Depolarization of rat basophilic leukemia cells inhibits calcium uptake and exocytosis

We have investigated the unusual observation that depolarization of rat basophilic leukemia cells in high potassium not only fails to induce secretion, but also inhibits the secretion induced when receptors for IgE are aggregated by antigen. Antigen-stimulated 45Ca uptake and the rise in cytoplasmic free ionized calcium measured with the fluorescent indicator quin2 were both inhibited in depolarized cells. 45Ca efflux, on the other hand, was unaffected, which confirms that IgE receptor activation was not impaired in high potassium. Unlike the large increase in total cell calcium seen when cells in normal saline solution were stimulated with antigen, there was a decrease in total cell calcium when depolarized cells were stimulated. This is consistent with our finding that 45Ca uptake was inhibited while 45Ca efflux was unaffected. Inhibition of 45Ca uptake and secretion closely paralleled the decrease in membrane potential, and could be overcome by increasing the extracellular calcium concentration. We conclude that changes in the electrochemical gradient for calcium are important in determining calcium influx and the magnitude of antigen-stimulated secretion from rat basophilic leukemia cells, while the release of calcium from intracellular stores is unaffected.     

Biochemical analysis of glucocorticoid-induced inhibition of IgE-mediated histamine release from mouse mast cells

Pretreatment of mouse mast cells with 10(-7) to 10(-6) M dexamethasone (DM) during overnight sensitization with mouse IgE antibody resulted in inhibition of antigen-induced histamine release and degranulation. The inhibition of both degranulation and histamine release increased linearly with the duration of the treatment; maximal inhibition was obtained after approximately 16 hr with DM. The addition of DM to sensitized mast cells immediately before antigen challenge did not affect the antigen-induced histamine release. DM interacted directly with mast cells by binding to DM-specific cytoplasmic receptors. The treatment of mast cells with DM did not affect the binding of IgE to mast cells or intracellular cAMP levels. Bridging of cell-bound IgE anti-DNP antibody on mouse mast cells either by multivalent DNP-HSA or by anti-IgE induced phospholipid methylation at the plasma membrane and Ca++ influx into the cells. Pretreatment of mast cells with DM inhibited the antigen-induced phospholipid methylation and Ca++ uptake but failed to affect histamine release by Ca++ ionophore A23187. The results suggest that DM treatment inhibits histamine release by the inhibition of the early stage of biochemical processes leading to opening Ca++ channels but does not affect the process distal to Ca++ influx or the binding of IgE molecules to IgE receptors.     

Cyclosporin A augments angiotensin II-stimulated rise in intracellular free calcium in vascular smooth muscle cells.

Pretreatment of rat vascular smooth muscle cells with the immunosuppressive drug cyclosporin A caused concentration- and time-dependent increases in both the amplitude and duration of the angiotensin II-induced rise in cytosolic free calcium, as measured with quin 2. Cyclosporin A had no significant effect on basal quin 2 fluorescence. However, cyclosporin A increased the basal 45Ca2+ influx. This stimulation of 45Ca2+ influx was not blocked by nifedipine (10(-6) M). Cyclosporin A also augmented the angiotensin II-stimulated influx and efflux of 45Ca2+. These results demonstrate that cyclosporin A increases the permeability of the plasma membrane for Ca2+ and also augments the angiotensin II-induced increases in cytosolic free calcium.     

Activation of rat basophilic leukemia cells. Temporal identification of the signal calcium influx mediated by the receptor-operated channel pathway

Antigen-induced 45calcium influx into rat basophilic leukemia (RBL) cells was examined with emphasis on the early time domain under conditions that exclude loss of the cation during the subsequent washing step. Such preparations demonstrate a distinct, temporally separate influx peaking at 3 min, followed by a substantial efflux. This internalized 45Ca2+ approaches millimolar total intracellular concentration and is therefore either sequestered or becomes bound to intracellular components (proteins). The amplitude of this influx is linearly proportional to IgE-receptor occupancy at low receptor occupancies, and is sensitive to the anti-allergic drug cromolyn. Furthermore, the timing of both the maximal uptake and the maximal susceptibility to cromolyn correlates with the Quin-2 signal in these cells, and the initial degranulation pattern bears some resemblance to trends in the 45Ca2+ uptake curve. These qualities suggest that the early peak at 2-3 min, rather than any later 45Ca2+ uptake, comprises the initial signalling Ca2+ pool. Maximal apparent inhibition by cromolyn for 45Ca2+ uptake was about 65% and required a 10-15-min preincubation with the cells. The inhibitory effect was limited to the peak at 3 min, suggesting that tracer incorporation beyond 5-6 min largely involves other pools or pathways, triggered by receptor aggregation, yet only indirectly related to channel activity or to the signal proper. A quantitative similarity was found between the early peak measured on intact cells and the single channel conductance measured on reconstituted planar bilayers containing the purified receptor for IgE and the purified cromolyn-binding protein [Corcia, A. et al. (1986) EM BO J. 5, 849-854]. This, as well as the effects of cromolyn, support the assumption that cromolyn-binding protein is a major constituent involved in this early influx, or that influx is a principal pathway for the signaling calcium mass.     

The mechanism of the calcium signal and correlation with histamine release in 2H3 cells

Rat basophil leukemic (2H3) cells ( Siraganian , R.P., McGivney , A., Barsumian , E. L., Crews, F. T., Hirata , F., and Axelrod , J. (1982) Fed. Proc. 41, 30-34) loaded with fluorescent Ca2+ indicator quin 2 ( Tsien , R. Y. (1980) Biochemistry 19, 2396-2404) showed a rapid increase in free cytosol calcium concentration [( Ca]i) when histamine release was induced. Intracellular quin 2 concentrations up to 7 mM did not affect release of histamine in response to antigen (aggregated ovalbumin) or concanavalin A with cells primed with antigen-specific monoclonal IgE, or in response to Ca2+ ionophores. The [Ca]i increased from approximately 105 nM to a maximum of approximately 1200 nM within 2 to 3 min after antigenic stimulation and then declined slowly over 30 min toward the level in unstimulated cells. Histamine release was most rapid as [Ca]i reached the maximum value and then decreased continuously with [Ca]i over the subsequent 30 min. Neither the Ca signal nor histamine release was observed when the Ca2+ concentration in the medium [( Ca]o) was less than 50 microM, but both responses were restored on readdition of Ca2+ to 1 mM. The maximal Ca signal was obtained when [Ca]o was approximately greater than 1 mM and was half-maximal at [Ca]o congruent to 0.4 mM. In marked contrast [Ca]i in unstimulated cells varied very little with [Ca]o from 0.1 to 1 mM. Maintenance of the Ca signal required the continuous presence of stimulating ligand, external Ca2+, and the maintenance of cellular ATP; metabolic inhibitors blocked or reversed the Ca signal. La+ ions also caused a rapid and reversible block of the Ca signal and histamine release. The data are interpreted in a model in which the Ca signal is generated by a La3+-sensitive signal influx pathway that is functionally independent of the normal Ca2+ influx pathway in unstimulated cells, and that allows a 10-fold or greater increase in rate of Ca2+ entry. The Ca signal is maintained dynamically by the balance between the increased Ca2+ influx and active Ca2+ efflux across the plasma membrane.     

Immunoglobulin E receptor cross-linking induces oscillations in intracellular free ionized calcium in individual tumor mast cells

Fura-2 fluorescence in single rat basophilic leukemia cells was monitored to study the rise in intracellular free ionized calcium ([Ca2+]i) produced by aggregation of immunoglobulin E receptors. Repetitive transient increases in [Ca2+]i were induced by antigen stimulation and were measured using digital video imaging microscopy at high time resolution. The [Ca2+]i oscillations were not dependent upon changes in the membrane potential of the cells and were observed in cells stimulated with antigen either with or without extracellular Ca2+. Transient oscillations in [Ca2+]i were also observed when calcium influx was blocked with La3+. These results suggested that during antigen stimulation of cells under normal physiological conditions, release of Ca2+ from intracellular stores makes an important contribution to the initial increase in [Ca2+]i. Oscillations in [Ca2+]i are not induced by elevating [Ca2+]i with the calcium ionophore ionomycin. Mitochondrial calcium buffering is not required for [Ca2+]i oscillations to occur. The results show that rat basophilic leukemia cells have significant stores of calcium and that release of calcium from these stores can participate in both the initial rise and the oscillations in [Ca2+]i.     

Role of extracellular calcium in the hyperthermic killing of CHL V79 cells

Two major questions are addressed by this study: Can an influx of calcium ion sensitize CHL V79 cells to hyperthermia, and, if so, does this occur during heating and does it play a crucial role in cell death? V79 cells are sensitized to hyperthermia by the calcium ionophore A23187 which also induces an influx of calcium at both 37 and 43 degrees C. Sensitization is at least partially dependent on the presence of extracellular calcium. In the absence of A23187, survival is independent of calcium concentration (from 0 to 25 mM) during heating, which differs from the behavior of hepatocytes which are sensitized to hyperthermia by 15 mM CaCl2. Calcium influx, as assayed by uptake of 45Ca measured after washing in LaCl3, is detectable in 3 mM CaCl2 only after 30 min at 45 degrees C, an exposure which reduces reproductive survival to less than 0.1%. Calcium uptake reaches 6 nmol/10(6) cells after 180 min at 45 degrees C. This is not due to a general loss of membrane permeability since there is no trypan blue staining during this time. In 15 mM CaCl2, influx occurs earlier (15 min) but still succeeds the loss of reproductive survival which is less than 1% at this time. Uptake is much higher in 15 mM CaCl2, reaching 10 nmol/10(6) cells by 30 min and 25 nmol/10(6) cells at 180 min, but the temporal pattern of uptake does not correlate with loss of reproductive survival. Thus, although A23187 sensitizes V79 cells to hyperthermia, probably by increased influx of calcium ion, and increased influx occurs during exposure to 45 degrees C, influx is not a crucial early event in the killing of V79 cells. This does not eliminate the possibility of intracellular calcium redistribution during hyperthermia.     

Release of calcium from intracellular stores in rat basophilic leukemia cells monitored with the fluorescent probe chlortetracycline.

Release of calcium from intracellular stores of rat basophilic leukemia cells was monitored using the fluorescent probe chlortetracycline. The ability of chlortetracycline to indicate release from intracellular calcium stores was initially validated. The decrease of chlortetracycline fluorescence upon antigen-stimulation was not the result of secretion of granule-associated dye or of changes in the properties of the membranes. The chlortetracycline fluorescence signal was not influenced by Ca2+ influx across the plasma membrane. Results obtained from these chlortetracycline fluorescence measurements corresponded well with 45Ca efflux data, an indirect measurement of release of calcium from stores. Chlortetracycline was used to examine the rate of antigen-induced release of calcium from stores, the depletion of intracellular calcium stores by EGTA, and the relationship between the antigen-stimulated release of stored calcium and exocytosis. Chlortetracycline was shown to be a useful qualitative indicator for the release of intracellular calcium with a relatively rapid response time.     

The relative contributions of extracellular and intracellular calcium to secretion from tumor mast cells. Multiple effects of the proton ionophore carbonyl cyanide m-chlorophenylhydrazone

The proton ionophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) inhibited antigen-stimulated secretion and calcium influx in rat basophilic leukemia cells. In a glucose-free solution the inhibitory effects of CCCP were due to a decrease in the intracellular ATP concentration; however, when glucose was present there was no decrease in ATP. Instead, we found that in a glucose-containing saline solution, CCCP inhibited antigen-stimulated calcium uptake because it depolarized the plasma membrane, which in rat basophilic leukemia cells inhibits antigen-stimulated calcium uptake. In the presence of glucose, relatively low concentrations of CCCP inhibited calcium uptake while higher concentrations were required to inhibit secretion. In contrast, the initial antigen-stimulated rise in cytoplasmic calcium, measured with the fluorescent calcium indicator quin2, was not inhibited by CCCP. This suggests that the release of calcium from intracellular stores might, in some cases, be sufficient to support antigen-stimulated secretion. In the presence of CCCP the pH gradient becomes important for regulating the membrane potential across the plasma membrane. When cells were depolarized with CCCP and the external pH was increased, the membrane potential returned to resting levels and antigen-stimulated calcium uptake was restored. Inhibition of antigen-stimulated secretion by higher concentrations of CCCP could also be reversed by increasing the external pH.     

Endothelin-1 increases intracellular calcium mobilization but not calcium uptake in rabbit vascular smooth muscle cells

Conflicting evidence has been reported regarding the role of endothelin-1, a potent vasconstrictor peptide, in stimulating extracellular calcium influx in rabbit vascular smooth muscle. The objective of this study was to elucidate the effects of endothelin-1 on transmembrane 45Ca2+ influx and intracellular calcium mobilization in cultured rabbit aortic smooth muscle cells. In calcium containing buffer, endothelin-1 induced a concentration-dependent 45Ca2+ efflux response over the range of 10 pM to 100 nM with an EC50 of approximately 60 pM. Maximum endothelin-stimulated 45Ca2+ efflux was not affected by the absence of extracellular calcium or the presence of 1 microM verapamil. Endothelin-1 did not induce transplasmalemmal 45Ca2+ uptake at times up to 30 min. These findings suggest that an alteration in intracellular calcium handling, rather than extracellular calcium influx, is responsible for the endothelin-stimulated increase in intracellular calcium concentration in rabbit aortic smooth muscle cells.     

Kinetic evidence for cytoplasmic calcium as an inhibitory messenger in parathyroid hormone release

A sudden change of extracellular Ca2+ from 0.5 to 3.0 mM resulted in a transient rise of the cytoplasmic Ca2+ concentration (Ca2+i) followed by a sustained increase in parathyroid cells loaded with the Ca2+-indicator fura-2. The initial transient could be eliminated by increasing the Ca2+ buffering capacity of the cytoplasm. Under such conditions the rise of Ca2+i exhibited kinetics reminiscent of those for 45Ca uptake and cell depolarization. Addition of 0.5 mM Mn2+ mimicked the effect of raising the extracellular Ca2+ concentration, since there was an initial Ca2+i transient followed by a slower entry of Mn2+ into the cells. This reaction pattern was different from that of pancreatic alpha 2-cells in which there was no substantial influx of Mn2+ before depolarization with arginine. When measuring the kinetics of parathyroid hormone (PTH) release it was apparent that Ca2+ inhibition of secretion followed Ca2+i and thus became substantially delayed after eliminating the initial transient. The results support the concept of a depolarizing Ca2+ permeability in the parathyroid cell membrane which can be activated by external Ca2+, and indicate that Ca2+i is an inhibitory messenger of importance for the physiological regulation of PTH release.     

Regulation of receptor-mediated calcium influx across the plasma membrane in a human leukemic T-cell line: evidence of its dependence on an initial calcium mobilization from intracellular stores

It has been repeatedly shown that stimulation of a human leukemic T-cell line, JURKAT, by lectins such as phytohaemagglutinin and anti-T3 antibody (OKT3) leads to an elevation in the concentration of cytosolic free Ca2. This Ca2+ transient results from both an intracellular mobilization and an influx of Ca2+ through specific membrane channels. The objective of this study was to investigate the mechanism by which receptor-mediated influx of Ca2+ is regulated in JURKAT cells, which demonstrably lack 'voltage-dependent calcium channels'. It was found that upon increased loading with quin2 or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) there was a pronounced decline of both phytohaemagglutinin-stimulated and OKT3-stimulated influx of 45Ca2+. Using 15 microM quin2/AM or 30 microM BAPTA/AM, agonist-stimulated 45Ca2+ influx was almost totally abolished. At these concentrations of both quin2/AM or BAPTA/AM, phytohaemagglutinin and OKT3 could still induce a rise of cytosolic free Ca2+ above 200 nM. In the presence of La3+ (200 microM), which completely inhibited the agonist-induced 45Ca2+ influx, both phytohaemagglutinin and OKT3 were able to raise the concentrations of cytosolic free Ca2+ to well above 200 nM by merely mobilizing Ca2+ from intracellular stores alone. The data suggest that an agonist-induced increase in the concentration of cytosolic free Ca2+, due to mobilization from intracellular stores, could either directly or indirectly, initiate receptor-mediated Ca2+ influx across the plasma membrane in JURKAT cells.     

Calcium dependence of the thrombin-induced increase in endothelial albumin permeability

We examined whether the increase in endothelial albumin permeability induced by alpha-thrombin is dependent on extracellular Ca2+ influx. Permeability of 125I-albumin across confluent monolayers of cultured bovine pulmonary artery endothelial cells was measured before and after the addition of 0.1 microM alpha-thrombin. In the presence of normal extracellular Ca2+ concentration ([Ca2+]o, 1000 microM), alpha-thrombin produced a 175 +/- 10% increase in 125I-albumin permeability. At lower [Ca2+]o (100, 10, 1, or less than 1 microM), alpha-thrombin caused a 140% increase in permeability (P less than 0.005). LaCl3 (1 mM), which competes for Ca2+ entry, blunted 38% of the increase in permeability. Preloading endothelial monolayers with quin2 to buffer cytosolic Ca2+ (Cai2+) produced a dose-dependent inhibition of the increase in 125I-albumin permeability. Preincubation with nifedipine or verapamil was ineffective in reducing the thrombin-induced permeability increase. A 60 mM K+ isosmotic solution did not alter base-line endothelial permeability. alpha-Thrombin increased [Ca2+]i in a dose-dependent manner and the 45Ca2+ influx rate. Extracellular medium containing 60 mM K+ did not increase 45Ca2+ influx, and nifedipine did not block the rise in 45Ca2+ influx caused by alpha-thrombin. Ca2+ flux into endothelial cells induced by alpha-thrombin does not occur through voltage-sensitive channels but may involve receptor-operated channels. In conclusion, the increase in endothelial albumin permeability caused by alpha-thrombin is dependent on Ca2+ influx and intracellular Ca2+ mobilization.     

Chemoattractant receptor promotion of Ca2+ influx across the plasma membrane of HL-60 cells. A role for cytosolic free calcium elevations and inositol 1,3,4,5-tetrakisphosphate production

The mechanisms by which the chemotactic peptide formyl-methyl-leucyl-phenyl-alanine stimulates Ca2+ influx across the plasma membrane were investigated in the human promyelocytic cell line HL-60, induced to differentiate with dimethyl sulfoxide. Ca2+ influx was determined: (a) from the initial rate of Mn2+ influx, apparent from the quenching of intracellular quin2 or fura-2 fluorescence; (b) from the rate of the elevation of cytosolic free calcium, [Ca2+]i, upon readdition of Ca2+ to cells previously stimulated in the absence of extracellular Ca2+. [3H]Inositol tris-, tetrakis-, and pentakisphosphates were analyzed by a high performance liquid chromatography procedure which was optimized for the separation of inositol tetrakisphosphates, yielding three predominant isomers: inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), inositol 1,4,5,6-tetrakisphosphate, and inositol 1,3,4, 6-tetrakisphosphate. Both the kinetics and agonist dose dependence of Ca2+ influx stimulation correlated closely with the corresponding receptor-mediated variations of [Ca2+]i either in the presence or in the absence of extracellular Ca2+. Of the different inositol phosphates determined in parallel and under the same conditions, accumulation of [3H]Ins(1,3,4,5)P4 correlated best with Ca2+ influx both temporally and in its dose dependence in the presence or in the absence of extracellular Ca2+; inositol 1,3,4-trisphosphate was also correlated but to a lesser extent. Attenuations of [Ca2+]i elevations by decreasing extracellular Ca2+ or by increasing the cytosolic Ca2+ buffering capacity with quin2 led to parallel inhibition of Ca2+ influx and Ins(1,3,4,5)P4 production. In conclusion: 1) activation of Ca2+ influx by formyl-methionyl-leucyl-phenylalanine depends on the elevation of [Ca2+]i, the latter being initiated by Ca2+ mobilization from intracellular stores; 2) Ins(1,3, 4,5)P4 is a strong candidate for maintaining receptor-mediated activation of Ca2+ influx in differentiated HL-60 cells.     

Role of calcium fluxes in the sustained phase of angiotensin II-mediated aldosterone secretion from adrenal glomerulosa cells

When angiotensin II stimulates aldosterone secretion, it causes a rapid but transient mobilization of calcium from an intracellular pool and a sustained increase in the influx of calcium in adrenal glomerulosa cells. The present studies were undertaken to determine the respective roles of the two angiotensin II-induced changes in cellular calcium metabolism in modulating events during the sustained phase of cellular response which is thought to be mediated by the C-kinase branch of the calcium messenger system. The sustained response to angiotensin II is only 50% of maximal in cells pretreated with dantrolene in a concentration sufficient to inhibit the angiotensin II-induced mobilization of intracellular calcium. Also, if A23187 is added to cells simultaneously with 1-oleoyl-2-acetylglycerol (OAG), the aldosterone secretory response is similar to that seen after angiotensin II. However, if A23187 is added first and the transient aldosterone secretory response allowed to decay, and OAG then added, the sustained aldosterone secretory response is only 45-50% of maximal. Addition of the calcium channel agonist, BAY K 8644, with OAG leads to an aldosterone secretory response which is only 50% of maximal, but if upon addition of OAG and BAY K 8644 the cells are also exposed for 5 min to media containing 8 mM K+, then the sustained secretory response is maximal. These data imply that the initial transient rise in the [Ca2+] of the cell cytosol plays a role in determining the extent to which C-kinase is shifted from its calcium-insensitive to its calcium-sensitive form. The second group of experiments examined the relationship between the sustained angiotensin II-induced increase in plasma membrane calcium influx and the sustained aldosterone secretory response. The results show that in the presence of 1 microM nitrendipine or 2 mM extracellular K+, angiotensin II causes no increase in calcium influx and only a transient rather than a sustained increase in the rate of aldosterone secretion indicating that the sustained phase of the response is dependent upon a continued high rate of Ca2+ influx which regulates the rate of turnover of the activated C-kinase.     

Modulation of cytosolic-free calcium transients by changes in intracellular calcium-buffering capacity: correlation with exocytosis and O2-production in human neutrophils

The intracellularly trapped fluorescent calcium indicator, quin 2, was used not only to monitor changes in cytosolic-free calcium, [Ca2+]i, but also to assess the role of [Ca2+]i in neutrophil function. To increase cytosolic calcium buffering, human neutrophils were loaded with various quin 2 concentrations, and [Ca2+]i transients, granule content release as well as superoxide [O2-] production were measured in response to the chemotactic peptide formyl-methionyl-leucyl- phenylalanine (fMLP) and the calcium ionophore ionomycin. Receptor- mediated cell activation induced by fMLP caused a rapid rise in [Ca2+]i. The extent of [Ca2+]i rise and granule release were inversely correlated with the intracellular concentration of quin 2, [quin 2]i. These effects of [quin 2]i were more pronounced in the absence of extracellular Ca2+. The initial rate and extent of fMLP-induced O2- production were also inhibited by [quin 2]i. The rates of increase of [Ca2+]i and granule release elicited by ionomycin were also inversely correlated with [quin 2]i in Ca2+-containing medium. As the effects of ionomycin, in contrast to those of fMLP, are sustained, the final increase in [Ca2+]i and granule release were not affected by [quin 2]i. A further reduction of fMLP effects was seen when intracellular calcium stores were depleted by incubating the cells in Ca2+-free medium with ionomycin. The specificity of quin 2 effects on cellular calcium were confirmed by loading the cells with Anis/AM, a structural analog of quin 2 with low affinity for calcium which did not inhibit granule release. In addition, functional responses to phorbol myristate acetate (PMA), which stimulates neutrophils without raising [Ca2+]i, were not affected by [quin 2]i. The findings indicate that rises in [Ca2+]i control the rate and extent of granule exocytosis and O2-generation in human neutrophils exposed to the chemotactic peptide fMLP.     

IgE receptor-mediated depolarization of rat basophilic leukemia cells measured with the fluorescent probe bis-oxonol

Receptor-mediated changes in plasma membrane potential were recorded in rat basophilic leukemia (RBL) cells with the potential-sensitive fluorescent indicator bis-oxonol. Depolarization of the mitochondria with metabolic inhibitors was not detected by bis-oxonol, suggesting that only potential changes across the plasma membrane were being measured. The resting membrane potential of RBL cells was largely generated by the equilibrium distribution of K+ and not through electrogenic activity of the sodium pump. Depolarization was maintained as long as IgE receptors remained aggregated. We believe that at physiologic calcium concentrations a large portion of the measured potential change may be due to calcium influx across the plasma membrane. Prevention of calcium influx by lanthanum, disruption of aggregated receptors, or prior depolarization in a high K+ saline solution completely inhibited the antigen-induced depolarization. The time course of the antigen-stimulated increase in bis-oxonol fluorescence was similar, but not identical, to the antigen-stimulated rise in cytoplasmic free ionized calcium measured with fura-2. Antigen-stimulated depolarization was inhibited by removing both calcium and sodium and could be restored by the addition of either ion. Reduction of total cellular adenosine triphosphate inhibited depolarization in response to antigen stimulation.     

Role of calcium in histamine release from rat mast cells activated by various secretagogues; intracellular calcium mobilization correlates with histamine release

Anti-IgE, con A or antigen caused an increase in the intracellular calcium concentration, [Ca2+]i, of mast cells. The increase occurred in two stages: a rapid initial rise caused by Ca-mobilization from intracellular Ca-stores and a second sustained rise caused by an influx of extracellular calcium (White, J.R., Pluznik, D.V., Ishizaka, K. & Ishizaka, T. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 8193-8197). The rapid initial rise was followed by a release of histamine, which seemed to coincide with the second rise. A23187 and compound 48/80 induced a rapid initial rise in [Ca2+]i, followed by a gradual decrease in [Ca2+]i, GMCHA-OPhBut, a specific pH 7 tryptase inhibitor (Muramatu, M., Ito, T., Takei, M. & Endo, K. (1988) Biol. Chem. Hoppe-Seyler 369, 617-625), strongly inhibited both the initial and second rises of [Ca2+]i, as well as histamine release by these secretagogues, and its effects on the initial rise were closely correlated with those on histamine release. Addition of GMCHA-OPhBut immediately after the initial rise strongly inhibited the second rise, thereby decreasing the final [Ca2+]i. These results strongly suggested a possible involvement of pH 7 tryptase, not only in Ca-mobilization leading to the initial rise in [Ca2+]i, but also in the second rise. Trapping of extracellular calcium by 3mM EGTA decreased both the initial rise in [Ca2+]i and histamine secretion induced by anti-IgE or con A; the magnitude of this effect depended on the time between induction and EGTA addition. Histamine release was closely correlated with the initial rise in [Ca2+]i. Similar results were obtained with A23187, but even 5 min after the addition of EGTA an initial rise of [Ca2+]i could still be induced, and histamine (30% of total histamine) was still released. However, A23187 did not induce a rise in [Ca2+]i in mast cells which had been exhaustively washed with Tyrode/Hepes solution containing 3mM EGTA, followed by suspension in the same solution. Even at 20 min after depletion of the extracellular calcium, compound 48/80 still caused an initial rise in [Ca2+]i to above half the maximal value, and histamine secretion was even less affected. The above results indicated that the initial rise in [Ca2+]i, due to Ca-mobilization, correlates with the histamine release promoted by the secretagogues described. On the other hand, isoproterenol strongly induced histamine secretion with no change of [Ca2+]i, while EGTA treatment prior to isoproterenol stimulation had no effect on histamine release, indicating a different secretion mechanism.     

Extracellular pH modifies mitochondrial control of capacitative calcium entry in Jurkat cells

It was found that a collapse of the mitochondrial calcium buffering caused by the protonophoric uncoupler CCCP, antimycin A plus oligomycin, or the inhibitor of the mitochondrial Ca2+/Na+ exchanger led to a strong inhibition of thapsigargin-induced capacitative Ca2+ entry (CCE) into Jurkat cells suspended in a medium at pH 7.2. The effect of these inhibitors was markedly less significant at higher extracellular pH. Moreover, dysfunction of the mitochondrial calcium handling greatly decreased CCE sensitivity to extracellular Ca2+ when the pH of extracellular solution was 7.2 (apparent Kd toward extracellular Ca2+ rose from 2.3 +/- 0.6 mm in control cells to 11.0 +/- 1.7 mM in CCCP-treated cells) as compared with pH 7.8 (apparent Kd toward extracellular Ca2+ increased from 1.3 +/- 0.4 mM in control cells to 2.4 +/- 0.4 mM in uncoupler-treated cells). Changes in intracellular pH triggered by methylamine did not influence Ca2+ influx. This suggests that, in Jurkat cells, store-operated calcium channels sense extracellular pH change as a parameter that modifies their sensitivity to intracellular Ca2+. In contrast, in human osteosarcoma cells, changes in extracellular pH as well as mitochondrial uncoupling did not exert any inhibitory effects on CCE.     

Release of histamine and arachidonate from mouse mast cells induced by glycosylation-enhancing factor and bradykinin

Stimulation of normal rat splenic T cells with pertussigen (lymphocytosis-promoting factor from Bordetella pertussis) resulted in the release of a soluble factor that enhanced the assembly of N-linked oligosaccharides to IgE-binding factors during their biosynthesis. The glycosylation-enhancing factor (GEF) is a kallikrein-like enzyme and is purified by absorption to p-aminobenzamidine-Agarose followed by elution with benzamidine. Incubation of normal mouse mast cells with affinity-purified GEF or bradykinin, a product of cleavage of kininogen by kallikrein, resulted in the release of histamine and arachidonate from the cells. Passive sensitization of mast cells with mouse IgE antibody, followed by pretreatment of the cells with a suboptimal concentration of GEF, resulted in an enhancement of antigen-induced histamine release. It was found that GEF and bradykinin induced the same biochemical events in mast cells as those induced by bridging of IgE receptors. Both GEF and bradykinin induced phospholipid methylation and an increase in intracellular cyclic AMP (cAMP). Incorporation of 3H-methyl groups into phospholipids and intracellular cAMP levels both reached a maximum 30 sec after challenge with GEF or bradykinin, and then declined to base-line levels within 2 to 3 min. These biochemical events were followed by 45Ca influx and histamine release; 45Ca uptake reached a plateau value at 2 min, and histamine release reached a maximum at 5 to 8 min. The initial rise in cAMP induced by GEF (or bradykinin) was not inhibited by indomethacin, indicating that the activation of adenylate cyclase is not the result of prostaglandin synthesis. In both IgE-mediated and GEF-induced histamine release, inhibitors of methyltransferases, such as 3-deaza adenosine and L-homocysteine thiolactone, inhibited not only phospholipid methylation but also the cAMP rise and subsequent Ca2+ uptake and histamine release. The results indicate that GEF induces activation of methyltransferases and that phospholipid methylation is involved in the cAMP rise, Ca2+ uptake, and histamine release. The induction of the same biochemical events in the same sequence by bridging of IgE receptors and by GEF (bradykinin) supports the hypothesis that receptor bridging induces the activation of serine protease(s) and cleavage products of this enzyme in turn activate methyltransferases in mast cells.