Regulation of calcium influx by second messengers in rat mast cells

3,5,3'-Tri-iodo-L-thyronine acutely regulates a protein kinase C-sensitive, Ca2+-independent, branch of the hepatic alpha1-adrenoreceptor signalling pathway

This work aimed to investigate the acute effect of the thyroid hormone 3,5,3'-tri-iodo-L-thyronine (T3) in regulating the hepatic metabolism either directly or by controlling the responsiveness to Ca2+-mobilizing agonists. We did not detect any acute metabolic effect of T3 either in perfused liver or in isolated liver cells. However, T3 exerted a powerful inhibitory effect on the alpha1-adrenoreceptor-mediated responses. The promptness of this T3 effect rules out that it was the result of rate changes in gene(s) transcription. T3 inhibited the alpha1-adrenoreceptor-mediated sustained stimulation of respiration and release of Ca2+ and H+, but not the glycogenolytic or gluconeogenic responses, in perfused liver. In isolated liver cells, T3 enhanced the alpha1-agonist-induced increase in cytosolic free Ca2+ and impeded the intracellular alkalinization. Since T3 also prevented the alpha1-adrenoreceptor-mediated activation of protein kinase C, its effects on pH seem to be the result of a lack of activation of the Na+/H+ exchanger. The failure of T3 to prevent the alpha1-adrenergic stimulation of gluconeogenesis despite the inhibition of protein kinase C activation indicates that the elevation of cytosolic free Ca2+ is a sufficient signal to elicit that response. T3 also impaired some of the angiotensin-II-mediated responses, but did not alter the effects of PMA on hepatic metabolism, indicating, therefore, that some postreceptor event is the target for T3 actions. The differential effect of T3 in enhancing the alpha1-adrenoreceptor-mediated increase in cytosolic free Ca2+ and preventing the activation of protein kinase C, provides a unique tool for further investigating the role of each branch of the signalling pathway in controlling the hepatic functions. Moreover, the low effective concentrations of T3 (<= 10 nM) in perturbing the alpha1-adrenoreceptor-mediated response suggests its physiological significance.

Substance P-related inhibitors of mast cell exocytosis act on G-proteins or on the cell surface

[p-Glu5,D-Trp(7,9,10)]substance P-(5-11) inhibited mastoparan-stimulated GTPase activity in homogenized rat peritoneal mast cells and decreased histamine secretion induced by mastoparan from streptolysin O-permeabilized mast cells (IC50 of about 30 microM), but not from intact cells. In contrast, [D-Pro4,D-Trp(7,9,10)]substance P-(4-11) inhibited the secretion from intact cells (IC50 of about 10 microM) but had no effect on histamine secretion from permeabilized cells, suggesting that this peptide exerts its inhibitory effect on the plasma membrane, whereas [p-Glu5,D-Trp(7,9,10)]substance P-(5-11) interacts with G proteins. Pretreatment of mast cells with neuraminidase led to an inhibition of the secretory response to mastoparan and related triggers. This response was restored following cell permeabilization, demonstrating the role of the cell surface on the entry of mastoparan and related triggers and on their ability to reach G proteins sensitive to pertussis toxin and [p-Glu5,D-Trp(7,9,10)]substance P-(5-11).

Non-capacitative calcium entry in Chinese hamster ovary cells expressing the platelet-derived growth factor receptor

Platelet-derived growth factor (PDGF) is believed to produce intracellular calcium (Ca2+i) transients by inositol trisphosphate (InsP3)-mediated release of intracellular Ca2+ stores followed by "capacitative" Ca2+ entry due to emptying of these stores. We examined the roles for the phospholipase Cgamma-InsP3 pathway and the emptying of InsP3-dependent intracellular Ca2+ stores in PDGF-mediated Ca2+ entry. Intracellular Ca2+ release and Ca2+ entry were measured with fluorometric methods in Chinese hamster ovary cells expressing wild type or mutant PDGF receptors. Activation of the wild type PDGF receptor caused both intracellular "Ca2+ release, " measured in nominally 0 Ca2+ extracellular medium, and "Ca2+ entry, " measured upon addition of 2 mM Ca2+ medium. Both phases were absent in Chinese hamster ovary cells expressing a PDGF receptor mutant (Y977F,Y989F) that fails to bind phospholipase Cgamma. Blockade of the InsP3 receptor, by microinjection of single cells with low molecular weight heparin (5-50 mg/ml), blocked only Ca2+i release (following PDGF or flash photolysis of caged InsP3) and had no effect on PDGF-induced Ca2+ entry. In whole cell patch-clamp experiments, intracellular heparin also failed to block PDGF-evoked ion currents. Release of InsP3-dependent intracellular Ca2+ stores, by flash photolysis of caged InsP3, was apparently not sufficient to maximally activate Ca2+ entry. Intracellular InsP3 caused significantly less Ca2+ entry than PDGF alone. These data suggest that InsP3 alone is not sufficient to maximally activate Ca2+ entry by the capacitative pathway and that products of phosphatidylinositol 4,5-bisphosphate breakdown other than InsP3 probably play a role in PDGF-mediated Ca2+ entry.

Fc epsilon RI-mediated chloride uptake by rat mast cells: modulation by chloride transport inhibitors in relation to histamine secretion

1. We have examined the role of extracellular chloride in the mast cell secretion process. The immunologically-directed ligand, antibody to IgE (anti-IgE) required extracellular chloride ions for optimum secretion from rat peritoneal mast cells. In contrast, replacement of extracellular chloride did not alter the mast cell secretory response to compound 48/80, calcium ionophore A23187 or substance P. 2. Anti-IgE-stimulation of mast cells evoked a significant uptake of chloride ions compared to non-stimulated cells. The magnitude of chloride uptake correlated with the magnitude of stimulated histamine secretion. 3. Compound 48/80, substance P and A23187 did not alter the rate of chloride ion uptake, although these agents caused significant histamine secretion. 4. The Na+/K+/2Cl- cotransport inhibitor, furosemide, reduced the rate of anti-IgE-stimulated chloride uptake at a relatively high concentration (700 microM). However, the more potent Na+/K+/2Cl- cotransport inhibitors, bumetanide (100 microM) and piretanide (100 microM) had no effect on the stimulated chloride uptake. 5. Furosemide inhibited anti-IgE-induced histamine secretion, bumetanide potentiated the response and piretanide had no effect. This suggests that their respective action on histamine secretion are unrelated to inhibition of the Na+/K+/2Cl- carrier. 6. The chloride channel blocker, 5-nitro-2-((3-phenylpropyl)-amino)-benzoic acid (NPPB), reduced both anti-IgE-stimulated chloride uptake and the corresponding histamine secretion in a dose-dependent manner. The magnitude of the inhibitory action of the drug on these two cellular processes was comparable, implying that chloride channel activity is related to the mechanism of histamine secretion. 7. It is concluded that chloride uptake has a role in the control of Fc epsilon RI-mediated histamine secretion from rodent mast cells.

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.

The effects of tachykinins on inflammatory and immune cells

The aim of this article is to provide an up-dated overview of the available information on the role played by tachykinins in recruiting/regulating the function of immune/inflammatory cells, an issue which has received considerable input from the recent availability of potent and selective antagonists for tachykinin receptors. It appears that NK1 receptors play a role in mediating the extravascular migration of granulocytes into inflamed tissues in response to various inflammatory stimuli, although this effect may not be due to the expression of NK1 receptors by granulocytes themselves. Several data also imply a role for NK1 and NK2 receptors in regulating immune function. No data are available to suggest the expression of NK3 receptors by inflammatory/immune cells. Mast cell degranulation by substance P appears to be a non-receptor dependent response which may take place in vivo during intense stimulation. An emerging concept in the field relates to the ability of certain immune cell types to synthesize and possibly release tachykinins. Immune cells could represent an additional source of tachykinins in inflamed tissues, providing a non-neurogenic tachykininergic contribution to the local inflammatory process.

Bursting firing of action potentials in central snail neurons elicited by d-amphetamine: role of cytoplasmic second messengers

The role of the intracellular second messengers on the bursting firing of action potentials in central snail neurons elicited by d-amphetamine was studied in the identified RP4 neuron of the African snail Achatina fulica Ferussac. Oscillation of membrane potential and bursting firing of action potentials were elicited by d-amphetamine in a concentration dependent manner. The bursting firing of action potentials was decreased following extracellular application of (1) H8 (N-(2-methyl-amino) ethyl-3-isoquinoline sulphonamide dihydrochloride), a specific protein kinase A inhibitor and (2) anisomycin, a protein synthesis inhibitor. However, the bursting firing of action potentials were not affected after (1) extracellular application of H7 (1,(5-isoquinoline-sulphonyl)-2-methylpiperasine dihydrochloride), a specific protein kinase C (PKC) inhibitor, or (2) intracellular application of GDPbetaS, a G protein inhibitor. The oscillation of membrane potential of the bursting activity was blocked after intracellular injection of 3'-deoxyadenosine, an adenylyl-cyclase inhibitor. These results suggested that the bursting firing of action potentials elicited by d-amphetamine in snail neurons may be associated with the cyclic adenosine monophosphate (cAMP) second messenger system: on the other hand it may not be associated with the G protein and protein kinase C activity.

A possible role of inwardly rectifying K+ channels in chick myoblast differentiation

We examined the developmental change of inwardly rectifying K+ channels (IRK) and its possible role in myogenesis. Northern blot analysis revealed an increase in the level of IRK mRNA during myogenesis. Accordingly, IRK current was not detectable in replicating myoblasts but first appeared in aligned myoblasts that were competent for fusion and gradually increased thereafter. The time course change of IRK activity was closely related to the increase in resting membrane potential during myogenesis. Application of 0.5 mM Ba2+ to the bath depolarized the membrane and blocked IRK currents dramatically but not outwardly rectifying K+ currents. Myoblasts devoid of IRK had low resting K+ permeability, whereas myotubes that possess IRK had high resting K+ permeability. In some aligned myoblasts, anomalous hyperpolarization was elicited by increasing extracellular K+ concentration, which may be attributable to the increased conductance of IRK. Noteworthy was the fact that maximal fusion was obtained at this range of K+ concentration. These findings imply that IRK is responsible for the change in the K+ permeability during chick myogenesis, which may provide a larger driving force for Ca2+ influx that is a prerequisite for myoblast fusion.

Inositol 1,4,5-trisphosphate- and caffeine-sensitive Ca(2+)-storing organelle in bovine adrenal chromaffin cells

The uptake and release properties of Ca2+ by several subcellular fractions of the bovine adrenal medulla were investigated. Investigation by the 45Ca2+ tracer method showed that permeabilized cells and the fractions of mitochondria (MT) and microsomes (MC) caused ATP-dependent Ca2+ uptake in a Ca2+ concentration-dependent manner (pCa 8-4), whereas permeabilized cells and the fractions of secretory granules (SG) were able to accumulate a significant amount of Ca2+ even in the absence of ATP, which was completed by the addition of hexokinase and glucose. In these organelle fractions, Ca2+ uptake in the presence of ATP at pCa 7 and pCa 5.8 was well-correlated with the activity of the NADPH cytochrome c reductase (marker enzyme for the endoplasmic reticulum) and cytochrome c oxidase (marker enzyme for mitochondria), respectively. As detected by Fura-2 ratiometry, both inositol 1,4,5-trisphosphate (IP3) and caffeine caused concentration-dependent Ca2+ releases from permeabilized cells and MC, but not from MT and SG. In an ATP-depleted condition, homogenates still took up a significant amount of Ca2+ but was not able to respond to IP3 and caffeine. These results suggest that the endoplasmic reticulum is a major Ca(2+)-storing organelle, which releases Ca2+ in response to IP3 and caffeine in bovine adrenal chromaffin cells.

G-protein alpha subunit Gi(alpha)2 mediates erythropoietin signal transduction in human erythroid precursors

Erythropoietin induces a dose-dependent increase in cytosolic calcium in human erythroblasts that is mediated by a voltage-independent Ca2+ channel. Inhibition of this response to erythropoietin by pertussis toxin suggests involvement of guanine nucleotide-binding regulatory proteins (G-proteins). The role of G-proteins in regulation of the erythropoietin-modulated Ca2+ channel was delineated here by microinjection of G-protein modulators or subunits into human erythroid precursors. This is the first report on the use of microinjection to study erythropoietin signal transduction in normal precursor cells. Fura-2 loaded day-10 burst-forming units-erythroid-derived erythroblasts were used for microinjection and free intracellular calcium concentration ([Ca(i)]) was measured with digital video imaging. BCECF (1,2',7'-bis(2-carboxyethyl)-5-(and -6-)-carboxyfluorescein) was included in microinjectate, and an increase in BCECF fluorescence was evidence of successful microinjection. Cells were microinjected with nonhydrolyzable analogues of GTP, GTPgammaS or GDPbetaS, which maintain the alpha subunit in an activated or inactivated state, respectively. [Ca(i)] increased significantly in a dose-dependent manner after microinjection of GTPgammaS. However, injection of GDPbetaS blocked the erythropoietin-induced calcium increase, providing direct evidence that activation of a G-protein is required. To delineate which G-protein subunits are involved, alpha or betagamma transducin subunits were purified and microinjected as a sink for betagamma or alpha subunits in the erythroblast, respectively. Transducin betagamma, but not alpha, subunits eliminated the calcium response to erythropoietin, demonstrating the primary role of the alpha subunit. Microinjected antibodies to Gi(alpha)2, but not Gi(alpha)1 or Gi(alpha)3, blocked the erythropoietin-stimulated [Ca(i)] rise, identifying Gi(alpha)2 as the subunit involved. This was confirmed by the ability of microinjected recombinant myristoylated Gi(alpha)2, but not Gi(alpha)1 or Gi(alpha)3 subunits, to reconstitute the response of pertussis toxin-treated erythroblasts to erythropoietin. These data directly demonstrate a physiologic function of G-proteins in hematopoietic cells and show that Gi(alpha)2 is required in erythropoietin modulation of [Ca(i)] via influx through calcium channels.

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

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

Characterization of whole-cell currents in mucosal and connective tissue rat mast cells using amphotericin-B-perforated patches and temperature control

Rat mucosal type mast cells are thought to possess only a K+-selective inwardly rectifying (IRK) current in the resting state. We used rat-bone-marrow-derived mast cells (BMMCs) as a model of mucosal mast cells and recorded whole-cell membrane currents from cells perforated with amphotericin B. Under these conditions, both inwardly rectifying (IR) and outwardly rectifying (OR) currents were observed. The reversal potential and conductance of the IR current depended on the extracellular K+ concentration, indicating that the channel was K+ selective. The OR current was not affected by changes in extracellular K+ concentration, but lowering extracellular Cl- concentration reduced the conductance and shifted the reversal potential in a positive direction. The OR current was not affected by K+ channel blockers, but was reversibly blocked by the chloride channel blocker 4,4'-diisothiocyanato-2,2'-stilbenedisulphonate (DIDS), again indicating a Cl- conductance. The IRK current was also detected in the majority of cells using the conventional whole-cell recording configuration at room temperature. In contrast, the ORCl current was only observed in 7% of recordings made at room temperature with the conventional whole-cell voltage-clamp mode, but was detected in 66% of cells if the bath temperature was increased and the integrity of the cell's cytoplasm was preserved by using the perforated-patch technique. Under similar conditions, the ORCl current was also present in rat peritoneal mast cells, a connective tissue phenotype previously thought to have no whole-cell currents in the resting state. The role of this current and factors affecting its activation are discussed.

Nonhydrolyzable analogues of GTP activate a new Na+ current in a rat mast cell line

Whole-cell patch clamp experiments were performed to examine the effects of the nonhydrolyzable GTP analogue, guanosine 5'-3-O-(thio)triphosphate, on membrane currents in rat basophilic leukemia cells. Guanosine 5'-3-O-(thio)triphosphate activated an inward sodium current. This current had a new permeability sequence to monovalent cations and a different pharmacological profile to that of other characterized Na+ channels. Long hyperpolarizing steps revealed that the current declined during the pulse, and the decline was voltage-dependent. Activation of the current required Mg2+ and ATP. The nonhydrolyzable ATP analogues, adenosine 5'-O-(thio)triphosphate and adenosine 5'-(beta,gamma-imino)triphosphate, could not substitute for ATP. Soluble second messengers like cAMP, cGMP, inositol polyphosphates, and Ca2+ did not activate the Na+ current. These results suggest that nonhydrolyzable GTP analogues activate a Na+ current in rat basophilic leukemia cells that is new in terms of its selectivity, pharmacology, and activation mechanism. It may be the prototype for a new family of Na+ channels expressed in certain nonexcitable cells.

Ca2+ influx and activation of a cation current are coupled to intracellular Ca2+ release in peptidergic neurons of Aplysia californica

1. Stimulation of inputs to bag cell neurons in the abdominal ganglion of Aplysia californica causes an increase in their intracellular Ca2+ concentration ([Ca2+]i). We have used thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ pump, to analyse the effects of Ca2+ released from intracellular stores on the electrophysiological responses of bag cell neurons. 2. Using digital imaging of fura-2-loaded isolated bag cell neurons we found that thapsigargin rapidly evoked an increase in [Ca2+]i in somata, with smaller increases in neurites. Thapsigargin-induced elevation of [Ca2+]i peaked at about 1 microM within 5-10 min and then decayed to basal levels by 30 min. 3. Placement of an extracellular vibrating Ca(2+)-selective microelectrode to within 1 micron of somata revealed a relatively large steady-state Ca2+ efflux. Thapsigargin produced a rapid increase in Ca2+ influx. Changes in Ca2+ flux were not detected at neurites. 4. Thapsigargin produced a small depolarization in isolated bag cell neurons in artificial sea water (ASW). Sometimes enhanced depolarizations were observed when extracellular Na+ was replaced by TEA or Tris, but not N-methyl-D-glucamine (NMDG). The depolarization was not blocked by 100 microM tetrodotoxin (TTX), removal of extracellular Ca2+ (0.5 mM EGTA) or addition of 10 mM Co2+ to the bath solution. 5. In voltage-clamp experiments, thapsigargin induced an inward current (ITg) that was recorded in Ca(2+)-free media containing TEA or Tris substituted for Na+. The apparent reversal potential of ITg was -16.8 +/- 1.2 mV in TEA-ASW. Induction of ITg was inhibited in neurons that were microinjected with the Ca2+ chelator BAPTA-Dextran70 or treated with the membrane-permeant analogue BAPTA AM. Activation of ITg was not observed when Na+ was replaced with NMDG. Manipulation of [Na+]o and [K+]o produced shifts in the reversal potential of ITg consistent with the underlying channels being permeable to both Na+ and K+. 6. Thapsigargin did not alter the amplitude or kinetics of voltage-activated Ba2+ currents, but in some experiments it did increase the amplitude of a component of outward K+ current. 7. Thapsigargin neither induced bag cell neurons within the intact ganglion to depolarize and fire spontaneously, nor did it alter the frequency or duration of firing of an electrically stimulated bag cell after-discharge. 8. We conclude that thapsigargin-sensitive Ca2+ pools are present predominantly in the somata of bag cell neurons. Ca2+ that is released from thapsigargin-sensitive Ca2+ stores activates a non-selective cation current that may help sustain depolarization of the somata, but does not by itself trigger an after-discharge.

Lymphocyte apoptosis: mediation by increased type 3 inositol 1,4,5-trisphosphate receptor

B and T lymphocytes undergoing apoptosis in response to anti-immunoglobulin M antibodies and dexamethasone, respectively, were found to have increased amounts of messenger RNA for the inositol 1,4,5-trisphosphate receptor (IP3R) and increased amounts of IP3R protein. Immunohistochemical analysis revealed that the augmented receptor population was localized to the plasma membrane. Type 3 IP3R (IP3R3) was selectively increased during apoptosis, with no enhancement of type 1 IP3R (IP3R1). Expression of IP3R3 antisense constructs in S49 T cells blocked dexamethasone-induced apoptosis, whereas IP3R3 sense, IP3R1 sense, or IP3R1 antisense control constructs did not block cell death. Thus, the increases in IP3R3 may be causally related to apoptosis.

An electrophysiological study of calcium entry during normal human T-lymphocyte activation

Our aim was to observe whether normal human T-cells respond to mitogenic stimulation with large whole-cell inward currents (composed of identifiable single-channel contributions) when [Ca2+]i is not markedly lowered but instead kept normal or moderately low, as has been reported in human leukaemic Jurkat T-cell line and T-cell clones [Kuno et al. (1986) Nature 323, 269-73; Kuno and Gardner (1987) Nature 326, 301-304; Gardner (1990) Annu. Rev. Immunol. 8, 231-252]. Whole-cell patch recordings showed no such currents in cells otherwise normally responding to depolarisation with the macroscopic IK described in T-lymphocytes and thus deemed viable, in agreement with the notion that Ca2+ influx in normal T-cells enterily depends on depletion of internal stores [Putney (1986) Cell Calcium 7, 1-12; Putney (1990) Cell Calcium 11, 611-624].

Inositol 1,4,5-trisphosphate-gated calcium transport through plasma membranes in nerve terminals

We developed new biochemical approaches to demonstrate the presence of inositol 1,4,5-triphosphate (InsP3)-gated calcium channels in presynaptic plasma membranes (SPM) and their involvement in the presynaptic receptor-mediated Ca2+ influx into nerve terminals. In perfusion experiments using SPM vesicles preloaded with 45Ca2+, InsP3 elicited the release of 45CA2+ into perfusates in a saturable manner. The InsP3- evoked 45Ca2+ release from resealed SPM vesicles was more potent than that from resealed vesicles using any other subcellular fractions. Here we also report the involvement of InsP3-gated mechanisms in the presynaptic receptor-mediated Ca2+ influx into synaptosomes (nerve terminals) by use of such resealed vesicles reconstituted with purified Gi1.

Presence of two Ca2+ influx components in internal Ca2+-pool-depleted rat parotid acinar cells

The molecular mechanism(s) involved in mediating Ca2+ entry into rat parotid acinar and other non-excitable cells is not known. In this study we have examined the kinetics of Ca2+ entry in fura-2-loaded parotid acinar cells, which were treated with thapsigargin to deplete internal Ca2+ pools (Ca2+-pool-depleted cells). The rate of Ca2+ entry was determined by measuring the initial increase in free cytosolic [Ca2+] ([Ca2+]i) in Ca2+-pool-depleted, and control (untreated), cells upon addition of various [Ca2+] to the medium. In untreated cells, a low-affinity component was detected with KCa = 3. 4 +/- 0.7 mM (where KCa denotes affinity for Ca2+) and Vmax = 9.8 +/- 0.4 nM [Ca2+]i /s. In thapsigargin-treated cells, two Ca2+ influx components were detected with KCa values of 152 +/- 79 microM (Vmax = 5.1 +/- 1.9 nM [Ca2+]i/s) and 2.4 +/- 0.9 mM (Vmax = 37.6 +/- 13.6 nM [Ca2+]i/s), respectively. We have also examined the effect of Ca2+ and depolarization on these two putative Ca2+ influx components. When cells were treated with thapsigargin in a Ca2+-free medium, Ca2+ influx was higher than into cells treated in a Ca2+-containing medium and, while there was a 46% increase in the Vmax of the low-affinity component (no change in KCa), the high-affinity component was not clearly detected. In depolarized Ca2+-pool-depleted cells (with 50 mM KCl in the medium) the high-affinity component was considerably decreased while there was an apparent increase in the KCa of the low-affinity component, without any change in the Vmax. These results demonstrate that Ca2+ influx into parotid acinar cells (1) is increased (four- to five-fold) upon internal Ca2+ pool depletion, and (2) is mediated via at least two components, with low and high affinities for Ca2+.

Intracellular ADP modulates the Ca2+ release-activated Ca2+ current in a temperature- and Ca2+-dependent Way

The rat basophilic cell line RBL-1 is known to express high levels of the Ca2+ current activated by store depletion, known as Ca2+ release-activated Ca2+ current (ICRAC), the main Ca2+ influx pathway so far identified in nonexcitable cells. We show here that, as reported in other cell types, metabolic drugs strongly inhibit the Ca2+ influx operated by store depletion in RBL-1 cells also. We have tested the hypothesis that intracellular adenine and/or guanine nucleotide levels act as coupling factors between ICRAC and cell metabolism. Using the whole cell configuration of the patch-clamp technique, we demonstrate that addition of ADP to the intracellular solution significantly reduces ICRAC induced by inositol 1,4,5-trisphosphate. This phenomenon differs from other regulatory pathways of ICRAC, since it is highly temperature-dependent, is observable only in the presence of low intracellular Ca2+ buffering capacity, and requires a cytosolic factor(s) which is rapidly lost during cell dialysis. Moreover, the inhibition is specific for ADP and is partially mimicked by ADPbetaS and AMP, but not by GDP or GTP.

Fluorescent analysis in polarized MDCK cell monolayers: intracellular pH and calcium interactions after apical and basolateral stimulation with arginine vasopressin

Intracellular calcium ([Ca2+]i) and hydrogen ion concentrations (pHi) are important regulators of cell function. Those ions also may interact and it is important, therefore, to measure their concentrations simultaneously. In the present studies we used a system developed for that purpose, a fluorescent emission ratio technique for simultaneous analysis of calcium (Indo-1) and pH (SNARF-1) in single cells at video rates, and determined if arginine vasopressin (AVP, 12.5 mumol/l) evoked [Ca2+]i and pHi signals interact in MDCK cells. We also employed a simple system for analysing the side specific (basolateral or apical) application of agonist to polarized cell layers on permeable membranes. AVP is found to evoke simultaneous changes in both pHi and [Ca2+]i. Basolateral application induced transient acidification, followed by partial recovery, and a [Ca2+]i transient with kinetic pattern similar to that of the pHi. Apical application also caused a mirror image pHi and [Ca2+]i pattern but of smaller magnitude (no peak). Selective removal of extracellular calcium ([Ca2+]e) or sodium ([Na+]e) dissociated the pHi and [Ca2+]i responses in both cases. Na+e removal abolished the pHi changes, but not the [Ca2+]i transients. [Ca2+]e removal abolished the [Ca2+]i changes and reduced, but did not abolish, the pHi responses. Thus, AVP induces pHi changes which are modified by calcium while calcium signalling is not modified by changes in pHi.

Mitoxantrone induces nonimmunological histamine release from rat mast cells

The antineoplastic drug mitoxantrone (MTX) elicits a fast noncytotoxic and nonimmunological histamine release from peritoneal and pleural rat mast cells. The non specific phosphodiesterase inhibitor isobuthyl-methylxantine (1 mM) decreases the potency of MTX. Theophylline (10 mM) decreases both the potency and the efficacy of MTX-induced histamine secretion. The protein kinase C (PKC) activator, tetradecanoyl-phorbol-13-acetate (50 ng/mL), enhances the effect of MTX, whereas the non specific PKC inhibitor trifluoperazine (10 microM) exerts no effect. Histamine release was also unaffected by substances acting on G-proteins, namely pertussis toxin (200 ng/mL), cholera toxin (300 mg/mL) and benzalkonium chloride (10 micrograms/ mL). The inhibition of protein phosphatases 1 and 2A by okadaic acid (1 microM) does not modify the response. The results indicate that mitoxantrone elicits the exocytosis in mast cells by a mechanism similar to the parent compound adriamycine, but different to the polyamine compound 48/80.

Role of mitogen-induced calcium influx in the control of the cell cycle in Balb-c 3T3 fibroblasts

The role of mitogen-activated calcium influx from the extracellular medium in the control of cell proliferation was studied in Balb-c 3T3 fibroblasts. Stimulation of serum-deprived, quiescent cells with 10% foetal calf serum (FCS) induced a long-lasting (up to 70 min) elevation of intracellular free calcium concentration ([Ca2+]i). Both the sustained [Ca2+]i increase and the related inward current, described in a previous paper [Lovisolo D. Munaron L. Baccino FM. Bonelli G. (1992) Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts. Biochim. Biophys. Acta, 1104, 73-82], could be abolished either by chelation of extracellular calcium with EGTA or by SK&F 96365, an imidazole derivative that can block receptor-activated calcium channels. The effect of the abolition of these ionic signals on FCS-induced proliferation was investigated by adding either EGTA or SK&F 96365 to the culture medium during the first hours of stimulation of quiescent cells with 10% FCS. As measured after 24 h, a 22% inhibition of growth was observed when SK&F 96365 was added for the first hour, and stronger inhibitions, up to 56%, were obtained by adding the blocker for the first 2 or 4 h. Similar effects were observed with addition of 3 mM EGTA, though the inhibition was less marked for the 4 h treatment. By contrast, incubation with either substance in the next 4 h of serum stimulation did not influence cell growth, except for a slight inhibition observed when SK&F 96365 was applied from the 4th to the 8th hour. The reduction in growth resulting from the abolition of the early calcium influx was paralleled by an accumulation of cells in the G2/M phase. Both growth inhibition and G2/M accumulation were reversible, since after further 24 h in 10% FCS cells had fully recovered the exponential growth. These data indicate that the early calcium influx seen in response to mitogen stimulation develops on a timescale long enough to play a significant role in cell cycle progression, and that its block in the early G1 phase can lead to a reduction of proliferation by arresting cells in later stages of the cycle.

Suppression of a nonselective cation conductance by substance P in cochlear outer hair cells

Substance P (SP) hyperpolarizes outer hair cells (OHCs) of guinea pig cochlea. The cellular mechanisms of the SP response were investigated with the whole cell patch-clamp technique. SP induced outward currents in a dose-dependent manner at a holding potential of -60 mV in a concentration range between 3 x 10(-6) and 10(-4) M. SP decreased slope conductance between -60 and +20 mV. Ion substitutions in the external medium revealed that SP suppresses nonselective cation conductance with high permeability for Ca2+. The relative ion permeability of the channel modulated by SP was as follows: Ca2+ > Li+ approximately Cs+ approximately Na+ > Tris+. The potency of the agonist action was as follows: SP >> neurokinin A > neurokinin B. Peptide antagonists induced currents similar to those of SP. CP-96345, a selective nonpeptide antagonist for the neurokinin type 1 receptor, did not inhibit the SP-induced current. Intracellular dialysis of guanosine 5'-O-(2-thiodiphosphate) and pertussis toxin (PTX) suggests that a PTX-insensitive G protein is involved in the SP response. Neither the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid nor staurosporine (10(-6) M) affects the SP response. Local application of SP by a puffer pipette indicates that the SP receptors are distributed along the side of the OHC. These results suggest the possibility that the action of SP on the OHCs may not be mediated by the tachykinin receptors but rather by a tachykinin receptor-independent pathway. It is proposed that SP suppresses the nonselective cation conductance in the lateral wall of OHCs via a PTX-insensitive G protein.

Role of tyrosine phosphorylation in potassium channel activation. Functional association with prolactin receptor and JAK2 tyrosine kinase

Chinese hamster ovary (CHO) cells, stably transfected with the long form of the prolactin (PRL) receptor (PRL-R) cDNA, were used for PRL-R signal transduction studies. Patch-clamp technique in whole cell and cell-free configurations were employed. Exposure of transfected CHO cells to 5 nM PRL led to the increase of Ca(2+)- and voltage-dependent K+ channel (KCa) activity. The effect was direct as it was observed also in excised patch experiments. A series of tyrosine kinase inhibitors was studied to investigate the possible involvement of protein tyrosine kinases in KCa functioning and its stimulation by PRL. Genistein, lavendustin A, and herbimycin A decreased in a concentration and time-dependent manner the amplitude of the KCa current in whole cell and the open probability of KCa channels in cell-free experiments. The subsequent application of PRL was ineffective. The protein tyrosine phosphatase inhibitor orthovanadate (1 mM) stimulated KCa channel activity in excised patches, indicating that channels can be modulated in opposite directions by protein tyrosine kinase and protein tyrosine phosphatase. Moreover, in whole cell experiments as well as in excised patch recordings, anti-JAK2 tyrosine kinase antibody decreased the KCa conductance and the open probability of the KCa channels. Subsequent application of PRL was no longer able to stimulate KCa conductance. Immunoblotting studies using the same anti-JAK2 antibody, revealed the constitutive association of JAK2 kinase with PRL-R. Preincubation of anti-JAK2 antibody with the JAK2 Immunizing Peptide abolished the effects observed using anti-JAK2 antibody alone in both electrophysiological and immunoblotting studies. We conclude from these findings that these KCa channels are regulated through tyrosine phosphorylation/dephosphorylation; JAK2 tyrosine kinase, constitutively associated with PRL-R, is implicated in PRL stimulation of KCa channels.

Capacitative Ca2+ influx and a Ca2+-dependent nonselective cation pathway are discriminated by genistein in mouse pancreatic acinar cells

We have investigated the effect of genistein on the hormone-stimulated Ca2+ influx and on a 28pS nonselective cation channel in mouse pancreatic acinar cells using the Ca2+ indicator fluo3 and the patch-clamp technique. The identity of the Ca2+ influx pathway has not been established in this cell type so far. Therefore we have investigated the Ca2+-dependent nonselective cation channel as a potential pathway for Ca2+ influx. Capacitative Ca2+ entry was induced by depletion of intracellular Ca2+ stores with 500nM acetylcholine or with the Ca2+ ATPase inhibitor 2,5di-tert- butylhydroquinone. In the presence of 100microM genistein, Ca2+ release was unimpaired, whereas Ca2+ influx was reversibly suppressed. Patch-clamp experiments demonstrated that genistein had no effect on Ca2+-activated nonselective cation channels, the activity of which was measured in excised membrane patches (inside/out) or in the whole-cell configuration. Therefore we conclude that this 28pS nonselective cation channel does not contribute to Ca2+ influx into mouse exocrine pancreatic cells. With the exception of genistein and tyrphostin 25, other tyrosine kinase inhibitors such as methyl-2,5-dihydroxycinnamate, lavendustin A, herbimycin A, and tyrphostin B56 were without effect on Ca2+ signalling. Thus, the involvement of tyrosine phosphorylation in the activation of the Ca2+ entry mechanism in mouse pancreatic acinar cells is unclear.

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

Convergent and parallel activation of low-conductance potassium channels by calcium and cAMP-dependent protein kinase

K+ channels, which have been linked to regulation of electrogenic solute transport as well as Ca2+ influx, represent a locus in hepatocytes for the concerted actions of hormones that employ Ca2+ and cAMP as intracellular messengers. Despite considerable study, the single-channel basis for synergistic effects of Ca2+ and cAMP on hepatocellular K+ conductance is not well understood. To address this question, patch-clamp recording techniques were applied to a model liver cell line, HTC hepatoma cells. Increasing the cytosolic Ca2+ concentration ([Ca2+]i) in HTC cells, either by activation of purinergic receptors with ATP or by inhibition of intracellular Ca2+ sequestration with thapsigargin, activated low-conductance (9-pS) K+ channels. Studies with excised membrane patches suggested that these channels were directly activated by Ca2+. Exposure of HTC cells to a permeant cAMP analog, 8-(4-chlorophenylthio)-cAMP, also activated 9-pS K+ channels but did not change [Ca2+]i. In excised membrane patches, cAMP-dependent protein kinase (the downstream effector of cAMP) activated K+ channels with conductance and selectivity identical to those of channels activated by Ca2+. In addition, cAMP-dependent protein kinase activated a distinct K+ channel type (5 pS). These data represent the differential regulation of low-conductance K+ channels by signaling pathways mediated by Ca2+ and cAMP. Moreover, since low-conductance Ca(2+)-activated K+ channels have been identified in a variety of cell types, these findings suggest that differential regulation of K+ channels by hormones with distinct signaling pathways may provide a mechanism for hormonal control of solute transport and Ca(2+)-dependent cellular functions in the liver as well as other nonexcitable tissues.

Hormone-regulated Ca2+ channel in rat hepatocytes revealed by whole cell patch clamp

An inward current responsible for hormone regulated Ca2+ entry has been identified in cultured rat hepatocytes using whole cell patch clamp. Addition of 20 nM vasopressin or of 100 microM ATP induced the inward current, which could be observed more clearly after blocking an outward K+ current. This large outward K+ current, which appeared after addition of vasopressin or ATP, could be blocked either by replacing K+ with Cs+ in the external medium and in the pipette solution, or by simply including 0.5 microM apamin in the K(+)-containing external medium. The outward current appears to be carried by a Ca2+ activated K+ channel. In the presence of apamin, hepatocytes pretreated with vasopressin in a Ca(2+)-free media reveal an inward current on addition of external Ca2+ (5 mM). The current could also be elicited by addition of vasopressin when cells are preincubated in the presence of 5 mM external Ca2+. No current is seen on addition of Ca2+ in the absence of vasopressin. Initially, the inward current was ca 200-300 pA at -60 mV, but it declined rapidly over 3 min to ca 20 pA. The current approached zero, as an asymptote at positive potential, and appeared to be somewhat inwardly rectifying. Additions of 5 mM Mn2+ or 5 mM Ba2+ in place of Ca2+ produced little or no current. An inhibitor of ER Ca(2+)-ATPase, thapsigargin, could also trigger the cascade of events leading to plasma membrane conductance of Ca2+. The data suggest that hormone-stimulated Ca2+ entry into hepatocytes is mediated by a Ca(2+)-release activated channel highly specific for Ca2+. This is the first demonstration of such a channel in hepatocytes, though similar ones have been described in mast cells, in vascular endothelial cells and T-lymphocytes.

Inositol phosphates modulate human red blood cell Ca(2+)-adenosine triphosphatase activity in vitro by a guanine nucleotide regulatory protein

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] inhibits human red blood cell (RBC) Ca(2+)-stimulable, Mg(2+)-dependent adenosine triphosphatase (Ca(2+)-ATPase) activity in vitro. Because we have previously shown that adrenergic receptors exist on the human mature RBC membrane and can modulate Ca(2+)-ATPase activity, we examined the possibility that a guanine nucleotide regulatory protein (G protein) mediated the Ins(1,4,5)P3 effect. Guanosine 5'-O-(3-thiotrisphosphate) (GTP gamma S) 10(-4) mol/L also inhibited RBC Ca(2+)-ATPase activity. Pertussis toxin 200 ng/mL blocked the effects of both Ins(1,4,5)P3 and GTP gamma S on Ca(2+)-ATPase activity. In separate studies, pertussis toxin-catalyzed adenosine diphosphate (ADP) ribosylation was shown to occur in RBC membranes under conditions in which measurements of Ca(2+)-ATPase activity were performed. When Ins(1,4,5)P3 10(-7) mol/L and GTP gamma S 10(-6) mol/L were added to membranes concurrently, their inhibitory actions on the enzyme were additive. At greater concentrations of Ins(1,4,5)P3 (10(-6) to 10(-5) mol/L) and GTP gamma S (10(-4) mol/L), the inositol phosphate reversed the inhibitory effect of GTP gamma S. These observations indicate that the novel effect of Ins(1,4,5)P3 on the activity of a plasma membrane Ca(2+)-ATPase depends at least in part on the action of a pertussis toxin-susceptible G protein.

Voltage-dependent ion channels on human basophils: do they exist?

The presence of voltage-dependent ion channels (particularly Ca2+ channels) on the surface of 'non excitable' cells such as human basophils is a matter of debate. Indeed, in basophils, Ca2+ entry or mobilization is not sufficient by itself to trigger secretion, although enhanced cytosolic Ca2+ concentration increases it. In order to address this question, we used a two-signal model and we report here experiments which suggest the presence of voltage-dependent structures directly or indirectly linked to membrane Ca2+ pathways. Indeed, it is known that, in the presence of PMA at threshold concentration (1st signal), elevation of cytosolic Ca2+ (2nd signal) induces histamine release. We observed that a depolarizing external solution (high K+) induced a Ca(2+)-dependent release of histamine from PMA-treated human basophils. High K+ alone did not induce histamine release. Although the voltage-sensitive component and the physiological relevance of this mechanism remain to be defined, these results suggest that this voltage-dependent Ca2+ influx in the human basophil could contribute to the up-regulation of histamine release.

IP3 receptor purified from liver plasma membrane is an (1,4,5)IP3 activated and (1,3,4,5)IP4 inhibited calcium permeable ion channel

The IP3 receptor is involved in Ca2+ mobilization from intracellular stores. Recently, we purified an inositol (1,4,5)-trisphosphate receptor from rat liver plasma membrane (LPM-IP3R) [Schäfer R. Hell K. Fleischer S. (1993) Purification of an IP3 receptor from liver plasma membrane. Biophys. J. 66, A146]. The purified LPM-IP3 receptor was incorporated into vesicle derived planar bilayers and its channel properties characterized. The receptor displayed ion channel activity that was activated by inositol (1,4,5)-trisphosphate [(1,4,5)IP3] (1 microM) and inhibited by inositol (1,3,4,5)-tetrakisphosphate (IC50 approximately 1 microM) and by heparin (IC50 approximately 20 micrograms/ml). The channel displays a unitary conductance of 9 pS, and 13 pS in symmetrical 100 mM and 500 mM KCl, respectively, and in symmetrical 250 mM cesium methanesulfonate the slope conductance is 11 pS. Activation by (1,4,5)IP3 is specific to the cis-side of the chamber, equivalent to the cytoplasmic face. The receptor is a Ca2+ permeable ion channel based on ion selectivity (Ca2+ > K+ > Na+ >> Cl). The LPM-IP3 receptor was also permeable to Cs (Cs+ > or = K+), similar to other intracellular Ca2+ release channels, i.e. the IP3 receptor from brain and smooth muscle (IP3R-1) and the ryanodine receptor from skeletal muscle (RyR-1) and heart (RyR-2). Channel activity is not voltage dependent (+/- 100 mV applied voltage). The channel is activated by ATP and Ca2+. The open probability of the (1,4,5)IP3 activated channel activity displays a bell shaped response to cis Ca2+ ion concentration of our system. The LPM-IP3 receptor differs from intracellular IP3R-1 in that the Ca2+ and ATP concentration required for maximum activation is about 10 times higher as compared with IP3R-1 from brain cerebellum and smooth muscle. We conclude that the LPM-IP3 receptor is an (1,4,5)IP3 activated Ca2+ permeable ion channel. The implication of our studies is that in liver, (1,4,5)IP3 regulates Ca2+ influx via the plasma membrane.

Chloride is required for receptor-mediated divalent cation entry in mesangial cells

Agonists which stimulate the inositol 1,4,5 trisphosphate ([1,4,5]-IP3)-dependent mobilization of Ca2+ from intracellular stores also stimulate entry of divalent cations across the cell membrane. Under appropriate experimental conditions, divalent cation entry across the cell membrane can be monitored as the rate at which the intracellular fluorescence of divalent cation indicators is quenched by the addition of Mn2+ to the extracellular medium. We report that addition of vasopressin to fura-2-loaded glomerular mesangial cells in culture markedly accelerated the rate at which Mn2+ quenched fura-2 fluorescence at its Ca(2+)-insensitive wavelength in the presence of extracellular NaCl, but that this quench response was attenuated when Cl- was removed from the extracellular medium by equimolar substitution with impermeant anions (gluconate, methanesulfonate, acetate, lactate). Similarly, loss of agonist-induced quench also occurred when Cl- was substituted with gluconate in K(+)-containing media. Addition of the Cl- channel inhibitor, 5-nitro-2-(3-phenylpropylaminobenzoic acid) (NPPB), also inhibited Mn(2+)-induced quench of fura-2 fluorescence following vasopressin addition. In contrast, in the presence of gramicidin to provide an alternate conductance pathway to accompany divalent cation entry, agonist-dependent Mn2+ quench occurred even in the absence of extracellular Cl-, indicating that the requirement for Cl- was not the result of cotransport on a common transporter nor the result of Cl- serving as a necessary cofactor for divalent cation entry. A similar dependence on extracellular Cl- was observed for other Ca(2+)-mobilizing agonists such as endothelin, as well as the intracellular Ca2+ ATPase inhibitor, thapsigargin. Extracellular Cl- dependence for agonist-induced divalent cation entry was also reflected in a corresponding extracellular Cl- dependence for agonist-induced mesangial cell contraction. It has been previously shown by ourselves (Kremer et al., 1992a, Am. J. Physiol., 262:F668-F678) and others that agonist-stimulated calcium mobilization in mesangial cells is accompanied by inhibition of K+ conductance and increased Cl- conductance. Accordingly, we conclude that the current findings suggest that activation of Cl- conductance provides regulated charge compensation for receptor-mediated divalent cation entry in response to Ca(2+)-mobilizing vasoconstrictor agonists in mesangial cells.

Role for tyrosine kinases in carbachol-regulated Ca entry into colonic epithelial cells

We studied a possible role of tyrosine kinases in the regulation of Ca entry into colonic epithelial cells HT-29/B6 using digital image processing of fura 2 fluorescence. Both carbachol and thapsigargin increased Ca entry to a similar extent and Ca influx was reduced by the tyrosine kinase inhibitor genistein (50 microM). Further experiments were performed in solutions containing 95 mM K to depolarize the membrane potential, and the effects of different inhibitors on influx of Ca, Mn, and Ba were compared. Genistein, but not the inactive analogue daidzein nor the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2- methylpiperazine, decreased entry of all three divalent cations by 47-59%. In high-K solutions, carbachol or thapsigargin both caused intracellular Ca to increase to a plateau of 223 +/- 19 nM. This plateau was reduced by the tyrosine kinase inhibitors genistein (to 95 +/- 8 nM), lavendustin A (to 155 +/- 17 nM), and methyl-2,5-dihydroxycinnamate (to 39 +/- 3 nM). Orthovanadate, a protein tyrosine phosphatase inhibitor, prevented the inhibitory effect of genistein. Ca pumping was unaffected by genistein. Carbachol increased tyrosine phosphorylation (immunoblots with anti-phosphotyrosine antibodies) of 110-, 75-, and 70-kDa proteins, and this phosphorylation was inhibited by genistein. We conclude that carbachol and thapsigargin increase Ca entry, and tyrosine phosphorylation of some key proteins may be important for regulating this pathway.

Substance P induces degranulation of mast cells and leukocyte adhesion to venular endothelium

Substance P (SP), one of the established neurotransmitters, evokes an immunoinflammatory response involving leukocyte adhesion to venular endothelium and the degranulation of mast cells. The pathogenetic relationship between these responses, however, remains unresolved. In this study, we propose to examine the changes associated with the activation of mast cells, as well as leukocyte adhesion to venular endothelium by in vivo observation of the rat mesentery. The use of an in vitro assay for intracellular Ca2+ mobilization and the degranulation of mast cells demonstrated the significant upper shift of concentration response to SP (10(-4)-10(-5) M). In vivo experiments on the mesenteric microcirculation also showed that SP induced a significant increase in the number of degranulated mast cells as well as in the number of leukocytes adherent to the venular wall. Tranilast, a mast cell stabilizer, as well as SP antagonist (CP-96,345) significantly attenuated the extent of mast cell degranulation and leukocyte adhesion elicited by SP. Although an immunoneutralization against CD18 by WT-3 significantly attenuated the leukocyte adhesion, it had no influence on the mast cell degranulation after SP superfusion. These separate in vivo observations show that SP induces leukocyte adhesion to the venular endothelium, possibly through the degranulation of mast cells.

A heterogeneous electrophysiological profile of bone marrow-derived mast cells

Electrophysiological properties of mouse bone marrow-derived mast cells (BMMC) were studied under the whole-cell clamp configuration. About one third of the cells were quiescent, but others expressed either inward or outward currents. Inwardly rectifying (IR) currents were predominant in 14% of the cells, and outwardly rectifying (OR) currents in 24%. The rest (22%) of the cells exhibited both inward and outward currents. The IR currents were eliminated by 1 mM Ba2+, and were partially inhibited by 100 microM quinidine. The reversal potential was dependent on extracellular K+, thereby indicating that K+ mediated the IR currents. The negative conductance region was seen at potentials positive to EK. The OR currents did not apparently depend on the extracellular K+ concentration, but were reduced by lowering the extracellular Cl- concentration. The OR currents were partially blocked by 1 mM Ba2+, and were further blocked by a Cl- channel blocker, 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS). In addition, the reversal potential of the OR currents was positively shifted by decreasing the ratio of external and internal Cl- concentrations, suggesting that Cl- was a major ion carrier. In cells exhibiting IR currents, the membrane potential varied among cells and tended to depolarize by elevating the external K+ concentration. In cells with OR currents, the resting potential was hyperpolarized in association with an increase in conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigen and thapsigargin promote influx of Ca2+ in rat basophilic RBL-2H3 cells by ostensibly similar mechanisms that allow filling of inositol 1,4,5-trisphosphate-sensitive and mitochondrial Ca2+ stores

In single, Fura 2-loaded RBL-2H3 cells, antigen and thapsigargin depleted the same intracellular pool of Ca2+ in the absence of external Ca2+; provision of external Ca2+ induced immediate increases in levels of free Ca2+ ([Ca2+]i). These increases were dependent on the presence of external Ca2+ and, presumably, on influx of Ca2+ across the cell membrane. Both stimulants enhanced intracellular accumulation of 45Ca2+ through ostensibly similar mechanisms because accumulation was blocked to similar extents by various multivalent cations or by depolarization with K+. Because thapsigargin blocked reuptake of Ca2+ into inositol 1,4,5-trisphosphate sensitive stores, uptake occurred independently of the refilling of these stores. Uptake was dependent instead on sequestration of 45Ca2+ in a pool of high capacity that was insensitive to thapsigargin, caffeine, GTP and inositol 1,4,5-trisphosphate but sensitive to ionomycin and mitochondrial inhibitors. The existence of an inositol 1,4,5-trisphosphate-insensitive pool was also apparent in permeabilized cells; at 0.1 microM [Ca2+]i, uptake of 45Ca2+ was largely confined (> 80%) to the inositol 1,4,5-trisphosphate-sensitive pool, but at 2 microM [Ca2+]i uptake was largely (> 60%) into the inositol 1,4,5-trisphosphate-insensitive pool. Provision of mitochondrial inhibitors along with thapsigargin to block uptake into both pools, did not impair the thapsigargin-induced increase in [Ca2+]i or influx of Ca2+, as indicated by changes in Fura 2 fluorescence, but did block the intracellular accumulation of 45Ca2+. The studies illustrate the utility of simultaneous measurements of [Ca2+]i and 45Ca2+ uptake for a full accounting of Ca2+ homoeostasis as exemplified by the ability to distinguish between influx and mitochondrial uptake of Ca2+.

Characterization of tachykinin mediated increases in [Ca2+]i in Chinese hamster ovary cells expressing human tachykinin NK3 receptors

The nature of the senktide response of the human NK3 receptor expressed in Chinese hamster ovary cells was characterised using the Ca2+ sensitive dye Fura-2 and imaging methods. Application of the NK3 receptor agonist senktide caused an increase in [Ca2+]i in the cells. The profile for NK3 receptor agonists was that senktide was more potent than [beta-Ala8]neurokinin A-(4-10) which was more potent than [Sar9,Met(O2)11]substance P. SR 48968 was a poor antagonist of the senktide response in intact cells confirming the weak affinity of this agent for the NK3 receptor (IC50 of approximately 1 microM) shown in binding assays. The NK3 receptor mediated increase in intracellular Ca2+ was independent of [Ca2+]o, blocked by the microsomal Ca2+ ATPase inhibitor thapsigargin and the phospholipase C inhibitor U73122 but not by ryanodine. Thus the source of the Ca2+ was probably a ryanodine insensitive, inositol triphosphate sensitive intracellular store.

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

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

Subtypes of inositol 1,4,5-trisphosphate receptor in human hematopoietic cell lines: dynamic aspects of their cell-type specific expression

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ signaling plays important roles in cellular responses to extracellular stimuli. We recently succeeded in cloning human counterparts of the three subtypes derived from separate genes. Using the cDNA sequences type-specific to these subtype receptors, we here analyzed the expression profile of IP3R subtypes in stimulated and unstimulated human hematopoietic cell lines representing T cells, B cells, neutrophils, macrophages, erythrocytes and megakaryocytes. Northern and dot blot analysis showed that each IP3R subtype is expressed differently in these cells and that the expression profile in each cell is dynamically changed upon stimuli which induce differentiation. Moreover, most of these cells were found to simultaneously express at least two different subtype receptors.

Picomolar doses of substance P trigger electrical responses in mast cells without degranulation

The nervous and immune systems may communicate through the action of neurotransmitters on mast cells. We used patchclamp electrophysiology to assess the responses of rat peritoneal mast cells (PMC) to low levels of substance P (SP), which are likely to occur in situ. SP at 50 nM, or even 10,000 times reduced to 5 pM, triggered an outwardly rectified Cl- current (50 nM: 10 of 10 cells; 5 pM: 10 of 11 cells), although degranulation never occurred. Electrical responses were delayed (mean 102.6 s for 5 pM SP), appearing as brief current pulses. Reapplication of SP resulted in peak current augmentation (mean 15.3 pA before exposure to SP, 47.3 pA after 1st exposure, and 116.0 pA after 2nd exposure to 5 pM SP). Cells repetitively exposed to SP degranulated 5-15 min and > 25 min after the second exposure to 50 nM SP (10 of 10 cells) or 5 pM SP (5 of 9 cells), respectively. This effect was reduced by 10 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid or when extracellular Ca2+ was removed, indicating a dependence on Cl- conductance and extracellular Ca2+. We propose that whole cell current oscillations in the absence of degranulation are the functional correlate of priming, a process that increases cellular responsiveness for the subsequent stimulation.

Calcium channels activated by depletion of internal calcium stores in A431 cells

Depletion of intracellular calcium stores induces transmembrane Ca2+ influx. We studied Ca(2+)- and Ba(2+)-permeable ion channels in A431 cells after store depletion by dialysis of the cytosol with 10 mM BAPTA solution. Cell-attached patches of cells held at low (0.5 microM) external Ca2+ exhibited transient channel activity, lasting for 1-2 min. The channel had a slope conductance of 2 pS with 200 mM CaCl2 and 16 pS with 160 mM BaCl2 in the pipette. Channel activity quickly ran down in excised inside-out patches and was not restored by InsP3 and/or InsP4. Thapsigargin induced activation in cells kept in 1 mM external Ca2+ after BAPTA dialysis. These channels represent one Ca2+ entry pathway activated by depletion of internal calcium stores and are clearly distinct from previously identified calcium repletion currents.