Rapid transient elevations of cytosolic calcium triggered by thyrotropin releasing hormone in individual cells of the pituitary line GH3B6

Investigating heterogeneity of intracellular calcium dynamics in anterior pituitary lactotrophs using a combined modelling/experimental approach

Cell responses are commonly heterogeneous, even within a subpopulation. In the present study, we investigate the source of heterogeneity in the Ca(2+) response of anterior pituitary lactotrophs to a Ca(2+) mobilisation agonist, thyrotrophin-releasing hormone. This response is characterised by a sharp increase of cytosolic Ca(2+) concentration as a result of mobilisation of Ca(2+) from intracellular stores, followed by a decrease to an elevated plateau level that results from Ca(2+) influx. We focus on heterogeneity of the evoked Ca(2+) spike under extracellular Ca(2+) free conditions. We introduce a method that uses the information provided by a mathematical model to characterise the source of heterogeneity. This method compares scatter plots of features of the Ca(2+) response obtained experimentally with those made from the mathematical model. The model scatter plots reflect random variation of parameters over different ranges, and matching the experimental and model scatter plots allows us to predict which parameters are most variable. We find that a large degree of variation in Ca(2+) efflux is a likely key contributor to the heterogeneity of Ca(2+) responses to thyrotrophin-releasing hormone in lactotrophs. This technique is applicable to any situation in which the heterogeneous biological response is described by a mathematical model.

Distinct pools of cAMP centre on different isoforms of adenylyl cyclase in pituitary-derived GH3B6 cells

Microdomains have been proposed to explain specificity in the myriad of possible cellular targets of cAMP. Local differences in cAMP levels can be generated by phosphodiesterases, which control the diffusion of cAMP. Here, we address the possibility that adenylyl cyclases, the source of cAMP, can be primary architects of such microdomains. Distinctly regulated adenylyl cyclases often contribute to total cAMP levels in endogenous cellular settings, making it virtually impossible to determine the contribution of a specific isoform. To investigate cAMP dynamics with high precision at the single-isoform level, we developed a targeted version of Epac2-camps, a cAMP sensor, in which the sensor was tagged to a catalytically inactive version of the Ca(2+)-stimulable adenylyl cyclase 8 (AC8). This sensor, and less stringently targeted versions of Epac2-camps, revealed opposite regulation of cAMP synthesis in response to Ca(2+) in GH(3)B(6) pituitary cells. Ca(2+) release triggered by thyrotropin-releasing hormone stimulated the minor endogenous AC8 species. cAMP levels were decreased by inhibition of AC5 and AC6, and simultaneous activation of phosphodiesterases, in different compartments of the same cell. These findings demonstrate the existence of distinct adenylyl-cyclase-centered cAMP microdomains in live cells and open the door to their molecular micro-dissection.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

Prolactin secretion and intracellular Ca(2+) change in rat lactotroph subpopulations stimulated by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) may stimulate lactotrophs to increase intracellular Ca(2+) and to secrete prolactin (PRL). In this study, PRL contents in lactotrophs were determined by the sequential cell immunoblot assay (SCIBA) and their changes in intracellular Ca(2+) was analyzed by confocal microscopy. Significant correlations were found in the corresponding parameters between TRH treatments with a recovery interval of 2 h. Measuring the PRL contents after the first TRH treatment and then determining the intracellular Ca(2+) changes after the second TRH treatment revealed four lactotroph subpopulations. Type I cells (51%) showed significant responses of both PRL secretion and intracellular Ca(2+) concentration. Type II cells (22%) increased in PRL secretion, but without changes in intracellular Ca(2+). Type III cells (17%) have increased in intracellular Ca(2+), but without changes in PRL secretion. Type IV cells (10%) did not show changes in PRL secretion and intracellular Ca(2+).

Dampening of cytosolic Ca2+ oscillations on propagation to nucleus

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

TRH signal transduction in melanotrope cells of Xenopus laevis

TRH is a neuropeptide that activates phospholipase C and, when acting on secretory cells, usually induces a biphasic response consisting of a transitory increase in secretion (due to IP(3) mobilization of Ca(2+) from intracellular stores), followed by a sustained plateau phase of stimulated secretion (by protein kinase C-dependent influx of extracellular Ca(2+) through voltage-operated Ca(2+) channels). The melanotrope cell of the amphibian Xenopus laevis displays a unique secretory response to TRH, namely a broad transient but no sustained second phase, consistent with the observation that TRH induces a single Ca(2+) transient rather than the classic biphasic increase in [Ca(2+)](i). The purpose of the present study was to determine the signal transduction mechanism utilized by TRH in generating this Ca(2+) signaling response. Our hypothesis was that the transient reflects the operation of only one of the two signaling arms of the lipase (i.e., either IP(3)-induced mobilization of internal Ca(2+) or PKC-dependent influx of external Ca(2+)). Using video-imaging microscopy it is shown that the TRH-induced Ca(2+) transient is dramatically attenuated under Ca(2+)-free conditions and that thapsigargin has no noticeable effect on the TRH-induced transient. These observations indicate that an IP(3)-dependent mechanism plays no important role in the action of TRH. PKC also does not seem to be involved because an activator of PKC did not induce a Ca(2+) transient and an inhibitor of PKC did not affect the TRH response. Experiments with a bis-oxonol membrane potential probe showed that the TRH response also does not underlie a PKC-independent mechanism that would induce membrane depolarization. We conclude that the action of TRH on the Xenopus melanotrope does not rely on the classical phospholipase C-dependent mechanism.

Correlation between electrical activity and intracellular Ca2+ oscillations in GH3 rat anterior pituitary cells

Simultaneous measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch current-clamped individual GH3 cells. Both in cells showing brief (<100 ms) and long action potentials (APs), we found a good correlation between the averaged intracellular Ca2+ concentration ([Ca2+]i) and AP frequency, but not between the mean [Ca2+]i and AP duration. Nevertheless, the magnitude of spontaneous Ca2+ oscillations was highly dependent on the size and duration of the APs. The decay of the Ca2+ transients was not slowed when the size of the oscillations was varied either spontaneously or after elongation of the AP with the K+ channel blocker tetraethyl ammonium. Furthermore, the recovery from Ca2+ loads similar to those induced by the APs was slightly retarded after treatment of the cells with intracellular store Ca2+-ATPase inhibitors. Among previous results showing that caffeine-induced [Ca2+]i increases are secondary to electrical activity enhancements in GH3 cells, these data indicate that the Ca2+ entry triggered via APs is the primary determinant of the [Ca2+]i variations, and that Ca2+-induced Ca2+ release has a minor contribution to Ca2+ oscillations recorded during spontaneous activity. They also point to modulation of electrical activity patterns as a crucial factor regulating spontaneous [Ca2+]i signalling, and hence pituitary cell functions in response to physiological secretagogues.

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

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

alpha-MSH potentiates the responsiveness of mammotropes by increasing Ca2+ entry

It is well known that the suckling stimulus renders mammotropes considerably more responsive to prolactin (PRL)-releasing stimuli, and the neurointermediate lobe peptide alpha-melanocyte-stimulating hormone (alpha-MSH) has been proposed to play a pivotal role in this priming. The objectives of the present study were to determine whether alpha-MSH could act directly on pituitary cells to potentiate PRL release in response to two physiologically relevant PRL secretagogues, thyrotropin-releasing hormone (TRH) and ATP, and, if so, to identify the mechanism by which this priming phenomenon is manifested. To this end, we cultured anterior pituitary cells from lactating rats overnight and then subjected them to a reverse hemolytic plaque assay for PRL to evaluate their responses to various test agents. We found that alpha-MSH, which had no effect on PRL export when tested alone, augmented by more than threefold the secretory responses to TRH and ATP. Next, we utilized digital-imaging fluorescence microscopy of fura 2 to evaluate the role of intracellular Ca2+ in this process. We found that PRL export induced by pharmacological activation of L-type voltage-operated calcium channels was also potentiated by alpha-MSH, as was Ca2+ entry induced by TRH. Our results indicate that alpha-MSH acts as a mammotrope-priming agent on a subset of mammotropes by increasing Ca2+ entry induced by PRL secretagogues.

Uncoupling of calcium mobilization and entry pathways in endothelin-stimulated pituitary lactotrophs

In cells expressing Ca2+-mobilizing receptors, InsP3-induced Ca2+ release from intracellular stores is commonly associated with extracellular Ca2+ influx. Operation of these two Ca2+ signaling pathways mediates thyrotropin-releasing hormone (TRH) and angiotensin II (AII)-induced prolactin secretion from rat pituitary lactotrophs. After an initial hyperpolarization induced by Ca2+ mobilization from the endoplasmic reticulum (ER), these agonists generated an increase in the steady-state firing of action potentials, further facilitating extracellular Ca2+ influx and prolactin release. Like TRH and AII, endothelin-1 (ET-1) also induced a rapid release of Ca2+ from the ER and a concomitant spike prolactin secretion during the first 3-5 min of stimulation. However, unlike TRH and AII actions, Ca2+ mobilization was not coupled to Ca2+ influx during sustained ET-1 stimulation, as ET-1 induced a long-lasting abolition of action potential firing. This lead to a depletion of the ER Ca2+ pool, a prolonged decrease in [Ca2+]i, and sustained inhibition of prolactin release. ET-1-induced inhibition and TRH/AII-induced stimulation of Ca2+ influx and hormone secretion were reduced in the presence of the L-type Ca2+ channel blocker, nifedipine. Basal [Ca2+]i and prolactin release were also reduced in the presence of nifedipine. Furthermore, TRH-induced Ca2+ influx and secretion were abolished by ET-1, as TRH was unable to reactivate Ca2+ influx and prolactin release in ET-1-stimulated cells. Depolarization of the cells during sustained inhibitory action of ET-1, however, increased [Ca2+]i and prolactin release. These results indicate that L-type Ca2+ channel represents a common Ca2+ influx pathway that controls basal [Ca2+]i and secretion and is regulated by TRH/AII and ET-1 in an opposite manner. Thus, the receptor-mediated uncoupling of Ca2+ entry from Ca2+ mobilization provides an effective control mechanism in terminating the stimulatory action of ET-1. Moreover, it makes electrically active lactotrophs quiescent and unresponsive to other calcium-mobilizing agonists.

Characterization of the calcium response to thyrotropin-releasing hormone in lactotrophs and GH cells

Thyrotropin-releasing hormone (TRH) acts via a G-protein-coupled receptor on lactotrophs to increase the intracellular free calcium ion concentration, [Ca(2+)](i). The [Ca(2+)](i) response depends on both TRH concentration and the duration of TRH exposure. An initial, short-lived [Ca(2+)](i) spike results from release of Ca(2+) from intracellular stores, whereas a later sustained [Ca(2+)](i) increase, often characterized by [Ca(2+)](i) oscillations, results from an influx of extracellular Ca(2+) through both voltage-gated and non-voltage-gated, store-operated Ca(2+) channels. The initial spike phase predominates at high doses of TRH, whereas the plateau phase predominates at low doses. The mechanisms underlying the complex [Ca(2+)](i) response to TRH are discussed.

Caffeine enhancement of electrical activity through direct blockade of inward rectifying K+ currents in GH3 rat anterior pituitary cells

Treatment of rat anterior pituitary GH3 cells with caffeine causes a reversible enhancement of electrical activity superimposed over a depolarization of the plasma membrane potential. Similar results are obtained with theophylline, but not with isobutylmethylxanthine or forskolin. The effects of caffeine are not related to Ca2+ liberation from intracellular stores since they are not affected by incubation of the cells with ryanodine or thapsigargin. Furthermore, caffeine-induced hyperpolarization of the membrane is not detectable even in cells in which Ca2+ liberation from inositol 1,4,5-trisphosphate-sensitive compartments produces a prominent transient hyperpolarization in response to thyrotropin-releasing hormone. Reductions of Ca2+-dependent K+ currents caused by partial block of L-type Ca2+ channels by caffeine are not sufficient to explain the effects of the xanthine, since the results obtained with caffeine are not mimicked by direct blockade of Ca2+ channels with nisoldipine. GH3 cell inwardly rectifying K+ currents are inhibited by caffeine. Studies on the voltage dependence of the caffeine-induced effects indicate a close correlation between alterations of electrical parameters and reported values of steady-state voltage dependence of inactivation of these currents. We conclude that, as previously shown for thyrotropin-releasing hormone, modulation of inwardly rectifying K+ currents plays a major role determining the firing rate of GH3 cells and its enhancement by caffeine.

Caffeine-induced oscillations of cytosolic Ca2+ in GH3 pituitary cells are not due to Ca2+ release from intracellular stores but to enhanced Ca2+ influx through voltage-gated Ca2+ channels

Caffeine, a well known facilitator of Ca2+-induced Ca2+ release, induced oscillations of cytosolic free Ca2+ ([Ca2+]i) in GH3 pituitary cells. These oscillations were dependent on the presence of extracellular Ca2+ and blocked by dihydropyridines, suggesting that they are due to Ca2+ entry through L-type Ca2+ channels, rather than to Ca2+ release from the intracellular Ca2+ stores. Emptying the stores by treatment with ionomycin or thapsigargin did not prevent the caffeine-induced [Ca2+]i oscillations. Treatment with caffeine occluded phase 2 ([Ca2+]i oscillations) of the action of thyrotropin-releasing hormone (TRH) without modifying phase 1 (Ca2+ release from the intracellular stores). Caffeine also inhibited the [Ca2+]i increase induced by depolarization with high-K+ solutions (56% at 20 mM), suggesting direct inhibition of the Ca2+ entry through voltage-gated Ca2+ channels. We propose that the [Ca2+]i increase induced by caffeine in GH3 cells takes place by a mechanism similar to that of TRH, i.e. membrane depolarization that increases the firing frequency of action potentials. The increase of the electrical activity overcomes the direct inhibitory effect on voltage-gated Ca2+ channels with the result of increased Ca2+ entry and a rise in [Ca2+]i. Consideration of this action cautions interpretation of previous experiments in which caffeine was assumed to increase [Ca2+]i only by facilitating the release of Ca2+ from intracellular Ca2+ stores.

Influence of thyrotropin-releasing hormone on experimental pancreatitis in rats

The main purpose of this study was to investigate the influence of thyroid releasing hormone on acute sodium-taurocholate-induced pancreatitis in rats. Thyroid-releasing hormone did not change the survival rate, serum amylase, glucose calcium, liver transaminases levels or the degree of pancreatic damage, but reduced lactate dehydrogenase. Our findings suggest that the use of thyroid-releasing hormone has no beneficial effect on the course of acute experimental pancreatitis.

Rapid desensitization of the thyrotropin-releasing hormone receptor expressed in single human embryonal kidney 293 cells

This study uses fluorescence microscopy combined with dynamic video imaging to examine the events associated with the rapid desensitization of the thyrotropin-releasing hormone receptor (TRH-R). In single non-pituitary human embryonic kidney 293 (HEK-293) cells, expressing either the rat or human TRH-Rs, TRH produced a rapid dose-dependent monophasic rise in [Ca2+]i. This Ca2+ transient was completely abolished by pretreatment of cells with the intracellular Ca2+ antagonists thapsigargin or cyclopiazonic acid, but not EGTA, the voltage-operated Ca2+ channel (VOCC) antagonist nifedipine or the second-messenger-operated Ca2+ channel antagonist SK&F 96365. These results suggest that TRH causes the mobilization of Ca2+ from thapsigargin/cyclopiazonic acid-sensitive intracellular Ca2+ stores but not the influx of extracellular Ca2+. HEK-293 cells also failed to respond to KCl or the slow Ca(2+)-channel activator BAY K 8644, suggesting that they lack L-type VOCCs. Rat and human TRH-Rs are highly conserved except at the C-terminus where the sequence differs. The C-terminus is believed to be important in receptor desensitization. Despite differences in this region, rat and human TRH-Rs expressed in HEK-293 cells underwent rapid (within 1 min) desensitization. This desensitization was dose-dependent and did not involve receptor loss. Similarly the bradykinin receptor endogenous to HEK-293 cells also displays a rapid desensitization. We conclude that in TRH-R-expressing non-pituitary HEK-293 cells, TRH mobilizes intracellular Ca2+ resulting in a monophasic Ca2+ transient. The rat and human TRH-Rs as well as the endogenous bradykinin receptor also displayed rapid receptor desensitization.

Capacitative Ca2+ entry contributes to the Ca2+ influx induced by thyrotropin-releasing hormone (TRH) in GH3 pituitary cells

Treatment of GH3 cells with either hypothalamic peptide thyrotropin-releasing hormone (TRH), the endomembrane Ca2+-ATPase inhibitor thapsigargin or the Ca2+ ionophore ionomycin mobilized, with different kinetics, essentially all of the Ca2+ pool from the intracellular Ca2+ stores. Any of the above- described treatments induced a sustained increase in intracellular Ca2+ concentration ([Ca2+]i), which was dependent on extracellular Ca2+ and was prevented by Ni2+ but not by dihydropyridines (DHPs), suggesting that it was due to capacitative Ca2+ entry via activation of a plasma membrane pathway which opened upon the emptying of the intracellular Ca2+ stores. The increase of the plasma membrane permeability to Ca2+ correlated negatively with the filling degree of the intracellular Ca2+ stores and was reversed by refilling of the stores. The mechanism of capacitative Ca2+ entry into GH3 cells differed from similar mechanisms described in several types of blood cells in that the pathway was poorly permeable to Mn2+ and not sensitive to cytochrome P450 inhibitors. In GH3 cells, TRH induced a transient [Ca2+]i increase due to Ca2+ release from the stores (phase 1) followed by a sustained [Ca2+]i increase due to Ca2+ entry (phase 2). At the single-cell level, phase 2 was composed of a DHP-insensitive sustained [Ca2+]i increase, due to activation of capacitative Ca2+ entry, superimposed upon which DHP- sensitive [Ca2+]i oscillations took place. The two components of the TRH-induced Ca2+ entry differed also in that [Ca2+]i oscillations remained for several minutes after TRH removal, whereas the sustained [Ca2+]i increase dropped quickly to prestimulatory levels, following the same time course as the refilling of the stores. The drop was prevented when the refilling was inhibited by thapsigargin. It is concluded that, even though the mechanisms of capacitative Ca2+ entry may show differences from cell to cell, it is also present and may contribute to the regulation of physiological functions in excitable cells such as GH3. There, capacitative Ca2+ entry cooperates with voltage-gated Ca2+ channels to generate the [Ca2+]i increase seen during phase 2 of TRH action. This contribution of capacitative Ca2+ entry may be relevant to the enhancement of prolactin secretion induced by TRH.

Effect of glucocorticoids on the paradoxical growth hormone response to thyrotropin-releasing hormone in patients with acromegaly

It has been hypothesized that in acromegalic patients, as well as in normal subjects, acute increases in serum cortisol levels may cause an enhancement of hypothalamic somatostatin secretion, which in turn may be responsible for the glucocorticoid-mediated growth hormone (GH) inhibition. The aim of this study was to investigate short-term effects of an intravenous (i.v.) infusion of hydrocortisone on the GH response to thyrotropin-releasing hormone (TRH) in acromegaly. We studied six adult patients with active acromegaly. The group was composed of four women and two men with a mean age of 55.8 +/- 6.4 years (range, 27 to 68) and a mean body mass index of 26.7 +/- 1 kg/m2 (range, 23.3 to 30). All patients underwent the following treatments: (1) hydrocortisone alone: a bolus i.v. injection of hydrocortisone succinate 100 mg in 2 mL saline at time -60 minutes, followed by a 120-minute i.v. infusion of hydrocortisone succinate 250 mg in 250 mL saline from -60 to 60 minutes; (2) TRH+hydrocortisone: a bolus i.v. injection of TRH 200 micrograms 60 minutes after initiation of a 2-hour hydrocortisone infusion; (3) TRH alone: a bolus i.v. injection of TRH at time 0, 60 minutes after initiation of a 2-hour saline infusion. In all six patients, TRH induced large GH increases (absolute peak GH level, 58.1 +/- 23.2 micrograms/L; maximum % GH change with respect to baseline, 1,397.8% +/- 807.8%; range, 205% +/- 5,219%).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in the frequency and shape of Ca2+ fluctuations in GH4C1 cells induced by thyrotropin-releasing hormone and Bay K 8644

We have examined statistically the actions of thyrotropin-releasing hormone (TRH) and Bay K 8644, an L-type Ca(2+)-channel agonist, on the frequency and shape of cytosolic Ca2+ spikes in individual GH4C1 rat pituitary cells. TRH induced a brief (0-40 s) suppression of Ca2+ spikes followed by a period (40-200 s) of increased spike frequency. TRH treatment reduced the rate of rise and amplitude of Ca2+ spikes, and increased the rate of fall, relative to spontaneous spikes before treatment. TRH had no significant effect on the correlation between spike amplitude and the spike decay time constant tau, suggesting that the increased rate of fall was due to enhanced Ca2+ extrusion and not to decreased Ca(2+)-induced Ca2+ release. Bay K rapidly (t1/2 = 9-13 s) induced a 2-fold increase in the rate of rise of spikes with no change in the total rise time, leading to an increase in spike amplitude. It increased by 2-fold the fall time of spikes, as predicted solely by the previously observed relationship between spike amplitude and fall time. Bay K therefore appeared to increase the number of Ca2+ channels participating in each spike event without altering the kinetics of channel activation or deactivation, and without influencing Ca2+ extrusion. After addition of Bay K, the interval between spikes gradually (t1/2 approximately 100 s) increased, whereas the rate of rise remained constant and maximal. To explain these actions of TRH and Bay K, we postulate that a fraction of L-type Ca2+ channels are inactivated during each spike and must be re-activated in order to participate in a subsequent spike. We conclude further that the changes in spike frequency and profiles induced by these secretagogues are most consistent with a model in which TRH induces increases in both Ca2+ influx and efflux while Bay K induces a large increase in Ca2+ influx but has little effect on efflux.

GHRP-6 induces a biphasic calcium response in rat pituitary somatotrophs

The mechanism of action of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6), a synthetic peptide which specifically induces the secretion of growth hormone (GH) in rat somatotrophs, is still poorly understood. We have studied the effects of GHRP-6 on the cytosolic free calcium concentration ([Ca2+]i) of somatotrophs in primary culture. [Ca2+]i was monitored in individual somatotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short application of GHRP-6 (10(-5) M, 10 s) induced a biphasic Ca2+ response in most cells (44%), which consisted in a rapid and large rise in [Ca2+]i followed by sustained oscillations. This response is dose dependent in a range of concentrations from 10(-10) to 10(-5) M. The first phase of the GHRP-6 response persisted in the absence of Ca2+ in the extracellular medium, whereas the second phase was inhibited. The application of Ca2+ channel blockers like cadmium chloride (200 microM) or PN-200-110 (200 nM) also prevented the second phase. Conversely, when the cells were pretreated with thapsigargin (TG) (100 nM), the first phase of the GHRP-6 Ca2+ response was abolished, whereas the second phase alone was preserved. When the cells were depleted in PKC by incubation with 10(-6) M PMA for 24 h, the second phase of the GHRP-6 response was inhibited, and only the first phase was maintained. These results were corroborated by using phloretin, a PKC inhibitor. These data show that GHRP-6 induces a biphasic elevation of the [Ca2+]i in rat somatotrophs. The first phase is probably due to mobilization of the intracellular Ca2+ stores, whereas the second phase is a PKC-dependent process.

The role of the inwardly rectifying K+ current in resting potential and thyrotropin-releasing-hormone-induced changes in cell excitability of GH3 rat anterior pituitary cells

Exposure of GH3 rat anterior pituitary cells to cholera toxin for 2-4 h significantly increased the thyrotropin-releasing-hormone(TRH)-induced inhibition of the inwardly rectifying K+ current studied in patch-perforated voltage-clamped cells. On the other hand, the current reduction became almost totally irreversible after washout of the neuropeptide. Comparison of the effects elicited by the toxin with those of 8-(4-chlorophenylthio)-cAMP or forskolin plus isobutylmethylxanthine indicated that, although the irreversibility may be due, at least in part, to elevations of cAMP levels, the enhancement of the TRH-induced inhibition of the current is not mediated by the cyclic nucleotide. Only reductions on the inwardly rectifying K+ current, but not those elicited by TRH on voltage-dependent Ca2+ currents, were increased by the treatment with cholera toxin. In current-clamped cells showing similar rates of firing, the second phase of enhanced action-potential frequency induced by TRH was also significantly potentiated by cholera toxin. Measurements of [Ca2+]i oscillations associated with electrical activity, using video imaging with fura-2-loaded cells, demonstrated that cholera toxin treatment causes a clear reduction of spontaneous [Ca2+]i oscillations. However, this did not prevent the stimulatory effect of TRH on oscillations due to the action potentials. In cholera-toxin-treated cells, the steady-state, voltage dependence of inactivation of the inward rectifier was shifted by nearly 20 mV to more negative values. These data suggest that the inwardly rectifying K+ current plays an important role in maintenance of the resting K+ conductance in GH3 cells. Furthermore, the TRH-induced reductions on this current may be an important factor contributing to the increased cell excitability promoted by the neuropeptide.

Protein phosphatase 2A reverses inhibition of inward rectifying K+ currents by thyrotropin-releasing hormone in GH3 pituitary cells

Thyrotropin-releasing hormone (TRH) reduces an inwardly rectifying K+ current in whole-cell voltage-clamped GH3 rat anterior pituitary cells. The TRH effect depends on the maintenance of a background level of Ca2+ in the pipette buffer, and is rapidly minimized by the intracellular dialysis produced under whole-cell conditions. Introduction of ADP-NH-P, a non-hydrolizable ATP analog, in the pipettes, nearly abolishes the TRH-evoked inhibition. The TRH-induced reduction of the inwardly rectifying current is significantly enhanced by incubation of cells 2-4 h with cholera toxin, but not by inclusion of 1 mM cyclic AMP in the pipettes. Under control whole-cell conditions, the reduction caused by TRH is not reversed upon washout of the neuropeptide. However, this effect is readily reversed by addition of purified catalytic subunits of protein phosphatase 2A (PP-2Ac) but not PP-1c to the buffer used to fill the patch pipettes. Among previous results with PP inhibitors, these data indicate that PP2A is involved in the phosphorylation/dephosphorylation mechanism(s) that regulate the delayed TRH effects on GH3 cell excitability.

Thyrotropin-releasing hormone and gonadotropin-releasing hormone-associated peptide modulation of [Ca2+]i in human lactotrophs

The effect of thyrotropin-releasing hormone (TRH) and gonadotropin-releasing hormone-associated peptide (GAP) was studied on both secretion and intracellular free Ca2+ concentrations ([Ca2+]i) in human pituitary cells cultured from prolactin (PRL)-secreting tumors. Secretion was measured during a 30-min incubation period and we used a microspectrofluorimetric method in individual cells and indo-1 as the fluorescent probe. TRH (10(-8) M) significantly increased PRL release in five out of the six cell populations. In these five cases, more than 68% of individual cells responded to TRH by an increase in [Ca2+]i. No significant increase in PRL secretion was found in another culture in which TRH increased [Ca2+]i in only 37% of the cells. The effect of GAP (10(-7) M) was studied in five cell populations. In three of them, a decrease of 20% to 51% of the PRL basal secretory rate was observed under GAP. GAP inhibited [Ca2+]i in respectively 59%, 46% and 94% of the cells from these cultures. The inhibitory effect of GAP was blocked by a pertussis toxin (PT) pretreatment which demonstrates the involvement of a PT-sensitive G-protein in GAP action. In two other cultures, GAP did not significantly alter PRL secretion or individual cell [Ca2+]i. These observations suggest that GAP might play a role in the control of PRL secretion in the human.

Activation of CCK-B receptors elevates cytosolic Ca2+ levels in a pituitary cell line

Cytosolic Ca2+ levels ([Ca2+]i) in GH3 cells, a rat anterior pituitary tumor cell line, were monitored with fura-2 by fluorescence measurements. Cholecystokinin octapeptide (CCK-8) produced a transient elevation of [Ca2+]i. The elevation of [Ca2+]i by CCK-8 was inhibited by L-365,260, but not by devazepide. It was still observed when extracellular Ca2+ was eliminated, indicating that CCK-8 mobilizes Ca2+ from intracellular storage sites after interaction with CCK-B receptors. Cholecystokinin octapeptide increased the turnover of phosphatidylinositol, but it did not affect cyclic AMP levels. A possible involvement of phosphatidylinositol breakdown and calcium mobilization in the transduction system of CCK-B receptors in GH3 cells is suggested.

Mechanism of spontaneous intracellular calcium fluctuations in single GH4C1 rat pituitary cells

Individual unstimulated GH4C1 cells exhibited spontaneous dynamic fluctuations in cytosolic free Ca2+ concentration ([Ca2+]i). Either chelation of extracellular Ca2+ with EGTA or treatment with nifedipine inhibited spontaneous [Ca2+]i fluctuations, indicating that the [Ca2+]i profile was dependent on the entry of extracellular Ca2+ via voltage-operated Ca2+ channels (VOCC). Spontaneous [Ca2+]i fluctuations did not resume immediately after exposure of EGTA-pretreated cells to extracellular Ca2+, supporting the hypothesis that the complex [Ca2+]i profiles observed in unstimulated cells required filling of an intracellular Ca2+ pool. BAY K 8644 elicited large rapid oscillations in [Ca2+]i. After chelation of extracellular Ca2+, however, re-addition of Ca2+ plus BAY K 8644 did not result in [Ca2+]i oscillations. The intracellular Ca2+ pool necessary for BAY K-induced oscillations was not the same Ins(1,4,5)P3-sensitive pool stimulated by thyrotropin-releasing hormone (TRH), because the TRH-stimulated Ins(1,4,5)P3-induced [Ca2+]i spike and the BAY K 8644-induced oscillations were differentially sensitive to chelation of extracellular Ca2+ and thapsigargin. Caffeine caused an increase in [Ca2+]i fluctuations in quiescent cells, supporting a role for Ca(2+)-induced Ca2+ release (CICR) in the generation of spontaneous [Ca2+]i fluctuations. In conclusion, the complex spontaneous changes in [Ca2+]i observed in single GH4C1 cells depend on both the influx of extracellular Ca2+ through VOCC and the action of an intracellular Ca2+ pool that increases [Ca2+]i through a CICR-like mechanism.

Antidepressants inhibit spontaneous oscillations of intracellular Ca2+ concentration in rat cortical cultured neurons

In rat cortical cultured neurons, spontaneous oscillations in the intracellular Ca2+ concentration ([Ca2+]i) were observed by direct [Ca2+]i measurements using the fluorescent indicator fura-2. These [Ca2+]i responses were eliminated by the removal of extracellular Ca2-, L-type Ca2+ channel blockers or a glutamate receptor antagonist. Three antidepressants (amitriptyline, imipramine and mianserin) suppressed the [Ca2+]i oscillations in the range of 1-50 microM. In addition, amitriptyline inhibited high K(+)-and glutamate-induced [Ca2+]i increases with IC50 values of 19 and 27 microM, respectively. Imipramine and mianserin also inhibited the high K(+)-induced [Ca2+]i increases with IC50 values of 45 and 24 microM, respectively. These results suggest that blocking actions by the antidepressants on voltage-dependent Ca2+ influx and glutamate receptor-mediated Ca2+ influx may be involved in the suppression of the [Ca2+]i oscillations.

Characteristics and modulation by thyrotropin-releasing hormone of an inwardly rectifying K+ current in patch-perforated GH3 anterior pituitary cells

Hyperpolarization of patch-perforated GH3 rat anterior pituitary cells in high-K+ Ca(2+)-free medium reveals an inwardly rectifying K+ current. This current showed potential-dependent activation and inactivation kinetics, complete inactivation during strong hyperpolarization and rectification at depolarized potentials. The current was blocked by millimolar concentrations of external Cs+, Ba2+, Cd2+ and Co2+, but it was almost insensitive to tetraethylammonium, 4-aminopyridine and two dihydropyridines, nisoldipine and nitrendipine. Verapamil and methoxyverapamil produced a strong and reversible inhibition of the current. In the presence of 100 nM thyrotropin-releasing hormone (TRH), the current was reduced. This reduction was increased by holding the cell at more negative potentials and was accompanied by a shift in steady-state voltage dependence of inactivation towards more positive voltages. Furthermore, the current slowly returned to the initial levels upon washout. Treatment of the cell with the protein phosphatase inhibitor okadaic acid increased the magnitude of the inhibition caused by TRH. Moreover, the current did not return towards the control level during a 30-min washout period. It is concluded that protein phosphatases participate in modulation of the GH3 cell inwardly rectifying K+ channels by TRH. Furthermore, these data indicate that either a protein phosphatase or a factor necessary for its activation is lost under whole-cell mode, which could account for the permanent reduction of the current in response to TRH.

Modulation of Ca2+ influx by protein phosphorylation in single intact clonal pituitary cells

In pituitary cells, electrical activity generates characteristic oscillations of the cytosolic free Ca2+ concentration, [Ca2+]i. These oscillations are controlled by activators as well as by inhibitors of secretion. We studied, in single fura-2-loaded cells, the role of protein phosphorylation in modulating [Ca2+]i oscillations, using either okadaic acid, an inhibitor of protein phosphatases, or activators of protein kinases A and C. Okadaic acid always increased rapidly both the frequency and amplitude of [Ca2+]i oscillations. In contrast, activation of protein kinases A or C generated more complex kinetic [Ca2+]i patterns: phosphorylation due to both kinases resulted in a sustained activation of [Ca2+]i oscillations in about one-third of the cells, whereas two-thirds of the cells responded by an arrest of [Ca2+]i oscillations. This transient phase of arrest was followed, after a few minutes, by a recovery of [Ca2+]i oscillations, often with enhanced frequency. During the arrest, depolarizing the cells with an external microelectrode could not trigger an increase in [Ca2+]i. We conclude that: (i) the fine regulation between phosphorylation/dephosphorylation events is crucial for the modulation of [Ca2+]i oscillations, and (ii) protein kinases A and C can control Ca2+ influx bidirectionally.

Single and dual hormone secretors in GH3 cultures respond differently to hypothalamic factors

Recent studies using both normal and tumoral pituitary cell cultures have demonstrated that growth hormone (GH) and prolactin (PRL) secreting populations contain cells which release either one or both of these hormones. In order to determine whether these two cell types can be differentially regulated by hypothalamic factors we performed the following study employing plaque assays for GH and PRL. Using cultures of GH3 cells, a rat tumor cell line which contains both of these cell types, we found that the hypothalamic factors vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) when used together had a greater influence on plaque formation than when each was used individually. This suggested that cells were present in culture that responded to one peptide but not the other. Estradiol-treated cultures (which contain only dual-secreting cells) were then evaluated for VIP and TRH responsiveness and found to respond to TRH but not VIP. Finally, we assessed the peptide sensitivity of cultures that were exposed to a conjugate of VIP and the A-chain of ricin (a potent cytotoxin). In addition to eliminating VIP-responsive cells, this treatment markedly reduced the proportions of cells secreting GH-only while having no appreciable influence on dual-hormone secretors. When taken together, our findings indicate that single and dual secretors respond differently to at least two hypothalamic secretagogues and suggest that regulatory differences between these cell types may be important in the control of GH and PRL secretion.

Thyrotropin-releasing hormone induces opposite effects on Ca2+ channel currents in pituitary cells by two pathways

Thyrotropin-releasing hormone (TRH) stimulates pituitary secretion by steps involving a cytosolic Ca2+ rise. We examined various pathways of Ca2+ elevation in pituitary GH3 cells. By using the patch clamp technique in the whole-cell configuration and Ba2+ as divalent charge carrier through Ca2+ channels, TRH (1 microM) reversibly reduced the current by about 55%. This hormonal effect was prevented by infusing guanine 5'-[beta-thio]diphosphate (GDP[beta S]) intracellularly but not by pretreating the cell with pertussis toxin (PT). Since PT-insensitive guanine nucleotide-binding regulatory (G) proteins are known to mediate a hormone-stimulated inositol trisphosphate-mediated Ca2+ release from intracellular stores, we assume that the inhibitory effect of TRH on Ba2+ currents through Ca2+ channels is caused by the increased intracellular Ca2+. To prevent a Ca(2+)-release-dependent inhibition of Ca2+ channels, we preincubated GH3 cells in a medium free of divalent charge carriers and measured the Na+ current through Ca2+ channels. When fura-2 was used as indicator for the cytosolic Ca2+, TRH induced a release from intracellular stores only once and had no effect on the intracellular Ca2+ concentration during further applications. In line with this observation, TRH initially reduced the Na+ current through Ca2+ channels but stimulated it during subsequent applications. The stimulation was sensitive to GDP[beta S] and was abolished by pretreatment with PT, suggesting that the stimulatory action of TRH is mediated by a G protein different from the one that functionally couples the receptor to phosphatidylinositol 4,5-bisphosphate hydrolysis. In conclusion, the present data suggest that TRH increases the intracellular Ca2+ concentration by two interacting pathways, that release from intracellular stores causes a secondary blockage of Ca2+ channels, and that, especially with empty intracellular Ca2+ stores, Ca2+ channels are stimulated by a PT-sensitive G protein.

Inhibition of Ca2+ inflow causes an abrupt cessation of growth-factor-induced repetitive free Ca2+ transients in single NIH-3T3 cells

In single NIH-3T3 fibroblasts loaded with fura-2, bombesin induced one of three patterns of increase in the concentration of intracellular free Ca2+ [( Ca2+]i): a single transient increase, a sustained increase, or repetitive transient increases in [Ca2+]i. Foetal-calf serum and ATP also gave these three patterns of response, although a lower proportion of cells gave repetitive Ca2+ transients in response to ATP. An increase in the concentration of bombesin from 1 to 25 nM increased the proportion of cells which exhibited repetitive Ca2+ transients. At 25 nM-bombesin, the proportion of cells which exhibited repetitive Ca2+ transients increased as the extracellular Ca2+ (Ca2+o) concentration was increased from 1 to 5 mM. Removal of Ca2+o by addition of EGTA, or inhibition of Ca2+ inflow by treatment of cells incubated in the presence of Ca2+o with verapamil or an activator of protein kinase C, abruptly terminated repetitive Ca2+ transients, with only one transient observed after the cessation of Ca2+ inflow. Repetitive Ca2+ transients were not observed in cells incubated in the absence of Ca2+o and in the presence of EGTA. Addition of Ca2+o to cells previously incubated in the presence of EGTA caused a resumption of repetitive Ca2+ transients. Addition of thapsigargin alone induced a large transient increase in [Ca2+]i, whereas much smaller transient increases in [Ca2+]i were induced in about 30% of cells tested by caffeine or carbonyl cyanide m-chlorophenylhydrazone (CCCP) plus oligomycin. Thapsigargin or the combination of CCCP plus oligomycin completely inhibited bombesin-induced repetitive Ca2+ transients, whereas caffeine had no effect. It is concluded from the studies of the role of Ca2+o that NIH-3T3 cells differ from other cell types in the anatomical or chemical links between extracellular Ca2+ and the intracellular stores involved in the generation of Ca2+ transients, whereas the results of the experiments with inhibitors indicate that the generation of repetitive Ca2+ transients in NIH-3T3 cells is unlikely to involve Ca(2+)-induced Ca2+ release from caffeine-sensitive stores.

Simultaneous monitoring of cytosolic free calcium and exocytosis at the single cell level

Abstract Quinacrine, a fluorescent basic molecule, accumulates in secretory granules of pituitary cells, as was revealed by its colocalization with immunoreactive prolactin. Thus quinacrine fluorescence may be used to monitor secretory activity at the single cell level. Rat pituitary cells in primary culture were loaded with quinacrine and stimulated with physiological secretagogues, such as thyrotrophin-releasing hormone or bradykinin, which induced a multiphasic lowering of fluorescence, corresponding to the loss of quinacrine contained in exocytosed granules. Quinacrine was further used in combination with the fluorescent calcium probe fura-2, in order to monitor simultaneously exocytosis and variations in the cytosolic free calcium concentration, [Ca(2+)](i). With an appropriate selection of the excitation wavelengths, in dual excitation microfluorimetry experiments, it was possible to distinguish between fluorescence changes due to altered [Ca(2+)](i) versus quinacrine exocytosis. Transient elevations of [Ca(2+)](i) were provoked in individual pituitary cells by enhancing calcium influx through voltage gated channels. In part of the cells an initial increase in [Ca(2+)](i) coincided with stimulated quinacrine release. The approach was also applied to cells of the neuroblastoma line NCB20, where stimulation with bradykinin caused a transient rise in [Ca(2+)](i), concomitantly with enhanced exocytosis. No increase in exocytosis was ever detected without an elevation of [Ca(2+)](i), suggesting that in both cellular systems, an increase in [Ca(2+)](i), is absolutely necessary, but not sufficient to induce secretion.

Effect of calcium on membrane potential behavior in a rat pituitary cell line (GH3)

The effects of intracellular calcium buffering and increasing bath Ca2+ on spontaneous membrane depolarizations expressed by the clonal rat pituitary cell line GH3 were examined by use of the whole-cell patch-clamp technique. Increasing intracellular calcium buffering capacity caused the duration of spontaneous depolarizations to increase without altering other parameters of membrane potential activity. Increasing bath Ca2+ caused a decrease in duration. These results suggest that the duration of spontaneous membrane depolarization in GH3 cells is regulated by the accumulation of free intracellular Ca2+. The behavior of spontaneous depolarizations measured with the perforated-patch variation of whole-cell patch-clamp techniques closely resembled that obtained in standard whole-cell patch-clamp measurements with an intracellular calcium buffer of 200 microM EGTA with free Ca2+ adjusted to 100 nM.

Adenosine A1 receptor-induced inhibition of Ca2+ transients linked to action potentials in clonal pituitary cells

Adenosine is a potent paracrine/autocrine feedback inhibitor of cell activation in a variety of tissues. Adenosine action was studied in pituitary cells, in which spontaneous electrical activity causes characteristic oscillations of the cytosolic free Ca2+ concentration, [Ca2+]i. Cells of the GH3B6 rat pituitary tumor line were studied by microspectrofluorimetry using the Ca2+ probes indo-1 and fura-2, in part in combination with electrophysiological tight seal whole cell recordings, obtained with the novel approach of patch perforation. It was demonstrated that adenosine receptor activation by N6-(R-phenyl-isopropyl)-adenosine (PIA) caused a block of electrical activity and abolished the ensuing alterations in [Ca2+]i. PIA mimicked the inhibitory action of somatostatin. Adenosine effects are mediated by A1 receptors in these cells and are antagonized by IBMX, an adenosine receptor blocker. PIA also suppressed action potentials that were elicited by the activation of protein kinase C with the phorbol ester PMA, or during the second phase of TRH action. In contrast, no interference was notable on TRH-induced intracellular Ca2+ mobilization. In addition to the abolition of Ca2+ transients, PIA lowers basal [Ca2+]i in some cells. It is proposed that in addition to the inhibition of adenylate cyclase, A1 receptor action on [Ca2+]i is an important element in the control of excitable pituitary cells.

Primary action of endothelin on Ca release in bovine coronary artery smooth muscle cells

Intracellular free Ca concentrations (Cai) were determined by fura-2 microfluorometry in single freshly dispersed cells to differentiate endothelin (ET)-induced Ca release from Ca influx through voltage-gated Ca channels (VGCC). In physiological solution ET (10(-8) M) significantly (P less than 0.05) increased Cai 23 +/- 3% (+/- SE) above baseline; this increase was not significantly attenuated by 2 x 10(-4) M lanthanum, a blocker of VGCC, or Ca-free solution. When the sarcoplasmic reticulum was depleted of Ca by prolonged treatment with 5 x 10(-3) M caffeine, depolarization with 80 mM K (80K; or 30K) plus ET did not increase Cai above that induced by 80K (or 30K) in caffeine alone. In contrast, 10(-6) M BAY K 8644, instead of ET in the protocol, significantly (P less than 0.05) increased Cai above that induced by 80K (or 30K). ET released Ca from the caffeine-sensitive internal store but was not rapid and transient like caffeine-induced release, which elicited a peak Cai increase in less than 1 min; instead, release was more gradual and prolonged with Cai peaking in greater than 2 min, thus resembling the response to 10(-5) M ryanodine. With two ET exposures, either a transient nonrepeatable increase in Cai or a delayed, but sustained, increase in Cai resulted, similar to the response to ryanodine. These data indicate that in freshly dispersed bovine cells the predominant mechanism by which ET increases Cai is release of Ca from the sarcoplasmic reticulum; if any increase in L-type voltage-gated Ca influx occurred, it was minimal and matched by efflux.

Effects of hypothalamic peptides on electrical activity and membrane currents of 'patch perforated' clamped GH3 anterior pituitary cells

'Perforated-patch' recordings of rat anterior pituitary GH3 cells allow long and stable monitoring of electrical activity and membrane currents. Under current clamp conditions, the biphasic effect of thryotropin releasing hormone (TRH) consisting of a transient hyperpolarization followed by a longer phase of increased action potential frequency is fully preserved. Somatostatin suppresses action potential activity and antagonizes the second phase of enhanced spiking caused by TRH. Voltage clamp records of isolated currents indicate that TRH affects calcium-dependent potassium currents, but does not alter either voltage-dependent potassium or calcium currents at times and concentrations at which the electrical activity is increased.

Rapid 'on-line' image processing as a tool in the evaluation of kinetic and morphological aspects of receptor-induced cell activation

Transmembrane signalling involves rapid and spatially well defined changes in cytosolic free Ca2+, [Ca2+]i. Specific technologies involving image processing permit the analysis of kinetic and morphological aspects of [Ca2+]i at the subcellular level with the fluorescent Ca2+ probe fura-2. Fluorescence excitation wavelengths (340 nm or 380 nm) are alternated in synchrony with the acquisition at video rate of images captured with an intensified CCD camera. Images are digitized, recursively filtered, divided, and displayed after calibration of the 'ratio' image into a numerical [Ca2+]i scale. The image processor IMAGINE (Synoptics Ltd., UK) permits these operations at video rate. This produces 'on-line' [Ca2+]i images in real time which are stored on video tapes for subsequent analysis. The present communication summarizes the rationale for the selection of our current technologies. A comparison with alternative solutions should highlight the particular advantages and drawbacks of our approach. The present text thus should serve as a help for investigators who try to assemble image processing tools for work in the receptor and cellular signalling field.

Galanin evokes a cytosolic calcium bursting mode and hormone release in GH3/B6 pituitary cells

The effects of galanin on secretion and cytosolic free Ca2+ concentration ([Ca2+]i) have been studied in GH3/B6 pituitary cells. Prolactin (PRL) and growth hormone (GH) release was measured in column perifusion experiments; [Ca2+]i was monitored in single cells by dual emission microspectrofluorimetry using indo-1 as intracellular Ca2+ probe. Galanin (0.1-1 nM) caused PRL and GH release coincident with a modest rise in [Ca2+]i. The increase in [Ca2+]i comprises the establishment of characteristic long-lasting bursts of [Ca2+]i transients. Galanin acts on Ca2+ entry through voltage-gated Ca2+ channels since there was no response to the peptide when Cd2(+)-a Ca2+ channel blocker-was added to the bath solution. The stimulation of bursting activity by galanin may provide a fine Ca2(+)-signalling mechanism which maximally stimulates hormone release while avoiding refractory periods.

Simultaneous recordings of cytosolic Ca2+ level and membrane potential and current during the response to thyroliberin in clonal rat anterior pituitary cells

The response to thyroliberin in prolactin-producing rat GH4C1 clonal cells was studied using fura-2 to monitor the cytosolic Ca2+ level ([Ca2+]i) in single cells, combined with recordings of membrane potential and current. The average value of [Ca2+]i was 109 nM (mean +/- SD, n = 112), and evoked action potentials caused transient elevations of about 100 nM. At higher firing frequencies these transients merged to a sustained elevation. In 100% of the cells thyroliberin caused an instant rise in [Ca2+]i, peaking at 795 +/- 300 nM (n = 112). This first phase of the thyroliberin response was associated with hyperpolarization in current clamp and outward current in voltage clamp, caused by the opening of Ca2(+)-activated K+ channels. In 75% of the cells the initial peak in [Ca2+]i was followed by a prolonged plateau phase at 247 +/- 76 nM (n = 84). In current clamp the second-phase elevation of [Ca2+]i was linked to either a modest depolarization in combination with enhanced firing frequency or a more pronounced depolarization in silent cells. This elevation of [Ca2+]i was reversed by hyperpolarizing current injection. No second-phase elevation of [Ca2+]i was observed during voltage clamp at a holding potential of -50 mV. Short exposure to Ca2(+)-free conditions eliminated the second-phase elevation in [Ca2+]i, whereas the first phase remained intact. Our experiments show a direct relationship between electrical activity and [Ca2+]i in the GH4C1 cells. The second-phase elevation of [Ca2+]i caused by thyroliberin is the result of influx through voltage-sensitive Ca2+ channels, without involving agonist-gated channels.

Participation of transient-type Ca2+ channels in the sustained increase of Ca2+ level in GH3 cells

Participation of two types of Ca2+ channels (T- and L-types) in the sustained increase of cytosolic-free Ca2+ concentration [( Ca2+]i) was studied in thyrotropin-releasing hormone (TRH)-stimulated clonal GH3 pituitary cells. The effects of Ca2+ channel blockers were analyzed by measuring Ca2+ channel current and [Ca2+]i, using whole-cell voltage-clamp and Fura-2 fluorometry, respectively. Phenytoin (100 microM) and Ni2+ (100 microM) selectively blocked T-type Ca2+ channels and suppressed the TRH-induced sustained [Ca2+]i increase in single cells. Synthetic omega-conotoxin (omega-CgTX, 2 microM) preferentially blocked L-type Ca2+ channels, but it did not suppress the TRH-induced sustained [Ca2+]i increase. The present results suggest that the sustained elevations of [Ca2+]i triggered by TRH may be mediated by T-type Ca2+ channels in GH3 cells.

Thyrotropin-releasing hormone activates a [Ca2+]i-dependent K+ current in GH3 pituitary cells via Ins(1,4,5)P3-sensitive and Ins(1,4,5)P3-insensitive mechanisms

The role of Ins(1,4,5)P3 in receptor-induced Ca2+ mobilization in pituitary cells was studied at the single-cell level. Experimental strategies were developed which allowed a comparative analysis of the effects of Ins(1,4,5)P3 with those of receptor activation under identical conditions. These include microfluorimetry as well as a novel technique which permits the controlled and rapid application of intracellular messenger molecules to individual cells. This latter approach is based on the tight-seal whole-cell recording (WCR) technique, and utilizes two patch-clamp micropipettes, one for electrical recording and the second for the controlled pressure injection. Ins(1,4,5)P3, when applied with this dual-WCR (DWCR) technique, leads rapidly to a marked rise in cytosolic free Ca2+ [( Ca2+]i) and a concomitant stimulation of Ca2(+)-activated K+ current; Ins(1,4,5)P3 can thus mimic the effects of thyrotropin-releasing hormone (TRH) in the same cells under identical conditions. In cells dialysed intracellularly with heparin, a potent antagonist of Ins(1,4,5)P3 action, the rapid response to extracellular stimulation with TRH was abolished, as were the effects of intracellular application of Ins(1,4,5)P3. Heparin, which abolished Ins(1,4,5)P3 action completely, blocked responses to TRH in some cells only partially, revealing that Ca2+ mobilization response to TRH is in part slower in onset than the response to Ins(1,4,5)P3. It is concluded (1) that Ins(1,4,5)P3 is an essential element for the action of TRH, providing a rapid mechanism for Ca2+ mobilization induced by the releasing hormone and (2) that TRH action in mobilizing intracellular Ca2+ is sustained by a slower mechanism which is independent of Ins(1,4,5)P3.

Thapsigargin, but not caffeine, blocks the ability of thyrotropin-releasing hormone to release Ca2+ from an intracellular store in GH4C1 pituitary cells

Thapsigargin stimulates an increase of cytosolic free Ca2+ concentration [( Ca2+]c) in, and 45Ca2+ efflux from, a clone of GH4C1 pituitary cells. This increase in [Ca2+]c was followed by a lower sustained elevation of [Ca2+]c, which required the presence of extracellular Ca2+, and was not inhibited by a Ca2(+)-channel blocker, nimodipine. Thapsigargin had no effect on inositol phosphate generation. We used thyrotropin-releasing hormone (TRH) to mobilize Ca2+ from an InsP3-sensitive store. Pretreatment with thapsigargin blocked the ability of TRH to cause a transient increase in both [Ca2+]c and 45Ca2+ efflux. The block of TRH-induced Ca2+ mobilization was not caused by a block at the receptor level, because TRH stimulation of InsP3 was not affected by thapsigargin. Rundown of the TRH-releasable store by Ca2(+)-induced Ca2+ release does not appear to account for the action of thapsigargin on the TRH-induced spike in [Ca2+]c, because BAY K 8644, which causes a sustained rise in [Ca2+]c, did not block Ca2+ release caused by TRH. In addition, caffeine, which releases Ca2+ from intracellular stores in other cell types, caused an increase in [Ca2+]c in GH4C1 cells, but had no effect on a subsequent spike in [Ca2+]c induced by TRH or thapsigargin. TRH caused a substantial decrease in the amount of intracellular Ca2+ released by thapsigargin. We conclude that in GH4C1 cells thapsigargin actively discharges an InsP3-releasable pool of Ca2+ and that this mechanism alone causes the block of the TRH-induced increase in [Ca2+]c.

Adenosine triphosphate, bradykinin, and thyrotropin-releasing hormone regulate the intracellular Ca2+ concentration and the 45Ca2+ efflux of human thyrocytes in primary culture

The hormonal stimulation of phospholipase C and the consequent activation of the Ca2+-phosphatidylinositol cascade in eukaryotic cells is associated with modifications of the [Ca2+]i (intracellular Ca2+ concentration) which modulates cellular functions. In this study, these modifications were investigated in primary cultures of human thyroid cells. The mean apparent basal [Ca2+]i of human thyrocytes measured using the intracellularly trapped fluorescent indicator Quin-2 was found to be 89 +/- 16 nM (n = 49). ATP and, to a lesser extent, ADP, but not AMP or adenosine, elicited a concentration-dependent biphasic rise in human thyrocytes [Ca2+]i and increased their 45Ca2+ efflux. The first transient phase of the [Ca2+]i rise induced by ATP was resistant to extracellular Ca2+ depletion, whereas the second sustained phase was abolished in these conditions. This suggests that although the first phase of this response involves a release of Ca2+ from intracellular stores, the second phase requires extracellular Ca2+ influx. The response of human thyrocytes to analogs of ATP is compatible with a P2-purinergic effect of ATP on these cells. Bradykinin and TRH affected the human thyrocyte [Ca2+]i and 45Ca2+ efflux similarly to ATP. The human thyrocyte [Ca2+]i and the 45Ca2+ efflux were not modified by carbachol, a nonhydrolyzable analog of acetylcholine. The present results suggest the presence of P2-purinergic receptors to ATP and of receptors to TRH and bradykinin on human follicular thyroid cells. They also confirm that the Ca2+-phosphatidylinositol cascade is present in these cells and suggest that this cascade is modulated by ATP, TRH, and bradykinin. As this cascade is involved in the regulation of protein iodination, and therefore of thyroid hormones synthesis, these agents might have an important role in the regulation of the thyroid function.