Depolarization of the membrane potential decreases the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores in rat thyroid FRTL-5 cells

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Interaction of holothurian triterpene glycoside with biomembranes of mouse immune cells

The in vitro interactions between triterpene glycoside, cucumarioside A(2)-2, isolated from the Far-Eastern holothurian Cucumaria japonica, and mouse splenocyte and peritoneal macrophage biomembranes were studied. Multiple experimental approaches were employed, including determination of biomembrane microviscosity, membrane potential and Ca(2+) signaling, and radioligand binding assays. Cucumarioside A(2)-2 exhibited strong cytotoxic effect in the micromolar range of concentrations and showed pronounced immunomodulatory activity in the nanomolar concentration range. It was established that the cucumarioside A(2)-2 effectively interacted with immune cells and increased the cellular biomembrane microviscosity. This interaction led to a dose-dependent reversible shift in cellular membrane potential and temporary biomembrane depolarization; and an increase in [Ca(2+)](i) in the cytoplasm. It is suggested that there are at least two binding sites for [(3)H]-cucumarioside A(2)-2 on cellular membranes corresponding to different biomembrane components: a low affinity site match to membrane cholesterol that is responsible for the cytotoxic properties, and a high affinity site corresponding to a hypothetical receptor that is responsible for immunostimulation.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

The role of adenosine A(1) receptors in the ATP-evoked Ca(2+) response in rat thyroid FRTL-5 cells

The effect of adenosine A(1) receptor activation on the ATP-induced increase in intracellular free Ca(2+) was studied in control and protein kinase C down-regulated Fisher rat thyroid (FRTL-5) cells. Long-term phorbol ester treatment, which leads to protein kinase C down-regulation, enhanced the ATP-evoked extracellular Ca(2+) influx. The increased Ca(2+) influx was antagonized by the adenosine A(1) receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). [3H]DPCPX binding studies revealed that phorbol ester-treatment increased the number of adenosine A(1) receptors. The adenosine A(1) receptor-mediated inhibition of the cyclic AMP formation was not affected by the increased receptor number. We conclude that the enhanced ATP-evoked Ca(2+) influx in protein kinase C down-regulated cells is mediated by adenosine formed by hydrolysis of ATP, and that this adenosine interacts with the increased number of A(1) receptors. The mechanism by which adenosine enhances Ca(2+) entry is not known. Thus, the larger number of adenosine A(1) receptors broadens the spectrum of adenosine A(1) receptor affected signaling systems in FRTL-5 cells.

Thyroid transcription factor 1 is calcium modulated and coordinately regulates genes involved in calcium homeostasis in C cells

Thyroid transcription factor 1 (TTF-1) was identified for its critical role in thyroid-specific gene expression; its level in the thyroid is regulated by thyrotropin-increased cyclic AMP levels. TTF-1 was subsequently found in lung tissue, where it regulates surfactant expression, and in certain neural tissues, where its function is unknown. Ligands or signals regulating TTF-1 levels in lung or neural tissue are unknown. We recently identified TTF-1 in rat parafollicular C cells and parathyroid cells. In this report, we show that TTF-1 is present in the parafollicular C cells of multiple species and that it interacts with specific elements on the 5'-flanking regions of the extracellular Ca2+-sensing receptor (CaSR), calmodulin, and calcitonin genes in C cells. When intracellular Ca2+ levels are increased or decreased in C cells, by the calcium ionophore A23187, by physiologic concentrations of the P2 purinergic receptor ligand ATP, or by changes in extracellular Ca2+ levels, the promoter activity, RNA levels, and binding of TTF-1 to these genes are, respectively, decreased or increased. The changes in TTF-1 inversely alter CaSR gene and calcitonin gene expression. We show, therefore, that TTF-1 is a Ca2+-modulated transcription factor that coordinately regulates the activity of genes critical for Ca2+ homeostasis by parafollicular C cells. We hypothesize that TTF-1 similarly coordinates Ca2+-dependent gene expression in all cells in which TTF-1 and the CaSR are expressed, i. e., parathyroid cells, neural cells in the anterior pituitary or hippocampus, and keratinocytes.

Thapsigargin-induced calcium entry in FRTL-5 cells: possible dependence on phospholipase A2 activation

Stimulating rat thyroid FRTL-5 cells with agonists that activate the inositol phosphate cascade results in the release of sequestered calcium and influx of extracellular calcium. In addition, phospholipase A2 (PLA2) is activated. Since PLA2 is a calcium-dependent enzyme we wanted to investigate the interrelationships between PLA2 activity and the entry of calcium. Stimulating 3H-arachidonic acid (3H-AA)-labelled cells with thapsigargin resulted in a substantial release of 3H-AA. This release was totally abolished in a calcium-free buffer. Pretreatment of Fura 2 loaded cells with 4-bromophenacyl bromide, an inhibitor of PLA2 activity, decreased the thapsigargin-induced entry of calcium, suggesting a role for PLA2 in the regulation of calcium entry. In cells treated with nordihydroguaiaretic acid (NDGA), clotramizole, or econazole, compounds with lipoxygenase and cytochrome P-450 inhibitory actions, the thapsigargin-induced entry of calcium was decreased in a dose-dependent manner. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the thapsigargin-induced calcium entry. We also showed that stimulation of the cells with arachidonic acid released sequestered calcium, apparently from the same intracellular pool as did thapsigargin. The results suggested that the calcium-induced PLA2 activation and the metabolism of the produced arachidonic acid by a noncyclooxygenase pathway may be of importance in maintaining calcium entry after releasing sequestered Ca2+ in FRTL-5 cells.

Importance of arachidonic acid metabolites in regulating ATP-induced calcium fluxes in thyroid FRTL-5 cells

Stimulating rat thyroid FRTL-5 cells with the purinergic agonist ATP activates both the inositol phosphate signal-transduction pathway and the phospholipase A2 pathway. In the present study we wanted to investigate the possible inter-relationships between these two systems during ATP-induced changes in intracellular free calcium ([Ca2+]i). Pretreatment of Fura-2 loaded cells with 4-bromophenylacyl, an inhibitor of phospholipase A2, had no effect on the ATP-induced entry of Ca2+ but inhibited the release of sequestered Ca2+. Nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, and 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of cytochrome P-450 enzymes, attenuated the ATP-evoked transient increase in [Ca2+]i. Furthermore, the capacitative entry of Ca2+ was also attenuated in NDGA- and ETYA-treated cells stimulated with ATP. Similar results were obtained using econazole, an inhibitor of cytochrome P-450 enzymes. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the ATP-evoked response in [Ca2+]i. We also showed that stimulation of intact or permeabilized FRTL-5 cells with arachidonic acid released sequestered calcium. This calcium originated, at least in part, from an IP3 sensitive calcium pool. In addition, arachidonic acid rapidly acidified the cytosol. The results suggest that metabolism of arachidonic acid by a non-cyclooxygenase pathway is of importance in supporting agonist-induced calcium fluxes evoked via stimulation of the inositol phosphate pathway in FRTL-5 cells. Furthermore, arachidonic acid per se may modify agonist-induced calcium fluxes in these cells.

Norepinephrine, unlike ATP, induces all-or-none increase in cytosolic calcium in thyroid cells. The role of inositol-trisphosphate-sensitive stores and calcium channels

The mechanism of action of norepinephrine and ATP has been analyzed in single FRTL5 cells (a normal thyroid cell line), loaded with the fluorescent Ca2+ probe Fura2. ATP increased the cytosolic Ca2+ in an apparently concentration-dependent manner with a maximal effect at 10 microM (413 +/- 26% over basal levels of 135 +/- 7 nM). In contrast, the norepinephrine-induced increase (198 +/- 5% over basal) was concentration independent in individual cells, the minimal effective concentration being 1 nM. However, the number of cells responding to norepinephrine was concentration dependent. The ATP-induced Ca2+ rise was biphasic, consisting of a rapid rise (2-4 s, 252 +/- 15%), resembling the effect of norepinephrine, followed by a slower and longer component, which reached a plateau in 0.5-2 min. The second component appeared to be related to the opening of a channel, since it required extracellular Ca2+ and was abolished by SC38249, an inhibitor of the second-messenger-operated and voltage-operated channels. Moreover, it was inhibited by 4 beta-phorbol 12-myristate 13-acetate, suggesting that protein kinase C might be involved in the modulation of this Ca2+ channel.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

ATP-induced entry of calcium in thyroid FRTL-5 cells. Studies with phorbol myristate acetate and thapsigargin

Receptor-mediated Ca2+ entry was investigated in fura-2-loaded thyroid FRTL-5 cells. Activation of protein kinase C (PKC) by phorbol myristate acetate (PMA) attenuated the ATP-induced increase in intracellular free Ca2+ ([Ca2+]i). In PKC down-regulated cells, the ATP-induced increase in [Ca2+]i was increased compared with control cells. This enhanced increase in [Ca2+]i was apparently dependent on extracellular Ca2+, as no difference was observed between control cells and PKC down-regulated cells in Ca(2+)-free buffer. Addition of Ca2+ to cells stimulated with ATP in Ca(2+)-free buffer rapidly increased [Ca2+]i. The increase was blocked by PMA. However, PKC down-regulation had no effect on the [Ca2+]i response. Stimulating FRTL-5 cells with thapsigargin increased [Ca2+]i. Addition of ATP after thapsigargin had almost no effect on [Ca2+]i. In PKC down-regulated cells, addition of ATP after thapsigargin evoked a substantial increase in [Ca2+]i which was dependent on extracellular Ca2+. The results indicate that PKC has a modulatory effect on the ATP-induced entry of Ca2+ in FRTL-5 cells.

Activation of calcium entry by cyclopiazonic acid in thyroid FRTL-5 cells

The aim of the present study was to investigate whether the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA) could empty intracellular Ca2+ stores and activate Ca2+ influx in thyroid FRTL-5 cells. Addition of CPA to Fura-2 loaded cells rapidly increased intracellular free Ca2+ ([Ca2+]i) which then stabilized at a new elevated steady state level. The initial increase was mainly dependent on the release of sequestered Ca2+, but was decreased in Ca(2+)-free buffer and in depolarized cells. The plateau phase was totally dependent on extracellular Ca2+. Addition of Ca2+ to cells exposed to CPA in Ca(2+)-free buffer rapidly increased [Ca2+]i. This influx was decreased in depolarized cells and inhibited by SKF 96365. Addition of CPA to cells prior to stimulating the cells with ATP totally abolished the ATP-induced increase in [Ca2+]i. In Ca(2+)-free buffer, addition of ATP prior to CPA decreased the response in [Ca2+]i evoked by CPA. The results show that emptying intracellular Ca2+ stores with CPA rapidly activates influx of Ca2+ in FRTL-5 cells. Furthermore, ATP and CPA appear to release Ca2+, at least in part, from the same intracellular Ca2+ store in these cells.

Modulatory effect of protein kinase C on thapsigargin-induced calcium entry in thyroid FRTL-5 cells

The aim of the present study was to investigate the regulation of calcium influx in thyroid FRTL-5 cells. Stimulating Fura 2-loaded cells with thapsigargin rapidly increased the cytosolic Ca2+ concentration ([Ca2+]i), which then stabilized at a new elevated plateau level. The initial increase in [Ca2+]i consisted mainly of the release of sequestered Ca2+. The plateau phase was totally dependent on extracellular Ca2+. The influx of Ca2+ was blocked by Ni2+ and was decreased in depolarized cells. The importance of protein kinase C in regulating influx of Ca2+ was then evaluated. Addition of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate prior to thapsigargin significantly decreased the influx of extracellular Ca2+. Studies with bisoxonol to measure membrane potential showed that TPA depolarized the plasma membrane in FRTL-5 cells. In cells where protein kinase C was downregulated or was inhibited by staurosporine, the thapsigargin-induced influx of Ca2+ was enhanced. The results indicate that emptying intracellular Ca2+ pools is sufficient to induce influx of Ca2+ in FRTL-5 cells, and that protein kinase C has a modulatory effect on this process.

Calcium fluxes in rat thyroid FRTL-5 cells. Evidence for a functional Na+/Ca2+ exchange mechanism

The effect of extracellular Na+ on cytosolic free Ca2+ and on influx and efflux of Ca2+ was investigated in FRTL-5 thyroid cells. Stimulating the cells with the purinergic agonist ATP induced a rapid efflux of 45Ca2+ from cells loaded with 45Ca2+. Replacement of extracellular Na+ with choline+, significantly decreased the adenosine triphosphate-induced efflux of 45Ca2+. Furthermore, adenosine triphosphate-induced uptake of 45Ca2+ was increased when extracellular Na+ was replaced with choline+, compared with the uptake seen in Na+ buffer. Replacing extracellular Na+ with choline+, increased resting levels of cytosolic free Ca2+ from 50 +/- 2 nM (mean +/- SE) to 81 +/- 3 nM (P less than 0.05) in Fura 2 loaded cells. In cells preincubated with 1 mM ouabain for 30 min, resting cytosolic free Ca2+ increased to 73 +/- 3 nM (P less than 0.05). In a Na+ buffer, the adenosine triphosphate-induced transient increase in cytosolic free Ca2+ was 872 +/- 59 nM, compared with 1070 +/- 63 nM in choline+ buffer (P less than 0.05). The plateau level of cytosolic free Ca2+ in response to adenosine triphosphate was 130 +/- 16 nM in Na+ buffer, compared with 209 +/- 9 nM in choline+ buffer (P less than 0.05). Readdition of Na+ to the plateau phase decreased cytosolic free Ca2+ to 152 +/- 5 nM. Stimulating the cells with 10 microM of the Na(+)-selective monovalent ionophore monensin increased cytosolic free Ca2+ from 53 +/- 9 nM to 124 +/- 16 nM (P less than 0.05). This increase in cytosolic free Ca2+ was dependent on both extracellular Na+ and extracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Depolarization of the membrane potential decreases the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores in rat thyroid FRTL-5 cells

The aim of the present study was to investigate the effect of membrane depolarization on ATP-induced changes in intracellular Ca2+ ([Ca2+]i) and the refilling of intracellular Ca2+ stores in thyroid follicular FRTL-5 cells. Depolarizing the cells with 50 mM K+, an amount sufficient to almost totally depolarize the cells as determined by bisoxonal, significantly reduced the ATP-induced uptake of 45Ca2+. This effect was not dependent on an enhanced efflux of Ca2+, as no difference in the ATP-induced efflux of 45Ca2+ was obtained between control cells and depolarized cells. The ATP-induced transient increase in [Ca2+]i in Fura-2 loaded cells was not altered by depolarization, whereas the ATP-induced plateau in [Ca2+]i was decreased compared with control cells. Furthermore, in cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ after the termination of the ATP response induced a decreased response in [Ca2+]i in depolarized cells. Refilling of intracellular Ca2+ stores was investigated by first stimulating the cells with noradrenaline (NA). The effect of NA was then terminated with prazosin, and the cells restimulated with ATP. In cells depolarized with high K+, the response to ATP was decreased compared with that seen in control cells. The results thus suggest that both the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores is decreased in depolarized FRTL-5 cells.