ with BAPTA. However, prolonged exposure to a Ca(2+)-free medium suppressed the transient component while leaving the oscillating component unaffected. We therefore propose that gonadotrophs employ Ca(2+)-sequestering pools, whose maintenance depends on a slow Ca(2+)-entry, to give an amplitude-coded Ca2+ rise in response to a short GnRH stimulation.

Calcium signaling and secretory responses in agonist-stimulated pituitary gonadotrophs

In cultured pituitary gonadotrophs, gonadotropin-releasing hormone (GnRH) caused dose-dependent and biphasic increases in cytoplasmic calcium concentration ([Ca2+]i) and LH release. Both extra- and intracellular calcium pools participate in GnRH-induced elevation of [Ca2+]i and LH secretion. The spike phase of the [Ca2+]i response represents the primary signal derived predominantly from the rapid mobilization of intracellular Ca2+. In contrast, the prolonged phase of the Ca2+ signal depends exclusively on Ca2+ entry from the extracellular pool. The influx of Ca2+ occurs partially through dihydropyridine-sensitive calcium channels. Both [Ca2+]i and LH responses to increasing concentrations of GnRH occur over very similar time scales, suggesting that increasing degrees of receptor occupancy are transduced into amplitude-modulated Ca2+ responses, which in turn activate exocytosis in a linear manner. However, several lines of evidence indicated the complexity over the relationship between Ca2+ signaling and LH exocytosis. In contrast to [Ca2+]i measurements in cell suspension, single cell Ca2+ measurements revealed the existence of a more complicated pattern of Ca2+ response to GnRH, with a biphasic response to high agonist doses and prominent oscillatory responses to lower GnRH concentrations, with a log-linear correlation between GnRH dose and the frequency of Ca2+ spiking. In addition, analysis of the magnitudes of the [Ca2+]i and LH responses of gonadotrophs to a wide range of GnRH concentrations in the presence and absence of extracellular Ca2+, and to K+ and phorbol ester stimulation, showed non-linearity between these parameters with amplification of [Ca2+]i-mediated exocytosis. Studies on cell depleted of protein kinase C under conditions that did not change the LH pool suggested the participation of protein kinase C in this amplification, especially during the plateau phase of the secretory response to GnRH.

Age-related changes in the mechanisms of LHRH-stimulated LH release from pituitary cells in vitro

In vitro release of LH in response to LHRH, phorbol myristate acetate (PMA), the ionophore A23187, and nifedipine was evaluated in primary cultures of anterior pituitary cells from intact mature (6 to 7 month) and old (23 to 24 month) male Wistar rats. LH release from pituitary cells is reduced approximately 30% and 60% after 4 and 48 h of 10(-7) M LHRH stimulation in cells of old rats, respectively. This impairment may be secondary to a loss of LHRH receptors. LHRH-stimulated LH release from cells of mature rats was inhibited 70% by the voltage-gated calcium channel blocker, nifedipine (10(-6) M), whereas LHRH-stimulated LH release from cells of old rats was too low to detect the effects of this drug. Age changes can be partially reversed by A23187 and PMA during 4 h, but not 48 hrs of stimulation. It therefore appears that short- and long-term (4 h and 48 h, respectively) stimulation of LH release may proceed through separate mechanisms that are differentially affected by aging.

Nitrendipine and omega-conotoxin modulate gonadotropin release and gonadotrope [Ca2+]i

We have examined the pharmacology of the voltage-sensitive Ca2+ channels (VSCCs) that mediate gonadotropin secretion from primary cultures of rat pituitary cells, stimulated by either cell depolarization or by binding of gonadotropin-releasing hormone (GnRH). We also measured single-cell [Ca2+]i transients using fura-2 in gonadotropes identified by a reverse hemolytic plaque assay employing an antiserum to luteinizing hormone (LH). Cell depolarization evoked by either 50 mM K+ or 30 microM veratridine induced 2- to 6-fold increases in gonadotropin secretion over basal levels. GnRH caused 6- to 20-fold increases in follicle-stimulating hormone (FSH) and LH secretion, respectively, with maximal stimulation at 100 nM GnRH. K(+)- or GnRH-induced FSH release was largely prevented by co-incubation with 1 mM CdCl. Tetrodotoxin (TTX, 5 microM) prevented the veratridine-, but not the K(+)- or GnRH-induced, stimulation of FSH secretion. Nitrendipine (Ntd, 1 microM) produced 35-50% inhibition (NS) of both FSH and LH release stimulated by either 50 mM K+ or 100 nM GnRH. Ntd also inhibited the K(+)-induced [Ca2+]i rise (greater than 90%), as well as the secondary, plateau phase of the GnRH-induced elevation of [Ca2+]i (100% inhibition). Omega-conotoxin (omega-CgTx, 100 nM) partially suppressed FSH and LH release (NS) due to both K+ (33% each) and GnRH (44% and 18%, respectively). omega-CgTx showed variable effects on [Ca2+]i transients evoked by K+ or GnRH ranging from clear inhibition to no effect. We conclude that influx of extracellular Ca2+ is one of several fundamental events underlying the depolarization- or receptor-activated release of LH and FSH, and that this influx can be inhibited by dihydropyridine-sensitive ('L') Ca2+ channels. Two classes of L-channels may exist in gonadotropes, that differ in their sensitivity to omega-CgTx.

Endothelin stimulation of cytosolic calcium and gonadotropin secretion in anterior pituitary cells

The presence of endothelin, a vasoconstrictor peptide, in the hypothalamus and posterior pituitary suggests that it also regulates neural and other nonvascular target cells. In pituitary gonadotrophs, low doses of endothelin evoked oscillations in the intracellular calcium concentration, and high doses induced a biphasic calcium response. Mobilization of intracellular calcium predominated during the spike phase of the calcium response to endothelin, whereas calcium entry through dihydropyridine-sensitive channels contributed to both the spike and plateau phases of the calcium response. Endothelin was a potent as hypothalamic gonadotropin-releasing hormone (GnRH) in stimulation of gonadotropin release in perifused pituitary cells. Endothelin bound specifically to pituitary cells with a dissociation constant of 70 picomolar, and induced rapid formation of inositol trisphosphate and diacyglycerol. Although intracellular calcium concentration and gonadotropin secretory responses to endothelin were independent to the GnRH receptor, endothelin and GnRH appeared to have a common signal transduction mechanism. These observations suggest that endothelin can act as a neuropeptide to regulate anterior pituitary function.

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.

Mechanism of action of gonadotropin releasing hormone upon gonadotropin secretion: involvement of protein kinase C as revealed by staurosporine inhibition and enzyme depletion

The role of protein kinase C (PKC) in the mechanism of action of gonadotropin-releasing hormone (GnRH) upon gonadotropin secretion is controversial and therefore was investigated in primary cultures of rat anterior pituitary cells. A relatively selective PKC inhibitor, staurosporine, inhibited both GnRH- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced luteinizing hormone (LH) release with half-maximal inhibition (IC50) of about 80 nM. Inhibition of GnRH action was not complete suggesting also a PKC-insensitive component in GnRH-induced gonadotropin release. Staurosporine had no effect on basal LH release, or on cellular LH content, neither did the drug interfere with the binding of [125I]iodo-[D-Ser(t-Bu)6]des-Gly10-GnRH N-ethylamide to its receptor in pituitary cells. When cultured pituitary cells were incubated with TPA (1 microM) for 24-48 h no measurable cellular PKC activity could be detected. The decrease in total PKC activity was accompanied by an increase in Ca2+, phosphatidylserine (PS), diacylglycerol (DG)-insensitive activity suggesting the release of a portion of the catalytic domain of PKC (M-kinase) by the phorbol ester treatment. TPA-induced LH release was nearly abolished in PKC-depleted cells and the response to GnRH was markedly reduced (40%). The stimulatory effect of the Ca2+ ionophore, ionomycin, was not impaired in PKC-depleted cells. Impaired responses to GnRH in PKC-depleted cells were only noticed at a later phase (2-4 h) of the exocytotic response of the neurohormone. The data strongly suggest a role for PKC during the second phase of GnRH-induced gonadotropin secretion.

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.

Calcium mobilization and influx during the biphasic cytosolic calcium and secretory responses in agonist-stimulated pituitary gonadotrophs

Stimulation of enriched pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) elicits dose-dependent biphasic elevations of cytosolic calcium ([Ca2+]i) and luteinizing hormone (LH) release, with rapid initial peaks followed by sustained plateaus during continued exposure to the agonist. A potent GnRH-antagonist, [N-acetyl-D-p-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10]GnRH, prevented the biphasic [Ca2+]i and LH responses when added before GnRH, and rapidly abolished both responses to GnRH when added during the plateau phase. In low Ca2+ medium the LH peak responses to GnRH were reduced and the subsequent sustained responses were almost completely abolished; reduction of extracellular Ca2+ during exposure to GnRH caused a prompt decline of LH release. The initial [Ca2+]i peak is derived largely from intracellular calcium mobilization with a partial contribution from calcium influx, while the sustained phase is dependent on the entry of extracellular Ca2+ through both L-type and dihydropyridine-insensitive channels. The presence of L-type voltage-sensitive calcium channels (VSCC) in pituitary gonadotrophs was indicated by the ability of elevated extracellular [K+] to stimulate calcium influx and LH release, and the sensitivity of these responses to dihydropyridine agonist and antagonist analogs. In cells pretreated with high [K+], the peak [Ca2+]i response to GnRH was enhanced but the subsequent plateau phase was markedly attenuated. This divergent effect of sustained membrane depolarization on the biphasic [Ca2+]i response suggests that calcium entry through VSCC initially potentiates agonist-induced mobilization of Ca2+ from intracellular storage sites. However, established Ca2+ entry through depolarization-activated VSCC cannot be further increased by agonist stimulation because both processes operate through the same channels, probably by changes in their activation-inactivation kinetics. Finally, the reciprocal potentiation by the dihydropyridine agonist, BK 8644, and GnRH of [Ca2+]i and LH responses confirms that both compounds act on the same type of channels, i.e., L-type VSCC, that participate in agonist-mediated calcium influx and gonadotropin secretion.

Characteristics of two types of calcium channels in rat pituitary gonadotrophs

The properties of Ca2+ channels in cultured rat pituitary gonadotrophs were analyzed by the patch-clamp technique. The inward Ca2+ currents, recorded in the presence of 5.2 mM Ca2+ or Ba2+, included a fast, transient component with activation-inactivation kinetics and a delayed component with slower activation. The midpoint of the activation curve lay at -30 mV for the transient component and at -12 mV for the delayed component. At the midpoint, changes in potential of 9.5 and 13 mV induced an e-fold change in the activation of the transient and delayed components, respectively. The rate of inactivation of the first component was strongly voltage dependent. At -43 mV, a 7.4-mV change in potential induced an e-fold change in the fraction of Ca2+ channels available to conduct Ca2+ current. During long-lasting (100-200 ms) low-frequency depolarizing voltage-clamp pulses, the size of the delayed component of the Ca2+ current remained constant. The differential effects of membrane potential on inactivation and the different time constants for activation of the two components of the Ca2+ conductance indicate the presence of two types of Ca2+ channels in the membrane of the gonadotroph: the rapidly inactivating current appears to be attributable to a T-type channel, and the noninactivating current corresponds to the L-type channel described in many other cell types.

Mechanism of action of GnRH: the participation of calcium mobilization and activation of protein kinase C in gonadotropin secretion

The role of protein kinase C in luteinizing hormone (LH) release was analyzed in studies on the secretory responses to gonadotropin releasing hormone (GnRH) and phorbol esters in pituitary cell cultures. 12-O-tetradecanoyl-phorbol 13-acetate (TPA), 4 beta-phorbol 12,13-bibenzoate, and 4 beta-phorbol 12,13-diacetate stimulated LH release with ED50s of 5, 10 and 1000 nM, respectively, and with about 70% of the efficacy of GnRH. Phorbol ester-stimulated LH secretion was decreased but not abolished by progressive reduction of [Ca2+] in the incubation medium, and the residual response was identical with that of GnRH in Ca2+-deficient medium. TPA increased [Ca2+]i to a peak after 30 s in normal medium but not in the absence of extracellular Ca2+, indicating that protein kinase C promotes calcium entry but can also mediate secretory responses without changes in calcium influx and [Ca2+]i. The extracellular Ca2+-dependent action of TPA on LH release was blocked by CoCl2 but not by nifedipine. The secretory actions of TPA and GnRH were additive at low doses and converged to a common maximum LH response at high concentrations of the agonists. TPA caused rapid translocation of cytosolic protein kinase C to the particulate fraction, followed by a progressive decrease in total enzyme activity to less than 10% after 6 h. Partial recovery of the cytosolic enzyme (to 20%) occurred after washing and reincubation for 15 h. Such kinase C-depleted cells showed prominent dose-dependent reductions in the actions of both GnRH and TPA on LH release in normal and Ca2+-deficient media. These observations show that the actions of kinase C on LH release include extracellular Ca2+-dependent and independent components, and support the hypothesis that protein kinase C participates in the LH secretory response to GnRH in pituitary gonadotrophs.

Calcium signaling and gonadotropin secretion

Agonist activation of pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) stimulates rapid InsP(3)-dependent peaks of calcium mobilization and luteinizing hormone (LH) release, followed by sustained increases in calcium-influx and hormone secretion. Receptor-mediated calcium entry through L-type and dihydropyridine-insensitive calcium channels accounts for the sustained elevation of cytosolic calcium during GnRH action, and for most of the gonadotropin secretory response. Protein kinase C contributes to the phase of sustained LH release from GnRH-stimulated gonadotrophs, and also to gonadotropin synthesis. Calcium-dependent inactivation of L channels occurs during GnRH action, and appears to be a primary factor in the onset of desensitization of gonadotropin secretion.

Ba2+ stimulation of luteinizing-hormone release demonstrates two mechanisms of Ca2+ entry in gonadotrope cells

Kinetic studies on gonadotropin-releasing-hormone (gonadoliberin, GnRH)-stimulated luteinizing-hormone (lutropin, LH) release in the cultured rat gonadotrope demonstrated a biphasic pattern of LH release. The first rapid phase of release was unaffected by the voltage-gated Ca2+-channel blockers methoxyverapamil (D600) and nifedipine [a dihydropyridine (DHP)], whereas the later second phase was partially inhibited by both drugs. These results suggested that the initial phase of LH release is independent of Ca2+ entry through dihydropyridine (DHP)-sensitive Ca2+ channels and might depend on entry of extracellular Ca2+ by another mechanism. These mechanisms were further studied by utilizing Ba2+ as a Ca2+ substitute. Ba2+, which freely permeates DHP-sensitive Ca2+ channels in the absence of GnRH, induced LH release which was sensitive to blockade by D600 and nifedipine. However, in the presence of the channel blockers, Ba2+-induced LH release could be elicited when GnRH was added to the system. This indicates that GnRH stimulates LH release by initially activating a DHP-insensitive Ca2+-entry mechanism and then a DHP-sensitive mechanism. The DHP-sensitive mechanism freely allows Ba2+ entry in the absence of GnRH-receptor occupancy, whereas the DHP-insensitive mechanism requires GnRH-receptor activation for Ba2+ entry.