Receptor-mediated calcium entry in fura-2-loaded human platelets stimulated with ADP and thrombin. Dual-wavelengths studies with Mn2+

Protein kinase C activates capacitative calcium entry in the insulin secreting cell line RINm5F

This study examines the calcium store-regulated (capacitative) calcium influx pathway in the endocrine pancreatic cell line RINm5F, utilizing thapsigargin. After preincubation of the cells with the phorbol ester TPA, thapsigargin induced a sustained elevation of cytosolic calcium as well as a sustained stimulation of manganese entry, the latter being used to assess calcium influx. Thapsigargin given alone provoked a smaller and only transient elevation of cytosolic calcium and stimulation of manganese entry. The protein kinase C inhibitor staurosporine antagonized the effect of the phorbol ester. Verapamil, nifedipine, or measures to hyperpolarize the cells exerted no inhibitory action against this effect, which excludes an involvement of voltage-dependent calcium channels. In conclusion, our data shows for the first time that protein kinase C stimulation activates the capacitative calcium influx pathway of endocrine pancreatic insulin-producing cells.

Characterization of receptor-mediated and store-regulated Ca2+ influx in human neutrophils

1. It is not known to what extent the emptying of intracellular Ca2+ stores participates in the mediation of chemoattractant-induced Ca2+ influx in human neutrophils. To study this question, we compared the properties of bivalent-cation influx in response to the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenyl-alanine (f-MLP) and to the microsomal Ca(2+)-ATPase inhibitor thapsigargin. 2. The influx pathway activated by f-MLP and thapsigargin had identical properties of permeation. Mn2+ influx became saturated at around 1 mM extracellular Mn2+, whereas Ca2+ influx did not become saturated up to concentrations of 10 mM. 3. The influx of the two bivalent cations, Mn2+ and Ca2+, was activated to a similar extent and with identical kinetics of activation. 4. The Mn2+ influx activated by f-MLP and thapsigargin was blocked, with identical dose-inhibition curves, by four imidazole analogues. 5. The same relationship between the emptying of Ca2+ stores and bivalent-cation influx was observed for f-MLP and thapsigargin, with a half-maximal activation of the influx at 40% emptying of intracellular stores. 6. In conclusion, neutrophils possess a single type of Ca(2+)-influx pathway that is activated by receptor agonists and by store depletion. Receptor agonists activate this influx pathway to a large extent, if not completely, through the depletion of intracellular Ca2+ stores.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Intracellular calcium mobilization is triggered by clustering of membrane glycoproteins in concanavalin A-stimulated platelets

Stimulation of human platelets with concanavalin A resulted in a significant increase in the concentration of cytoplasmic free Ca2+. This effect was due to two different processes: Ca2+ mobilization from internal stores and Ca2+ influx from the extracellular medium. Kinetic analysis revealed that the release of Ca2+ from internal storage sites occurred sooner than the opening of plasma membrane Ca2+ channels. The ability of concanavalin A to induce a sustained increase in cytoplasmic Ca2+ concentration was antagonized and reversed by methyl alpha-D-mannopyranoside, demonstrating that it was promoted by the interaction of the lectin with cell surface glycoproteins. Succinyl-concanavalin A, a dimeric derivative of the lectin, that does not promote patching/capping of the receptor, was able to bind to the platelet surface, and antagonized the effects of native concanavalin A. In addition, succinyl-concanavalin A, per se, was unable to induce Ca2+ mobilization in human platelets. Therefore, the action of the native concanavalin A was mediated by receptor clustering events. Concanavalin A mobilized Ca2+ from the same internal stores from which Ca2+ was mobilized in response to strong platelet agonists, such as thrombin and arachidonic acid. However, while thrombin was ineffective in inducing Ca2+ release after stimulation of platelets with ConA, ConA was able to cause a full discharge of Ca2+ from internal stores even in platelets previously stimulated with thrombin. These results demonstrate for the first time that the clustering of specific membrane glycoproteins can trigger platelet activation. The physiological implications during platelet aggregation are discussed.

Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Depletion of InsP3 stores activates a Ca2+ and K+ current by means of a phosphatase and a diffusible messenger

In non-excitable cells, release of Ca2+ from the inositol 1,4,5-trisphosphate (InsP3)-sensitive store can activate Ca2+ entry. Very little is known about the signal mechanism relating store emptying to plasma membrane Ca2+ influx. It has been suggested that the signal may be either a diffusible messenger like an inositol phosphate, or the InsP3 receptor itself, which, by physically coupling to some component of Ca2+ entry in the plasma membrane, may link store release to Ca2+ entry. The nature of the Ca2+ entry pathway is also unclear. Only in mast cells has a very selective Ca2+ current been observed after store emptying. Activation of exogenous 5-hydroxytryptamine (5-HT) receptors expressed in Xenopus oocytes or direct injection of InsP3 evokes Ca2+ entry activated by InsP3 pool depletion. Here we investigate the nature of this influx pathway and find a current activated by pool depletion. This has an unusual selectivity in that it is more permeable to Ca2+ ions than to other divalent cations (Ba2+, Sr2+ or Mn2+). Moreover, a K+ permeability is also stimulated after pool depletion. The activation of this store depletion current involves both a phosphatase and an unidentified diffusible messenger. Both the Ca2+ entry pathway and the activating factors found here may be relevant to pool-depleted Ca2+ entry in a variety of non-excitable cells.

Spatial localization of agonist-induced Ca2+ entry in bovine adrenal chromaffin cells. Different patterns induced by histamine and angiotensin II, and relationship to catecholamine release

The spatial organization of agonist-induced Ca2+ entry in single bovine adrenal chromaffin cells has been investigated using video-imaging techniques to visualize fura-2 quenching by the Ca2+ surrogate, Mn2+. The potent secretagogue histamine, in addition to releasing Ca2+ from intracellular stores, resulted in a large influx of external Mn2+ that occurred over the entire surface of the cell. The influx of Ca2+ that this mirrors was found to be an obligatory requirement for the triggering of catecholamine release by histamine, which suggests that such a global influx of Ca2+ into the cell probably underlies the ability of this agonist to stimulate a large secretory response. By contrast, the weaker secretagogue angiotensin II, which also acts through the second messenger inositol trisphosphate, produced a localized entry of external Mn2+ in 64% of cells. In these cells, localized Mn2+ entry always occurred at the pole of the cell in which the angiotensin II-induced rise in [Ca2+]i was largest. Since exocytosis in response to angiotensin II has previously been shown to be restricted to this same pole of the cell (Cheek et al. (1989). J. Cell Biol. 109, 1219–1227), these results suggest that localized influx of Ca2+ in response to angiotensin II could underlie the polarized exocytotic response observed with this stimulus. These results directly demonstrate that different agonists can induce different patterns of divalent cation influx in the same cells and, furthermore, suggest how these different patterns can have a direct influence on cellular function.

Cytosolic calcium homeostasis in bovine parathyroid cells and its modulation by protein kinase C

1. The effects of protein kinase C (PKC) activators and inhibitors on the mechanisms regulating cytosolic Ca2+ homeostasis in dissociated bovine parathyroid cells loaded with fura-2 were examined. 2. Stepwise increases in the concentration of extracellular Ca2+ (from 0.5 to 2 or 3 mM) elicited transient followed by sustained increases in the concentration of intracellular free Ca2+ ([Ca2+]i). Cytosolic Ca2+ transients reflected the mobilization of intracellular Ca2+ and influx of extracellular Ca2+ whereas sustained increases in [Ca2+]i resulted from the influx of extracellular Ca2+. Brief (1-2 min) pretreatment with phorbol myristate acetate (PMA) shifted the concentration-response curve for extracellular Ca(2+)-induced cytosolic Ca2+ transients to the right without affecting the maximal response. Cytosolic Ca2+ transients elicited by extracellular Mg2+ were similarly affected by PMA. 3. These effects of PMA were mimicked by various other activators of PKC with the rank order of potency PMA > phorbol dibutyrate > bryostatin , > (-)indolactam V > mezerein. Isomers or analogues of these compounds that do not alter PKC activity (4 alpha-phorbols and (+)indolactam V) did not alter [Ca2+]i. 4. PKC activators depressed evoked increases in [Ca2+]i when influx of extracellular Ca2+ was blocked with Gd3+. Cytosolic Ca2+ transients elicited by extracellular Mg2+ in the absence of extracellular Ca2+ were similarly inhibited by PKC activators. Activation of PKC thus inhibits the mobilization of intracellular Ca2+ elicited by extracellular divalent cations. 5. Increases in the concentration of extracellular Ca2+ caused corresponding increases in the formation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3). Pretreatment with PMA shifted the concentration-response curve for extracellular Ca(2+)-induced [3H]InsP3 formation to the right without affecting the maximal response. 6. PKC activators also caused some depression of steady-state increases in [Ca2+]i elicited by extracellular Ca2+. In contrast, PMA did not affect increases in [Ca2+]i elicited by ionomycin or thapsigargin. 7. Ba2+ was used to monitor divalent cation influx. PMA decreased the rate of rise of the fluorescent signal elicited by extracellular Ba2+. 8. All these effects of PKC activators on [Ca2+]i were blocked or reversed by staurosporine at concentrations (30-100 nM) that inhibited PKC activity in parathyroid cells. Staurosporine alone potentiated cytosolic Ca2+ responses evoked by submaximal concentrations of extracellular divalent cations. 9. PKC thus depresses both the mobilization of intracellular Ca2+ and the influx of extracellular Ca2+ in parathyroid cells. The effects on [Ca2+]i provide evidence for a Ca2+ receptor on the surface of parathyroid cells that uses transmembrane signalling mechanisms common to some other Ca(2+)-mobilizing receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

[Calcium and liver]

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

The Use of Fura-2 Fluorescence to Monitor the Movement of Free Calcium Ions into the Matrix of Plant Mitochondria (Pisum sativum and Helianthus tuberosus)

Purified mitochondria isolated from pea (Pisum sativum L. cv Alaska) stems and Jerusalem artichoke (Helianthus tuberosus L. cv OB1) tubers were loaded with the acetoxymethyl ester of the fluorescent Ca2+ indicator fura-2. This made possible the continuous monitoring of free [Ca2+] in the matrix ([Ca2+]m) without affecting the apparent viability of the mitochondria. Pea stem mitochondria contained an initial [Ca2+]m of approximately 60 to 100 nM, whereas [Ca2+]m was severalfold higher (400-600 nM) in mitochondria of Jerusalem artichoke tubers. At low extramitochondrial Ca2+ concentrations ([greater than or equal to]100 nM), there was an energy-dependent membrane potential increase in [Ca2+]m; the final [Ca2+]m was phosphate-dependent in Jerusalem artichoke but was phosphate-independent in pea stem mitochondria. The data presented indicate that (a) there is no absolute requirement for phosphate in Ca2+ uptake; (b) plant mitochondria can accumulate external free Ca2+ by means of an electrophoretic Ca2+ uniporter with an apparent affinity for Ca2+ (Km approximately 150 nM) that is severalfold lower than that measured by conventional methods (isotopes and Ca2+-sensitive electrodes); and (c) [Ca2+]m is within the regulatory range of mammalian intramitochondrial dehydrogenases.

Platelet activating factor-induced increase in cytosolic calcium and transmembrane current in human macrophages

Platelet-activating factor (PAF) is synthesized and secreted by macrophages in responding to inflammatory stimuli. When exogenously applied to human monocyte derived macrophages (HMDMs), PAF induces a rapid rise in cytosolic free calcium (Cai) believed to be an early triggering event in macrophage activation. We investigated PAF-induced Ca2+ signaling in HMDMs using the calcium indicator Fura-2, combining single cell ratio fluorimetry and digital video imaging with whole-cell recording techniques. Application of PAF (20 ng/ml) to adherent macrophages induced transient increases in Cai that were biphasic, consisting of an initial phase that could be observed in Ca(2+)-free solutions and a second phase that was critically dependent upon Ca2+ entry. When Mn2+ was applied to cells in the presence and absence of Ca2+, PAF increased the rate of Mn2+ entry rate only when Ca2+ was absent. PAF increased the rate of Ba2+ entry even when measured in the presence of external Ca2+. Ca2+ entry was reversibly inhibited in the presence of external La3+ (1 mM). Data obtained from simultaneous voltage-clamp/microfluorimetry experiments demonstrated the activation of a nonselective cation current which closely paralleled the rising phase of the Cai transient. We investigated whether the non-selective cation conductance provided for the bulk of the agonist-induced Ca2+ influx. Changes in Cai following removal of extracellular Ca2+ (Cao) during the agonist-induced Cai response were not associated with changes in whole-cell current. The inability to detect whole-cell current changes correlated with a decrease in Cao suggests that the bulk of the Ca2+ influx was not through the nonselective conductance and either does not occur through a conductance pathway or occurs via a parallel pathway consisting of channels which are both low conductance and highly Ca2+ selective.

Multiple mechanisms of manganese-induced quenching of fura-2 fluorescence in rat mast cells

Whole-cell patch-clamp recordings of membrane currents and fura-2 measurements of free intracellular calcium concentration ([Ca2+]i) were used to study Mn2+ influx in rat peritoneal mast cells. The calcium-selective current, activated by depletion of intracellular calcium stores (ICRAC for calcium release-activated calcium current), supports a small but measurable Mn2+ current. In the presence of intracellular BAPTA, a Mn2+ current through ICRAC was recorded in isotonic MnCl2 (100 mM) without a significant quenching of fura-2 fluorescence. Its amplitude was 10% of that measured in physiological solution containing 10 mM Ca2+. However, following store depletion, a significant quenching of fura-2 fluorescence could be measured only when intracellular BAPTA was omitted, so that all the incoming Mn2+ could be captured by the fluorescent dye. Two other ionic currents activated by receptor stimulation also induced Mn2+ quenching of fura-2 fluorescence: a small current through non-specific cation channels of 50-pS unitary conductance and a distinct cationic current of large amplitude. In addition to these influx mechanisms, Mn2+ was taken up into calcium stores and was subsequently co-released with Ca2+ by Ca(2+)-mobilizing agonists.

Thapsigargin-evoked changes in human platelet Ca2+, Na+, pH and membrane potential

1. In this work we explored the effect of thapsigarin on the intracellular Ca2+, pH, Na+ and membrane potential in human platelets. These parameters were monitored using the fluorescent probes fura-2, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein, sodium-binding benzofuran isophthalate, and 3,3'-dipropylthiadicarbocyanine iodide. 2. Thapsigargin caused an increase in the cytosolic Ca2+, coupled with cytosolic alkalinization. Thapsigargin-induced alkalinization was Na(+)-dependent, indicating that thapsigargin stimulated the Na(+)-H+ exchange. 3. Using Mn2+ as a Ca2+ surrogate, we showed that thapsigargin activated Ca2+ channels at relatively low levels of cytosolic Ca2+, suggesting that a rise in cytosolic free Ca2+ is not the signal for the activation of these channels. 4. Thapsigargin-induced increase in the cytosolic free Ca2+ was greater in Na(+)-containing medium than in Na(+)-free medium, suggesting that Na(+)-dependent mechanisms participate in the regulation of platelet cytosolic Ca2+. 5. Thapsigargin not only increased the cytosolic Ca2+, but also elevated the cytosolic free Na+. The latter effect was more pronounced in Ca(2+)-free medium, a finding that may indicate that some of the Na+ enters through Ca2+ entry pathways. 6. Finally, thapsigargin evoked sustained platelet hyperpolarization which was attenuated by charybdotoxin, indicating thapsigargin-induced stimulation of Ca(2+)-sensitive K+ channels. 7. Together these observations demonstrate a multifactorial effect of thapsigargin on platelets that can be utilized to further understand platelet ionic homeostasis.

Function of intracellular [Ca2+]i in exocytosis and transbilayer movement in human platelets surface-labeled with the fluorescent probe 1-(4-trimethylammonio)phenyl)-6-phenyl-1,3,5-hexatriene

Ellipsometry indicated that 1-(4-(trimethylammonio)phenyl-6-phenylhexa-1,3,5-triene (TMA-DPH) bound to platelets in a reversible and saturable way. Accordingly, the fluorescence intensity (F) of a suspension of TMA-DPH-labeled platelets was described as a quantity, determined by the amount of TMA-DPH bound to the platelet surface. Most platelet activators elevated F to a degree that correlate well with the secretion of serotonin evoked by these activators. The increase in F levels reflected the increase in outer membrane surface area following exocytosis. However, activators that evoked prolonged (> 2.5 min) and strong (> 600 nM) elevations of cytosolic [Ca2+]i increased F to levels that were much higher than expected from the maximal increase in surface area due to exocytosis. This high increase in F was caused by inward transbilayer movement of TMA-DPH over the plasma membrane and the subsequent labeling of cytosolic membrane sides. The kinetics of exocytosis and changes in cytosolic [Ca2+]i were studied by stopped-flow mixing of platelets with agonist. Thrombin-induced exocytosis had a delay of only 3 s, which was shortened when external CaCl2 or ADP was present. This correlated well with a faster rise in [Ca2+]i in the presence of CaCl2 or ADP, indicating that exocytosis was linked in time to elevation of [Ca2+]i. By itself, ADP was unable to evoke exocytosis and it elicited a [Ca2+]i transient of much shorter duration than thrombin, but with similar maximum. We concluded that both exocytosis and transbilayer movement were associated with elevation of [Ca2+]i: exocytosis required a moderate, relatively prolonged rise and transbilayer movement was accompanied by a stronger rise of even longer duration. Influx of external Ca2+ was essential for transbilayer movement, but not for exocytosis.

Cross-linking of IgG receptors inhibits membrane immunoglobulin-stimulated calcium influx in B lymphocytes

By cross-linking membrane immunoglobulins (mIg), the antigenic stimulation of B lymphocytes induces an increase in intracellular free calcium levels ([Ca2+]i) because of a combination of release from intracellular stores and transmembrane influx. It has been suggested that both events are linked, as in a number of other cases of receptor-induced increase in [Ca2+]i. Conversely, in B lymphocytes, type II receptors for the Fc fragment of IgG (Fc gamma RII) inhibit mIg-mediated signaling. Thus, we have investigated at the level of single cells if these receptors could act on specific phases of mIg Ca2+ signaling. Lipopolysaccharide-activated murine B splenocytes and B lymphoma cells transfected with intact or truncated Fc gamma RII-cDNA were used to determine the domains of Fc gamma RII implicated in the inhibition of the Ca2+ signal. [Ca2+]i was measured in single fura-2-loaded cells by microfluorometry. The phases of release from intracellular stores and of transmembrane influx were discriminated by using manganese, which quenches fura-2, in the external medium as a tracer for bivalent cation entry. The role of membrane potential was studied by recording [Ca2+]i in cells voltage-clamped using the perforated patch-clamp method. Cross-linking of mIgM or mIgG with F(ab')2 fragments of anti-Ig antibodies induced a sustained rise in [Ca2+]i due to an extremely fast and transitory release of Ca2+ from intracellular stores and a long lasting transmembrane Ca2+ influx. The phase of influx, but not that of release, was inhibited by membrane depolarization. The increase in [Ca2+]i occurred after a delay inversely related to the dose of ligand. Co-cross-linking mIgs and Fc gamma RII with intact anti-Ig antibodies only triggered transitory release of Ca2+ from intracellular stores but no Ca2+ influx, even when the cell was voltage-clamped at negative membrane potentials. These transitory Ca2+ rises had similar amplitudes and delays to those induced by cross-linking mIgs alone. Thus, our data show that Fc gamma RII does not mediate an overall inhibition of mIg signaling but specifically affects transmembrane Ca2+ influx without affecting the release of Ca2+ from intracellular stores. Furthermore, this inhibition is not mediated by cell depolarization. Thus, Fc gamma RII represents a tool to dissociate physiologically the phases of release and transmembrane influx of Ca2+ triggered through antigen receptors.

Ca2+ and Mn2+ influx through receptor-mediated activation of nonspecific cation channels in mast cells

Whole-cell patch-clamp recordings of membrane currents and Fura-2 measurements of free intracellular calcium concentration ([Ca2+]i) were used to study calcium influx through receptor-activated cation channels in rat peritoneal mast cells. Cation channels were activated by the secretagogue compound 48/80, whereas a possible concomitant Ca2+ entry through pathways activated by depletion of calcium stores was blocked by dialyzing cells with heparin. Heparin effectively suppressed the transient Ca2+ release induced by 48/80 and abrogated inositol 1,4,5-trisphosphate-induced calcium influx without affecting activation of 50-pS cation channels. There was a clear correlation between changes in [Ca2+]i and the activity of 50-pS channels. The changes in [Ca2+]i increased with elevation of extracellular Ca2+. At the same time, inward currents through 50-pS channels were diminished as more Ca2+ permeated. This effect was due to a decrease in slope conductance and a reduction in the open probability of the cation channels. In physiological solutions, 3.6% of the total current was carried by Ca2+. The cation channels were not only permeable to Ca2+ but also to Mn2+, as evidenced by the quench of Fura-2 fluorescence. Mn2+ current through 50-pS channels could not be resolved at the single-channel level. Our results suggest that 50-pS cation channels partially contribute to sustained increases of [Ca2+]i in mast cells following receptor activation.

Cyclic AMP enhances agonist-induced Ca2+ entry into endothelial cells by activation of potassium channels and membrane hyperpolarization

The mechanism underlying cyclic AMP (cAMP)-mediated amplification of agonist-induced Ca2+ responses in endothelial cells was investigated in pig endothelial cells. Forskolin, adenosine and isoprenaline, as well as the membrane-permeant cAMP analogue dibutyryl cAMP, enhanced bradykinin-induced rises in intracellular free Ca2+ as well as bradykinin-induced Mn2+ entry. These agents were also found to hyperpolarize endothelial cells without increasing intracellular Ca2+ by itself, i.e. in the absence of bradykinin. Both amplification of bradykinin effects and the hyperpolarizing action was blocked by the protein kinase inhibitor H-8. The involvement of K+ channels in the hyperpolarizing effects of forskolin was consequently studied in perforated outside-out vesicles. Two different types of K+ channels were recorded, one of which had a large conductance (170 pS) and was activated by forskolin. We suggest that stimulation of endothelial adenylate cyclase results in activation of large-conductance K+ channels and consequently in membrane hyperpolarization, which in turn enhances bradykinin-induced entry of Ca2+ by increasing its electrochemical gradient.

Effects of analogues of adenine nucleotides on increases in intracellular calcium mediated by P2T-purinoceptors on human blood platelets

1. By use of a number of analogues of adenine nucleotides, the structure-activity relationships of the human platelet receptor for adenosine 5'-diphosphate (ADP) mediating increases in intracellular calcium were investigated, and compared with the known structure-activity relationships for induction by ADP of platelet aggregation. 2. ADP, 2-methylthioadenosine 5'-diphosphate (2-methylthio-ADP), adenosine 5'-O-(1-thiodiphosphate) (ADP-alpha-S) and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S) each induced increases in intracellular calcium in a manner similar to their reported ability to induce human platelet aggregation. The effects of these agonists were antagonized by ATP, with a pA2 value in each case consistent with the inhibition by ATP of ADP-induced aggregation. In the case of ADP, the inhibition by ATP of increases in intracellular calcium was shown to be competitive by Schild analysis. 3. Of the analogues tested as inhibitors of the effect of ADP on intracellular calcium, 2-chloroadenosine 5'-triphosphate (2-chloro-ATP), adenosine 5'-O-(1-thiotriphosphate) (ATP-alpha-S), P1, P5-diadenosine pentaphosphate (Ap5A) and adenylyl 5'-(beta,gamma-methylene)diphosphonate (AMPPCP) were apparently competitive antagonists, although only one concentration of each antagonist was used. There was a good correlation between the pA2 values found here for these antagonists including ATP, and their pA2 values reported for inhibition of ADP-induced aggregation. Adenosine 5'-(alpha, beta-methylene)triphosphate (AMPCPP) and uridine 5'-triphosphate (UTP) (100 microM) were only very weak inhibitors of the effect of ADP on intracellular calcium, and this is consistent with their weak actions as inhibitors of aggregation. 2-Methylthioadenosine 5'-triphosphate (2-methylthio-ATP) (50 microM) non-competitively inhibited the effect of ADP on intracellular calcium, in a very similar way to its inhibition of ADP-induced aggregation.4. The good correspondence found for these analogues between their effect on intracellular calcium and on aggregation confirms that there is a causal relationship between these actions of ADP, and that they are mediated by the same receptor on platelets. These findings cast further doubt on the use of the affinity reagent 5'-fluorosulphonylbenzoyladenosine (FSBA) as an antagonist and label for the ADP receptor, as this compound has been reported to inhibit aggregation but not ADP-induced increases in intracellular calcium.

Relationships between serotonin induced elevation of intracellular Ca2+ concentration and stimulation of Ca2+ influx in blood platelets

Serotonin (5-HT) caused immediate elevation of intracellular Ca2+ concentration ([Ca2+]i) in blood platelets, and it was completely inhibited by 1 mM EGTA. In Ca2+ replenished platelets, however, 2 mM EGTA did not affect the 5-HT induced elevation of [Ca2+]i when EGTA was applied just before or during the stimulation by 5-HT. At the same concentration 5-HT was also found to enhance Ca2+ influx through the activation of 5-HT2 receptor, but with rather longer latent time. From these results it is suggested that 5-HT induced elevation of [Ca2+]i is caused by mobilization of Ca2+ from intracellular Ca2+ storage sites, but not by direct stimulation of Ca2+ influx. Depletion of such Ca2+ stores might impair the effect of 5-HT on [Ca2+]i. Thus, 5-HT induced augmentation of Ca2+ influx might be secondary to replenishment of the depleted Ca2+ stores which was caused by 5-HT induced internal release of Ca2+. It is concluded that the effects of 5-HT on [Ca2+]i and Ca2+ influx in platelets are manifested sequentially or independently.

Receptor-activated Ca influx in human airway smooth muscle: use of Ca imaging and perforated patch-clamp techniques

Previous studies have demonstrated a dihydropyridine-insensitive, receptor-activated calcium influx pathway in cultured human airway smooth muscle (ASM) cells. To further define the biophysical characteristics of this pathway, the relationship between membrane potential and cytosolic free calcium ([Ca2+]i) was studied with the combined methods of the patch-clamp technique and single cell calcium imaging. The nystatin perforated-patch method was used to maintain normal intracellular calcium buffering and receptor-activated signal transduction processes in voltage-clamped cells. Single voltage-clamped human ASM cells responded to exposure to histamine (200 microM) with an initial transient rise in [Ca2+]i followed by a secondary sustained elevation that was dependent on extracellular calcium. Before agonist activation, step changes in holding potential produced only slight changes in [Ca2+]i, whereas, after activation, cells developed a sustained rise in [Ca2+]i that showed a large variation as a function of membrane potential. Depolarization from -80 to 0 mV caused a fall in the steady-state [Ca2+]i to basal levels or slightly below. Repolarization to -80 mV caused the redevelopment of the sustained phase of the calcium response. When calcium was removed from the extracellular fluid by the addition of a stoichiometric excess of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), the voltage dependence of the sustained phase was abolished. In a series of experiments, agonist addition evoked a 54-fold increase in the voltage dependence of calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonselective cation channels: physiological and pharmacological modulations of channel activity

Cation channels play a major role in fast and sustained cellular responses to hormones and neurotransmitters. They contribute to depolarization of the membrane and--in most cases--to an increase in the intracellular Ca2+ concentration. Nonselective cation channels presumably form a large family of diverse channels which are modulated by various extracellular and intracellular signals. Structure and regulation of ligand-operated and cyclic nucleotide-activated nonselective cation channels found in synapses and sensory receptor cells, respectively, are well documented; none of the structures of other cation channels are known. Except for ligand-operated and stretch-activated channels, G-proteins form the link between the involved receptors and signalling cascades stimulating nonselective cation channels. Observed in numerous cellular systems is hormonal activation of cation channels by hormones or neurotransmitters interacting with heptahelical receptors inducing a phosphoinositide breakdown (PI response); several pathways stimulated within the PI response may generate signals involved in cation channel activation. Pharmacological modifications of nonselective cation channels by inorganic and organic blockers are so far extremely limited; various blockers have been described but unfortunately lack high specificity for these channels.

Indications from Mn-quenching of Fura-2 fluorescence in melanotrophs that dopamine and baclofen close Ca channels that are spontaneously open but not those opened by high [K+]O; and that Cd preferentially blocks the latter

In unstimulated melanotrophs, which secrete spontaneously, Mn caused a progressive quenching of Fura-2 fluorescence (F360) which was: (a) unaffected by tetrodotoxin to suppress spontaneous Na-action potentials; (b) slowed by lowering temperature to 23 degrees C; and (c) arrested by the Ca channel blocker, Ni. Mn quenching slowed on lowering [K+]O from 5 to 2 mM to hyperpolarize (indicating Mn entry through voltage-dependent channels) and accelerated on raising [K+]O to 50 or 100 mM to strongly depolarize (indicating recruitment of high threshold channels). The secreto-inhibitor, dopamine, arrested spontaneous Mn quenching and so, too, did the GABAB agonist, baclofen; and these effects like those of the two agonists on secretion and [Ca2+]i were blocked by the specific D2 and GABAB antagonists, sulpiride and CGP 35348, respectively, and were lost following exposure to pertussis toxin. By contrast, neither dopamine nor baclofen prevented Mn quenching in response to high K, although this was arrested by Ni. A second Ca channel blocker, Cd, in concentrations that inhibited the response to high K, failed to inhibit spontaneous entry of Mn. This preferential effect offers an explanation for observations made with Cd that have been interpreted as contrary to the notion of Ca-regulated secretion in the melanotroph. The results we have obtained are interpreted to mean that in the melanotroph secreting spontaneously some voltage-dependent Ca channels are in the open state; that this open state is not dependent on any Na spiking activity; and that these channels are preferentially closed by dopamine and baclofen which are without effect on Ca channels opened by strongly depolarizing concentrations of K.

Intracellular Ca2+ rise in human platelets induced by polymorphonuclear-leucocyte-derived cathepsin G

Cathepsin G, a serine protease released by polymorphonuclear-leucocyte azurophilic granules upon stimulation, activates human platelets, inducing an increase in intra-platelet Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner (50-200 nM). The [Ca2+]i rises elicited by low (50-80 nM) cathepsin G concentrations in fura-2-loaded platelets showed a biphasic mode, with a first small peak followed by a greater and more prolonged Ca2+ transient. Higher (100-200 nM) cathepsin G concentrations induced a monophasic increase in intracellular Ca2+. Acetylsalicylic acid, nordihydroguaiaretic acid and ketanserin did not affect platelet activation by cathepsin G, whereas the ADP-scavenger system phosphocreatine/creatine kinase significantly decreased Ca2+ mobilization, platelet aggregation and 5-hydroxytryptamine secretion by cathepsin G. Preventing cathepsin G-induced platelet aggregation with the synthetic peptide RGDSP (Arg-Gly-Asp-Ser-Pro) did not significantly affect cathepsin G-induced Ca2+ transients. Ni2+ (4 mM), a bivalent-cation-channel inhibitor, decreased the cathepsin G-induced fluorescence rise by more than 90%. This effect was reversed by either decreasing Ni2+ or increasing cathepsin G concentration. Preventing Ca2+ influx across the plasma membrane with 4 mM-EGTA totally abolished Ca2+ transients. However, EGTA also strongly decreased catalytic activity of cathepsin G, which is essential for platelet activation. Evidence of a rapid and sustained bivalent-cation channel opening in the platelet membrane was obtained by adding Mn2+ to the platelet suspension 30 s or 3 min after cathepsin G. No accumulation of InsP3 could be detected when platelets were stimulated with cathepsin G. All these data indicate that cathepsin G induces a [Ca2+]i increase mainly through an influx across the plasma membrane. This massive Ca2+ entry is probably due to opening of receptor-operated channels and is amplified by endogenous ADP release.

Isolation and characterization by cell density adjustment of a PC12 pheochromocytoma variant with altered Ca2+ homeostasis

Rat PC12 pheochromocytoma cells loaded with the fluorescent Ca2+ dye fluo-3 or indo-1 and scanned fluorimetrically on a cell sorter apparatus showed a rapid cell density-dependent increase in free cytosolic calcium concentration [Ca2+]i when maintained in suspension cultures. Cell adhesion, measured under a defined set of conditions, was low when cells were seeded at 1.5 x 10(4) cells/ml but reached maximal levels after addition of A23187 calcium ionophore. A six to sevenfold increase in cell density mimicked the effect of the ionophore. Densities above 2 x 10(6) cells/ml caused a decrease in cell adhesion, which was further reduced by the addition of A23187. BAPTA, AM (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) and nifedipine (10 microM each), partially inhibited cell attachment (34% and 44% reduction), but at 0.25 microM and 1 microM, respectively, they enhanced attachment (46% and 67% increase). The data suggest that a certain permissive level of [Ca2+]i, attained by either increasing cell density or by the presence of a calcium ionophore, is sufficient for maximal cell adhesion. Above the permissive level, manipulation of [Ca2+]i either by altering cell density or by the addition of calcium blocking agents in high concentrations results in a significant reduction in cell adhesion. Based on these observations, we were able to isolate a biochemically and morphologically distinct cell population. The variant, designated PC12ds (density selected), differed substantially from the original cells. Most notable was a relatively lower content of free [Ca2+]i in the PC12ds cells, as independently assayed by using fluo-3 and indo-1 dyes. In addition, the variant cells exhibited a significantly diminished rate of 45Ca2+ uptake, most likely due to less efficient L-type voltage-dependent calcium (VDC) channels. Addition of several calcium channels agonists and antagonists suggested that PC12ds cells contained relatively more N-type VDC channels, possibly indicating a shift to a neuronal phenotype.

Ca2+ influx induced by the Ca(2+)-ATPase inhibitors 2,5-di-(t-butyl)-1,4-benzohydroquinone and thapsigargin in bovine adrenal chromaffin cells

We have characterized the effect of the Ca(2+)-ATPase inhibitors 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBHQ) and thapsigargin on the concentration of cytosolic Ca2+ in single bovine adrenal chromaffin cells by video-imaging of fura-2-loaded cells. Addition of either inhibitor released Ca2+ from internal stores in the absence of external Ca2+. tBHQ was unable to stimulate further Ca2+ release after addition of thapsigargin, but thapsigargin could do so after release by tBHQ, indicating that the tBHQ-sensitive stores are a sub-set of those sensitive to thapsigargin. Angiotensin II was able to elicit Ca2+ release after application of tBHQ, indicating that at least part of the tBHQ-sensitive stores were distinct from those discharged by Ins(1,4,5)P3. In the presence of external Ca2+, both Ca(2+)-ATPase inhibitors produced a more prolonged rise in cytosolic Ca2+ consistent with stimulated Ca2+ entry. The ability of the inhibitors to activate a Ca(2+)-entry pathway was confirmed by monitoring quenching of fura-2 after stimulated entry of the Ca2+ surrogate Mn2+. These findings indicate that bovine adrenal chromaffin cells possess a mechanism by which Ca2+ entry can be activated, following emptying of certain internal stores, independently of receptor occupation.

Dual effects of oleic acid on Ca2+ mobilization and protein phosphorylation in human platelets in presence or absence of platelet activating factor

This laboratory demonstrated earlier that oleic acid inhibited platelet activating factor (PAF)-induced aggregation and serotonin release of rabbit platelets (M. Miwa, C. Hill, R. Kumar, J. Sugatani, M. S. Olson, and D. J. Hanahan, 1987, J. Biol. Chem. 262, 527-530). More recently, we reported that oleic acid caused a decrease in phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2), but did not affect the level of inositol-1,4,5-trisphosphate (IP3), in rabbit platelets (D. Nunez, J. Randon, C. Gandhi, A. Siafaka-Kapadai, M. S. Olson, and D. J. Hanahan, 1990, J. Biol. Chem. 265, 18330-18838). These results suggested that oleic acid did not stimulate phospholipase C. In contrast, PAF induced a decrease in PIP2 and an increase in PIP level and IP3. These effects were shown to be attenuated by oleic acid. In this current study, our experiments show that (a) oleic acid blocked PAF-induced rise in intracellular [Ca2+] (to provide a mechanism in agreement with our previous experiments which showed that oleic acid inhibited PAF-induced IP3 rise in platelets) and (b) oleic acid itself induced a gradual rise in [Ca2+]i, which would provide a mechanism for oleic acid-induced aggregation despite the fact that oleic acid did not cause the production of IP3 (Nunez et al., 1990). Oleic acid, in a dose-dependent manner, was shown to inhibit PAF-induced Ca2+ mobilization from intra- and extracellular sources. The inhibition was closely related to the suppressive effect of oleic acid on PAF-induced aggregation. Furthermore, oleic acid inhibited the PAF-stimulated phosphorylation of the 20- and 40-kDa proteins. At concentrations above 20 microM, oleic acid itself could induce platelet aggregation and Ca2+ mobilization, but the time sequence of these two responses in human platelets was significantly different from those obtained with PAF. Oleic acid alone, at 20 microM, caused a 1.4-fold increase in the cAMP level in platelets which was followed by a decline to a basal value at higher concentrations of this fatty acid. It seemed clear that elevation of adenylate cyclase activity was not associated with free fatty acid inhibition of platelet activation. Interestingly, both PAF and oleic acid added separately to human platelets induced protein-tyrosine phosphorylation, but oleic acid did not cause any inhibition of PAF-induced protein-tyrosine phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Qualitative differences in [Ca2+]i increases and InsP3 generation following stimulation of N1E-115 cells with micromolar and millimolar ATP

Addition of ATP (100 microM) to populations of the neuronal cell line N1E-115 caused a transient increase in intracellular calcium ([Ca2+]i) which rapidly reached a peak (maximum of 243 +/- 7 nM above basal) and returned to basal after approximately 50 sec. The response was concentration-dependent (EC50 21 +/- 4 microM) and was unchanged when calcium was omitted from the extracellular medium. Transient increases in D-myo-inositol-1,4,5-trisphosphate levels (InsP3) were also observed over a similar time period. Addition of millimolar ATP, however, produced characteristically different responses; [Ca2+]i again increased rapidly (reaching a maximum of 639 +/- 23 nM above basal) but returned to a new maintained plateau (274 +/- 34 nM) which was abolished by the inorganic calcium channel blocker, nickel (1 mM), and omission of calcium from the extracellular medium. InsP3 levels were also maintained but were, however, unaffected by nickel or removal of extracellular calcium. The qualitative difference in the mechanism of calcium elevation produced with millimolar ATP, compared with lower concentrations, suggests that the N1E-115 cells might also contain a low affinity P2 receptor coupled with a calcium channel.

Intracellular cations modulate noradrenaline-stimulated calcium entry into smooth muscle cells of rat portal vein

1. The action of noradrenaline (NA, 10 microM) was studied in single patch-clamped smooth muscle cells of rat portal vein where free internal Ca2+ concentration in the cell (Ca2+i) was estimated using the emission from the dye Indo-1. 2. In the presence of 50 microM D600, a blocker of voltage-dependent Ca2+ channels, NA applied to the cell evoked an initial peak in Ca2+i followed by a smaller sustained rise. The initial rise in Ca2+i was associated with the activation of Ca(2+)-dependent Cl- channels. 3. The maintained rise in Ca2+i induced by NA was enhanced by increasing the external Ca2+ concentration and was abolished in Ca(2+)-free solution. The transient rise was resistant to the absence of external Ca2+. 4. Following the transient rise in Ca2+i induced by NA, the mechanisms extruding and/or sequestering cytoplasmic Ca2+ were stimulated. This stimulation was measured during the sustained rise in Cai and was maintained for a few seconds after NA was removed. 5. Unlike the transient rise in Ca2+i, the sustained rise in Ca2+i produced by NA was affected by changing the cell membrane potential. 6. Changing the Na+ gradient showed that the Na(+)-Ca2+ exchange was not involved in the sustained rise in Ca2+i. 7. The sustained increase in Ca2+i produced by NA was modulated by intracellular cations. This phase could be observed with 130 mM Na+ or 130 mM K+ in the pipette solution, but was severely reduced when the only cation in the intracellular solution was Cs+ and abolished with NMDG (N-methyl-D-glucamine) or TEA. However, inclusion of only 10 mM Na+ or 50 mM K+ in the pipette solution was sufficient to obtain a sustained rise in Ca2+i of maximal amplitude, similar to that obtained with 130 mM Na+i or 130 mM K+i during NA application. 8. In portal vein smooth muscle cells, NA induced a two-phase increase in Ca2+i similar to the two phases which have been previously observed upon muscarinic receptor activation by carbachol in intestinal smooth muscle. The transient rise was due to Ca2+ store release whereas the sustained rise was due to an increased Ca2+ entry into the cell down its electrochemical gradient, but not through voltage-dependent Ca2+ channels. The Ca2+ permeability pathway involved in the sustained rise in Ca2+i induced by NA was modulated by the intracellular cations.

Calcium influx evoked by Ca2+ store depletion in human platelets is more susceptible to cytochrome P-450 inhibitors than receptor-mediated calcium entry

We have previously reported that a component of ADP-evoked Ca2+ entry in human platelets appears to be promoted following the release of Ca2+ from intracellular stores. Other agonists may employ a similar mechanism. Here we have further investigated the relationship between the state of filling of the Ca2+ stores and plasma membrane Ca2+ permeability in Fura-2-loaded human platelets. Ca2+ influx was promoted following store depletion by inhibitors of the endoplasmic reticulum Ca(2+)-ATPase, thapsigargin (TG) and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ). Divalent cation entry was confirmed by quenching of Fura-2 fluorescence with externally added Mn2+. It has been suggested that cytochrome P-450 may couple Ca2+ store depletion to an increased plasma membrane Ca2+ permeability. In apparent agreement with this, Mn2+ influx promoted by TG and tBuBHQ, or by preincubation of cells in Ca(2+)-free medium, was inhibited by the imidazole antimycotics, econazole and miconazole, which inhibit cytochrome P-450 activity. Agonist-evoked Mn2+ influx was only partially inhibited by these compounds at the same concentration (3 microM). Econazole (3 microM) reduced the Mn2+ quench evoked by ADP by 38% of the control value and that evoked by vasopressin, platelet activating factor (PAF) and thrombin no more than 15% of control, 20 s after agonist addition. Stopped-flow fluorimetry indicated that econazole had no detectable effect on the early time course of agonist-evoked Mn2+ entry or rises in [Ca2+]i. These data confirm the existence of a Ca2+ entry pathway in human platelets which is activated by depletion of the intracellular Ca2+ stores. Further, the results support the suggestion that cytochrome P-450 may participate in such a pathway. However, any physiological role for the cytochrome or its products in agonist-evoked events appears to be in the long-term maintenance or restoration of store Ca2+ content, rather than in promoting Ca2+ influx in the initial stages of platelet Ca2+ signal generation.

Adenosine inhibits divalent cation influx across human neutrophil plasma membrane via surface adenosine A2 receptors

Adenosine and its analogues inhibited increases in divalent cation influx stimulated by platelet-activating factor (PAF) and formyl-methionyl-leucyl-phenylalanine (FMLP) in a dose-dependent fashion. This effect was antagonized by theophylline, an adenosine receptor antagonist. When extracellular adenosine was removed by adenosine deaminase, the effect of adenosine was completely abolished. Two adenosine analogues with different affinities for adenosine receptor subtypes, 5'-N-ethylcarboxamideadenosine (NECA) and L-N6-phenylisopropyladenosine (PIA), also inhibited divalent cation influx, NECA being more potent than PIA. These results suggest that adenosine and its analogues inhibit divalent cation influx across neutrophil plasma membranes via surface adenosine A2 receptors. Adenosine had little effect on the initial peaks of intracellular free calcium rises induced by chemoattractants, but it inhibited the subsequent rise in free calcium. Since calcium influx through the divalent cation channels or neutrophil plasma membranes is responsible for maintaining free calcium concentration following the initial peaks, we suggest that adenosine modulates neutrophil function by interfering with this calcium influx.

Elevation of cytosolic [Ca2+] due to intracellular Ca2+ release retards carbachol stimulation of divalent cation entry in rat parotid gland acinar cells

This study examines the activation of divalent cation entry into rat parotid gland acinar cells by using Mn2+ as a Ca2+ surrogate cation. Following muscarinic-cholinergic stimulation of dispersed parotid acini with carbachol (10 microM), the onset of internal Ca2+ release (cytosolic [Ca2+], [Ca2+]i, increase) and the stimulation of Mn2+ entry (increase in fura2 quenching) are not simultaneously detected. [Ca2+]i elevation, due to intracellular release, is detected almost immediately following carbachol addition and peak [Ca2+]i increase occurs at 6.0 +/- 0.8 sec. However, there is an interval (apparent lag) between carbachol addition and the detection of stimulated Mn2+ entry. This apparent lag is decreased from 26 +/- 3.1 sec to 9.2 +/- 1.5 sec when external Mn2+ ([Mn2+]0) is increased from 12.5 to 500 microM. It is not decreased further with increase in [Mn2+]0 from 500 microM to 1 mM (9.8 +/- 2.1 sec), although both intracellular free Mn2+ and [Mn2+-fura2]/[fura2] increase. Thus, at [Mn2+]0 < 500 microM, the observed lag time is partially due to a limitation in the magnitude of Mn2+ entry. Furthermore, neither peak [Ca2+]i nor the time required to reach peak [Ca2+]i is significantly altered by [Mn2+]0 (12.5 microM to 1 mM). At every [Mn2+]0 tested (i.e., 12.5 microM-1 mM), the apparent lag is significantly greater than the time required to reach peak [Ca2+]i. However, when carbachol stimulation of the [Ca2+]i increase is attenuated by loading the acini with the Ca2+ chelator, 2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA), there is no detectable lag in carbachol stimulation of Mn2+ entry (with 1 mM [Mn2+]0). Importantly, in BAPTA-loaded acini, carbachol stimulates Mn2+ entry via depletion of the internal Ca2+ pool and not via direct activation of other divalent cation entry mechanisms. Based on these results, we suggest that the apparent lag in the detection of carbachol stimulation of Mn2+ entry into parotid acinar cells is due to a retardation of Mn2+ entry by the initial increase in [Ca2+]i, due to internal release, which most likely occurs proximate to the site of divalent cation entry.

Cyclic nucleotides and intracellular-calcium homeostasis in human platelets

The relationship between agonist-sensitive calcium compartments and those discharged by the Ca(2+)-ATPase inhibitor thapsigargin were studied in human platelets. In this context, calcium mobilization from intracellular pools and manganese influx was investigated in relation to the effect of altered cyclic-nucleotide levels. For maximal calcium release from intracellular stores, thapsigargin, compared to a receptor agonist like thrombin, requires the platelet's self-amplification mechanism, known to generate thromboxane A2. With this lipid mediator formed, thapsigargin released calcium and stimulated manganese influx in a manner similar to thrombin. Blocking the thromboxane receptor by addition of sulotroban (BM13.177) or, alternatively, increasing platelet cAMP or cGMP using prostacyclin or sodium nitroprusside, dramatically reduced the ability of thapsigargin to release calcium from intracellular compartments. The same experimental conditions significantly reduced the rate of manganese influx initiated by thapsigargin compared to thrombin. The experiments indicate that thapsigargin-sensitive compartments play only a minor role in inducing manganese influx compared to the receptor-sensitive compartment. Cyclic nucleotides accelerate the redistribution of an agonist-elevated platelet calcium into the thapsigargin-sensitive compartment, from which calcium can be released by inhibition of the Ca(2+)-ATPase. In human platelets, thapsigargin-induced calcium increase and influx were responsible for only part the calcium release resulting from inhibition of the corresponding ATPase; another part results from the indirect effect of thapsigargin acting via thromboxane-A2-receptor activation. Cyclic nucleotides are therefore an interesting regulatory device which can modify the thapsigargin response by not allowing the self-amplification mechanism of platelets to operate.

Evidence for two pathways of receptor-mediated Ca2+ entry in hepatocytes

Receptor-mediated Ca2+ entry was studied in fura-2-loaded isolated hepatocytes. Emptying of internal Ca2+ stores by treatment with either the Ca(2+)-mobilizing hormone vasopressin or the inhibitors of the microsomal Ca2+ pump, 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) or thapsigargin, stimulated Ca2+ entry, as indicated by a rise in the cytosolic free Ca2+ concentration after Ca2+ was added to cells suspended in nominally Ca(2+)-free medium. The enhancement of Ca2+ entry was proportional to the degree of depletion of the intracellular Ca2+ pool and occurred also after removal of vasopressin from its receptor. In contrast, the stimulation of Mn2+ entry by vasopressin required the continuous presence of the agonist, since it was prevented by the addition of vasopressin receptor antagonist. This effect was observed under conditions where refilling of the agonist-sensitive pool was prevented by using nominally Ca(2+)-free medium. Unlike vasopressin, tBuBHQ or thapsigargin did not stimulate Mn2+ entry. These results suggest the existence of two pathways for receptor-mediated Ca2+ entry in hepatocytes, a 'capacitative' pathway that is sensitive to the Ca2+ content in the Ins(1,4,5)P3-sensitive Ca2+ pool and does not allow Mn2+ entry, and a second pathway that depends on receptor occupation, seems to require a second messenger for activation, and permits influx of Mn2+.

Trypanosoma brucei: the tumor promoter thapsigargin stimulates calcium release from an intracellular compartment in slender bloodstream forms

Maintenance of calcium homeostasis is a critical activity of eukaryotic cells. Homeostatic pathways stabilize intracellular free calcium concentrations ([Ca2+]i) at the resting level and provide the source of mobilized calcium for cellular activation. We have measured calcium release from intracellular pools within bloodstream forms of Trypanosoma brucei to better understand homeostatic pathways which operate in these organisms. Fura-2 and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein were used to quantitate [Ca2+]i and intracellular pH (pHi), respectively. We report that the tumor promoter, thapsigargin, elevated [Ca2+]i by 50-75 nM. Mn2+ quench experiments demonstrated that the source of calcium was intracellular. No change in pHi was associated with the release of calcium from this compartment. In contrast, nigericin released approximately three-fold more calcium than thapsigargin from a pH-sensitive, intracellular pool. The nigericin-sensitive pool was nonmitochondrial. The effects of thapsigargin and nigericin on [Ca2+]i were additive, regardless of the order in which the treatment was given. We conclude that at least two pools of exchangeable calcium occur in bloodstream forms of T. brucei. One pool is sensitive to thapsigargin and apparently resides within the endoplasmic reticulum, while the nigericin-sensitive pool is nonmitochondrial and is of unknown origin.

Thyrotropin-releasing hormone-mediated Mn2+ entry in perifused rat anterior pituitary cells

Receptor-mediated Ca2+ influx has been shown to exist in several cell types. Thyrotropin-releasing-hormone (TRH)-stimulated Ca2+ entry has also been postulated to exist in rat anterior pituitary cells, but direct evidence has been lacking. We have measured the fluorescence quenching of indo-1 caused by Mn2+ at a Ca(2+)-insensitive wavelength to investigate the actions of TRH on cation entry in dispersed perifused anterior pituitary cells. In indo-1-loaded cells perifused with Ca(2+)-free medium, Mn2+ caused fluorescence quenching in unstimulated cells; TRH caused further quenching. TRH-stimulated Mn2+ entry was transient, and levelled off within a few minutes in the presence of continuous TRH infusion. TRH-stimulated Mn2+ entry was dependent on the concentration of Mn2+ (50 microM-1 mM). TRH (1 microM) caused a larger effect than TRH (10 nM). La3+ and Ni2+ blocked the quenching stimulated by TRH. The rate of basal quenching was not blocked by dopamine, but TRH-stimulated Mn2+ entry was partially blocked by 1 microM-dopamine and almost completely abolished by 10 microM-dopamine. Thapsigargin (1-5 microM), a tumour promotor which depleted intracellular Ca2+ stores, had little effect on Mn2+. F- (20 mM), which activates G-proteins, also had little effect on Mn2+ entry. We conclude that TRH can transiently stimulate Ca2+ entry through a channel than can pass Mn2+ and be inhibited by dopamine. Depleting Ca2+ stores alone is not sufficient to stimulate Ca2+ entry, and so TRH must do so by other mechanisms.

Thromboxane receptor stimulation inhibits adenylate cyclase and reduces cyclic AMP-mediated inhibition of ADP-evoked responses in fura-2-loaded human platelets

Stimulation of human platelets with the thromboxane A2 analogue, U46619, after treatment with prostaglandin E1 or forskolin, reduced the inhibition of ADP-evoked Mn2+ influx and the release of Ca2+ from intracellular stores. U46619 decreased the elevated concentration of 3',5'-cyclic AMP in platelets that were pretreated with prostaglandin E1. These results suggest that occupation of prostaglandin H2/thromboxane A2 receptors, like those for other agonists, inhibits adenylate cyclase activity, which can contribute to the promotion of platelet activation.

Depletion of intracellular calcium stores activates a calcium current in mast cells

In many cell types, receptor-mediated Ca2+ release from internal stores is followed by Ca2+ influx across the plasma membrane. The sustained entry of Ca2+ is thought to result partly from the depletion of intracellular Ca2+ pools. Most investigations have characterized Ca2+ influx indirectly by measuring Ca(2+)-activated currents or using Fura-2 quenching by Mn2+, which in some cells enters the cells by the same influx pathway. But only a few studies have investigated this Ca2+ entry pathway more directly. We have combined patch-clamp and Fura-2 measurements to monitor membrane currents in mast cells under conditions where intracellular Ca2+ stores were emptied by either inositol 1,4,5-trisphosphate, ionomycin, or excess of the Ca2+ chelator EGTA. The depletion of Ca2+ pools by these independent mechanisms commonly induced activation of a sustained calcium inward current that was highly selective for Ca2+ ions over Ba2+, Sr2+ and Mn2+. This Ca2+ current, which we term ICRAC (calcium release-activated calcium), is not voltage-activated and shows a characteristic inward rectification. It may be the mechanism by which electrically nonexcitable cells maintain raised intracellular Ca2+ concentrations and replenish their empty Ca2+ stores after receptor stimulation.