Association of cytoplasmic free Ca2+ gradients with subcellular organelles

Endoplasmic Reticulum-Mitochondria Communication Through Ca2+ Signaling: The Importance of Mitochondria-Associated Membranes (MAMs)

The execution of proper Ca²⁺ signaling requires close apposition between the endoplasmic reticulum (ER) and mitochondria. Hence, Ca²⁺ released from the ER is "quasi-synaptically" transferred to mitochondrial matrix, where Ca²⁺ stimulates mitochondrial ATP synthesis by activating the tricarboxylic acid (TCA) cycle. However, when the Ca²⁺ transfer is excessive and sustained, mitochondrial Ca²⁺ overload induces apoptosis by opening the mitochondrial permeability transition pore. A large number of regulatory proteins reside at mitochondria-associated ER membranes (MAMs) to maintain the optimal distance between the organelles and to coordinate the functionality of both ER and mitochondrial Ca²⁺ transporters or channels. In this chapter, we discuss the different pathways involved in the regulation of ER-mitochondria Ca²⁺ flux and describe the activities of the various Ca²⁺ players based on their primary intra-organelle localization.

De novo expression of parvalbumin in ependymal cells in response to brain injury promotes ependymal remodeling and wound repair

The calcium-binding protein parvalbumin (PV) hallmarks subpopulations of interneurons in the murine brain. We serendipitously observed the de novo expression of PV in ependymal cells of the lateral ventricle wall following in vivo lesioning and brain slicing for the preparation of organotypic hippocampal slice cultures (OHSCs). In OHSCs, de novo PV-expression begins shortly after the onset of culturing, and the number of ependymal cells implicated in this process increases with time. PV-immunopositive ependymal cells aggregate and form compact cell clusters, which are characterized by lumen-formation and beating cilia. Scratches inflicted on such clusters with a sharp knife are rapidly closed. Exposure of OHSCs to NF-КB-inhibitors and to antioxidants reduces PV-expression in ependymal cells, thereby implicating injury-induced inflammation in this process. Indeed, in vivo stab injury enhances PV-expression in ependymal cells adjacent to the lesion, whereas neuraminidase denudation is without effect. PV-knock-out mice manifest an impaired wound-healing response to in vivo injury, and a reduced scratch-wound reparation capacity in OHSCs. Whole-transcriptome analysis of ependymal-cell clusters in OHSCs revealed down-regulation of genes involved in cytoskeletal rearrangement, cell motility and cell adhesion in PV-knock out mice as compared with wild-type mice. Our data indicate that the injury-triggered up-regulation of PV-expression is mediated by inflammatory cytokines, and promotes the motility and adhesion of ependymal cells, thereby contributing to leakage closure by the re-establishment of a continuous ependymal layer.

Involvement of the penta-EF-hand protein Pef1p in the Ca2+-dependent regulation of COPII subunit assembly in Saccharomyces cerevisiae

Although it is well established that the coat protein complex II (COPII) mediates the transport of proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus, the regulation of the vesicular transport event and the mechanisms that act to counterbalance the vesicle flow between the ER and Golgi are poorly understood. In this study, we present data indicating that the penta-EF-hand Ca²⁺-binding protein Pef1p directly interacts with the COPII coat subunit Sec31p and regulates COPII assembly in Saccharomyces cerevisiae. ALG-2, a mammalian homolog of Pef1p, has been shown to interact with Sec31A in a Ca²⁺-dependent manner and to have a role in stabilizing the association of the Sec13/31 complex with the membrane. However, Pef1p displayed reversed Ca²⁺ dependence for Sec13/31p association; only the Ca²⁺-free form of Pef1p bound to the Sec13/31p complex. In addition, the influence on COPII coat assembly also appeared to be reversed; Pef1p binding acted as a kinetic inhibitor to delay Sec13/31p recruitment. Our results provide further evidence for a linkage between Ca²⁺-dependent signaling and ER-to-Golgi trafficking, but its mechanism of action in yeast seems to be different from the mechanism reported for its mammalian homolog ALG-2.

Vesicular calcium regulates coat retention, fusogenicity, and size of pre-Golgi intermediates

This study establishes a role for luminal Ca²⁺ in ER/Golgi transport organelles and elucidates an effector mechanism involving the EF-hand protein ALG-2 and regulation of COPII coat retention.

The significance and extent of Ca²⁺ regulation of the biosynthetic secretory pathway have been difficult to establish, and our knowledge of regulatory relationships integrating Ca²⁺ with vesicle coats and function is rudimentary. Here, we investigated potential roles and mechanisms of luminal Ca²⁺ in the early secretory pathway. Specific depletion of luminal Ca²⁺ in living normal rat kidney cells using cyclopiazonic acid (CPA) resulted in the extreme expansion of vesicular tubular cluster (VTC) elements. Consistent with this, a suppressive role for vesicle-associated Ca²⁺ in COPII vesicle homotypic fusion was demonstrated in vitro using Ca²⁺ chelators. The EF-hand–containing protein apoptosis-linked gene 2 (ALG-2), previously implicated in the stabilization of sec31 at endoplasmic reticulum exit sites, inhibited COPII vesicle fusion in a Ca²⁺-requiring manner, suggesting that ALG-2 may be a sensor for the effects of vesicular Ca²⁺ on homotypic fusion. Immunoisolation established that Ca²⁺ chelation inhibits and ALG-2 specifically favors residual retention of the COPII outer shell protein sec31 on pre-Golgi fusion intermediates. We conclude that vesicle-associated Ca²⁺, acting through ALG-2, favors the retention of residual coat molecules that seem to suppress membrane fusion. We propose that in cells, these Ca²⁺-dependent mechanisms temporally regulate COPII vesicle interactions, VTC biogenesis, cargo sorting, and VTC maturation.

The effect of adenosine 5'-triphosphate on calcium mobilization and cell proliferation in cervical cancer cells

OBJECTIVE

To know the effect of adenosine 5'-triphosphate (ATP) on intracellular calcium level and cell proliferation in cervical cancer cells.

STUDY DESIGN

Four different human cervical cancer cell lines (Caski, C33A, HeLaS3 and SiHa) were used in this study. The change of intracellular calcium level, cell proliferation and the activity of proliferation- and calcium-related transcription factors by extracellular ATP were examined in these cell lines.

RESULTS

Extracellular ATP induced calcium mobilization, cell proliferation and the activation of NF-kappaB in all cell lines used.

CONCLUSION

These results suggest that calcium mobilization and NF-kappaB dependent signaling pathway play an important role in the cell proliferation by ATP in cervical cancer.

Calcium: a fundamental regulator of intracellular membrane fusion?

For many years, it has been known that an increase in cytosolic calcium triggers the fusion of secretory granules and synaptic vesicles with the plasma membrane. However, the role of calcium in the intracellular membrane-fusion reactions that coordinate the secretory and endocytic pathways has been less clear. Initially, there was accumulating evidence to indicate that a focally localized and transient calcium signal is required to trigger even those fusion events formerly classified as 'constitutive'-that is, those that normally occur in the absence of global cytosolic calcium increases. Therefore, calcium seemed to be a required fundamental co-factor underlying all biological membrane-fusion steps, perhaps with a conserved mechanism of action. However, although such unification would be gratifying, new data indicate that several intracellular fusion events do not require calcium after all. In this review, the evidence for calcium requirements and its modes of action in constitutive trafficking are discussed. As a challenging perspective, I suggest that the specific absence of calcium requirements for some transport steps in fact expands the function of calcium in trafficking, because divergent luminal calcium concentrations and requirements for fusion might increase the specificity with which intracellular membrane-fusion partners are determined.

MscCa Regulation of Tumor Cell Migration and Metastasis

The acquisition of cell motility is a required step in order for a cancer cell to migrate from the primary tumor and spread to secondary sites (metastasize). For this reason, blocking tumor cell migration is considered a promising approach for preventing the spread of cancer. However, cancer cells just as normal cells can migrate by several different modes referred to as "amoeboid," "mesenchymal," and "collective cell." Under appropriate conditions, a single cell can switch between modes. A consequence of this plasticity is that a tumor cell may be able to avoid the effects of an agent that targets only one mode by switching modes. Therefore, a preferred strategy would be to target mechanisms that are shared by all modes. This chapter reviews the evidence that Ca(2+) influx via the mechanosensitive Ca(2+)-permeable channel (MscCa) is a critical regulator of all modes of cell migration and therefore represents a very good therapeutic target to block metastasis.

Increases in mitochondrial reactive oxygen species trigger hypoxia-induced calcium responses in pulmonary artery smooth muscle cells

Mitochondria have been implicated as a potential site of O(2) sensing underlying hypoxic pulmonary vasoconstriction (HPV), but 2 disparate models have been proposed to explain their reaction to hypoxia. One model proposes that hypoxia-induced increases in mitochondrial reactive oxygen species (ROS) generation activate HPV through an oxidant-signaling pathway, whereas the other proposes that HPV is a result of decreased oxidant signaling. In an attempt to resolve this debate, we use a novel, ratiometric, redox-sensitive fluorescence resonance energy transfer (HSP-FRET) probe, in concert with measurements of reduced/oxidized glutathione (GSH/GSSG), to assess cytosolic redox responses in cultured pulmonary artery smooth muscle cells (PASMCs). Superfusion of PASMCs with hypoxic media increases the HSP-FRET ratio and decreases GSH/GSSG, indicating an increase in oxidant stress. The antioxidants pyrrolidinedithiocarbamate and N-acetyl-l-cysteine attenuated this response, as well as the hypoxia-induced increases in cytosolic calcium ([Ca(2+)](i)), assessed by the Ca(2+)-sensitive FRET sensor YC2.3. Adenoviral overexpression of glutathione peroxidase or cytosolic or mitochondrial catalase attenuated the hypoxia-induced increase in ROS signaling and [Ca(2+)](i). Adenoviral overexpression of cytosolic Cu, Zn-superoxide dismutase (SOD-I) had no effect on the hypoxia-induced increase in ROS signaling and [Ca(2+)](i), whereas mitochondrial matrix-targeted Mn-SOD (SOD-II) augmented [Ca(2+)](i). The mitochondrial inhibitor myxothiazol attenuated the hypoxia-induced changes in the ROS signaling and [Ca(2+)](i), whereas cyanide augmented the increase in [Ca(2+)](i). Finally, simultaneous measurement of ROS and Ca(2+) signaling in the same cell revealed that the initial increase in these 2 signals could not be distinguished temporally. These results demonstrate that hypoxia triggers increases in PASMC [Ca(2+)](i) by augmenting ROS signaling from the mitochondria.

The Ca2+-binding protein ALG-2 is recruited to endoplasmic reticulum exit sites by Sec31A and stabilizes the localization of Sec31A

The formation of transport vesicles that bud from endoplasmic reticulum (ER) exit sites is dependent on the COPII coat made up of three components: the small GTPase Sar1, the Sec23/24 complex, and the Sec13/31 complex. Here, we provide evidence that apoptosis-linked gene 2 (ALG-2), a Ca(2+)-binding protein of unknown function, regulates the COPII function at ER exit sites in mammalian cells. ALG-2 bound to the Pro-rich region of Sec31A, a ubiquitously expressed mammalian orthologue of yeast Sec31, in a Ca(2+)-dependent manner and colocalized with Sec31A at ER exit sites. A Ca(2+) binding-deficient ALG-2 mutant, which did not bind Sec31A, lost the ability to localize to ER exit sites. Overexpression of the Pro-rich region of Sec31A or RNA interference-mediated Sec31A depletion also abolished the ALG-2 localization at these sites. In contrast, depletion of ALG-2 substantially reduced the level of Sec31A associated with the membrane at ER exit sites. Finally, treatment with a cell-permeable Ca(2+) chelator caused the mislocalization of ALG-2, which was accompanied by a reduced level of Sec31A at ER exit sites. We conclude that ALG-2 is recruited to ER exit sites via Ca(2+)-dependent interaction with Sec31A and in turn stabilizes the localization of Sec31A at these sites.

Spectral imaging microscopy demonstrates cytoplasmic pH oscillations in glial cells

Glial cells exhibit distinct cellular domains, somata, and filopodia. Thus the cytoplasmic pH (pH(cyt)) and/or the behavior of the fluorescent ion indicator might be different in these cellular domains because of distinct microenvironments. To address these issues, we loaded C6 glial cells with carboxyseminaphthorhodafluor (SNARF)-1 and evaluated pH(cyt) using spectral imaging microscopy. This approach allowed us to study pH(cyt) in discrete cellular domains with high temporal, spatial, and spectral resolution. Because there are differences in the cell microenvironment that may affect the behavior of SNARF-1, we performed in situ titrations in discrete cellular regions of single cells encompassing the somata and filopodia. The in situ titration parameters apparent acid-base dissociation constant (pK'(a)), maximum ratio (R(max)), and minimum ratio (R(min)) had a mean coefficient of variation approximately six times greater than those measured in vitro. Therefore, the individual in situ titration parameters obtained from specific cellular domains were used to estimate the pH(cyt) of each region. These studies indicated that glial cells exhibit pH(cyt) heterogeneities and pH(cyt) oscillations in both the absence and presence of physiological HCO(3)(-). The amplitude and frequency of the pH(cyt) oscillations were affected by alkalosis, by acidosis, and by inhibitors of the ubiquitous Na(+)/H(+) exchanger- and HCO(3)(-)-based H(+)-transporting mechanisms. Optical imaging approaches used in conjunction with BCECF as a pH probe corroborated the existence of pH(cyt) oscillations in glial cells.

Angiostatin induces intracellular acidosis and anoikis in endothelial cells at a tumor-like low pH

Angiostatin inhibits angiogenesis by binding to endothelial cells (ECs) lining the vasculature of growing tumors. These cells are in a dynamic state during angiogenesis and are thus not firmly attached to the extracellular matrix. This makes them more vulnerable to anoikis, a process resulting in cell death initiated by or promoted by loss of attachment. Another potential source of EC vulnerability during tumor angiogenesis is that tumor extracellular pH is typically lower than in normal tissues. This presents an additional challenge to ECs in terms of maintaining ionic homeostasis. We report here that the lethality of angiostatin is significantly enhanced both by reduced matrix attachment during exposure and lowered extracellular pH (pH(e)). Another effect of angiostatin at reduced pH(e) is a decreased intracellular pH (pH(i)). These effects were observed in three model systems: aortic ring sprouts, ECs during tube formation, and ECs in a scratch/migration assay. In these three dynamic assays, angiostatin-induced cell death and intracellular acidification were clearly seen when pH(e) was reduced to 6.7. The intracellular acidification was far greater than that induced by pH(e) reduction alone. In contrast, the effect of angiostatin on pH(i) and on viability were not observed in a subconfluent monolayer in which the cells were allowed to attach to substrate for 48 h prior to exposure to angiostatin. These data suggest that low pH(e) and reduced adhesion to matrix play a role in the specificity of angiostatin for tumor neovasculature in contrast to wound healing and other normal angiogenic processes. The results also implicate roles for both pH(e) and pH(i) regulation in the mechanism of angiostatin action.

Different sensitivity to nephrotoxic agents and osmotic stress in proximal tubular and collecting duct cell lines derived from transgenic mice

We established six renal tubular cell lines from definite tubular areas of the kidney of transgenic mice harboring tsSV40 large T-antigen gene. Three are proximal tubular cell lines prepared from the S(1), S(2) and S(3) segments of the proximal tubule and the others are collecting duct cell lines obtained from cortical, outer medullary and inner medullary collecting ducts (CCD, OMCD and IMCD, respectively). To verify the growth properties of these cell lines under different temperature conditions (33 and 39 degrees C), two representative cells were chosen from the proximal tubule (S(1) cells) and from the collecting duct (IMCD cells). From these cells, a daily change in cell number was evaluated as a parameter of cell growth. As might be expected, cell numbers of these cells increased only at 33 degrees C. Similar patterns were also observed with the other cell lines. To observe the different sensitivity to nephrotoxic agents in proximal tubular cell lines, the cells were exposed to nephrotoxic agent, gentamicin, ochratoxin A or cisplatin. Gentamicin (1 mg/ml) dose-dependently decreased cellular ATP levels of the S(1) cells only. In contrast, the effect of ochratoxin A (10(-6) M) was most pronounced in the S(2) cells, and that of cisplatin (10 microg/ml) in the S(3) cells. To characterize collecting duct cell lines, a hyperosmotic challenge of 700 or 1100 mOsm/l was applied to the cells. At an isoosmotic condition of 300 mOsm/l, the number of cells from the collecting ducts, regardless of their origin, increased continuously during the culture period of 4 days. At an osmotic concentration of 700 mOsm/l, the number of CCD cells decreased, while OMCD cells showed a gradual but a significant increase in cell numbers throughout the culture period. IMCD cells, however, proliferated even at a concentration as high as 1100 mOsm/l, although an initial decrease in cell number was noted on the first day of culture. For confirmation of intracellular free calcium ([Ca(2+)](i)) mobilization, cells were treated with ATP and bradykinin. The [Ca(2+)](i) was increased significantly and immediately by ATP (10(-4) M) in S(1) cells and bradykinin (10(-7) M) in IMCD cells. From the results obtained, it is indicated that renal tubular cell lines from transgenic mice have different sensitivities to nephrotoxic or osmotic stress showing the conservation of the functional characters of the definite part it originated from.

A role for calcium in stabilizing transport vesicle coats

Calcium has been implicated in regulating vesicle fusion reactions, but its potential role in regulating other aspects of protein transport, such as vesicle assembly, is largely unexplored. We find that treating cells with the membrane-permeable calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM), leads to a dramatic redistribution of the vesicle coat protein, coatomer, in the cell. We have used the cell-free reconstitution of coat-protomer I (COPI) vesicle assembly to characterize the mechanisms of this redistribution. We find that the recovery of COPI-coated Golgi vesicles is inhibited by the addition of BAPTA to the cell-free vesicle budding assay. When coatomer-coated membranes are incubated in the presence of calcium chelators, the membranes "uncoat," indicating that calcium is necessary for maintaining the integrity of the coat. This uncoating is reversed by the addition of calcium. Interestingly, BAPTA, a calcium chelator with fast binding kinetics, is more potent at uncoating the coatomer-coated membrane than EGTA, suggesting that a calcium transient or a calcium gradient is important for stabilizing COPI vesicle coat. The primary target for the effects of calcium on coatomer recruitment is a step that occurs after ADP-ribosylation factor binding to the membrane. We suggest that a calcium gradient may serve to regulate the timing of vesicle uncoating.

Purinoceptor-mediated calcium mobilization and proliferation in HaCaT keratinocytes

To investigate the effect of nucleotides on cytosolic free calcium mobilization and proliferation activity in HaCaT keratinocytes, nucleotides-induced intracellular free calcium concentration ([Ca(2+)](i)) and cell proliferation observed. [Ca(2+)](i) to the extracellular nucleotides was determined using Ca(2+) sensitive indicator, Fura-2/AM with digital video fluorescence imaging microscopy, and cell proliferation was evaluated by counting of cell number. An adenosine 5'-triphosphate (ATP)-induced [Ca(2+)](i) increase was observed from the concentration of 10(-8) M and was more conspicuous at higher concentrations in a concentration-dependent manner. Additionally, other nucleotides such as ADP, UTP, and 2-me-S-ATP also induced a [Ca(2+)](i) increase in a concentration-dependent manner. However, adenosine induced a slight increase of [Ca(2+)](i) only at 10(-3) M. alpha,-methylene-ATP did not evoke any rise in [Ca(2+)](i). The maximal response observed occurred with ATP and UTP at a concentration of 10(-4) M. The ATP-induced transient [Ca(2+)](i) increase was attenuated by the pretreatment with phospholipase C (PLC) inhibitor, U-73122 (10 microM) for 30 min. ATP-induced [Ca(2+)](i) increase and cell proliferation were inhibited by putative P2Y receptor antagonist, suramin (10(-4) M). When the HaCaT cells were stimulated with nucleotides on a concentration of 10(-4) M and cultured for 5 days, the order of effect on cell proliferation was observed to be ATP>UTP>ADP>2-me-S-ATP. Based on these results, we suggest that extracellular ATP stimulate HaCaT keratinocytes proliferation via purinoceptor-mediated [Ca(2+)](i) mobilization

ER calcium and the functions of intracellular organelles

Cytosolic calcium has long been known as a second messenger of major significance. Recently it has become apparent that calcium stored in cellular organelles can also be an important regulator of cellular functions. The endoplasmic reticulum (ER) is usually the largest store of releasable calcium in the cell. The diverse signalling functions of calcium populating the endoplasmic reticulum and its interactions with other organelles are illustrated in Figure ?? and described in this paper.

Purinoceptor-mediated calcium mobilization and cellular proliferation in cultured bovine corneal endothelial cells

In the present study, we investigated the effect of adenosine triphosphate (ATP) on cytosolic free calcium mobilization and mitogenic activity in cultured bovine corneal endothelial cells (BCEC). The [Ca2+]i was determined using a Ca2+ sensitive indicator, Fura-2/AM, and cell proliferation was evaluated by counting the cell number. ATP, its metabolites and analogs caused transient increase in [Ca2+]i in a concentration-dependent manner (10(-7) M-10(-3) M) and the potency of agonists was ordered as follows: 2-methylthio-ATP > uridine triphosphate > ATP > adenosine diphosphate. Adenosine monophosphate and adenosine did not affect [Ca2+]i. ATP (10(-4) M) also promoted the accumulation of inositol trisphosphate (IP3). The ATP-induced transient [Ca2+]i increase and IP3 accumulation were attenuated by pretreatment with a phospholipase C inhibitor, U-73122 (5 microM), for 30 min. ATP (10(-5) M) significantly enhanced the proliferation of BCEC. ATP-induced [Ca2+]i increase and cell proliferation were inhibited by a purinoceptor antagonist, suramin (10(-4) M). Thus, the present study indicates that BCEC contain P2 purinoceptors that regulate their proliferation.

Cell proliferation is associated with enhanced capacitative Ca(2+) entry in human arterial myocytes

Depletion of Ca(2+) stores in the sarcoplasmic reticulum (SR) activates extracellular Ca(2+) influx via capacitative Ca(2+) entry (CCE). Here, CCE levels in proliferating and growth-arrested human pulmonary artery smooth muscle cells (PASMCs) were compared by digital imaging fluorescence microscopy. Resting cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)) in proliferating PASMCs was twofold higher than that in growth-arrested cells. Cyclopiazonic acid (CPA; 10 microM), which inhibits SR Ca(2+)-ATPase and depletes inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores, transiently increased [Ca(2+)](cyt) in the absence of extracellular Ca(2+). The addition of 1.8 mM Ca(2+) to the extracellular solution in the presence of CPA induced large increases in [Ca(2+)](cyt), indicative of CCE. The CPA-induced SR Ca(2+) release in proliferating PASMCs was twofold higher than that in growth-arrested cells, whereas the transient rise of [Ca(2+)](cyt) due to CCE was fivefold greater in proliferating cells. CCE was insensitive to nifedipine but was significantly inhibited by 50 mM K(+), which reduces the driving force for Ca(2+) influx, and by 0.5 mM Ni(2+), a putative blocker of store-operated Ca(2+) channels. These data show that augmented CCE is associated with proliferation of human PASMCs and may be involved in stimulating and maintaining cell growth.

Connexin-aequorin chimerae report cytoplasmic calcium environments along trafficking pathways leading to gap junction biogenesis in living COS-7 cells

The cytoplasmic calcium environments along membrane trafficking pathways leading to gap junction intercellular communication channels at the plasma membrane were studied. Connexins, the constitutive proteins of gap junctions, were fused at their carboxyl terminus to the calcium-sensitive photoprotein aequorin. The cellular location of the chimeric proteins was determined by immunolocalization and subcellular fractionation. The generation of functional gap junctions by the connexin chimerae was monitored by the ability of the cells to exchange small dyes. Although aequorin fused to connexin-26 was nonfunctional, its ability to report Ca2+ and to form functional gap junctions was rescued by replacement of its cytoplasmic carboxyl tail with that of connexin-43. In COS-7 cells expressing these connexin-aequorin chimerae, calcium levels below the plasma membrane were higher (approximately 5 microM) than those in the cytoplasm (approximately 100 nM); gap junctions were able to transfer dyes under these conditions. Cytoplasmic levels of free calcium surrounding the ERGIC/Golgi reported by connexin-43 chimera (approximately 420 nM) were twice those measured by connexin-32 chimera (approximately 200 nM); both chimerae measured calcium levels substantially higher than those reported by a connexin-26 chimera (approximately 130 nM). Dispersion of the ERGIC and Golgi complex by brefeldin A led to a marked reduction in calcium levels. The results show that the various connexin chimerae were located in spatially different subcellular stores and that the ERGIC/Golgi regions of the cell maintain heterogeneous cytoplasmic domains of calcium. The implications of the subplasma-membrane Ca2+ levels on the gating of gap junctions are discussed.

A role for local calcium gradients upon hypoxic injury in human umbilical vein endothelial cells (HUVEC)

Upon hypoxic injury, bleb formation is an early event of cell damage observed in a variety of cell types. Although a rise in cytosolic free Ca2+ ([Ca2+]i) has been considered to be involved in this process, the exact relationship between these phenomena remains ill-defined. In order to examine the relationship between bleb formation, and [Ca2+]i or nuclear free Ca2+ ([Ca2+]n), we analyzed [Ca2+]i and [Ca2+]n in HUVEC during hypoxic injury using confocal laser scanning microscopy. [Ca2+]i and [Ca2+]n were measured using Fluo-3, and cell viability and mitochondrial membrane potential were assessed by the exclusion of propidium iodide (PI) and rhodamine 123, respectively. After the initiation of hypoxia, [Ca2+]i and [Ca2+]n rose gradually up to 15 min reaching peak values of 447 +/- 62 and 516 +/- 105 nM, respectively, which was accompanied by a decrease in rhodamine 123 fluorescence and an increase in PI-stained cells. Bleb formation was observed after [Ca2+]i and [Ca2+]n had reached their peak values and the number of blebs increased thereafter. Confocal z-sectioning images revealed a localized increase in [Ca2+]i at the bleb forming site and this localized elevation in [Ca2+]i was observed before bleb formation in the corresponding area. In conclusion, bleb formation induced by hypoxic stress appears to involve Ca(2+)-dependent reactions that are linked to a regional elevation of [Ca2+]i.

Molecular basis and function of voltage-gated K+ channels in pulmonary arterial smooth muscle cells

K(+)-channel activity-mediated alteration of the membrane potential and cytoplasmic free Ca2+ concentration ([Ca2+]cyt) is a pivotal mechanism in controlling pulmonary vasomotor tone. By using combined approaches of patch clamp, imaging fluorescent microscopy, and molecular biology, we examined the electrophysiological properties of K+ channels and the role of different K+ currents in regulating [Ca2+]cyt and explored the molecular identification of voltage-gated K+ (KV)- and Ca(2+)-activated K+ (KCa)-channel genes expressed in pulmonary arterial smooth muscle cells (PASMC). Two kinetically distinct KV currents [IK(V)], a rapidly inactivating (A-type) and a noninactivating delayed rectifier, as well as a slowly activated KCa current [IK(Ca)] were identified. IK(V) was reversibly inhibited by 4-aminopyridine (5 mM), whereas IK(Ca) was significantly inhibited by charybdotoxin (10-20 nM). K+ channels are composed of pore-forming alpha-subunits and auxiliary beta-subunits. Five KV-channel alpha-subunit genes from the Shaker subfamily (KV1.1, KV1.2, KV1.4, KV1.5, and KV1.6), a KV-channel alpha-subunit gene from the Shab subfamily (KV2.1), a KV-channel modulatory alpha-subunit (KV9.3), and a KCa-channel alpha-subunit gene (rSlo), as well as three KV-channel beta-subunit genes (KV beta 1.1, KV beta 2, and KV beta 3) are expressed in PASMC. The data suggest that 1) native K+ channels in PASMC are encoded by multiple genes; 2) the delayed rectifier IK(V) may be generated by the KV1.1, KV1.2, KV1.5, KV1.6, KV2.1, and/or KV2.1/KV9.3 channels; 3) the A-type IK(V) may be generated by the KV1.4 channel and/or the delayed rectifier KV channels (KV1 subfamily) associated with beta-subunits; and 4) the IK(Ca) may be generated by the rSlo gene product. The function of the KV channels plays an important role in the regulation of membrane potential and [Ca2+]cyt in PASMC.

Calcium-containing organelles display unique reactivity to chemical stimulation in cultured hippocampal neurons

Cultured rat hippocampal neurons grown on glass coverslips for 1-3 weeks were loaded with the calcium-sensitive fluorescent dye Fluo-3 and viewed with a confocal laser scanning microscope. Large pyramidal-shaped neurons were found to contain dye-accumulating organelles in their somata, primarily around nuclei and near the base of their primary dendrites. These organelles varied in size and increased in density over weeks in culture, and were not colocalized with the endoplasmic reticulum or with mitochondria. The Fluo-3 fluorescence in these calcium-containing organelles (CCOs) was transiently quenched by exposure to Mn2+, indicating that the dye is a genuine [Ca2+] reporter and is not just a site of accumulating Fluo-3 dye. Recovery of fluorescence in the CCOs after washout of Mn2+ involved activation of a thapsigargin-sensitive process. CCOs responded to stimuli that evoke a rise of cytosolic [Ca2+] ([Ca]i) in a unique manner; perfusion of caffeine caused a prolonged rise of [Ca] in the CCOs ([Ca]C), whereas it caused only a transient rise of [Ca]i. Pulse application of caffeine also caused a faster effect on [Ca]C than on [Ca]i. Glutamate caused a transient rise of both [Ca]i and [Ca]C, followed by a prolonged fall of only [Ca]C to below rest level. This fall was blocked by preincubation with thapsigargin. Ryanodine blocked the cytosolic effects of caffeine but not its effect on [C]C. A clear distinction between CCOs and the known calcium stores was seen in digitonin-permeabilized cells; in these, remaining Fluo-3 reported changes in store calcium, i.e., caffeine caused a reduction in Fluo-3 fluorescence in permeabilized cells, whereas it still caused an increase in [Ca]C. A possible role of CCOs in regulation of release of calcium from ryanodine-sensitive stores was indicated by the observation that CCO-containing cells exhibited a larger and faster response to caffeine than cells that did not have them. We propose that CCOs constitute a unique functional compartment involved in release of calcium from calcium-sensitive stores.

Norepinephrine-mediated calcium signaling is altered in vascular smooth muscle of diabetic rat

We studied the influence of diabetes on norepinephrine (NE)-induced changes in intracellular free Ca2+ levels (receptor-mediated Ca2+ signaling) in single tail artery vascular smooth muscle (VSM) cells. VSM cells from 12-16 week streptozotocin-induced diabetic (SID) rats showed an increase in sensitivity to NE when compared to control VSM cells in that the concentration of NE needed to elicit half maximal response of the initial Ca2+ transient was reduced more than 4-fold though the maximal response attained was apparently reduced. In addition, the slope factor (steepness) of the dose-response relation was lowered 4-fold. Moreover, VSM cells of diabetic animals had a higher incidence of NE-induced Ca2+ oscillatory responses. The shift of the dose-response curve to the left, coupled with a higher incidence of oscillations, indicate that the noradrenergic receptor-mediated Ca2+ signaling pathways in tail artery VSM of diabetic rat may be altered.

Dynamic properties of an inositol 1,4,5-trisphosphate- and thapsigargin-insensitive calcium pool in mammalian cell lines

The functional characteristics of a nonacidic, inositol 1,4,5-trisphosphate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ pool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensitive store with an agonist coupled to phospholipase C activation and/or thapsigargin. The amount of Ca2+ trapped within this pool increased significantly after a prolonged elevation of intracellular Ca2+ concentration elicited by activation of Ca2+ influx. This pool was affected neither by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distribution across the cytosol, as revealed by confocal microscopy, and its insensitivity to brefeldin A make localization within the Golgi complex unlikely. A proton gradient as the driving mechanism for Ca2+ uptake was excluded since ionomycin is inefficient in releasing Ca2+ from acidic pools and Ca2+ accumulation/release in/from this store was unaffected by monensin or NH4Cl, drugs known to collapse organelle acidic pH gradients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, distinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cytological nature and functional role of this Ca2+ storage compartment are discussed.

Separate analysis of nuclear and cytosolic Ca2+ concentrations in human umbilical vein endothelial cells

Ca2+ concentration inside human umbilical vein endothelial cells was studied separately in cytosol and nucleus by a confocal laser scanning microscopy using fluo-3. The in vivo calibration curve for cytosol and nucleus showed good linearity between fluorescence intensity and Ca2+ concentration in cytosol ([Ca2+]i) and nuclei ([Ca2+]n). After calibration, [Ca2+]n was constantly higher than [Ca2+]i before and after the chelation of extracellular Ca2+ suggesting an active Ca2+ accumulation system on nuclear membrane. [Ca2+]n was also constantly higher than [Ca2+]i after the stimulation of thrombin (0.05 U/ml), FCS (10%), and thapsigargin (Tsg, 1 microM). The temporal change of [Ca2+]n and [Ca2+]i was identical, and [Ca2+]i gradient towards the nucleus and peripheral or central [Ca2+]n rise was observed after these stimulations. From these results, [Ca2+]n is not only regulated by the active Ca2+ accumulation system on nuclear membrane at rest but also the generation of inositol-triphosphate. FCS caused heterogeneous [Ca2+]n or [Ca2+]i rise from cell to cell; single spike or oscillatory change of [Ca2+]n and [Ca2+]i was observed in about 56% of cells, which were completely abolished by the chelation of extracellular Ca2+, suggesting that FCS stimulated [Ca2+]n and [Ca2+]i rise solely depending on Ca2+ influx from extracellular medium. The higher concentration of [Ca2+]n and heterogeneous [Ca2+]n rise may have important roles in nuclear-specific cellular responses.

Lasting effects of glutamate on nuclear calcium concentration in cultured rat hippocampal neurons: regulation by calcium stores

1. Changes in free intracellular calcium concentrations ([Ca2+]i) were measured in the nucleus and perinuclear regions of cultured rat hippocampal neurons using either fura-2 or fluo-3 calcium indicators. 2. Brief application of glutamate caused a transient rise of [Ca2+]i in all cell compartments, which recovered to pre-drug levels in all but the nuclear region. The new, higher level of nuclear calcium ([Ca2+]n) was sustained for as long as the cell was monitored. 3. The new level of [Ca2+]n was dependent on the magnitude of the calcium transient, and was higher in older cells in culture, but it did not affect responses to subsequent applications of glutamate. 4. The sustained elevation of [Ca2+]n was prevented by drugs which affect calcium stores (caffeine, ryanodine and Ruthenium Red), indicating that an extranuclear calcium store interacts with [Ca2+]n.

Modulation of two functionally distinct Ca2+ stores in astrocytes: role of the plasmalemmal Na/Ca exchanger

Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura-2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca(2+)-ATPase, induced a transient, four-fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca(2+)-sensitive stores, induced a two-fold increase in [Ca2+]cyt. The CPA- and CAF-sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA- and CAF-induced Ca2+ transients. The amount of Ca2+ releasable from the IP3(CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage-dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3- (CPA-) sensitive, and the smaller, Ca(2+)- (CAF-) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.

Modulation of two functionally distinct Ca2+ stores in astrocytes: role of the plasmalemmal Na/Ca exchanger

Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura-2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca(2+)-ATPase, induced a transient, four-fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca(2+)-sensitive stores, induced a two-fold increase in [Ca2+]cyt. The CPA- and CAF-sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA- and CAF-induced Ca2+ transients. The amount of Ca2+ releasable from the IP3(CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage-dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3- (CPA-) sensitive, and the smaller, Ca(2+)- (CAF-) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.

Fluorometric measurement of the intracellular free Ca(2+)-concentration in the ciliate Didinium nasutum using Fura-2

We developed an experimental approach to measure somatic and ciliary Ca(2+)-signals in the ciliate Didinium under voltage clamp conditions using the dye Fura-2. Intracellular pressure injection of Fura-2 molecules did not alter electrophysiological membrane properties besides an expected buffering effect. The intracellular free Ca(2+)-concentration was determined at 2 x 10(-7) M. During membrane excitation, this resting value increased in the cilia; a quantification was not feasible. Within the cell soma, however, the Ca(2+)-level was unchanged within the physiological range of the membrane potential (-70 mV to 0 mV). Increasing the driving force for Ca(2+)-ions via strong hyperpolarization (potentials negative to -200 mV) a centripetal increase in the somatic Ca2+ concentration was found. Our results support the hypothesis that Ca2+ is the intracellular messenger in rapid electromotor coupling in ciliates.

Imaging of caffeine-inducible release of intracellular calcium in cultured embryonic mouse telencephalic neurons

To gain a better understanding of Ca(2+)-induced Ca2+ release in central neurons, we have studied the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by application of caffeine to cells cultured from embryonic mouse telencephalon (hippocampus or cortex). The magnitudes and distributions of changes in [Ca2+]i in neuron somata were measured by quantitative video microscopy. We observed that application of caffeine to pyramidally shaped neurons typically initiated an increase in [Ca2+]i in the cytoplasmic region between the nucleus and the base of a major dendrite. [Ca2+] in this region increased over a period of 3 to 6 s and was followed by, with a slight delay, a surge of Ca2+ that moved across the soma and into or over the nucleus. Similar Ca2+ responses to caffeine were observed in Ca(2+)-containing and nominally Ca(2+)-free external solutions, suggesting that caffeine was inducing Ca2+ release from intracellular stores. Ca2+ responses to caffeine were potentiated by inducing a tonic Ca2+ influx through N-methyl-D-aspartate (NMDA)-type glutamate receptors activated by 0.3 microM glutamate and multiple responses to caffeine could be elicited by using this Ca2+ influx to refill the intracellular stores. Ryanodine inhibition of caffeine-induced Ca2+ release was use- and concentration-dependent; the median effective concentration EC50 for ryanodine declined from 22 microM for the first application of caffeine to 20 nM for the fourth. We conclude, based on these responses to caffeine, that ryanodine-sensitive mechanisms of intracellular Ca2+ release are active in hippocampal and cortical neurons and may be involved in generation of directed Ca2+ waves that engulf the nucleus.

Changes in either cytosolic or nucleoplasmic inositol 1,4,5-trisphosphate levels can control nuclear Ca2+ concentration

The free nucleoplasmic Ca2+ concentration ([Ca2+]n) may regulate many nuclear events, such as gene transcription. Since the nucleus may possess the enzymes necessary to generate the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), and because the nuclear envelope may enclose an Ins(1,4,5)P3-releasable Ca2+ store, we tested the hypothesis that nuclear and/or cytosolic levels of Ins(1,4,5)P3 can control [Ca2+]n. To assay [Ca2+]n, we measured the fluorescence of the Ca2+ indicator fluo 3 in the nucleus of Xenopus oocytes by confocal microscopy. When we injected Ins(1,4,5)P3 into the cytosol, [Ca2+]n increased. This increase in [Ca2]n still occurred when heparin was present in the nucleus, but was abolished when heparin was present in the cytosol, indicating that cytosolic Ins(1,4,5)P3 levels could control [Ca2+]n. When we injected Ins(1,4,5)P3 directly into the nucleus, [Ca2+]n increased, even when heparin was present in the cytosol, indicating that Ins(1,4,5)P3 could control [Ca2+]n from within the nucleus. These results provide functional evidence for Ins(1,4,5)P3 receptors facing the nucleoplasm and raise the possibility that a phosphoinositide cycle situated at the nuclear membranes can control Ca(2+)-dependent nuclear functions.

Relationship between [Ca2+] changes in nucleus and cytosol

The free calcium concentration in nucleus ([Ca2+]n) and in cytoplasm ([Ca2+]c) of single cells were estimated by confocal laser microscopy using the Ca(2+)-indicator Indo-1. It is shown that in various cell types a nucleo-cytosolic Ca(2+)-gradient is present at rest and during stimulation. The direction and the extent of the nucleo-cytosolic Ca(2+)-gradient may vary with the cell type, differentiation status, phosphorylation conditions and also with the type of agonist. Evidence is given for the role of extra- and intranuclear storage sites as well as for Ca(2+)-influx. Finally potential artefactual interference with the measurements is discussed.

The nucleus is insulated from large cytosolic calcium ion changes

Extracellular events regulate functions in the cell nucleus by means of calcium ions acting through effector enzymes. Recently, the traditional view of the nuclear pore as freely permeable to small ions has been questioned as a result of reports that nuclear calcium can be regulated independently of cytosolic calcium. We have used confocal microscopy of fluorescent Ca2+ indicators to investigate the Ca2+ dynamics between cytosol and nucleus in neurons. We find that a previously reported amplification of Ca2+ changes in the nucleus is a measurement artefact. Small changes of cytosolic Ca2+ cause equally rapid changes in nuclear Ca2+, consistent with the free diffusion of Ca2+ through nuclear pores. In contrast, large cytosolic Ca2+ increases (above 300 nM) are attenuated in the nucleus. Our results show the nuclear envelope shapes but does not block the passage of Ca2+ signals from cytosol to nucleus.

Intracellular calcium mobilization by inositol 1,4,5-trisphosphate: intracellular movements and compartmentalization

Intracellular calcium ion (Ca2+) changes in NIH-3T3 fibroblasts responding to inositol 1,4,5-trisphosphate (IP3) injections have been monitored using high resolution digital imaging of the calcium indicator Fura-2. Ester loaded and microinjected indicator report radically different patterns of Ca2+ change during the IP3 response. These differences arise from intracellular compartmentalization of the ester loaded indicator which can seriously distort reported Ca2+ levels. Prominent among these aberrant responses is a signal in which Ca2+ levels in the cell nucleus appear to exceed those in the rest of the cell, and an apparent slowing of the Ca2+ recovery time-course throughout the cell when temperature is increased. Similar behavior is observed in other cell types. Judicious use of both loading techniques can provide information on Ca2+ movements into organelles that might otherwise escape detection. The Ca2+ rise normally measured in bulk or integrated single cell measurements is a complex mix of cytosol/nucleus and organellar changes. Much, if not all, of the observable organellar change is an accumulation, not release, of Ca2+ following the IP3 injection. The Golgi apparatus is a conspicuous early site for this accumulation, and mitochondria show a large, temperature sensitive uptake that is capable of limiting the maximal Ca2+ change during the response.

Intracellular free Ca2+ in the cell cycle in human fibroblasts: transitions between G1 and G0 and progression into S phase

Intracellular free calcium ([Ca2+]i) has been proposed to play an important part in the regulation of the cell cycle. Although a number of studies have shown that stimulation of quiescent cells with growth factors causes an immediate rise in [Ca2+]i (Rabinovitch et al., 1986; Vincentini and Villereal, 1986; Hesketh et al., 1988; Tucker et al., 1989, Wahl et al., 1990), a causal relationship between the [Ca2+]i transient and the ability of the cells to reenter the cell cycle has not been firmly established. We have found that blocking the mitogen-induced elevation of [Ca2+]i with the cytoplasmic [Ca2+]i buffer dimethyl BAPTA (dmBAPTA) also blocks subsequent entry of cells into S phase. The dose response curves for inhibition of serum stimulation of [Ca2+]i and DNA synthesis by dmBAPTA are virtually identical including an anomalous stimulation observed at low levels of dmBAPTA. Reversal of the [Ca2+]i buffering effect of dmBAPTA by transient exposure of the cells to the Ca2+ ionophore ionomycin also reverses the inhibition of DNA synthesis 20-24 h later. Ionomycin by itself does not stimulate DNA synthesis. These data are consistent with the conclusion that a transient increase in [Ca2+]i occurring shortly after serum stimulation of quiescent fibroblasts is necessary but not sufficient for subsequent entry of the cells into S phase. This study is the first to show a direct relationship between early serum stimulated Cai2+ increase and subsequent DNA synthesis in human cells. It also goes beyond recent studies on BALB/3T3 cells by providing dose response data and demonstrating reversibility, which are strong indications of a cause and effect relationship.

Dissociation of store release from transmembrane influx of calcium in human neutrophils

Release of Ca2+ from intracellular stores was visualised in individual neutrophils in the presence of the Mn2+ or SKF 96365. Influx of Mn2+ quenched fura-2 close to the plasma membrane but did not quench fura-2 at the site of store release. The size and location of the 'cloud' of elevated Ca2+ was unaffected by the channel blocker SKF 96365. Furthermore, the size and location was unaffected by the presence of extracellular Ca2+. This dissociation of transmembrane influx from store release demonstrates that the entry of Ca2+ into the cytosol of neutrophils occurs directly into the cytosol and not via the store site.

Histamine H1 receptors in UC-11MG astrocytes and their regulation of cytoplasmic Ca2+

Experiments were carried out on UC-11MG human astrocytoma cells, a continuous cell line that expresses a broad range of the biochemical and electrophysiological properties found in well-differentiated astrocytes. Because of a number of recent reports that astrocytes may express receptors for a variety of neuro-active substances, we measured the effects of 12 different neurotransmitters on intracellular free Ca2+ (Ca2+i) in UC-11MG cells. Of these neurotransmitters only histamine was found to have a significant effect. Further characterization of the nature of the histamine response showed that UC-11MG cells express mepyramine-sensitive H1 receptors the activation of which causes both mobilization of Ca2+ from intracellular stores and entry of Ca2+ from the extracellular solution. No evidence was found for the presence of H2 receptors. The Ca2+i response was maximal at 300 microM histamine and was attenuated by increasing cell density. We suggest that this neurotransmitter may play a role in astrocytic function in the human CNS.