Effect of signal transduction pathways on the action of thapsigargin on rat mast cells. Crosstalks between cellular signalling and cytosolic pH

PKC and cAMP positively modulate alkaline-induced exocytosis in the human mast cell line HMC-1

We study in HMC-1 the activation process, measured as histamine release. We know that ammonium chloride (NH(4)Cl) and ionomycin release histamine, and the modulatory role of drugs targeting protein kinase C (PKC), adenosine 3',5'-cyclic monophosphate (cAMP), tyrosine kinase (TyrK) and phosphatidylinositol 3-kinase (PI3K) on this effect. We used Gö6976 (100 nM) and low concentration of GF 109203X (GF) (50 nM) to inhibit Ca(2+)-dependent PKC isozymes. For Ca(2+)-independent isozymes, we used 500 nM GF and 10 microM rottlerin (specifically inhibits PKCdelta). Phorbol 12-myristate 13-acetate (PMA) (100 ng/ml) was used to stimulate PKC, and genistein (10 microM) and lavendustin A (1 microM) as unspecific TyrK inhibitors. STI571 10 microM was used to specifically inhibit the activity of Kit, the receptor for stem cell factor, and 10 nM wortmannin as a PI3K inhibitor. Activation of PKC with PMA enhances histamine release in response to NH(4)Cl and ionomycin. PMA increases NH(4)Cl-induced alkalinization and ionomycin-induced Ca(2+) entry. Inhibition of PKCdelta strongly inhibits Ca(2+) entry elicited by ionomycin, but failed to modify histamine release. The effect of cAMP-active drugs was explored with the adenylate cyclase activator forskolin (30 microM), the inhibitor SQ22,536 (1 microM), the cAMP analog dibutyryl cAMP (200 microM), and the PKA blocker H89 (1 microM). Forskolin and dibutyryl cAMP do increase NH(4)Cl-induced alkalinization, and potentiate histamine release elicited by this compound. Our data indicates that alkaline-induced exocytosis is modulated by PKC and cAMP, suggesting that pH could be a modulatory signal itself.

Calcium-pH Crosstalks in the human mast cell line HMC-1: Intracellular alkalinization activates calcium extrusion through the plasma membrane Ca2+-ATPase

The human mast cell line (HMC-1) has been used to study the relationship between intracellular pH and cytosolic calcium (Ca2+) in mast cells. Thapsigargin (TG) caused store-operated Ca2+ entry, that is enhanced by the PKC activator PMA. NH4Cl-induced alkalinization showed an inhibitory effect on TG-sensitive stores depletion (not on TG-insensitive stores), and also on final cytosolic Ca2+ levels reached in response to both TG and the ionophore ionomycin. Loperamide, a positive modulator of store-operated channels, induced a slight Ca2+ entry by itself, and also increased TG-induced Ca2+ entry. This enhancement was not enough to reverse the inhibitory effect of NH4Cl-induced alkalinization. When comparing the effect of NH4Cl-induced alkalinization on Ca2+ levels, with those observed using Ca2+ channel blockers (namely Ni2+ and SKF-96365), cytosolic profiles for this ion are different, either in modified saline solution or in HCO3(-)-free medium. Thus, it seems unlikely that the inhibitory effect of NH4Cl-induced alkalinization on Ca2+ is taking place by blockage of Ca2+ entry. Furthermore, inhibition of the plasma membrane Ca2+-ATPase (an important mechanism for Ca2+ efflux) with sodium orthovanadate (SO) matches with the inhibition of the negative effect on Ca2+ levels elicited by NH4Cl. Data indicate that NH4Cl-induced alkalinization might be activating Ca2+ efflux from the cell, by stimulation of the plasma membrane Ca2+-ATPase, and also confirm our previous finding that Ca2+ is a secondary signal to activate HMC-1 cells.

The use of okadaic acid to elucidate the intracellular role(s) of protein phosphatase 2A: Lessons from the mast cell model system

In recent years a heightened appreciation has emerged for the role(s) that phosphatases play in regulating signal transduction pathways and other cellular processes. The tumor-promoting agent okadaic acid (OA) has been an invaluable tool in efforts aimed at delineating the contributions of the most abundant mammalian serine/threonine phosphatase, protein phosphatase 2A (PP2A), to intracellular signaling and cell function. PP2A, which is ubiquitous and vital in virtually every cell system studied, continues to be the focus of much research on phosphorylation control machinery. Mast cells represent an excellent in vitro model for the study of protein phosphorylation events because they possess a number of distinct signaling pathways that lead to the production and/or release of discreet mediators in response to different stimuli. The utility of OA in analyzing PP2A function has been demonstrated in mast cells across several species. Results of these studies have contributed to the current recognition that PP2A plays a crucial role in the biology of mast cells and other cell types.

Mast cell exocytosis can be triggered by ammonium chloride with just a cytosolic alkalinization and no calcium increase

A human mast cell line (HMC-1) has been used to study the effect of cytosolic alkaline pH in exocytosis. Compound 48/80, concanavalin A, and thapsigargin do not induce histamine release in HMC-1 cells. Although thapsigargin does not activate histamine release, it does show a large increase in cytosolic Ca(2+), and no change in cytosolic pH. However, when HMC-1 cells were activated with ionomycin, a significant histamine release takes place, and this effect is higher in the presence of thapsigargin. Both drugs show an additive effect on cytosolic Ca(2+) levels. Ammonium chloride (NH(4)Cl) does activate cytosolic alkalinization and histamine release, with no increase in cytosolic Ca(2+). NH(4)Cl does block the release of internal Ca(2+) by thapsigargin, not by ionomycin, and decreases Ca(2+) influx stimulated by these drugs. Under conditions in which the alkalinization induced by NH(4)Cl is blocked by acidification with sodium propionate, histamine release is inhibited. The release of histamine is also observed when NH(4)Cl is added after propionate addition, regardless of the final pH value attained. Our results show that a shift in pH alkaline values, even with final pH below 7.2 is enough to activate histamine release. A shift to less acidic values is a sufficient signal to activate the cells.

The Cl-HCO3- exchanger slows the recovery of acute pHi changes in rat mast cells

The CO(2)/HCO(3)(-) buffering system is one of the main mechanisms implicated in cytosolic pH (pH(i)) regulation. We studied this pH(i)-regulatory system in rat mast cells using a fluorescent dye. Mast cells had a more alkaline pH(i) in the presence of HCO(3)(-) than in its absence. The recovery from an acid load was faster in HCO(3)(-)-free conditions than in HCO(3)(-)-containing media. In HCO(3)(-)-buffered conditions the increase of the recovery rate of an acidification in 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-incubated cells suggested the implication of a Na(+)-independent Cl(-)/HCO(3)(-) exchanger. This HCO(3)(-) transport acidified the cytosol and was also partially responsible for the recovery of intracellular alkalinizations. Moreover, regulation of the recovery rate of an acidification by protein kinase C and calcium signaling pathways depended on the presence or absence of HCO(3)(-). The presence of HCO(3)(-) limits the recovery of acute intracellular acidifications probably through the Na(+)-independent Cl(-)/HCO(3)(-) exchanger and modulates the regulation of pH(i) by protein kinase C and calcium.

Ouabain-induced enhancement of rat mast cells response. Modulation by protein phosphorylation and intracellular pH

The digitalic glicoside ouabain induces potentiation of rat mast cell histamine release in response to several stimuli, which is mediated by Na+/Ca2+ exchanger. In this work, we studied the effect of ouabain on cytosolic calcium, intracellular pH and histamine release with Ca2+ ionophore A23187 in conditions designed to maximize ouabain-induced potentiation of rat mast cells response. The effect of protein kinase C (PKC), cAMP and phosphatase inhibition was also tested. Ouabain induced an enhancement in histamine release, cytosolic calcium and intracellular pH. The adenylate cyclase activator forskolin reduced the effect of ouabain on histamine release and intracellular pH, but enhanced the effect on cytosolic calcium. PKC activator PMA enhanced the effect of ouabain on histamine release and cytosolic calcium, without affecting intracellular pH. A PKC inhibitor, GF-109203X, reduced ouabain-induced enhancement of histamine release and intracellular pH, but increased the enhancement on cytosolic calcium. Finally, inhibition of protein phosphatases 1 and 2A with okadaic acid, increased the effect of ouabain on histamine release and intracellular pH, but reduced cytosolic calcium in presence of ouabain. This result suggest that ouabain-induced potentiation of rat mast cell histamine release with A23187 is modulated by kinases, and this modulation may be carried out by changes in intracellular alkalinization. However, the mechanism underlying cellular alkalinization remains to be elucidated.

HCO(3)(-) ions modify the role of PKC isoforms in the modulation of rat mast cell functions

PKC and the intracellular calcium signal are two well-known intracellular signaling pathways implicated in the induction of mast cell exocytosis. Both signals are modified by the presence or absence of HCO(3)(-) ions in the external medium. In this work, we studied the regulation of the exocytotic process by PKC isozymes and its relationship with HCO(3)(-) ions and PKC modulation of the calcium entry. The calcium entry, induced by thapsigargin and further addition of calcium, was inhibited by PMA, a PKC activator, and enhanced by 500 nM GF109203X, which inhibits Ca(2+)-independent PKC isoforms. PMA inhibition of the Ca(2+) entry was reverted by 500 and 50 nM GF109203X, which inhibit Ca(2+)-independent and Ca(2+)-dependent isoforms, respectively, and Gö6976, a specific inhibitor of Ca(2+)-dependent PKCs. Thus, activation of Ca(2+)-dependent and Ca(2+)-independent PKC isoforms inhibit Ca(2+) entry in rat mast cells, either in a HCO(3)(-)-buffered or a HCO(3)(-)-free medium. PMA, GF109203X, Gö6976 and rottlerin, a specific inhibitor of PKC delta, were also used to study the role of PKC isoforms in the regulation of exocytosis induced by thapsigargin, ionophore A23187 and PMA. The results demonstrate that Ca(2+)-dependent PKC isoforms inhibit exocytosis in a HCO(3)(-)-dependent way. Moreover, Ca(2+)-independent PKC delta was the main isoform implicated in promotion of Ca(2+)-dependent mast cell exocytosis in the presence or absence of HCO(3)(-). The role of PKC isoforms in the regulation of mast cell exocytosis depends on the stimulus and on the presence or absence of HCO(3)(-) ions in the medium, but it is independent of PKC modulation of the Ca(2+) entry.

HCO3(-) ions increase mast cell sensitivity to thapsigargin-induced Ca(2+) entry

In rat mast cells Ca(2+) entry is modified by the presence or absence of other ions in the external medium. HCO3(-) ions, which modify mast cell degranulation, seemed to modulate the Ca(2+) entry elicited by the intracellular Ca(2+)-ATPase inhibitor thapsigargin. In this work we studied the regulation of the Ca(2+) entry by HCO3(-) and its relationship with exocytosis. The Ca(2+) entry was activated by thapsigargin and Ca(2+) in mast cells bathed by a HCO3(-)-buffered medium or a HCO3(-)-free medium. Both Ca(2+) entry and exocytosis were enhanced by the presence of HCO3(-) ions. Nondegranulated mast cells showed a low Ca(2+) entry either in the presence or absence of HCO3(-). Thus, mast cells with a high [Ca(2+)](i) increase in a HCO3(-)-buffered medium undergo degranulation. In the same cells a second Ca(2+) entry was significantly higher than the first Ca(2+) entry in a HCO3(-)-free medium, while in a HCO3(-)-buffered medium the first and second Ca(2+) entries reached similar [Ca(2+)](i) levels. Although the second Ca(2+) entry is high in a HCO3(-)-free medium, degranulation is still low. Our results demonstrate that HCO3(-) ions increase the capacitative Ca(2+) entry and the sensitivity of mast cells to intracellular Ca(2+) in order to induce degranulation.

Calcium-pH crosstalks in rat mast cells: cytosolic alkalinization, but not intracellular calcium release, is a sufficient signal for degranulation

The aim of this work was to study the relationship between intracellular alkalinization, calcium fluxes and histamine release in rat mast cells. Intracellular alkalinization was induced by nigericin, a monovalent cation ionophore, and by NH(4)Cl (ammonium chloride). Calcium cytosolic and intracellular pH were measured by fluorescence digital imaging using Fura-2-AM and BCECF-AM. In rat mast cells, nigericin and NH(4)Cl induce a dose-dependent intracellular alkalinization, a dose-dependent increase in intracellular calcium levels by releasing calcium from intracellular pools, and an activation of capacitative calcium influx. The increase in both intracellular calcium and pH activates exocytosis (histamine release) in the absence of external calcium. Under the same conditions, thapsigargin does not activate exocytosis, the main difference being that thapsigargin does not alkalinize the cytosol. After alkalinization, histamine release is intracellular-calcium dependent. With 2.5 mM EGTA and thapsigargin the cell response decreases by 62%. The cytosolic alkalinization, in addition to the calcium increase it is enough signal to elicit the exocytotic process in rat mast cells.

Effect of low temperatures on compound 48/80-induced intracellular Ca2+ changes and exocytosis of rat peritoneal mast cells

It has been well documented that compound 48/80-induced exocytosis of mast cells is accompanied by changes in intracellular Ca2+ concentration ([Ca2+]i) showing a biphasic pattern: an initial phase which constitutes an abrupt increase, followed by a plateau phase. The former is caused by Ca2+ release from intracellular Ca2+ stores, and the latter is the result of secondary Ca2+ influx. Low temperatures lead to the inhibition of exocytosis, but the precise mechanism remains unclear. The present study aims to reveal whether [Ca2+]i changes are affected by the environmental temperature. To this end, we developed a novel imaging method to record [Ca2+]i changes and exocytotic processes simultaneously. Rat peritoneal mast cells were loaded by Indo-1/AM or Fluo-3/AM for measuring [Ca2+]i, and the exocytosed granule matrices were stained by sulforhodamine-B. Cells were stimulated by compound 48/80, and [Ca2+]i changes and exocytosis were recorded by means of a real-time confocal microscope. At 37 degrees C, [Ca2+]i changes in stimulated mast cells showed a sustained plateau phase. Granule discharge was observed at the cell surface, and, in addition, most of the intracellular granule matrices were involved in compound exocytosis. The granule discharge and compound exocytosis proceeded over a period of a few minutes. At 4 degrees C, the plateau phase of [Ca2+]i changes declined rapidly, although the initial phase was not suppressed. Granule discharge occurred at the cell surface, but compound exocytosis ceased within a few minutes. These findings indicate that a low temperature inhibits compound exocytosis which can be caused by Ca2+ influx. The present imaging method represents a powerful tool for investigating the stimulus-secretion coupling of mast cells.

Functional compartments in rat mast cells for cAMP and calcium on histamine release

The crosstalk between 3', 5'-cyclic adenosine monophosphate (cAMP), intracellular calcium, and histamine release in rat mast cells using the stimulatory effect of three different drugs, thapsigargin, sodium fluoride (NaF), and compound 48/80 were studied. Each of these drugs induces histamine release by different mechanisms. The transducting pathways modulating cAMP and intracellular calcium levels were modified by using, cholera toxin (CTX) which ADP-rybosylates Gs-protein, pertussis toxin (PTX) which ADP-rybosylates Gi-protein, and okadaic acid (OA) which inhibits phosphatases 1 and 2a. Our results show that CTX increased cAMP levels and inhibited histamine release elicited by thapsigargin and compound 48/80. The inhibitory effect of CTX on histamine release was potentiated by OA in the presence of compound 48/80 but was decreased in the presence of thapsigargin. Calcium uptake was stimulated by NaF and compound 48/80. The previous treatment with OA increased calcium uptake when combined with compound 48/80 but not with NaF. Treatment with NaF highly stimulated calcium uptake and cAMP levels only when combined with OA and CTX. These results suggest that the modulatory effect of intracellular calcium and cAMP on histamine release depend more on the crosstalk of the activated signal transducting pathway than on the final level of calcium or cAMP, further supporting the theory that rat mast cells are divided into functionally distinct compartments.

Modulatory effect of HCO3- on rat mast cell exocytosis: cross-talks between bicarbonate and calcium

HCO-3 modulation of histamine release and its relationship with the Ca2+ signal were studied in serosal rat mast cells. Histamine release was induced by Ca2+ mobilizing stimuli, namely compound 48/80, thapsigargin, Ca2+ chelators, ionophore A23187, and PMA and ionophore A23187 in a HCO-3-buffered medium or a HCO-3-free medium. The presence of HCO-3 reduced histamine release by 48/80, Ca2+ chelators, A23187, and PMA/A23187, but increased histamine release induced by thapsigargin. Histamine release by PMA was significantly higher in a HCO-3-free medium than in a HCO-3-free medium, as it was the PMA potentiation of histamine release by A23187. [Ca2+]i changes induced by these drugs were measured in fura-2-loaded mast cells. In thapsigargin and EGTA or BAPTA preincubated mast cells [Ca2+]i increase was higher in a HCO-3-buffered medium than in a HCO-3-free medium in the presence of Ca2+. On the contrary, in compound 48/80 and PMA/A23187 activated mast cells the [Ca2+]i increase is the same both in the presence and in the absence of HCO-3. The effect of HCO-3 on histamine release in serosal rat mast cells depends on the stimulus, but it is not related to the presence of Cl-. In thapsigargin-stimulated mast cells the effect of HCO-3 on histamine release may be related to the Ca2+ signal, but in compound 48/80, EGTA, and PMA/A23187-activated mast cells there is no relationship between intracellular Ca2+ and the inhibitory effect of HCO-3 on histamine release. Additionally, the PKC pathway is implicated in the inhibitory effect of HCO-3 on histamine release, the higher the chelation of calcium rendering the higher the enhancement of the response after adding calcium in the absence of HCO-3.

Membrane potential changes associated with calcium signals in human lymphocytes and rat mast cells

Human lymphocytes and rat mast cells, two non-excitable cellular models, were used to investigate membrane potential changes accompanying Ca2+ signals. Cells were stimulated with agents known to induce both Ca2+ release from internal stores and influx of extracellular Ca2+, namely thapsigargin, ionomycin and compound 48/80. Thapsigargin and ionomycin were used to activate lymphocytes, while compound 48/80 was used to stimulate mast cells. Membrane potential changes and Ca2+ concentration were monitored with the fluorescent dyes bis-oxonol and fura-2, respectively. In lymphocytes, thapsigargin induced a hyperpolarization temporally correlated with the increase in intracellular Ca2+ concentration. This hyperpolarization is due to activation of a K+ conductance which consists of two phases, a first phase independent on external Ca2+ and a second one blocked in a Ca2+-free medium. Ionomycin induced a Ca2+-dependent depolarization attributed to a massive influx of external Ca2+. On the other hand, stimulation of mast cells with compound 48/80 produced a fast hyperpolarization and an increase in intracellular Ca2+ levels. Besides different time-courses, this hyperpolarization differs from that induced by thapsigargin in lymphocytes in two aspects, it is mainly due to a Cl(-)-entry current and exit of K+ and it is completely inhibited in the absence of extracellular Ca2+. Compound 48/80-induced histamine release is not related to membrane potential changes.

A highly temperature-sensitive proton current in mouse bone marrow-derived mast cells

Proton (H+) conductive pathways are suggested to play roles in the regulation of intracellular pH. We characterized temperature-sensitive whole cell currents in mouse bone marrow-derived mast cells (BMMC), immature proliferating mast cells generated by in vitro culture. Heating from 24 to 36 degrees C reversibly and repeatedly activated a voltage-dependent outward conductance with Q10 of 9.9 +/- 3.1 (mean +/- SD) (n = 6). Either a decrease in intracellular pH or an increase in extracellular pH enhanced the amplitude and shifted the activation voltage to more negative potentials. With acidic intracellular solutions (pH 5.5), the outward current was detected in some cells at 24 degrees C and Q10 was 6.0 +/- 2.6 (n = 9). The reversal potential was unaffected by changes in concentrations of major ionic constituents (K+, Cl-, and Na+), but depended on the pH gradient, suggesting that H+ (equivalents) is a major ion species carrying the current. The H+ current was featured by slow activation kinetics upon membrane depolarization, and the activation time course was accelerated by increases in depolarization, elevating temperature and extracellular alkalization. The current was recorded even when ATP was removed from the intracellular solution, but the mean amplitude was smaller than that in the presence of ATP. The H+ current was reversibly inhibited by Zn2+ but not by bafilomycin A1, an inhibitor for a vacuolar type H(+)-ATPase. Macroscopic measurements of pH using a fluorescent dye (BCECF) revealed that a rapid recovery of intracellular pH from acid-load was attenuated by lowering temperature, addition of Zn2+, and depletion of extracellular K+, but not by bafilomycin A1. These results suggest that the H+ conductive pathway contributes to intracellular pH homeostasis of BMMC and that the high activation energy may be involved in enhancement of the H+ conductance.

Inhibition of rat peritoneal mast cell exocytosis by frusemide: a study with different secretagogues

It has been reported that the loop diuretic frusemide can prevent exercise induced asthma, and that this effect may be due to the inhibition of mast cells in the airway. By using various mast cell secretagogues which increase intracellular calcium via different routes, this study attempted to elucidate the mechanism of the mast cell stabilizing action of frusemide. As well as confirming that immunologically induced histamine release from rat peritoneal mast cells was dose dependently inhibited by frusemide (10(-3) - 10(-5) M), the present study has extended the observation to histamine release induced by compound 48/80. The inhibitory potency was however less in the case of compound 48/80 induced release. Frusemide induced inhibition by the two secretagogues was decreased by drug preincubation. In contrast, histamine release induced by ionophore A23187 and thapsigargin was not inhibited by frusemide. The prototype antiallergic compound disodium cromoglycate (DSCG) demonstrated a similar specificity pattern against the various secretagogues. Another loop diuretic, bumetanide, did not show the same results as frusemide on rat peritoneal mast cell degranulation. Hence it is concluded that frusemide does not inhibit immunological activation of mast cells via its diuretic Na+/K+/Cl- co-transporter capacity. Instead, it protects mast cells in a similar manner to DSCG.

Influence of the anti-allergic drug oxatomide on the signal transduction mechanism in a mast cell model

In a mast cell model, oxatomide displays inhibition of mediator release which is not related to its histamine H1 receptor antagonistic activity. From a previous study it appeared that especially early steps in the signal transduction leading to exocytosis were influenced by oxatomide. We now studied effects of oxatomide on those early steps in more detail. The antigen- and thapsigargin-mediated exocytosis in rat basophilic leukemia (RBL-2H3) cells were both inhibited by oxatomide. After aggregation of high affinity receptors for immunoglobulin E (Fc epsilon RI), protein tyrosine phosphorylation is induced. Oxatomide caused remarkable changes in the tyrosine phosphorylation pattern in resting cells. Also after antigen and thapsigargin activation, changes in the tyrosine phosphorylation of cellular proteins are observed. In addition, Ca2+ fluxes were studied by means of the net influx of 45Ca2+ and by measuring intracellular free Ca2+ concentrations ([Ca2+]) with the fluorescent probe fura-2. Oxatomide inhibited the 45Ca2+ influx and the increase in [Ca2+]i upon antigen and thapsigargin activation of the cells. Neither the release of Ca2+ from internal stores nor the efflux of Ca2+ over the plasma membrane seems to be affected. The effect of oxatomide on Ca2+ influx was further characterized by studying Ba2+ influx in the absence of extracellular free Ca2+. We conclude that inhibition of mediator release is mainly caused by inhibition of influx of extracellular Ca2+, via plasma membrane Ca2+ channels that are activated by depletion of intracellular Ca2+ stores. The molecular mechanism with which oxatomide might interfere with these channels is discussed.

Dual regulation of the Na+/H(+)-exchange in rat peritoneal mast cells: role of protein kinase C and calcium on pHi regulation and histamine release

1. The purpose of this study was to compare the actions of phorbol 12-myristate 13-acetate (PMA) and ionomycin on Na+/H+ exchange activation and histamine release to that of compound 48/80 in order to study the possible relationship between pHi and secretion of histamine in rat peritoneal mast cells. 2. Resting pHi in mast cells suspended in a bicarbonate-free physiological salt solution amounted to 6.73 +/- 0.05 (mean +/- s.d., n = 52). 3. PMA (20 nM) induced a substantial but rather slow increase in pHi. This response was very sensitive to inhibition by staurosporine, very sensitive to inhibition by 5-(N,N-hexamethylene)amiloride (HMA), insensitive to the absence of extracellular calcium (without EGTA), and sensitive to partial depletion of intracellular calcium with EGTA. 4. Ionomycin (1 microM) induced a biphasic change in pHi that was sensitive to inhibition by HMA, insensitive to staurosporine. In the absence of extracellular calcium using EGTA, the biphasic response disappeared, leaving only a slow, and diminished change in pHi. 5. The effects of ionomycin and PMA on pHi were additive. 6. Addition of the secretagogue compound 48/80 (1 microgram ml-1) increased pHi, substantially, delta pHi amounting to 0.29 +/- 0.05 pH-units (n = 4). The biphasic pHi-response was insensitive to the absence of extracellular calcium (without EGTA). The initial fast response in pHi was, however, inhibited by HMA, not staurosporine. 7. The finding that staurosporine and HMA each inhibited approximately half of the compound 48/80-induced pHi-response, whereas both inhibitors completely abolished the compound 48/80-induced pHi-response seems to indicate that two independent pathways for the activation of the Na+/H+ exchange were stimulated by compound 48/80. 8. The histamine release induced via both PKC activation (using PMA) and calcium (using ionomycin) were much larger than the sum of each activation pathway, whereas in the absence of extracellular calcium using EGTA, the histamine release in response to PMA and ionomycin was completely abolished. 9. The compound 48/80-induced histamine release was partially sensitive to inhibition by HMA (approximately 30% inhibition) and partially sensitive to inhibition by staurosporine (approximately 50% inhibition). Preincubation with staurosporine and HMA before stimulation with compound 48/80 showed the same degree of inhibition as observed after staurosporine alone, even though this combination of drugs completely inhibited the pHi-response. Furthermore, compound 48/80-induced histamine release was not dependent on the presence of extracellular calcium (with and without EGTA). 10. In spite of the similarities in second messenger pathways for pHi regulation and histamine release, it is, however, not very likely that these two processes are directly related. It is, however, possible, that an increase in pHi plays a permissive, rather than an essential role for histamine release in rat peritoneal mast cells. This hypothesis was supported by the finding that preincubation with the Na+/H+ exchange-inhibitor HMA inhibited 30% of the compound 48/80-induced histamine secretion.

Effects of hydroquinone-type and phenolic antioxidants on calcium signals and degranulation of RBL-2H3 cells

We previously reported that a hydroquinone-type antioxidant, 2,5-di(tert-butyl)-1,4-hydroquinone (DTBHQ), increases intracellular free Ca2+ concentration ([Ca2+]i), causes degranulation together with a protein kinase C activator, phorbol 12-myristate 13-acetate (TPA), and increases antigen-induced degranulation in rat basophilic leukemia (RBL-2H3) cells. In this study, the effects of five-hydroquinone-type and phenolic antioxidants (2,5-di(tert-amyl)-1,4-hydroquinone [DTAHQ], 2-tert-butyl-1,4-hydroquinone [MTBHQ], 3,5-di(tert-butyl)-4-hydroxytoluene [BHT], 3,5-di(tert-butyl)-4-hydroxyanisole [DTBHA], and 3-tert-butyl-4-hydroxyanisole [MTBHA]) on [ca2+]i and degranulation (beta-hexosaminidase release) were examined and compared with that of DTBHQ. DTAHQ (> or = 3 microM) showed effects similar to those of DTBHQ (10 microM) on [Ca2+]i elevation, induction of degranulation with TPA, and increase of antigen-induced degranulation. BHT (50 microM) and DTBHA (50 microM) caused [Ca2+]i elevation and increased degranulation in the presence of TPA or antigen, but their effects were less than those of DTBHQ and DTAHQ. MTBHQ and MTBHA had no effect on [Ca2+]i and degranulation, even at 50 microM. The degree of Ca2+ response caused by the compounds correlated well with the increase in degranulation, but not with their antioxidant activity estimated with the first oxidation potential. From these results, it is suggested that the increasing effects of six antioxidants on degranulation in the presence of TPA or antigen were dependent on [Ca2+]i increase caused by the compounds, probably through their ability to inhibit endoplasmic reticulum Ca2+-ATPase.

Effects of 2,5-di(tert-butyl)-1,4-hydroquinone on intracellular free Ca2+ levels and histamine secretion in RBL-2H3 cells

The effects of 2,5-di(tert-butyl)-1,4-hydroquinone (DTBHQ) on the intracellular free Ca2+ level ([Ca2+]i) and histamine secretion of rat basophilic leukemia (RBL-2H3) cells were examined. DTBHQ (0.1-10 mumol/l) alone induced rapid and sustained increases in [Ca2+]i in a concentration-dependent manner. In cells sensitized with anti-dinitrophenyl IgE, DTBHQ (10 mumol/l) further increased the antigen (dinitrophenylated BSA)-induced Ca2+ response. In the absence of external Ca2+ with addition of 1 mmol/l EGTA, both DTBHQ (10 mumol/l) and the antigen (10 microgram/ml) induced transient increase in [Ca2+]i. In sensitized cells, both DTBHQ (10 mumol/l) and antigen (10 micrograms/ml) elicited histamine secretion, although the response was far stronger in the latter case. The DTBHQ-induced histamine secretion was markedly enhanced by addition of the protein kinase C activator, phorbol 12-myristate 13-acetate (TPA) (10 ng/ml) whereas TPA alone did not cause any increase. Moreover, DTBHQ enhanced the antigen-induced histamine secretion. The results suggest that DTBHQ increases [Ca2+]i and enhances antigen-induced histamine secretion while DTBHQ alone does not cause as much histamine secretion as antigen, which support the idea that calcium signals are necessary but are not sufficient for maximum histamine secretion in RBL-2H3 cells.