Cholera toxin increases the rate of antigen-stimulated calcium influx in rat basophilic leukemia cells

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

cAMP-Independent Activation of the Unfolded Protein Response by Cholera Toxin

Cholera toxin (CT) is an AB₅ protein toxin that activates the stimulatory alpha subunit of the heterotrimeric G protein (Gsα) through ADP-ribosylation. Activation of Gsα produces a cytopathic effect by stimulating adenylate cyclase and the production of cAMP. To reach its cytosolic Gsα target, CT binds to the plasma membrane of a host cell and travels by vesicle carriers to the endoplasmic reticulum (ER). The catalytic CTA1 subunit then exploits the quality control mechanism of ER-associated degradation to move from the ER to the cytosol. ER-associated degradation is functionally linked to another quality control system, the unfolded protein response (UPR). However, the role of the UPR in cholera intoxication is unclear. We report here that CT triggers the UPR after 4 h of toxin exposure. A functional toxin was required to induce the UPR, but, surprisingly, activation of the adenylate cyclase signaling pathway was not sufficient to trigger the process. Toxin-induced activation of the UPR coincided with increased toxin accumulation in the cytosol. Chemical activation of the heterotrimeric G protein or the UPR also enhanced the onset of CTA1 delivery to the cytosol, thus producing a toxin-sensitive phenotype. These results indicate there is a cAMP-independent response to CT that activates the UPR and thereby enhances the efficiency of intoxication.

Calcium: an important second messenger in mast cells

Recently there has been considerable controversy over the mechanism(s) by which intracellular Ca2+ is elevated when receptors for IgE on the surface of mast cells are aggregated by antigen. The central role played by calcium in the initiation of secretion from these cells has also been called into question. In a mast cell line which has been widely used to study stimulus-secretion coupling in non-excitable cells it is now clear that calcium is indeed important in the physiological response of the cells but that other intracellular messengers are also involved. In addition it has been shown that while the increase in intracellular Ca2+ probably originates from intracellular stores it can only be sustained by the influx of calcium across the plasma membrane. The nature of the Ca2+ permeability pathway has yet to be elucidated although a number of candidates for the calcium channel in mast cells have been proposed. Significant oscillations and spatial gradients of Ca2+ are often seen when the responses of individual antigen-stimulated cells are measured using digital imaging microscopy. The complexity of these responses highlights the importance of single-cell measurements in elucidating the relationship between IgE receptor activation, Ca2+ movements and exocytosis.

Phospholipases D1 and D2 regulate different phases of exocytosis in mast cells

The rat mast cell line RBL-2H3 contains both phospholipase D (PLD)1 and PLD2. Previous studies with this cell line indicated that expressed PLD1 and PLD2 are both strongly activated by stimulants of secretion. We now show by use of PLDs tagged with enhanced green fluorescent protein that PLD1, which is largely associated with secretory granules, redistributes to the plasma membrane in stimulated cells by processes reminiscent of exocytosis and fusion of granules with the plasma membrane. These processes and secretion of granules are suppressed by expression of a catalytically inactive mutant of PLD1 or by the presence of 50 mM 1-butanol but not tert-butanol, an indication that these events are dependent on the catalytic activity of PLD1. Of note, cholera toxin induces translocation of PLD1-labeled granules to the plasma membrane but not fusion of granules with plasma membrane or secretion. Subsequent stimulation of calcium influx with Ag or thapsigargin leads to rapid redistribution of PLD1 to the plasma membrane and accelerated secretion. Also of note, PLD1 is recycled from plasma membrane back to granules within 4 h of stimulation. PLD2, in contrast, is largely confined to the plasma membrane, but it too participates in the secretory process, because expression of catalytically inactive PLD2 also blocks secretion. These data indicate a two-step process: translocation of granules to the cell periphery, regulated by granule-associated PLD1, and a calcium-dependent fusion of granules with the plasma membrane, regulated by plasma membrane-associated PLD2 and possibly PLD1.

Potentiation of Fcepsilon receptor I-activated Ca(2+) current (I(CRAC)) by cholera toxin: possible mediation by ADP ribosylation factor

Antigen-evoked influx of extracellular Ca(2+) into mast cells may occur via store-operated Ca(2+) channels called calcium release-activated calcium (CRAC) channels. In mast cells of the rat basophilic leukemia cell line (RBL-2H3), cholera toxin (CT) potentiates antigen-driven uptake of (45)Ca(2+) through cAMP-independent means. Here, we have used perforated patch clamp recording at physiological temperature to test whether cholera toxin or its substrate, Gs, directly modulates the activity of CRAC channels. Cholera toxin dramatically amplified (two- to fourfold) the Ca(2)+ release-activated Ca(2+) current (I(CRAC)) elicited by suboptimal concentrations of antigen, without itself inducing I(CRAC), and this enhancement was not mimicked by cAMP elevation. In contrast, cholera toxin did not affect the induction of I(CRAC) by thapsigargin, an inhibitor of organelle Ca(2+) pumps, or by intracellular dialysis with low Ca(2+) pipette solutions. Thus, the activity of CRAC channels is not directly controlled by cholera toxin or Gsalpha. Nor was the potentiation of I(CRAC) due to enhancement of phosphoinositide hydrolysis or calcium release. Because Gs and the A subunit of cholera toxin bind to ADP ribosylation factor (ARF) and could modulate its activity, we tested the sensitivity of antigen-evoked I(CRAC) to brefeldin A, an inhibitor of ARF-dependent functions, including vesicle transport. Brefeldin A blocked the enhancement of antigen-evoked I(CRAC) without inhibiting ADP ribosylation of Gsalpha, but it did not affect I(CRAC) induced by suboptimal antigen or by thapsigargin. These data provide new evidence that CRAC channels are a major route for Fcin receptor I-triggered Ca(2+) influx, and they suggest that ARF may modulate the induction of I(CRAC) by antigen.

Two separate plasma membrane Ca2+ carriers participate in receptor-mediated Ca2+ influx in rat hepatocytes

The plasma membrane Ca2+ carrier system involved in receptor-mediated Ca2+ entry was studied. Using the Ca2+ readdition protocol, the rate of cytosolic free Ca2+ concentration ([Ca2+]i) increase in vasopressin-pretreated hepatocytes was significantly higher than in thapsigargin- or 2,5-di(tert-butyl)hydroquinone-pretreated cells. The addition of Mn2+ to unstimulated hepatocytes resulted in a biphasic quench of fura-2 fluorescence. After an initial phase that was fast in rate but of short duration, the rate of fura-2 quench by Mn2+ became much slower and lasted until all the cellular fura-2 was quenched. Pretreatment of the cells with vasopressin only accelerated the rate of the latter phase but not of the initial one. In agonist-stimulated cells, acidification of the extracellular medium or the presence of ruthenium red, econazole or SK&F 96365 decreased the rates of both [Ca2+]i increase and Mn2+ entry upon addition of the respective cation. By contrast, neomycin and N-tosyl-L-phenylalanine chloromethyl ketone markedly decreased the rate of [Ca2+]i increase upon Ca2+ readdition but had no effect on vasopressin-stimulated Mn2+ entry. None of the treatments affected the ability of vasopressin and thapsigargin to mobilize the internal Ca2+ store. It is concluded that in hepatocytes the two pathways of receptor-mediated Ca2+ entry control two distinct yet pharmacologically related cation carriers.

Cholera toxin directly stimulates pregnenolone generation with increasing Ca2+ efflux in bovine adrenocortical mitochondria

The present experiments demonstrated that the holotoxin as well as the A- and B-subunits of cholera toxin were able to directly enhance pregnenolone synthesis when isolated intact mitochondria, prepared from bovine adrenocortical tissue, were incubated; they were not, however, able to enhance pregnenolone synthesis when the inner mitochondrial fraction was similarly incubated, suggesting that the conformational structure of mitochondria is very important for activation of cholesterol side-chain cleavage by cholera toxin. Data are also presented demonstrating that cholera toxin can enhance Ca2+ release from isolated mitochondria, while pertussis toxin could activate neither pregnenolone generation nor increase Ca2+ efflux from mitochondria. Thus it is suggested that cholera toxin may activate pregnenolone synthesis by regulating Ca2+ movement in mitochondria.

Cyclic AMP- and Ins(1,4,5)P3-induced Ca2+ fluxes in permeabilised cells of Dictyostelium discoideum: cGMP regulates Ca2+ entry across the plasma membrane

Transduction of chemotactic signals in Dictyostelium discoideum apparently involves a precise regulation of the cytosolic Ca2+ concentration. Cyclic AMP stimulation causes Ca2+ influx followed by Ca2+ extrusion, the magnitude of extrusion depending on the state of differentiation. Here, we show that the cAMP receptor controls Ca2+ influx both at the level of entry across the plasma membrane and at the level of transport into Ca2+-sequestering organelles. The use of permeabilised cells allowed us to discriminate between both fluxes. Permeabilised cells still showed the cAMP-induced Ca2+ uptake. The flux across the plasma membrane was more sensitive to Ba2+ and Mn2+, respectively, than Ca2+ sequestration. We have shown previously, using stmF mutants, that cGMP regulates Ca2+ influx. We confirmed this result with the membrane-permeant cGMP-analogue, Sp-8-Br-cGMPS, which enhanced the cAMP-induced Ca2+ influx in intact cells but not the uptake in permeabilised cells, indicating that cGMP regulates Ca2+ influx across the plasma membrane. Occasionally, a fast transient Ca2+ efflux, preceding the influx, occurred in intact cells. A small cAMP-induced Ca2+ release was also found in permeabilised cells. A similarly sized Ca2+ release was elicited by Ins(1,4,5)P3 and could be substituted for by GTP or GTPgammaS. This result suggests that rapid Ca2+ release can be mediated by Ins(1,4,5)P3.

Fc epsilon RI and the T cell receptor for antigen activate similar signalling pathways in T cell-RBL cell hybrids

In order to investigate the functional similarities of the high affinity receptor for IgE (Fc epsilon RI) and the T cell receptor for antigen, we have developed a high efficiency polyethylene glycol-mediated fusion method to make somatic hybrids between cells from a mast cell line (RBL-2H3) and cells from T lymphoma cell lines (Jurkat and HPB-ALL). Using flow cytometry to select for the heterologously fused cells, we demonstrated that aggregation of the T cell receptor results in the efficient secretion of [3H]5-hydroxytryptamine from RBL cell-derived granules. In addition, both receptors mediate Ca2+ mobilization in the hybrid cells that is insensitive to inhibition by the protein kinase C activator phorbol-12-myristoyl-13-acetate (PMA). In contrast, Ca2+ mobilization caused by aggregation of Fc epsilon RI in the parent RBL cells is completely inhibited by PMA. The results indicate that these two different receptors for foreign antigen can substitute for each other to trigger responses in the hybrid cells that are unique to each cell type. The methodology employed has general utility for studying signal transduction mediated by mammalian cell surface receptors.

Tenidap: a novel inhibitor of calcium influx in a mast cell line

The anti-inflammatory agent tenidap has previously been shown to inhibit antigen-induced secretion in tumor mast cells. We have investigated the possibility that this effect is due to modulation of the Ca2+ response in mast cells and in particular that tenidap might be an inhibitor of the Ca2+ influx pathway or channel in these and other non-excitable cells. Tenidap inhibited the antigen-induced increase in intracellular Ca2+ measured both in cell suspensions and at the single cell level using digital imaging of Fura-2 fluorescence. Tenidap also inhibited both antigen- and thapsigargin-induced 45Ca influx across the plasma membrane at concentrations similar to those required for the inhibition of secretion. Somewhat unexpectedly, the compound itself caused some release of calcium from intracellular stores; however, this effect did not appear to be related to the inhibition of calcium influx or secretion. In mouse pituitary tumour (AtT-20) cells, tenidap inhibited depolarization-induced increases in intracellular Ca2+ suggesting that this compound also inhibits Ca2+ influx through voltage-sensitive calcium channels. We conclude that tenidap has a number of interesting effects on calcium handling which makes it a potentially valuable tool for the study of calcium movements particularly in non-excitable cells.

Biosynthesis of paf-acether in cultured-mouse mast cells: the role of calcium and G proteins

We examined the potential role of a guanine nucleotide-binding protein in the biosynthesis of paf-acether (paf) and the release of beta-hexosaminidase during antigenic stimulation of cultured mouse bone marrow-derived mast cells. Unlike pertussis toxin, cholera toxin treatment enhanced the antigen-stimulated production of paf and calcium mobilisation without affecting acetyltransferase activation and cell degranulation. The level of intracellular cAMP doubled in cholera toxin-treated cells. Our data suggest that a cholera toxin-sensitive guanine nucleotide-binding protein is involved in the IgE receptor-mediated signal transduction leading to paf production most probably at the level of Ca2+ influx.

Adenosine receptors in rat basophilic leukaemia cells: transductional mechanisms and effects on 5-hydroxytryptamine release

1. The presence of adenosine receptors linked to adenylate cyclase activity and their functional role in calcium-evoked 5-hydroxytryptamine (5-HT) release was investigated in rat basophilic leukaemia (RBL) cells, a widely used model for studying the molecular mechanisms responsible for stimulus-secretion coupling. 2. In [3H]-5-HT-loaded cells triggered to release by the calcium ionophore A23187, a biphasic modulation of 5-HT secretion was induced by adenosine analogues, with inhibition of stimulated release at nM and potentiation at microM concentrations, suggesting the presence of adenosine receptor subtypes mediating opposite effects on calcium-dependent release. This was also confirmed by results obtained with other agents interfering with adenosine pharmacology, such as adenosine deaminase and the non-selective A1/A2 antagonist 8-phenyl-theophylline. 3. Similar biphasic dose-response curves were obtained with a variety of adenosine analogues on basal adenylate cyclase activity in RBL cells, with inhibition and stimulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production at nM and microM concentrations, respectively. The rank order of potency of adenosine analogues for inhibition and stimulation of adenylate cyclase activity and the involvement of G-proteins in modulation of cyclic AMP levels suggested the presence of cyclase-linked A1 high-affinity and A2-like low-affinity adenosine receptor subtypes. However, the atypical antagonism profile displayed by adenosine receptor xanthine antagonists on cyclase stimulation suggested that the A2-like receptor expressed by RBL cells might represent a novel cyclase-coupled A2 receptor subtype.4. Micromolar concentrations of adenosine analogues could also increase inositol phospholipid hydrolysis and inositol tris-phosphate formation in both unstimulated cells and in cells triggered to release by the calcium ionophore. The stimulation was constant, small and additive to that exerted by the calcium ionophore.5. It is concluded that RBL cells express both A1 and A2-like adenosine receptors which exert opposite effects on 5-HT release and intracellular cyclic AMP levels. However, besides modulation of cyclic AMP levels, additional transduction pathways, such as modulation of phospholipase C activity, may contribute to the release response evoked by adenosine analogues in this cell-line.

Impaired secretion and increased insolubilization of IgE-receptor complexes in mycophenolic acid-treated (guanine nucleotide-depleted) RBL-2H3 mast cells

In RBL-2H3 rat leukemic mast cells, cross-linking anti-DNP IgE-receptor complexes with multivalent antigen (DNP-BSA) activates a signal transduction pathway leading to Ca2+ influx and secretion. Cross-linking IgE-receptor complexes also stimulates a pathway that inactivates (desensitizes) receptors; this pathway becomes important at high concentrations of cross-linking antigen. Recent evidence that antigen-induced secretion is impaired by mycophenolic acid (MPA), an inhibitor of guanine nucleotide synthesis de novo, has implicated a GTP-binding protein (G protein) in the signaling pathway. Other recent studies have indicated that the conversion of cross-linked receptors to a detergent-insoluble (cytoskeleton-associated) form at high antigen concentrations is correlated with the loss of signaling activity. Here we show that secretion elicited by an optimal concentration of antigen (0.05 micrograms/ml DNP-BSA) is only inhibited by about 25% in guanine nucleotide-depleted cells, whereas secretion elicited by 5 micrograms/ml DNP-BSA, a concentration in the range that causes the high-dose inhibition of secretion, is inhibited by more than 60%. We also show that IgE-receptor complexes are insolubilized in response to 5 but not 0.05 micrograms/ml DNP-BSA in both control and guanine nucleotide-depleted cells. Importantly, the extent of insolubilization elicited by 5 micrograms/ml DNP-BSA is increased by more than 60% in the guanine nucleotide-depleted samples. These results raise the possibility that guanine nucleotide depletion reduces the secretory response to high antigen concentrations in two ways: by inhibiting the G protein-coupled signaling pathway and by increasing the availability of receptors to the pathway leading to receptor insolubilization and inactivation.

Transmembrane signalling and paf-acether biosynthesis

Expression of lyso paf-acether (lyso paf):acetyl-CoA acetyltransferase and its activation above basal levels by specific agonists controls the rate of paf biosynthesis in proinflammatory cells. Acetyltransferase activation in these cells is due to the rapid postranslational modification of an inactive precursor by phosphorylation, most probably catalyzed by a cAMP-dependent kinase. However, the possibility exists that a calcium/calmodulin-dependent kinase can be implicated as well. Unlike murine cultured mast cells, human neutrophils form paf when stimulated with phorbol myristate acetate (PMA) or diacylglycerol. In both cell types, acetyltransferase is activated by PMA. Controversy exists as to whether PMA activates the remodeling pathway, i.e. the activation of phospholipase A2 and acetyltransferase, or the de novo route through CDPcholine cholinephosphotransferase action on alkylacetylglycerol. There is some indication that PKC might regulate paf biosynthesis. The implication of a GTP-regulated protein has also been postulated in signal transduction leading to paf formation in endothelial cells, neutrophils, and mast cells. The topography of paf formation is discussed in light of the subcellular distribution of acetyltransferase in neutrophils and Krebs II cells.

Variety of Ca(2+)-permeable channels in human carcinoma A431 cells

Patch-clamp methods were used to search for and characterize channels that mediate calcium influx through the plasma membrane of human carcinoma A431 cells. Here we present four Ca(2+)-permeable channel types referred to as SG, G, 1 and BI. With 105 mM Ca2+ as the charge carrier, at 30-33 degrees C their mean unitary conductances (in pS) are: 1.3 (SG), 2.4 (G), 3.7 (I) and 12.8 (BI). SG and G channels are activated by nonhydrolyzable analogues of guanosine 5-triphosphate (GTP) applied to the inside of the membrane, suggesting an involvement of G-proteins in the control of their activity. I and BI channels are activated by inositol 1,4,5-trisphosphate (InsP3). G, I, BI and possibly SG channels are activated from the extracellular side of the membrane by epidermal growth factor (EGF) and histamine. It is assumed that all identified Ca2+ channels take part in the generation of the agonist-induced intracellular Ca2+ signal. The variety of Ca-channel types seems to be necessary to tune cell responses according to the respective type and level of an external signal, on the one hand, and to the functional state of the cell, on the other.

Tyrosine kinase-dependent phosphatidylinostiol turnover and functional responses in the Fc epsilon R1 signalling pathway

In RBL-2H3 rat basophilic leukemia cells, Fc epsilon R1 crosslinking by multivalent antigen stimulates phosphatidylinositol (PI) turnover and Ca2+ influx and causes functional responses that include secretion, membrane ruffling and actin polymerization. Here, we show that the tyrosine kinase inhibitor, genistein, inhibits antigen-induced PI turnover, determined from assays of 1,4,5-inositol trisphosphate production, and impairs receptor-mediated secretion, ruffling and actin polymerization. Genistein has little effect on several functional responses to stimuli that bypass PI hydrolysis (ionomycin-induced secretion, phorbol ester-induced ruffling) but it inhibits phorbol ester-induced actin polymerization. These data implicate a common tyrosine kinase-dependent event, most likely the activation of phospholipase C gamma, in the Fc epsilon R1-mediated stimulation of PI turnover, secretion and ruffling. There may be additional tyrosine kinase-mediated events in the actin assembly pathway.

Relationship of IgE receptor topography to secretion in RBL-2H3 mast cells

In RBL-2H3 rat leukemic mast cells, cross-linking IgE-receptor complexes with anti-IgE antibody leads to degranulation. Receptor cross-linking also stimulates the redistribution of receptors on the cell surface, a process observed here by labeling the anti-IgE with 15 nm protein A-gold particles that are visible by back-scattered electron imaging in the scanning electron microscope. We report that anti-IgE binding stimulates the redistribution of IgE-receptor complexes at 37 degrees C from a dispersed topography to distributions dominated sequentially by short chains, small clusters, and large aggregates of cross-linked receptors. Cells incubated with 1 microgram/ml anti-IgE, a concentration that stimulates maximum net secretion, redistribute receptors into chains and small clusters during a 15 min incubation period. At 3 and 10 micrograms/ml anti-IgE, net secretion is reduced and the majority of receptors redistribute rapidly into clusters and large aggregates. The addition of Fab fragments with the high anti-IgE concentrations, to reduce cross-linking, delays receptor aggregation and enhances secretion. The progression of receptors from small clusters to large aggregates is prevented in cells treated with dihydrocytochalasin B to prevent F-actin assembly. These results establish that characteristic patterns of receptor topography are correlated with receptor activity. In particular, they link the formation of large receptor aggregates to reduced signalling activity. Cytoskeleton-membrane interaction is implicated in the formation or stabilization of the large receptor clusters.

Regulation of IgE receptor-mediated secretion from RBL-2H3 mast cells by GTP binding-proteins and calcium

In RBL-2H3 rat basophilic leukemia cells, cholera toxin does not per se stimulate secretion but it enhances secretion stimulated by antigens that crosslink IgE receptors, by the Ca2+ ionophore, ionomycin, and by thapsigargin, a tumor promoter that releases cytoplasmic Ca2+ stores. Calmodulin inhibitors reduce both the basal and cholera toxin-enhanced secretory responses to antigen and Ca2(+)-mobilizing agents. These synergistic effects suggest that the activation of a Gs-like GTP-binding protein, together with a (probably calmodulin-dependent) event activated by an increase in cytoplasmic Ca2+ levels, may jointly provide a sufficient signal for secretion. Antigen-stimulated secretion is inhibited by depleting cells of GTP with mycophenolic acid but is maximal in cells treated with mycophenolic acid plus cholera toxin. The simplest explanation is that cholera toxin selectively reactivates the Gs-coupled pathway leading to secretion in GTP-depleted cells without restoring the activity of a separate GTP-binding protein(s) that constrains antigen-stimulated secretion.

Microfilaments regulate the rate of exocytosis in rat basophilic leukemia cells

Disruption of microfilaments in rat basophilic leukemia (RBL) cells by exposure to cytochalasin B is observed to potentiate the rate of antigen-stimulated secretion from these cells. Under these conditions, cytochalasin B is without effect on the antigen-stimulated production of inositol phosphates or 45Ca2(+)-influx. In streptolysin-O-permeabilized RBL cells, cytochalasin B is observed to potentiate the rate of secretion in response both to guanosine 5'-(2-thio)-O-triphosphate (GTP gamma S) and to Ca2+ (buffered between 0.1 and 10 microM). However, under these conditions, cytochalasin B does not affect to antigen-stimulated production of inositol phosphates. Consistent with these data, microfilaments are proposed to regulate a terminal step in exocytosis, in a physiologically relevant manner.

Characterization of a new type of variant of rat basophilic leukemia 2H3 cells presenting a different pattern of calcium signal

We selected a new type of variant (designated 3C7) derived spontaneously from parental RBL-2H3 cells. 3C7 cells showed lower contact inhibition, anchorage dependency, and serotonin release activity than those of RBL-2H3 cells. We conclude that 3C7 cells are a transformant of RBL-2H3 cell with greater malignancy. The production of inositol bisphosphate and the release of Ca2+ from intracellular stores induced by IgE-antigen stimulation were enhanced in 3C7. Oscillation of [Ca2+]i in individual 3C7 cells was observed by a digital imaging microscopic technique. We propose that 3C7 cells are a useful model system for studies on the mechanisms of stimulus-secretion coupling and the relationships between malignant alterations and disorders of signal transduction.