Developmental changes in capacitative Ca(2+) entry in mouse mammary epithelial cells

Developmental changes in capacitative Ca(2+) entry and Ca(2+) release from intracellular stores were measured using fura-2 fluorescence method during the pregnancy period (day 3-;18) in mouse mammary epithelial cells. Ca(2+) release was identified with the transient intracellular Ca(2+) ([Ca(2+)](i)) increase induced by thapsigargin addition in a Ca(2+)-free solution. Capacitative Ca(2+) entry was measured by the transient [Ca(2+)](i) increase induced by re-addition of extracellular Ca(2+) after depletion of Ca(2+) stores by thapsigargin. The capacitative Ca(2+) entry was greatest at the early stage of pregnancy (i.e. day 3 of pregnancy) and decreased as pregnancy progressed, while Ca(2+) release remained unchanged throughout the developmental stages. These findings indicate that in contrast to Ca(2+) release, a close correlation exists between capacitative Ca(2+) entry and pregnancy-induced development in mammary epithelial cells.     

Novel effects of propranolol. Release of internal Ca(2+) followed by activation of capacitative Ca(2+) entry in Madin Darby canine kidney cells

The effect of propranolol on Ca(2+) signalling in Madin Darby canine kidney (MDCK) cells was investigated by using fura-2 as a Ca(2+) probe. Propranolol increased cytosolic free Ca(2+) levels ([Ca(2+)](i)) in a concentration-dependent manner between 0.1 and 1 mM. The response was partly inhibited by external Ca(2+) removal. In Ca(2+)-free medium pretreatment with 0.2 mM propranolol partly inhibited the [Ca(2+)](i) rise induced by 1 microM thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+) pump; but pretreatment with thapsigargin abolished propranolol-induced Ca(2+) release. Addition of 3 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment with 0.2 mM propranolol in Ca(2+)-free medium. Propranolol (0.2 mM) inhibited 25% of thapsigargin-induced capacitative Ca(2+) entry. Suppression of 1,4,5-trisphosphate (IP(3)) formation by 2 microM U73122, a phospholipase C inhibitor, did not alter 0.2 mM propranolol-induced internal Ca(2+) release. Propranolol (1 mM) also increased [Ca(2+)](i) in human neutrophils. Collectively, we have found that 0.2 mM propranolol increased [Ca(2+)](i) in MDCK cells by releasing Ca(2+) from thapsigargin-sensitive Ca(2+) stores in an IP(3)-independent manner, followed by Ca(2+) influx from external space. Independently, propranolol was able to inhibit thapsigargin-induced capacitative Ca(2+) entry.     

Epidermal growth factor-induced depletion of the intracellular Ca2+ store fails to activate capacitative Ca2+ entry in a human salivary cell line

Epidermal growth factor (EGF) is a multifunctional factor known to influence proliferation and function of a variety of cells. The actions of EGF are mediated by EGF receptor tyrosine kinase pathways, including stimulation of phospholipase Cgamma and mobilization of intracellular Ca(2+) ([Ca(2+)](i)). Generally, agonist-mediated Ca(2+) mobilization involves both Ca(2+) release from internal stores and Ca(2+) influx activated by store depletion (i.e. capacitative or store-operated Ca(2+) influx). However, the role of capacitative Ca(2+) entry in EGF-mediated Ca(2+) mobilization is still largely unknown. In this study, we compared [Ca(2+)](i) signals elicited by EGF with those induced by agents (the muscarinic receptor agonist carbachol and thapsigargin (Tg)) known to activate capacitative Ca(2+) entry. Unlike carbachol and Tg, EGF (5 nm) elicited a transient [Ca(2+)](i) signal without a plateau phase in the presence of extracellular Ca(2+) and also failed to accelerate Mn(2+) entry. Repletion of extracellular Ca(2+) to cells stimulated with EGF in the absence of Ca(2+) elicited an increase in [Ca(2+)](i), indicating that EGF indeed stimulates Ca(2+) influx. However, the influx was activated at lower EGF concentrations than those required to stimulate Ca(2+) release. Interestingly, the phospholipase C inhibitor completely inhibited Ca(2+) release induced by both EGF and carbachol and also reduced Ca(2+) influx responsive to carbachol but had no effect on Ca(2+) influx induced by EGF. EGF-induced Ca(2+) influx was potentiated by low concentrations (<5 ng/ml) of oligomycin, a mitochondrial inhibitor that blocks capacitative Ca(2+) influx in other systems. Transient expression of the hTRPC3 protein enhanced Ca(2+) influx responsive to carbachol but did not increase EGF-activated Ca(2+) influx. Both EGF and carbachol depleted internal Ca(2+) stores. Our results demonstrate that EGF-induced Ca(2+) release from internal stores does not activate capacitative Ca(2+) influx. Rather, EGF stimulates Ca(2+) influx via a mechanism distinct from capacitative Ca(2+) influx induced by carbachol and Tg.     

Effects of chronic hypoxia on Ca(2+) stores and capacitative Ca(2+) entry in human neuroblastoma (SH-SY5Y) cells.

Microfluorimetric measurements of intracellular calcium ion concentration [Ca(2+)](i) were employed to examine the effects of chronic hypoxia (2.5% O(2), 24 h) on Ca(2+) stores and capacitative Ca(2+) entry in human neuroblastoma (SH-SY5Y) cells. Activation of muscarinic receptors evoked rises in [Ca(2+)](i) which were enhanced in chronically hypoxic cells. Transient rises of [Ca(2+)](i) evoked in Ca(2+)-free solutions were greater and decayed more slowly following exposure to chronic hypoxia. In control cells, these transient rises of [Ca(2+)](i) were also enhanced and slowed by removal of external Na(+), whereas the same manoeuvre did not affect responses in chronically hypoxic cells. Capacitative Ca(2+) entry, observed when re-applying Ca(2+) following depletion of intracellular stores, was suppressed in chronically hypoxic cells. Western blots revealed that presenilin-1 levels were unaffected by chronic hypoxia. Exposure of cells to amyloid beta peptide (1-40) also increased transient [Ca(2+)](i) rises, but did not mimic any other effects of chronic hypoxia. Our results indicate that chronic hypoxia causes increased filling of intracellular Ca(2+) stores, suppressed expression or activity of Na(+)/Ca(2+) exchange and reduced capacitative Ca(2+) entry. These effects are not attributable to increased amyloid beta peptide or presenilin-1 levels, but are likely to be important in adaptive cellular remodelling in response to prolonged hypoxic or ischemic episodes.     

Suppression of EGF-induced cell proliferation by the blockade of Ca2+ mobilization and capacitative Ca2+ entry in mouse mammary epithelial cells

The role of intracellular Ca2+ stores and capacitative Ca2+ entry on EGF-induced cell proliferation was investigated in mouse mammary epithelial cells. We have previously demonstrated that EGF enhances Ca2+ mobilization (release of Ca2+ from intracellular Ca2+ stores) and capacitative Ca2+ entry correlated with cell proliferation in mouse mammary epithelial cells. To confirm their role on EGF-induced cell cycle progression, we studied the effects of 2,5-di-tert-butylhydroquinone (DBHQ), a reversible inhibitor of the Ca2+ pump of intracellular Ca2+ stores, and SK&F 96365, a blocker of capacitative Ca2+ entry, on mitotic activity induced by EGF. Mitotic activity was examined using an antibody to PCNA for immunocytochemistry. SK&F 96365 inhibited capacitative Ca2+ entry in a dose-dependent manner (I50: 1-5 microM). SK&F 96365 also inhibited EGF-induced cell proliferation in the same range of concentration (I50: 1-5 microM). DBHQ suppressed [Ca2+]i response to UTP and thus depleted completely Ca2+ stores at 5 microM. DBHQ also inhibited EGF-induced cell proliferation at an I50 value of approximately 10 microM. The removal of these inhibitors from the culture medium increased the reduced mitotic activity reversibly. Using a fluorescent assay of DNA binding of ethidium bromide, no dead cells were detected in any of the cultures. These results indicate that the inhibitory effects of SK&F 96365 and DBHQ on cell proliferation were due to the inhibition of capacitative Ca2+ entry and Ca2+ mobilization suggesting the importance of capacitative Ca2+ entry and Ca2+ mobilization in the control of EGF-induced cell cycle progression in mouse mammary epithelial cells.     

Upregulated TRP and enhanced capacitative Ca(2+) entry in human pulmonary artery myocytes during proliferation

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) due to Ca(2+) release from intracellular Ca(2+) stores and Ca(2+) influx through plasmalemmal Ca(2+) channels plays a critical role in mitogen-mediated cell growth. Depletion of intracellular Ca(2+) stores triggers capacitative Ca(2+) entry (CCE), a mechanism involved in maintaining Ca(2+) influx and refilling intracellular Ca(2+) stores. Transient receptor potential (TRP) genes have been demonstrated to encode the store-operated Ca(2+) channels that are activated by Ca(2+) store depletion. In this study, we examined whether CCE, activity of store-operated Ca(2+) channels, and human TRP1 (hTRP1) expression are essential in human pulmonary arterial smooth muscle cell (PASMC) proliferation. Chelation of extracellular Ca(2+) and depletion of intracellularly stored Ca(2+) inhibited PASMC growth in media containing serum and growth factors. Resting [Ca(2+)](cyt) as well as the increases in [Ca(2+)](cyt) due to Ca(2+) release and CCE were all significantly greater in proliferating PASMC than in growth-arrested cells. Consistently, whole cell inward currents activated by depletion of intracellular Ca(2+) stores and the mRNA level of hTRP1 were much greater in proliferating PASMC than in growth-arrested cells. These results suggest that elevated [Ca(2+)](cyt) and intracellularly stored [Ca(2+)] play an important role in pulmonary vascular smooth muscle cell growth. CCE, potentially via hTRP1-encoded Ca(2+)-permeable channels, may be an important mechanism required to maintain the elevated [Ca(2+)](cyt) and stored [Ca(2+)] in human PASMC during proliferation.     

Relation between phosphatidylserine exposure and store-operated Ca(2+) entry in stimulated cells

A significant increase in intracellular Ca(2+) is required to trigger the remodeling of the cell plasma membrane. Scott syndrome is an extremely rare inherited disorder of the transmembrane migration of phosphatidylserine toward the exoplasmic leaflet in blood cells. We have recently reported a reduced capacitative Ca(2+) entry in Scott cells [Martínez et al. (1999) Biochemistry 38, 10092-10098]. We have investigated here the links between defective phosphatidylserine exposure and Ca(2+) signaling in Scott cells by focusing on the Ca(2+) entry following the emptying of intracellular stores. After depletion of caffeine- or thapsigargin-sensitive stores, Ca(2+) entry was lower in Scott compared to control lymphoblasts. However, the simultaneous depletion of both types of stores restored a normal Ca(2+) influx across the plasma membrane in Scott cells and phosphatidylserine externalization ability was improved concomitantly with capacitative Ca(2+) entry. These observations point to the essential role of capacitative Ca(2+) entry in the control of phosphatidylserine exposure of stimulated cells.     

Activation of muscarinic acetylcholine receptors induces Ca(2+) mobilization in FRT cells

The effect of the muscarinic receptors agonist carbachol (Cch) on intracellular calcium concentration ([Ca(2+)](i)) and cAMP level was studied in polarized Fischer rat thyroid (FRT) epithelial cells. Cch provoked a transient increase in [Ca(2+)](i), followed by a lower sustained phase. Thapsigargin, a specific microsomal Ca(2+)-ATPase inhibitor, caused a rapid rise in [Ca(2+)](i) and subsequent addition of Cch was without effect. Removal of extracellular Ca(2+) reduced the initial transient response and completely abolished the plateau phase. Ryanodine, an agent that depletes intracellular Ca(2+) stores through stimulation of ryanodine receptors (RyRs), had no effect on [Ca(2+)](i). However, the transitory activation of [Ca(2+)](i) was dose-dependently attenuated in cells pretreated with U73122, a specific inhibitor of phospholipase C (PLC). These data suggest that the Cch-stimulated increment of [Ca(2+)](i) required IP(3) formation and binding to its specific receptors in Ca(2+) stores. Further studies were performed to investigate whether the effect of Cch on Ca(2+) entry into FRT cells was via L-type voltage-dependent Ca(2+) channels (L-VDCCs). Nicardipine, a nonspecific L-type Ca(2+) channel blocker, decreased Cch-induced increase on [Ca(2+)](i), while Bay K-8644, an L-type Ca(2+) channel agonist, slightly increased [Ca(2+)](i) in FRT cells. These data indicate that Ca(2+) entry into these nondifferentiated thyroid cells occurs through an L-VDCC, and probably through another mechanism such as a capacitative pathway. Cch did not affect the intracellular cAMP levels, but its effects on [Ca(2+)](i) were significantly reduced when cells were pretreated with forskolin, suggesting the existence of an intracellular cross-talk between PLC and cAMP mechanisms in the regulation of intracellular Ca(2+) mobilization in neoplastic FRT cells.     

Activation of a capacitative Ca(2+) entry pathway by store depletion in cultured hippocampal neurones

Intracellular Ca(2+) ([Ca(2+)](i)) changes were measured in cell bodies of cultured rat hippocampal neurones with the fluorescent indicator Fluo-3. In the absence of external Ca(2+), the cholinergic agonist carbachol (200 microM) and the sarcoendoplasmic reticulum Ca(2+) pump inhibitor thapsigargin (0.4 microM) both transiently elevated [Ca(2+)](i). A subsequent addition of Ca(2+) into the bathing medium caused a second [Ca(2+)](i) change which was blocked by lanthanum (50 microM). Taken together, these experiments indicate that stores depletion can activate a capacitative Ca(2+) entry pathway in cultured hippocampal neurones and further demonstrate the existence of such a Ca(2+) entry in excitable cells.     

Discriminating between capacitative and arachidonate-activated Ca(2+) entry pathways in HEK293 cells.

We have recently questioned whether the capacitative or store-operated model for receptor-activated Ca(2+) entry can account for the influx of Ca(2+) seen at low agonist concentrations, such a those typically producing [Ca(2+)](i) oscillations. Instead, we have identified an arachidonic acid-regulated, noncapacitative Ca(2+) entry mechanism that appears to be specifically responsible for the receptor-activated entry of Ca(2+) under these conditions. However, it is unclear whether these two systems reflect the activity of distinct entry pathways or simply different mechanisms of regulating a common pathway. We therefore used the known selectivity of the Ca(2+)-stimulated type VIII adenylyl cyclase for Ca(2+) entry occurring via the capacitative pathway (Fagan, K. A., Mahey, R., and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444) to attempt to discriminate between these two entry mechanisms in HEK293 cells. Consistent with the earlier reports, we found that thapsigargin induced an approximate 3-fold increase in adenylyl cyclase activity that was unrelated to global changes in [Ca(2+)](i) or to the release of Ca(2+) from internal stores but was specifically dependent on the induced capacitative entry of Ca(2+). In marked contrast, the arachidonate-induced entry of Ca(2+) completely failed to affect adenylyl cyclase activity despite producing a substantially greater rate of entry than that induced by thapsigargin. These data demonstrate that the arachidonate-activated entry of Ca(2+) occurs via an entirely distinct influx pathway.     

Modulation of intracellular Ca(2+) via alpha(1B)-adrenoreceptor signaling molecules,G alpha(h) (transglutaminase II) and phospholipase C-delta 1

We characterized the alpha(1B)-adrenoreceptor (alpha(1B)-AR)-mediated intracellular Ca(2+) signaling involving G alpha(h) (transglutaminase II, TGII) and phospholipase C (PLC)-delta 1 using DDT1-MF2 cell. Expression of wild-type TGII and a TGII mutant lacking transglutaminase activity resulted in significant increases in a rapid peak and a sustained level of intracellular Ca(2+) concentration ([Ca(2+)](i)) in response to activation of the alpha(1B)-AR. Expression of a TGII mutant lacking the interaction with the receptor or PLC-delta 1 substantially reduced both the peak and sustained levels of [Ca(2+)](i). Expression of TGII mutants lacking the interaction with PLC-delta 1 resulted in a reduced capacitative Ca(2+) entry. Reduced expression of PLC-delta 1 displayed a transient elevation of [Ca(2+)](i) and a reduction in capacitative Ca(2+) entry. Expression of the C2-domain of PLC-delta 1, which contains the TGII interaction site, resulted in reduction of the alpha(1B)-AR-evoked peak increase in [Ca(2+)](i), while the sustained elevation in [Ca(2+)](i) and capacitative Ca(2+) entry remained unchanged. These findings demonstrate that stimulation of PLC-delta 1 via coupling of the alpha(1B)-AR with TGII evokes both Ca(2+) release and capacitative Ca(2+) entry and that capacitative Ca(2+) entry is mediated by the interaction of TGII with PLC-delta 1.     

Effects of capsaicin on Ca(2+) release from the intracellular Ca(2+) stores in the dorsal root ganglion cells of adult rats

We have investigated the effect of capsaicin on Ca(2+) release from the intracellular calcium stores. Intracellular calcium concentration ([Ca(2+)](i)) was measured in rat dorsal root ganglion (DRG) neurons using microfluorimetry with fura-2 indicator. Brief application of capsaicin (1 microM) elevated [Ca(2+)](i) in Ca(2+)-free solution. Capsaicin-induced [Ca(2+)](i) transient in Ca(2+)-free solution was evoked in a dose-dependent manner. Resiniferatoxin, an analogue of capsaicin, also raised [Ca(2+)](i) in Ca(2+)-free solution. Capsazepine, an antagonist of capsaicin receptor, completely blocked the capsaicin-induced [Ca(2+)](i) transient. Caffeine completely abolished capsaicin-induced [Ca(2+)](i) transient. Dantrolene sodium and ruthenium red, antagonists of the ryanodine receptor, blocked the effect of capsaicin on [Ca(2+)](i). However, capsaicin-induced [Ca(2+)](i) transient was not affected by 2-APB, a membrane-permeable IP(3) receptor antagonist. Furthermore, depletion of IP(3)-sensitive Ca(2+) stores by bradykinin and phospholipase C inhibitors, neomycin, and U-73122, did not block capsaicin-induced [Ca(2+)](i) transient. In conclusion, capsaicin increases [Ca(2+)](i) through Ca(2+) release from ryanodine-sensitive Ca(2+) stores, but not from IP(3)-sensitive Ca(2+) stores in addition to Ca(2+) entry through capsaicin-activated nonselective cation channel in rat DRG neurons.     

Cytosolic [Ca2+] transients in dictyostelium discoideum depend on the filling state of internal stores and on an active sarco/endoplasmic reticulum calcium ATPase (SERCA) Ca2+ pump

Stimulation of Dictyostelium discoideum with cAMP evokes a change of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). We analyzed the role of the filling state of Ca(2+) stores for the [Ca(2+)] transient. Parameters tested were the height of the [Ca(2+)](i) elevation and the percentage of responding amoebae. After loading stores with Ca(2+), cAMP induced a [Ca(2+)](i) transient in many cells. Without prior loading, cAMP evoked a [Ca(2+)](i) change in a few cells only. This indicates that the [Ca(2+)](i) elevation is not mediated exclusively by Ca(2+) influx but also by Ca(2+) release from stores. Reducing the Ca(2+) content of the stores by EGTA preincubation led to a cAMP-activated [Ca(2+)](i) increase even at low extracellular [Ca(2+)]. Moreover, the addition of Ca(2+) itself elicited a capacitative [Ca(2+)](i) elevation. This effect was not observed when stores were emptied by the standard technique of inhibiting internal Ca(2+) pumps with 2,5-di-(t-butyl)-1,4-hydroquinone. Therefore, in Dictyostelium, an active internal Ca(2+)-ATPase is absolutely required to allow for Ca(2+) entry. No influence of the filling state of stores on Ca(2+) influx characteristics was found by the Mn(2+)-quenching technique, which monitors the rate of Ca(2+) entry. Both basal and cAMP-activated Mn(2+) influx rates were similar in control cells and cells with empty stores. By contrast, determination of extracellular free Ca(2+) concentration ([Ca(2+)](e)) changes, which represent the sum of Ca(2+) influx and efflux, revealed a higher rate of [Ca(2+)](e) decrease in EGTA-treated than in control amoebae. We conclude that emptying of Ca(2+) stores does not change the rate of Ca(2+) entry but results in inhibition of the plasma membrane Ca(2+)-ATPase. Furthermore, the activities of the Ca(2+) transport ATPases of the stores are of crucial importance for the regulation of [Ca(2+)](i) changes.     

Capacitative Ca(2+) entry and tyrosine kinase activation in canine pulmonary arterial smooth muscle cells

We investigated the role of capacitative Ca(2+) entry and tyrosine kinase activation in mediating phenylephrine (PE)-induced oscillations in intracellular free Ca(2+) concentration ([Ca(2+)](i)) in canine pulmonary arterial smooth muscle cells (PASMCs). [Ca(2+)](i) was measured as the 340- to 380-nm ratio in individual fura 2-loaded PASMCs. Resting [Ca(2+)](i) was 96 +/- 4 nmol/l. PE (10 micromol/l) stimulated oscillations in [Ca(2+)](i), with a peak amplitude of 437 +/- 22 nmol/l and a frequency of 1.01 +/- 0.12/min. Thapsigargin (1 micromol/l) was used to deplete sarcoplasmic reticulum (SR) Ca(2+) after extracellular Ca(2+) was removed. Under these conditions, a nifedipine-insensitive, sustained increase in [Ca(2+)](i) (140 +/- 7% of control value) was observed when the extracellular Ca(2+) concentration was restored; i.e., capacitative Ca(2+) entry was demonstrated. Capacitative Ca(2+) entry also refilled SR Ca(2+) stores. Capacitative Ca(2+) entry was attenuated (32 +/- 3% of control value) by 50 micromol/l of SKF-96365 (a nonselective Ca(2+)-channel inhibitor). Tyrosine kinase inhibition with tyrphostin 23 (100 micromol/l) and genistein (100 micromol/l) also inhibited capacitative Ca(2+) entry to 63 +/- 12 and 85 +/- 4% of control values, respectively. SKF-96365 (30 micromol/l) attenuated both the amplitude (15 +/- 7% of control value) and frequency (50 +/- 21% of control value) of PE-induced Ca(2+) oscillations. SKF-96365 (50 micromol/l) abolished the oscillations. Tyrphostin 23 (100 micromol/l) also inhibited the amplitude (17 +/- 7% of control value) and frequency (45 +/- 9% of control value) of the oscillations. Genistein (30 micromol/l) had similar effects. Both SKF-96365 and tyrphostin 23 attenuated PE-induced contraction in isolated pulmonary arterial rings. These results demonstrate that capacitative Ca(2+) entry is present and capable of refilling SR Ca(2+) stores in canine PASMCs and may be involved in regulating PE-induced Ca(2+) oscillations. A tyrosine kinase is involved in the signal transduction pathway for alpha(1)-adrenoreceptor activation in PASMCs.     

Three distinct Ca(2+) influx pathways couple acetylcholine receptor activation to catecholamine secretion from PC12 cells

Amperometry and microfluorimetry were employed to investigate the Ca(2+)-dependence of catecholamine release induced from PC12 cells by cholinergic agonists. Nicotine-evoked exocytosis was entirely dependent on extracellular Ca(2+) but was only partly blocked by Cd(2+), a nonselective blocker of voltage-gated Ca(2+) channels. Secretion and rises of [Ca(2+)](i) observed in response to nicotine could be almost completely blocked by methyllycaconitine and alpha-bungarotoxin, indicating that such release was mediated by receptors composed of alpha7 nicotinic acetylcholine receptor subunits. Secretion and [Ca(2+)](i) rises could also be fully blocked by co-application of Cd(2+) and Zn(2+). Release evoked by muscarine was also fully dependent on extracellular Ca(2+). Muscarinic receptor activation stimulated release of Ca(2+) from a caffeine-sensitive intracellular store, and release from this store induced capacitative Ca(2+) entry that could be blocked by La(3+) and Zn(2+). This Ca(2+) entry pathway mediated all secretion evoked by muscarine. Thus, activation of acetylcholine receptors stimulated rises of [Ca(2+)](i) and exocytosis via Ca(2+) influx through voltage-gated Ca(2+) channels, alpha7 subunit-containing nicotinic acetylcholine receptors, and channels underlying capacitative Ca(2+) entry.     

Arachidonic acid in astrocytes blocks Ca(2+) oscillations by inhibiting store-operated Ca(2+) entry,and causes delayed Ca(2+) influx

ATP-elicited oscillations of the concentration of free intracellular Ca(2+) ([Ca(2+)](i)) in rat brain astrocytes were abolished by simultaneous arachidonic acid (AA) addition, whereas the tetraenoic analogue 5,8,11,14-eicosatetraynoic acid (ETYA) was ineffective. Inhibition of oscillations is due to suppression by AA of intracellular Ca(2+) store refilling. Short-term application of AA, but not ETYA, blocked Ca(2+) influx, which was evoked by depletion of stores with cyclopiazonic acid (CPA) or thapsigargin (Tg). Addition of AA after ATP blocked ongoing [Ca(2+)](i) oscillations. Prolonged AA application without or with agonist could evoke a delayed [Ca(2+)](i) increase. This AA-induced [Ca(2+)](i) rise developed slowly, reached a plateau after 5 min, could be reversed by addition of bovine serum albumin (BSA), that scavenges AA, and was blocked by 1 microM Gd(3+), indicative for the influx of extracellular Ca(2+). Specificity for AA as active agent was demonstrated by ineffectiveness of C16:0, C18:0, C20:0, C18:2, and ETYA. Moreover, the action of AA was not affected by inhibitors of oxidative metabolism of AA (ibuprofen, MK886, SKF525A). Thus, AA exerted a dual effect on astrocytic [Ca(2+)](i), firstly, a rapid reduction of capacitative Ca(2+) entry thereby suppressing [Ca(2+)](i) oscillations, and secondly inducing a delayed activation of Ca(2+) entry, also sensitive to low Gd(3+) concentration.     

Low cytoplasmic [Ca(2+)] activates I(CRAC) independently of global Ca(2+) store depletion in RBL-1 cells

Release of Ca(2+) from inositol (1,4,5)-trisphosphate-sensitive Ca(2+) stores causes "capacitative calcium entry," which is mediated by the so-called "Ca(2+) release-activated Ca(2+) current" (I(CRAC)) in RBL-1 cells. Refilling of the Ca(2+) stores or high cytoplasmic [Ca(2+)] ([Ca(2+)](cyt)) inactivate I(CRAC). Here we address the question if also [Ca(2+)](cyt) lower than the resting [Ca(2+)](cyt) influences store-operated channels. We therefore combined patch clamp and mag fura-2 fluorescence methods to determine simultaneously both I(CRAC) and [Ca(2+)] within Ca(2+) stores of RBL-1 cells ([Ca(2+)](store)). We found that low [Ca(2+)](cyt) in the range of 30-50 nM activates I(CRAC) and Ca(2+) influx spontaneously and independently of global Ca(2+) store depletion, while elevation of [Ca(2+)](cyt) to the resting [Ca(2+)](cyt) (100 nM) resulted in store dependence of I(CRAC) activation. We conclude that spontaneous activation of I(CRAC) by low [Ca(2+)](cyt) could serve as a feedback mechanism keeping the resting [Ca(2+)](cyt) constant.     

Increase of [Ca(2+)](i) via activation of ATP receptors in PC-Cl3 rat thyroid cell line.

In PC-Cl3 rat thyroid cell line, ATP and UTP provoked a transient increase in [Ca(2+)](i), followed by a lower sustained phase. Removal of extracellular Ca(2+) reduced the initial transient response and completely abolished the plateau phase. Thapsigargin (TG) caused a rapid rise in [Ca(2+)](i) and subsequent addition of ATP was without effect. The transitory activation of [Ca(2+)](i) was dose-dependently attenuated in cells pretreated with the specific inhibitor of phospholipase C (PLC), U73122. These data suggest that the ATP-stimulated increment of [Ca(2+)](i) required InsP(3) formation and binding to its specific receptors in Ca(2+) stores. Desensitisation was demonstrated with respect to the calcium response to ATP and UTP in Fura 2-loaded cells. Further studies were performed to investigate whether the effect of ATP on Ca(2+) entry into PC-Cl3 cells was via L-type voltage-dependent Ca(2+) channels (L-VDCC) and/or by the capacitative pathway. Nifedipine decreased ATP-induced increase on [Ca(2+)](i). Addition of 2 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment of the cells with TG or with 100 microM ATP in Ca(2+)-free medium. These data indicate that Ca(2+) entry into PC-Cl3 stimulated with ATP occurs through both an L-VDCC and through a capacitative pathway. Using buffers with differing Na(+) concentrations, we found that the effects of ATP were dependent of extracellular Na(+), suggesting that a Na(+)/Ca(2+) exchange mechanism is also operative. These data suggest the existence, in PC-Cl3 cell line, of a P2Y purinergic receptor able to increase the [Ca(2+)](i) via PLC activation, Ca(2+) store depletion, capacitative Ca(2+) entry and L-VDCC activation.     

Mechanism of endothelin-1-induced cytosolic Ca(2+) mobility in cultured H9c2 myocardiac ventricular cells

The effect of endothelin-1 (ET-1) on the intracellular free Ca(2+) ([Ca(2+)](i)) mobility in cultured H9c2 myocardiac ventricular cells was studied after loading with fura-2-AM. In Ca(2+)-containing buffer, ET-1 induced [Ca(2+)](i) rise from 10(-7) to 10(-9) M. ET-1 induced [Ca(2+)](i), which was composed of a first small peak and a secondary persistent plateau. In Ca(2+)-free buffer, pretreatment with 10(-7) M ET-1 inhibited the thapsigargin and carbonylcyanide m-chlorophenylhydrazone (CCCP)-induced [Ca(2+)](i) increase. Meanwhile, pretreatment with thapsigargin and CCCP also inhibited ET-1-induced [Ca(2+)](i) rise. In Ca(2+)-containing buffer, the ET(A) receptor antagonist (BQ123) completely abolished the secondary rising peak and plateau. Conversely, the ET(B) receptor antagonist (BQ788) completely inhibited the first small peak and secondary peak plateau. Nifedipine and La(3+) also abolished the 10(-7) M ET-1-induced [Ca(2+)](i) in the first rising peak. The internal Ca(2+) release induced by ET-1 was inhibited by U73122 (phospholipase C inhibitor), propranolol (phospholipase D inhibitor) and aristolochic acid (phospholipase A2 inhibitor). After incubation of 10(-7) M ET-1 in Ca(2+)-free buffer, the addition of 5 mM CaCl(2) increased Ca(2+) influx, implying that release of Ca(2+) from internal stores further induces capacitative Ca(2+) entry. Taken together, these results suggest that both ET(A) and ET(B) receptors are involved in ET-1-induced [Ca(2+)](i) rise in H9c2 myocardiac ventricular cells. Whereas ET(B) receptor seems to mediate the initial Ca(2+) influx via L-type Ca(2+) channel, ET(A) receptor appears to be involved in the subsequent Ca(2+) release from endoplasmic reticulum and mitochondria Ca(2+) stores.