Sodium-potassium ATPase inhibition potentiates compound 48/80-induced histamine secretion from mast cells

Much More than a Cardiotonic Steroid: Modulation of Inflammation by Ouabain

Since the discovery of ouabain as a cardiotonic steroid hormone present in higher mammals, research about it has progressed rapidly and several of its physiological and pharmacological effects have been described. Ouabain can behave as a stress hormone and adrenal cortex is its main source. Direct effects of ouabain are originated due to the binding to its receptor, the Na⁺/K⁺-ATPase, on target cells. This interaction can promote Na⁺ transport blockade or even activation of signaling transduction pathways (e.g., EGFR/Src-Ras-ERK pathway activation), independent of ion transport. Besides the well-known effect of ouabain on the cardiovascular system and blood pressure control, compelling evidence indicates that ouabain regulates a number of immune functions. Inflammation is a tightly coordinated immunological function that is also affected by ouabain. Indeed, this hormone can modulate many inflammatory events such as cell migration, vascular permeability, and cytokine production. Moreover, ouabain also interferes on neuroinflammation. However, it is not clear how ouabain controls these events. In this brief review, we summarize the updates of ouabain effect on several aspects of peripheral and central inflammation, bringing new insights into ouabain functions on the immune system.

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.

Role of the Na+/K+-ATPase in regulating the membrane potential in rat peritoneal mast cells

1. The aim of this study was to investigate the effect of the Na+/K+-ATPase on the membrane potential of peritoneal mast cells isolated from male Sprague-Dawley SPF-rats. 2. Experiments were performed at 22-26 degrees C in the tight-seal whole-cell configuration of the patch-clamp technique by use of Sylgard-coated patch pipettes (3-6 M[omega]). High-resolution membrane currents were recorded with an EPC-9 patch-clamp amplifier controlled by the 'E9SCREEN' software. In addition, a charting programme on another computer synchronously recorded at low resolution (2 Hz) membrane potential and holding current (low-pass filtered at 500 Hz). 3. Na+/K+-ATPase activity was measured as the ouabain-sensitive change in the zero-current potential. The zero-current potential in rat peritoneal mast cells measured 2 min after obtaining whole-cell configuration amounted to 1.7 +/- 2.5 mV (n = 21). Ouabain (5 mM), a Na+/K+-ATPase-inhibitor, had only a very minor effect upon the membrane potential under resting conditions (n = 3). 4. When mast cells were superfused with nominal calcium-free external solution, the cells hyperpolarized (delta mV: 20.2 +/- 3.8 mV (n = 5)). In addition, when the mast cells were preincubated in nominal calcium-free external solution for 12 +/- 1.6 min before whole-cell configuration, the membrane potential amounted to -53.7 +/- 9.8 mV (n = 8). A subsequent superfusion with ouabain (5 mM) depolarized the membrane potential (ouabain-sensitive hyperpolarization (delta mV): 23.0 +/- 8.4 mV (n = 8)). 5. A high intracellular concentration of Na+ ([Na+]i) (26.6 mM) also resulted in hyperpolarization (delta mV: 20.2 +/- 9.1 mV (n = 7)), but only when ATP was present. A subsequent superfusion with ouabain (5 mM) repolarized these cells to -1.2 +/- 14 mV (ouabain-sensitive hyperpolarization (delta mV): 19.7 +/- 7.7 mV (n = 7)). 6. The size of the [Na+]i-dependent hyperpolarization was dose-dependent. Low [Na+]i (1 mM) had no effect on membrane potential and these cells were unaffected by superfusion with calcium-free external solution. 7. These data thus directly confirm that the stimulant effect of calcium-free external solutions on the ouabain-sensitive changes in the zero-current potential, and hence the Na+/K+-ATPase, is mediated through [Na+]i and that the activity of the Na+/K+-ATPase can have an important influence on the resting membrane potential in rat peritoneal mast cells.

Modulation of histamine release by sodium, potassium adenosine triphosphatase inhibition

BACKGROUND

Previous studies have shown that a sodium, potassium adenosine triphosphatase inhibitor is present in the plasma of allergic subjects in whom enhanced histamine releasability has also been reported.

PURPOSE

The purpose of this study was to determine the effect of in vitro sodium, potassium adenosine triphosphatase inhibition on whole blood histamine release.

METHODS

Samples obtained from 12 patients with allergic rhinitis and 12 nonallergic subjects were incubated in duplicate for 30 minutes with anti-IgE antibody (100 micrograms/mL) or control buffer following a 0-, 10-, 20-, 30-, 60-, 120- and 180-minute preincubation with ouabain (3.0 mM) or diluent. Cell supernatants were assayed for histamine by radioimmunoassay and results were expressed as a percentage of total histamine release.

RESULTS

Mean (+/- 1 SEM) anti-IgE induced release, in the presence and absence of ouabain, respectively, for allergic subjects was 27.5 +/- 5.5 and 21.0 +/- 4.5 (ten minutes, P < 0.05), 30.3 +/- 6.0 and 22.4 +/- 4.8 (20 minutes, P < .025), 28.9 +/- 5.2 and 23.5 +/- 4.1 (30 minutes), 33.8 +/- 7.1 and 26.7 +/- 5.4 (60 minutes, P < .05), 43.2 +/- 7.5 and 24.3 +/- 4.6 (120 minutes, P < .001), and 34.5 +/- 5.0 and 29.8 +/- 5.4% (180 minutes). Spontaneous histamine release in allergic subjects was also significantly increased by ouabain. Mean (+/- SEM) spontaneous release, in the presence and absence of ouabain, respectively, for allergic subjects was 2.0 +/- 0.5 and 0.9 +/- 0.2 (60 minutes, P < .025), 2.8 +/- 0.5 and 1.9 +/- 0.4 (120 minutes, P < .05), and 5.4 +/- 1.5 and 3.9 +/- 0.8% (180 minutes, P < .005). Ouabain did not significantly alter histamine release in non-allergic subjects.

CONCLUSIONS

These data show that ouabain induced a significant increase in both spontaneous and induced histamine release in allergic subjects. In vivo, sodium, potassium adenosine triphosphatase inhibition may have an effect on histamine release in allergic subjects.

Mast cell activation involves plasma membrane potential- and thapsigargin-sensitive intracellular calcium pools

The regulation and role of the intracellular Ca2+ pools were studied in rat peritoneal mast cells. Cytosolic free calcium concentration ([Ca2+]i) was monitored in fura-2 loaded mast cells. In the presence of Ca2+ and K+, compound 48/80 induced a biphasic increase in [Ca2+]i composed of a fast transient phase and an apparent sustained phase. The sustained phase was partially inhibited by the addition of Mn2+. DTPA, a cell-impermeant chelator of Mn2+, reversed this inhibition, suggesting that a quenching of fura-2 fluorescence occurs in the extracellular medium. In the absence of extracellular Ca2+, the transient phase, but not the sustained one, could be preserved, provided that mast cells were depolarized. The transient phase was completely abolished by thapsigargin, a microsomal Ca(2+)-ATPase inhibitor. Maximum histamine release induced by either compound 48/80 or antigen was obtained in the absence of added Ca2+ only when mast cells were depolarized. These histamine releases were inhibited by low doses (< 30 nM) of thapsigargin. Thapsigargin at higher doses induced histamine release which was unaffected by changing the plasma membrane potential, but was completely dependent on extracellular Ca2+, showing that a Ca2+ influx is required for thapsigargin-induced exocytosis. Together, these results suggest that the mobilization of Ca2+ from thapsigargin sensitive-intracellular pools induced by compound 48/80 or antigen is sufficient to trigger histamine release. The modulation of these pools by the plasma membrane potential suggest their localization is close to the plasma membrane.

Peptidergic pathway in human skin and rat peritoneal mast cell activation

The common pathway of heterogenous mast cell activation as mediated by antigens is through the cross-linking of IgE bound to Fc epsilon RI receptors. The peptidergic pathway of mast cell activation, achieved by cationic secretagogues, is restricted to "serosal" mast cells, the experimental models being rat peritoneal and human skin mast cells. Cationic secretagogues include positively charged peptides but also various amines such as compound 48/80 and natural polyamines. An early intracellular event of this pathway is the activation of pertussis toxin-sensitive G proteins. The correlation observed between the ability of basic compounds to trigger mast cell exocytosis and their potency to activate purified G proteins strongly suggests that cationic compounds activate mast cell G proteins via a receptor-independent but membrane-assisted process. In this paper, alternative mechanisms are discussed. The consequence of G protein stimulation is the activation of phospholipase C with an increase in inositol triphosphates. Natural polyamines are relatively poor triggers of mast cells (10(-4) to 10(-2) M). Neuropeptides such as substance P, neuropeptide Y or vasoactive intestinal peptide, peptidic hormones such as kinins, and venoms such as mastoparan and mast cell degranulating peptide, are all active in a concentration range from 10(-7) to 10(-4) M. The cationic anaphylatoxin C3a also stimulates mast cells at concentrations below precursor complement C3 blood levels. The component C3 of the complement system is one of only a few plasma proteins having activation fragments (i.e. C3a) that can be generated at micromolar levels. The effects of basic secretagogues defines a peptidergic pathway of mast cell activation, which represents a potentially toxic process considering the tissue effects caused by exogenous basic compounds such as venom peptides and certain amine containing drugs. Peptidergic activation of mast cells may also be a pathophysiological process having an important role in neurogenic inflammation and in diseases involving extensive activation of the blood complement cascade.

Effect of ouabain, digoxin and digitoxigenin on potassium uptake and histamine release from rat peritoneal mast cells

Rat peritoneal mast cells were used to study the effects of digitalis glycosides on potassium uptake and histamine release induced by compound 48/80, substance P and egg-albumin (immunological release). In the absence of calcium all glycosides inhibited potassium uptake. Ouabain and digoxin enhanced the histamine release while digitoxigenin either had no effect or was slightly inhibitory. In the presence of calcium, the glycosides only affected potassium uptake and histamine release slightly. In the presence of lithium or lanthanum the enhancement of the histamine release was counteracted. Hydrophilic digitalis glycosides seem to enhance histamine release secondary to an increase in intracellular sodium. Lipophilic glycosides have no effect on the release.

Activation of the Na+/K(+)-pump in rat peritoneal mast cells following histamine release: a possible role in cell recovery

1. The activity of the Na+/K(+)-pump in rat peritoneal mast cells was measured at various time intervals after induction of cellular histamine release by compound 48/80 or by the antigen-antibody reaction. The Na+/K(+)-pump activity was assessed as the ouabain-sensitive potassium uptake of the cells using 86Rb+ as a tracer for potassium (K+(86Rb+)-uptake). 2. Stimulation of the cells with compound 48/80 induced a time and concentration dependent increase of the Na+/K(+)-pump activity. The pump activity was maximal 2 min after stimulation of the cells. Then, the activity gradually decreased and reached a level not significantly different from the controls after 2 h of incubation. 3. When the cells were stimulated by the antigen-antibody reaction, there was also a rapid (within 5 min) stimulation of the Na+/K(+)-pump. In contrast to the stimulation with compound 48/80, the pump activity returned to the control level after 60 min of incubation with antigen. 4. The ouabain-resistant potassium uptake of the cells was increased after stimulation of the cells, regardless of the secretagogue used. This probably reflects the increased surface area of the cells present after the histamine release. 5. On the basis of the present results, we suggest a role for the Na+/K(+)-pump in the recovery process of the mast cell following histamine release.

Ouabain enhancement of compound 48/80 induced histamine secretion from rat peritoneal mast cells: dependence on extracellular sodium

Purified populations of rat peritoneal mast cells were used to study the effect of ouabain on compound 48/80-induced histamine secretion and on 86Rb+ uptake. 86Rb+ was used as a tracer for extracellular K+. The calculated value of the ouabain-sensitive uptake of K+ and 86Rb+ was considered a measure of the Na(+)-K+ pump activity of the cells. Ouabain caused an immediate inhibition of the pump activity and a time-dependent increase in histamine secretion in the absence of extracellular calcium. No effect on the secretion was observed in the presence of calcium. The effect of ouabain on the secretion occurs in the presence of sodium but not when sodium was replaced by lithium. Preservation by ouabain of a high intracellular sodium content in sodium-loaded cells was associated with preservation of the secretory response in a calcium-free medium. In the presence of lanthanum in a calcium-free medium, the pump activity was inhibited and the enhancement by ouabain of the secretion of histamine was blocked. A less marked inhibition of the pump was found in a calcium-free medium containing magnesium. The inhibition exerted by magnesium was concentration-dependent (0-5 mM) as was the counteraction of magnesium of the enhancement of ouabain of the secretion of histamine. These observations indicate that the enhancement by ouabain of the secretory response of mast cells preincubated in a calcium-free medium is associated with accumulation of sodium inside the cell. In addition to a decreased rate of sodium-calcium exchange caused by a decreased inward directed sodium gradient, the mechanism by which ouabain enhances the secretory response is likely to involve an increased binding of calcium to membrane binding sites.

Changes in rat mast cell responses in sodium-free media. Lack of demonstrable sodium channel activity

Exposure of rat mast cells to isotonic sucrose (employed as a sodium free medium) increased several-fold the sensitivity to calcium, which itself became a stimulus for exocytosis. Concentrations of the cation as low as 25 microM permitted maximal histamine release. Preincubation of cells in sucrose to allow sodium efflux before adding the ionophore A23187 led to a slower release of histamine. We postulate that sodium efflux can generate a membrane potential that causes the increased sensitivity to calcium and the delay in response after preincubation. The response to A23187 is somewhat unspecific since the ionophore can release histamine from internal calcium reservoirs. Saxitoxin or veratridine did not affect cell responses, so that sodium activity is not mediated through defined sodium channels.

Regulation of the Na(+)-K+ pump activity and estimation of the reserve capacity in intact rat peritoneal mast cells

Evidence is provided that regulation of the Na(+)-K+ pump activity in rat peritoneal mast cells occurs mainly through stimulation of the pump from inside the plasma membrane by sodium. It is demonstrated that there is a large reserve capacity for the exchange of intracellular sodium with extracellular potassium in these cells. The maximal pump activity was estimated to be 3230 pmol/10(6) cells per min and Km for extracellular potassium was 1.5 mM.

Inhibition of Na(+) -K+ pump activity by divalent cations in intact peritoneal mast cells of the rat

1. The inhibition by the divalent cations magnesium, barium and strontium and the trivalent ion lanthanum of the Na(+) -K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure mast cell populations by measurement of the ouabain-sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). 2. Exposure of the cells to magnesium induced a time- and concentration-dependent decrease in the ouabain-sensitive K+(86Rb+)-uptake of the cells without influencing the ouabain-resistant uptake. The time-dependent decrease was apparent after incubation of the cells for 10 min or more, but no decrease was observed after 2 min incubation when the cells are supposed to be loaded with sodium due to the cell isolation procedure. 3. Barium and strontium caused concentration-dependent decreases in the ouabain-sensitive K(+) -(86Rb+) -uptake of the cells but the ouabain-resistant uptake was not changed. Half maximum decrease in the ouabain-sensitive K+(86Rb+)-uptake was observed with 1.8 mM magnesium, 1.2mM barium and 0.7 mM strontium. 4. The trivalent ion lanthanum blocked almost completely the ouabain-sensitive K+(86Rb+)-uptake at a concentration of 1 microM as does 1 mM calcium. Combining either of these ions with magnesium had no further inhibitory effect on the ouabain-sensitive uptake. 5. In conclusion, in addition to the previously suggested modulation by calcium of the activity of the Na+ (-)K+ pump, evidence is provided in this investigation that the modulation may be a more general effect of divalent and polyvalent cations present in the extracellular space through their influence on the sodium permeability of the plasma membrane.

Changes in histamine secretion from mast cells caused by digitalis glycosides

Cardiotonic glycosides modify histamine secretion from rat mast cells in the following way. (1) Preincubation (30 min) of mast cells with liposoluble glycosides (10(-4) mol/l) increases the spontaneous histamine secretion by about 5%. (2) Preincubation of mast cells with 10(-4) mol/l liposoluble glycosides (digitoxin, digoxin, digitoxigenine) decreases histamine release induced by compound 48/80 in the presence of calcium, whereas the water soluble glycoside, strophanthin G, has no effect on the secretion. (3) Preincubation of mast cells in a calcium-free medium with the glycosides (10(-4) mol/l) has a dual-effect on histamine secretion induced by compound 48/80: water soluble glycosides (strophanthin G and K) potentiate histamine release, whereas the liposoluble glycosides (digitoxin, digitoxigenine) decrease the secretory response. The difference in the activity of different glycosides could be explained by their dual effects, namely an inhibition of Na+K(+)-ATPase which leads to an increase in histamine release, and intracellular action(s) of liposoluble glycosides leading to a decrease of histamine secretion.

Reversal by EGTA of the enhanced secretory responsiveness of mast cells due to treatment with ouabain

The effect of EGTA on the enhancement by ouabain of compound 48/80-induced secretion from mast cells was compared with the effect on the Na(+)-K+ pump activity. The time-dependent secretory enhancement by ouabain was blocked by addition of EGTA to the cell suspension concomitantly with the addition of ouabain, and EGTA caused a large increase in the pump activity. Addition of 10 microM EGTA to ouabain-treated cells stopped but did not reverse the enhancement. The experiments show that the effect of ouabain was due to changes in a calcium pool utilized in compound 48/80-induced secretion following changes in the Na+,K+ pump activity.

Direct activation of GTP-binding regulatory proteins (G-proteins) by substance P and compound 48/80

The neuropeptide substance P and the polyamine compound 48/80, both known to activate mast cell secretory processes, increased the rate of GTP S binding to G-proteins purified from calf brain (Go/Gi mixture). The GTPase activity of G-proteins was also increased by substance P and compound 48/80 in a dose-dependent and Mg2+-dependent way. These effects were similar to those of the wasp venom peptide mastoparan, another histamine releaser of rat peritoneal and human skin mast cells. This suggests that the secretory property of compound 48/80 and substance P is not due to a receptor-mediated process but, like mastoparan, results from a direct activation of G-proteins.

The mode of inhibition of the Na+-K+ pump activity in mast cells by calcium

1 The inhibition by calcium of the Na(+)-K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure populations of the cells by measuring the ouabain-sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). 2 Exposure of the cells to calcium induced a time- and concentration-dependent decrease in the ouabain-sensitive K+(86Rb+)-uptake of the cells without influencing the ouabain-resistant uptake. The development of the inhibition required the presence of potassium in the medium in the millimolar range (1.5-8.0 mM), and it did not occur at a concentration of potassium (0.24 mM) that is probably rate limiting for the pump activity. In the presence of 1 mM calcium full inhibition developed almost immediately and was not readily reversed. The inhibition was not significantly reduced by 15 min incubation with 1.2 mM EGTA. 3 The inhibitory action of calcium did not develop when the mast cells were incubated in a potassium-free medium, which is known to block Na(+)-K+ pump activity and allow accumulation of sodium inside the cells. Likewise, increasing the sodium permeability of the plasma membrane by monensin abolished the inhibition of the pump activity. In both cases, incubation of the cells with 4.7 mM potassium and tracer amounts of 86Rb+ resulted in a very large uptake of K+ (86Rb+) into the cells (up to 2 nmol per 10(6) cells min-1), indicating a high activity of the Na(+)-K+ pump. 4. These observations support the view that long-term incubation of rat peritoneal mast cells in a calcium-free medium increases the permeability of the plasma membrane to sodium, and the consequent increase in the intracellular concentration of sodium causes an increase in the activity of the pump. Addition of calcium to the cell suspension decreases the sodium permeability, and hence the pump activity. This hypothesis is supported by the stimulation of pump activity produced by monensin, which is not inhibited by calcium. The enhancement of pump activity after exposure of calcium-deprived cells to EGTA might be the result of a further increase in the sodium permeability of the plasma membrane.

Na+ -K+ pump activity in rat peritoneal mast cells: inhibition by extracellular calcium

1. Pure populations of rat peritoneal mast cells were used to study cellular potassium uptake. The radioactive potassium analogue, 86rubidium, was used as a tracer for potassium for measurements of the activity of the cellular potassium uptake process. 2. The ouabain-sensitive and the ouabain-resistant potassium (86rubidium) uptake of mast cells incubated in the presence of calcium, 1 mmol l-1, were very low, 52 and 147 pmol per 10(6) cells min-1. 3. Calcium-deprivation of the cells uncovered a large capacity ouabain-sensitive potassium (86rubidium) uptake mechanism. The activity of the uptake mechanism was decreased by reintroduction of calcium into the cell suspension, and it was dependent on cellular energy metabolism, temperature and pH. 4. The potassium (86rubidium) uptake of mast cells incubated in a calcium-free medium occurs through an active and ouabain-sensitive mechanism that has the nature of an enzyme, and it is mediated by the Na+ -K+ pump located in the plasma membrane. It is demonstrated that the activity of the Na+ -K+ pump mechanism is inhibited by low concentrations of extracellular calcium (0.1-1.2 mmol l-1). The possibility is discussed that calcium-deprivation may increase the pump activity by increasing the permeability of the plasma membrane for Na+.

The question of chymase in cultured muscle cells

An alkaline proteinase, identical to mast cell chymase, has been described by a number of laboratories as being associated with myofibrils extracted from adult rat skeletal muscle tissue. A more recent study has indicated that chymase may be an intrinsic protein in the rat myocyte. The present study of rat myogenic cell lines, using more stringent controls and a probe of more highly defined specificity, supports the view that (i) chymase originates from mast cells of the interstitium and (ii) chymase from mast cells becomes adsorbed to myofibrils of adult muscle during homogenization of this complex tissue.

Preservation of the secretory response of peritoneal mast cells in the absence of extracellular calcium

The transfer of rat peritoneal mast cells from balanced salt solution to calcium-free buffer led to a time-dependent decrease in their response to compound 48/80 and to ionophore A23187. The concomittant absence of potassium from the calcium-free buffer enabled the mast cells to retain their secretory response. The increase in potassium level, with a parallel decrease in sodium to maintain osmolarity, led to a slight potentiation of the response to 48/80 and to a large but transient potentiation of the response to A23187. Mast cells can be considered nonexcitable. The apparent dependency upon extracellular calcium of mast cell secretory responses might be related to the presumed tight equilibrium between endoplasmic reticulum calcium stores and extracellular calcium. The control of this equilibrium by transmembrane gradients of monovalent ions is proposed.

Transmembrane sodium and potassium gradients modulate histamine secretion induced by ionophore A23187

Histamine secretion was induced from rat peritoneal mast cells by calcium ionophore A23187 in the presence of various extracellular calcium concentrations. Transmembrane sodium and potassium gradients were altered by cold pretreatment of mast cells or through the inhibition of sodium-potassium ATPase by the use of ouabain or potassium-deprivation. Such pretreatments led to a parallel shift to the left of the extracellular calcium concentration-histamine secretion curve, i.e. to an apparent decrease of extracellular calcium requirement for the ionophore-induced histamine release. These effects were fully reversed by warming mast cells, by washing out ouabain or by adding potassium. Metabolic inhibition of mast cells prevented the ionophore-induced secretion in all the experimental conditions described. Secretion observed in the absence of added calcium was inhibited by short term treatment of cells with 5 X 10(-6) M EGTA or EDTA provided magnesium was absent from the assay medium. Data show that ionophore A23187 was able to induce secretion in the presence of micromolar concentrations of extracellular calcium, when the efficiency of the ionophore was not decreased by extracellular magnesium and when transmembrane sodium and potassium gradients were altered.

Plasma membrane fluidity measurements on whole living cells by fluorescence anisotropy of trimethylammoniumdiphenylhexatriene

Trimethylammoniumdiphenylhexatriene (TMA-DPH) is a hydrophobic fluorescent probe with a high quantum yield, which was shown earlier to have specific localization properties in the plasma membranes of whole living cells. This probe was used in aqueous suspensions of L929 mouse fibroblasts, rat mast cells and ReH6 leukemic lymphocytes for determining plasma membrane fluidity from fluorescence stationary anisotropy measurements. TMA-DPH was only partially incorporated into the membranes, most of it remained as a stable form in the buffer solution; the distribution was governed by an equilibrium. The measurements were influenced by unavoidable parasitic scattered light and an appropriate correction is described. A set of precautions for the proper use of the probe is proposed. The results indicated that the fluidity was considerably lower in whole cells than in isolated membranes from the same system.

Calcium dependent modulation of histamine release from mast cells by sodium and potassium

The inhibition of sodium-potassium ATPase by ouabain or potassium-deprivation led to a potentiation of histamine release induced by antigen or compound 48/80 from rat mast cells. The fullest potentiation required 30 min preincubation of cells with ouabain or without potassium before adding the triggering agent. The potentiation was observed provided that the calcium concentration was less than 0.5 mM. The effect of sodium-potassium ATPase blockade was reversed slowly by washing out ouabain or by adding potassium. Metabolic inhibition with deoxyglucose and dinitrophenol-blocked histamine release observed under all conditions described. Histamine release from rat mast cells was also elicited when the cells were incubated in sodium-deficient media where sodium was replaced by sucrose. Potassium-deprivation potentiated this secretion process previously shown to be dependent upon metabolic energy and sensitive to millimolar calcium concentrations.

Plasma membrane fluidity studies by fluorescence polarization in rat mast cells stimulated by compound 48/80

Plasma membrane fluidity measurements were performed on purified living mast cells using a novel non-permeant fluorescence polarization probe TMA-DPH, upon stimulation by compound 48/80. The fluorescence anisotropy increased rapidly after treatment by 48/80 in a dose-dependent way. The effect was found to be specific for mast cells; it was inhibited by the histamine release antagonist FR 7534 in a correlative manner. The role of calcium was examined. The results brought evidence for a plasma membrane fluidity decrease induced by 48/80; a biphasic mechanism was inferred for the histamine release process.

[Cellular histamine liberation and mediators of anaphylaxis]

A study of the release of histamine from basophils and mast cells is clearly important to understand the mechanisms of activation of these cells and the anti-allergic mechanisms of the drugs in use. Important information has been gained through the study of highly purified rodent mast cells as well as human mast cells and basophils of low purity, in many fields: morphology, triggering mechanisms, degranulation and mediator release. Recent findings obtained with purified human mast cells and basophils could better identify the specific potential mediators of these two cells. Many biochemical events have been associated with anaphylactic mediator release: recent evidence suggested that an elevation of intracellular calcium level was an essential event in the processes by which mediators are released.