Flufenamate and Gd3+ inhibit stimulated Ca2+ influx in the epithelial cell line CFPAC-1

Activation of store-operated I(CRAC) by hydrogen peroxide

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) play a role in both innate immunity as well as cellular injury. H2O2 induces changes in intracellular calcium ([Ca(2+)]i) in many cell types and this seems to be at least partially mediated by transient receptor potential melastatin 2 (TRPM2) in cells that express this channel. Here we show that low concentrations of H2O2 induce the activation of the Ca(2+)-release activated Ca(2+) current I(CRAC). This effect is not mediated by direct CRAC channel activation, since H2O2 does not activate heterologously expressed CRAC channels independently of stromal interaction molecule (STIM). Instead, I(CRAC) activation is partially mediated by store depletion through activation of inositol 1,4,5 trisphosphate receptors (IP3R), since pharmacological inhibition of IP3 receptors by heparin or molecular knock-out of all IP3 receptors in DT40 B cells strongly reduce H2O2-induced I(CRAC). The remainder of H2O2-induced I(CRAC) activation is likely mediated by IP3R-independent store-depletion. Our data suggest that H2O2 can activate Ca(2+) entry through TRPM2 as well as store-operated CRAC channels, thereby adding a new facet to ROS-induced Ca(2+) signaling.

Contribution of capacitative and non-capacitative Ca2+-entry to M3-receptor-mediated contraction of porcine coronary smooth muscle

We studied the contribution of store-operated or capacitative Ca2+-entry (SOCE or CCE, respectively) through store-operated Ca2+ channels (SOCCs) and the contribution of Ca2+-entry through receptor-operated, non-selective cation channels (ROCCs or NSCCs, respectively), on the M3-receptor-mediated (270 nM Ach) contractile response of porcine coronary smooth muscle strips by means of the respective inhibitors. In the presence of L-VOCC blockade (1 microM verapamil), LOE 908 (inhibition of NSCCs) decreased the contractile response to 75+/-5% (p<0.01, n=6), 2-APB (inhibition of SOCCs) and SK and F 96365 (inhibition of SOCCs and of NSCCs) decreased the response to 45+/-4% (p<0.001, n=10) and to 23+/-2% (p<0.001, n=5), respectively (control: Ach response in the presence of verapamil alone). In the absence of L-VOCC blockade, LOE 908 reduced the Ach-response to 49+/-7% (p<0.001, n=8) and SK and F 96365 to 3+/-2% (p<0.001, n=4) of control, whereas 2-APB transiently increased the response (peak effect: 130+/-11%; p<0.05, n=8). We conclude: (1) the main source of activator Ca2+ during the M3-receptor-mediated contractile response is the Ca2+ influx through L-VOCCs; (2) however, in the presence of L-VOCC blockade, the contractile response is mainly due to Ca2+-entry through SOCCs; (3) NSCCs may be considerably involved in M3-receptor-mediated contraction as they may serve to depolarize the membrane potential and, thus, to open L-VOCCs; (4) in primary tissue of vascular smooth muscle, both, SOCE and Ca2+-entry through NSCCs are activated during M3-receptor stimulation.

Contribution of store-operated Ca2+ entry to pHo-dependent changes in vascular tone of porcine coronary smooth muscle

Vascular smooth muscle contracts on increases of extracellular pH (pH(o)) and relaxes on pH(o) decreases possibly resulting from changes in transsarcolemmal Ca(2+) influx. Therefore, we studied store-operated Ca(2+) entry (SOCE; i.e. capacitative Ca(2+) entry (CCE)) during acidification (pH(o)=6.5) and alkalinization (pH(o)=8.0) in isolated porcine coronary smooth muscle cells (SMCs) by monitoring cytoplasmic Ca(2+) ([Ca(2+)](i)) and divalent cation entry (Mn(2+) quench) with fura-2/AM-fluorometry. Additionally, we evaluated the contribution of SOCE to pH(o)-dependent changes in isometric tension of porcine coronary smooth muscle strips. SOCE elicited in SMCs by the SERCA inhibitor BHQ was strongly modulated by pH(o) showing a decrease upon acidification and vice versa an increase upon alkalinization. BHQ-mediated tension of smooth muscle strips also revealed strong pH(o) dependence. In contrast, L-VOC-dependent tension ([K(+)](o)=20 and 40 mmol l(-1)) was remarkably less affected by pH(o) changes. Moreover, refilling of depleted Ca(2+) stores after repeated M(3)-cholinergic receptor stimulation could be almost completely inhibited by SKF 96365 and was markedly reduced by acidification and considerably enhanced by alkalinization pointing to a major role of SOCE in refilling. We conclude that vascular tone particularly responds to alterations in pH(o) whenever SOCE substantially contributes to the amount of activator Ca(2+) for contraction.

Active and passive behaviour in the regulation of stiffness of the lateral wall in outer hair cells of the guinea-pig

The stiffness of the outer hair cell (OHC) lateral wall, measured by the micropipette aspiration technique, is non-linear, decreasing from the ciliary pole (stiffness parameter Sp 1.83+/-0.13 nN/microm n=10) towards the cell base (Sp 1.14+/-0.16 nN/microm, n=10) irrespective of the cochleoapical or cochleobasal origin of the cells. The length of the aspirated lateral wall segment was related exponentially to the duration of the applied negative pressure (6 cm H2O) in the synaptic region of the OHCs whereas an active, sigmoid component was observed between 30 and 60 s in the supranuclear regions. A significant increase of the midlateral wall stiffness (to 1.91+/-0.23 nN/microm; n=10) was observed in calcium-free medium and the sigmoid component of the response of the lateral wall was abolished. Salicylate (5 mM) had no significant effect on the active sigmoid behaviour of the lateral wall (n=10). Gadolinium (5 mM), a non-specific cation channel blocker, increased the stiffness of the lateral wall and attenuated the active component (n=10). The motor protein prestin thus does not seem to be involved in the active stiffness regulation seen in this study. A role for the cortical cytoskeleton in the regulation of stiffness seems reasonable according to our model. The mechanism may involve calcium-dependent metabolic modification of cytoskeletal or membrane proteins.

Calcium-stimulated short-circuit currents in the canine proximal colonic epithelium: effects of DK-PGD2, a metabolite of prostaglandin D2

Prostaglandin D2 (PGD2) has marked inhibitory effects on the canine proximal colonic epithelium set up in Ussing chambers. These effects involved a receptor that is pharmacologically distinct from the classical DP, presumably the recently identified CRTH2/DP2 variety. The mechanism underlying these effects was studied using 13,14-dihydro-15-keto-PGD2 (DK-PGD2), a stable metabolite of the parent prostanoid. The metabolite quickly reversed short circuit currents (I(sc)) stimulated by diverse agonists. Greater inhibitory effects were seen with stimulants such as carbachol and cyclopiazonic acid (CPA) rather than with forskolin or protein kinase A activators. Since the same stimulants were differentially affected by removal and replacement of serosal Ca2+, we tested the possibility that the prostanoid inhibited basolateral Ca2+ entry. In the absence of serosal Ca2+, tissues primed with CPA demonstrated concentration-dependent increases in I(sc), to cumulative additions of Ca2+ or Sr2+, though the former was more potent. Cl- removal and pretreatment with bumetanide virtually abolished responses, suggesting that the increase in I(sc) reflected Ca2+ dependent Cl- secretion. Though responses were insensitive to the L-type channel antagonist, verapamil, a marked inhibition was seen in the presence of metal cations (Gd3+, Cd2+, and La3+). Pretreatment with DK-PGD2 inhibited responses to Ca2+ in CPA-primed tissues. Thus, basolateral Ca2+ entry via store-operated Ca2+ channels may be the locus for the inhibitory effects of PGD2 in this tissue. These results could indicate a potential transduction mechanism for the novel DP receptor variously called CRTH2 or DP2.

Pituitary adenylate cyclase-activating polypeptide induces a sustained increase in intracellular free Ca(2+) concentration and catechol amine release by activating Ca(2+) influx via receptor-stimulated Ca(2+) entry, independent of store-operated Ca(2+) channels, and voltage-dependent Ca(2+) channels in bovine adrenal medullary chromaffin cells

Characteristics of pituitary adenylate cyclase-activating polypeptide (PACAP)-induced increase of Ca(2+) entry and catecholamine (CA) release were studied in bovine adrenal medullary chromaffin cells. PACAP induced intracellular free Ca(2+) concentration ([Ca(2+)](i)), showing an initial transient [Ca(2+)](i) rise followed by a sustained rise and CA release, which were not blocked by the blocking agents for nicotinic acetylcholine receptor (nAChR) channel, the voltage-dependent Ca(2+) channel (VOC), or the Na(+) channel. The sarcoendoplasmic Ca(2+)-ATPase inhibitors thapsigargin and cyclopiazonic acid did not affect the PACAP-induced sustained rise of [Ca(2+)](i), but did inhibit the initial [Ca(2+)](i) rise. In cells pretreated with cyclopiazonic acid or membrane-permeable, low-affinity Ca(2+) chelator N',N',N',N'-tetrakis(2-pyridylmethyl)ethylenediamine, PACAP further stimulated the entry of Ca(2+) or Mn(2+), whereas these treatments masked [Ca(2+)](i) dynamics induced by bradykinin. PACAP-induced sustained [Ca(2+)](i) rise and Mn(2+) entry were enhanced by acidic extracellular solution and reduced by alkalinization, whereas thapsigargin-induced Mn(2+) entry was regulated by the opposite. PACAP-induced [Ca(2+)](i) rise and Mn(2+) entry were not affected by blockers of cAMP-dependent protein kinase, phospholipase C, or protein kinase C. All store-operated Ca(2+) channel (SOC) blocking agents tested inhibited thapsigargin-induced Mn(2+) entry. 1(beta-[3-(4-Methoxyphenyl)-propoxy]-4-methoxyphenylethyl)-1H-imidazole hydrochloride (SK&F 96365), (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl-N,N-di-[2-(2,3,4-trimethoxyphenyl)ethyl]-acetamide, and econazole inhibited PACAP-induced Ca(2+) or Mn(2+) entry, whereas GdCl(3), 7,8-benzoflavone, nor-dihydroguaiaretic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, fulfenamic acid, and niflumic acid did not. SK&F 96365 and econazole but not GdCl(3) inhibited PACAP-induced CA release. These results suggest that PACAP activates a novel Ca(2+) entry pathway associated with sustained CA release independent of the nAChR channel, VOC and SOC, activated by acid pH, with different sensitivity to blockers of SOC. This pathway may provide a useful model for the study of receptor-operated Ca(2+) entry.

Ca(2+) entry into primary cultured pig coronary smooth muscle cells after previous store depletion by repetitive P2Y purinoceptor stimulation

Store-operated Ca(2+) entry, stimulated by depletion of intracellular Ca(2+) pools, has not been fully elucidated in vascular smooth muscle cells of pig coronary arteries. Therefore, [Ca(2+)](i) was measured in cultured cells derived from extramural pig coronary arteries using the Fura-2/AM fluorometry. Divalent cation entry was visualized with the Fura-2 Mn(2+)-quenching technique. Ca(2+) stores were depleted either by repetitive stimulation of P2Y purinoceptors with ATP (10 micromol/L), or by the sarcoendoplasmic Ca(2+)-ATPase inhibitor 2,5-Di-(tert-butyl)-1,4-benzohydroquinone (BHQ; 1 micromol/L) in Ca(2+)-free medium (EGTA 1 mmol/L). Addition of Ca(2+)(1 mmol/L) induced refilling of ATP-sensitive Ca(2+) stores and an increase in [Ca(2+)](i) in the presence of BHQ. Both could be significantly diminished by Ni(2+)(5 and 1mmol/L), La(3+)(10 micromol/L), Gd(3+)(10 micromol/L), and Mg(2+)(5.1 mmol/L). In contrast to the BHQ-mediated rise in [Ca(2+)](i), refilling of ATP-depleted stores was affected by neither flufenamate (0.1 mmol/L), nor by nitrendipine, nifedipine, and nisoldipine (each 1 micromol/L). The data suggest that after store depletion in pig coronary smooth muscle cells ATP and BHQ may converge on a common, Ni(2+)-, La(3+)-, Gd(3+)-, and Mg(2+)- sensitive Ca(2+) entry pathway, i.e. on a store-operated Ca(2+) entry. An additional contribution of the Na(+)/Ca(2+) exchanger cannot be excluded. Flufenamate-sensitive non-selective cation channels and dihydropyridine-sensitive L-type Ca(2+) channels are not involved in refilling of Ca(2+) stores after previous depletion by repetitive P2Y purinoceptor stimulation. The store-operated Ca(2+) entry in-between repetitive purinoceptor stimulation, i.e. in the absence of the agonist, may be responsible for the maintenance of agonist-induced rhythmic Ca(2+) responses.

Store-operated calcium influx inhibits renin secretion

On the basis of evidence that changes in the extracellular concentration of calcium effectively modulate renin secretion from renal juxtaglomerular cells, our study aimed to determine the effect of calcium influx activated by depletion of intracellular calcium stores on renin secretion. For this purpose we characterized the effects of the endoplasmatic Ca(2+)-ATPase inhibitors thapsigargin (300 nM) and cyclopiazonic acid (20 microM) on renin secretion from isolated perfused rat kidneys. We found that Ca(2+)-ATPase inhibition caused a potent inhibition of basal renin secretion as well as renin secretion activated by isoproterenol, bumetanide, and by a fall in the renal perfusion pressure. The inhibitory effect of Ca(2+)-ATPase inhibition on renin secretion was reversed within seconds by lowering of the extracellular calcium concentration into the submicromolar range but was not affected by lanthanum, gadolinium, flufenamic acid, or amlodipine. These data suggest that calcium influx triggered by release of calcium from internal stores is a powerful mechanism to inhibit renin secretion from juxtaglomerular cells. The store-triggered calcium influx pathway in juxtaglomerular cells is apparently not sensitive to classic blockers of the capacitative calcium entry pathway.

Multiple action sites of flufenamate on ion transport across the rat distal colon

The antisecretory effects of flufenamate in the rat distal colon were investigated with the Ussing-chamber and the patch-clamp method as well as by measurements of the intracellular Ca(2+) concentration using fura-2-loaded isolated crypts. Flufenamate (5.10(-4) mol l(-1)) suppressed the short-circuit current (Isc) induced by carbachol (5.10(-5) mol l(-1)), forskolin (5.10(-6) mol l(-1)) and the Isc induced by the membrane-permeable analogue of cyclic AMP, CPT - cyclic AMP (10(-4) mol l(-1)). Indomethacin (10(-6) - 10(-4) mol l(-1)) did not mimic the effect of flufenamate, indicating that the antisecretory effect of flufenamate is not related to the inhibition of the cyclo-oxygenase. When the basolateral membrane was depolarized by a high K(+) concentration and a Cl(-) current was induced by a mucosally directed Cl(-) gradient, the forskolin-stimulated Cl(-) current was blocked by flufenamate, indicating an inhibition of the cyclic AMP-stimulated apical Cl(-) conductance. When the apical membrane was permeabilized by the ionophore, nystatin, flufenamate decreased the basolateral K(+) conductance and inhibited the Na(+) - K(+)-ATPase. Patch-clamp experiments revealed a variable effect of flufenamate on membrane currents. In seven out of 11 crypt cells the drug induced an increase of the K(+) current, whereas in the remaining four cells an inhibition was observed. Experiments with fura-2-loaded isolated crypts indicated that flufenamate increased the basal as well as the carbachol-stimulated intracellular Ca(2+) concentration. These results demonstrate that flufenamate possesses multiple action sites in the rat colon: The apical Cl(-) conductance, basolateral K(+) conductances and the Na(+) - K(+)-ATPase.

Polycations induce calcium signaling in glomerular podocytes

BACKGROUND

The neutralization of the polyanionic surface of the podocyte by perfusion of kidneys with polycations, such as protamine sulfate, leads to a retraction of podocyte foot processes and proteinuria. This study investigates the effects of protamine sulfate or anionic, neutral, or cationic dextrans on the cytosolic calcium activity ([Ca2+]i) in podocytes.

METHODS

[Ca2+]i was measured in single cultured differentiated mouse podocytes with the fluorescence dye fura-2/AM.

RESULTS

Protamine sulfate caused a concentration-dependent and partially reversible increase of [Ca2+]i (EC50 approximately 1.5 micromol/liter). Pretreatment of the cells with heparin (100 U/liter) inhibited the protamine sulfate-mediated increase of [Ca2+]i. Like protamine sulfate, diethylaminoethyl dextran (DEAE-dextran) concentration dependently increased [Ca2+]i in podocytes (EC50 approximately 20 nmol/liter), whereas dextran sulfate or uncharged dextran (both 10 micromol/liter) did not influence [Ca2+]i. A reduction of the extracellular Ca2+ concentration (from 1 mmol/liter to 1 micomol/liter) partially inhibited the protamine sulfate and the DEAE-dextran-induced [Ca2+]i response. Flufenamate (100 micromol/liter) or Gd3+ (10 micromol/liter), which are known to inhibit nonselective ion channels, did not influence the [Ca2+]i increase induced by protamine sulfate. In the presence of thapsigargin (50 nmol/liter), an inhibitor of the endoplasmic reticulum Ca2+-ATPase, both protamine sulfate and DEAE-dextran increased [Ca2+]i.

CONCLUSIONS

The data indicate that polycations increase podocyte [Ca2+]i. The increase of [Ca2+]i may be an early event in the pathogenesis of protamine sulfate-mediated retraction of podocyte foot processes.

Extracellular ATP increases [Ca2+]i in primarily cultured pig coronary smooth muscle cells via a P2Y purinoceptor subtype

In primarily cultured pig coronary smooth muscle cells, extracellular adenosine triphosphate (ATP; 10(-9) to 10(-3) M) dose-dependently increases intracellular calcium ([Ca2+]i). The [Ca2+]i transients measured by fura-2 fluorescence consist of peak and plateau phases with [Ca2+]i values of 191.84 +/- 5.67 nM (n = 10) and 91.67 +/- 1.89 nM, respectively. In Ca(2+)-free solution, the peak phases persisted, but there was a loss of the plateau response, indicating an initial ATP-stimulated intracellular Ca2+ release and a subsequent transarcolemmal Ca2+ entry. Various agonists have been used to characterize the P2 purinoceptor subtype involved in the ATP-induced Ca2+ transients. The rank order of potency was uridine triphosphate (UTP) > ATP >> 2-meSATP > beta,gamma-meATP = alpha,beta-meATP = adenosine = 0. To examine the refilling of ATP-sensitive stores, four repetitive 60-s ATP responses were produced throughout with a 5-min recovery period in between. Now the ATP peaks gradually declined in Ca(2+)-free solution, indicating the emptying of the stores. If, however, Ca2+ entry was allowed in the "refilling period" (i.e., between the ATP pulses), the Ca2+ peaks could be maintained or restored, respectively. The data suggest that the ATP-dependent [Ca2+]i transients may be mediated via a UTP > ATP-activated P2Y purinoceptor subtype, mediating both an intracellular Ca2+ release and a transarcolemmal Ca2+ influx. The refilling of Ca2+ stores may occur through the unstimulated membrane after agonist stimulation. A putative pathway may be a "capacitative" Ca2+ entry induced on depletion of intracellular Ca2+ stores.

Sequential activation of different Ca2+ entry pathways upon cholinergic stimulation in mouse pancreatic acinar cells

1. We have studied capacitative calcium entry (CCE) under different experimental conditions in fura-2-loaded mouse pancreatic acinar cells by digital microscopic fluorimetry. CCE was investigated during [Ca2+]i decay after cell stimulation with a supramaximal concentration of ACh (10 microM) or during Ca2+ readmission in Ca2+-depleted cells (pretreated with thapsigargin or ACh). 2. La3+ and Zn2+ (100 microM) inhibited CCE during Ca2+ readmission but had negligible effects during ACh decay. In contrast flufenamic acid (100 microM), an inhibitor of non-selective cation channels, genistein (10 microM), a broad-range tyrosine kinase inhibitor, and piceatannol (10 microM), an inhibitor specific for non-receptor Syk tyrosine kinase, inhibited CCE during ACh decay but not during Ca2+ reintroduction. 3. Simultaneous detection of Mn2+ entry and [Ca2+]i measurement showed that, in the presence of extracellular calcium, application of 100 microM Mn2+ during ACh decay resulted in manganese influx without alteration of calcium influx, whilst when applied during Ca2+ readmission, Mn2+ entry was significantly smaller and induced a clear inhibition of CCE. 4. Application of the specific protein kinase C inhibitor GF109293X (3 microM) reduced CCE in Ca2+-depleted cells, whereas the activator phorbol 12-myristate, 13-acetate (3 microM) increased Ca2+ entry. 5. Based on these results we propose that cholinergic stimulation of mouse pancreatic acinar cells induces Ca2+ influx with an initial phase operated by a non-specific cation channel, sensitive to flufenamic acid and tyrosine kinase inhibitors but insensitive to lanthanum and divalent cations, followed by a moderately Ca2+-selective conductance inhibited by lanthanum and divalent cations.

The role of K+ channels in colonic Cl- secretion

Cl- secretion in the rat colonic crypt base cell (bc) requires the coordinated (a) opening of Cl- channels in the luminal membrane; (b) activation of the Na+2Cl-K+ cotransporter; (c) enhanced conductive K+ exit from the cell; and (d) increased pumping by the (Na+ + K+)-ATPase. In this study we focus on the importance of conductive K+ exit. After stimulation with the cholinergic agonist carbachol (CCH, 0.1-10 mumol/l) bc respond with a marked increase in whole cell (wc) conductance and a hyperpolarization of the membrane voltage (Vm). This is paralleled by a marked increase in the (Cl- secretory) short-circuit current (Isc) in Ussing chamber studies of the intact distal colon. Current evidence favors the view that CCH, via IP3, enhances cytosolic Ca2+ activity, and that Ca2+ increases the open probability of Cl- channels indirectly and that of K+ channels directly. After stimulation with PGE2 bc also enhance the wc conductance, but this is paralleled by a marked depolarization of Vm. Again these effects correspond to a marked increase in (Cl- secretory) Isc. The depolarization and enhanced wc conductance is partly due to the activation of Cl- channels. However, current evidence suggests that these effects on Cl- channels are paralleled by an activation of K+ channels. The chromanol 293B, by inhibiting these K+ channels specifically, abolishes PGE2-induced Cl- secretion completely, but has no effect on basal K+ conductance or on CCH-induced Cl- secretion. CCH apparently activates a Ca(2+)-dependent K+ channel with a conductance of 10-20 pS, whilst PGE2 (or cAMP) activate a much smaller K+ channel. Only the latter K+ channel can be inhibited by 293B in excised patches. Noise analysis suggests that this K+ channel has a conductance of < 3 pS and fast kinetics. The complete 293B induced inhibition of Cl- secretion caused by PGE2 can be explained by the fact that PGE2 induces a marked depolarization and that this depolarization reduces the basal K+ conductance. Current evidence suggests that this inhibition of the basal K+ conductance is caused by a depolarization induced inhibition of Ca2+ entry.

Pertussis toxin- and PMA-insensitive calcium mobilization by sphingosine in CFPAC-1 cells: evidence for a phosphatidic acid-dependent mechanism

In a pancreatic duct adenocarcinoma cell line (CFPAC-1) sphingosine (10 microM) induced both mobilization of intracellular Ca2+ and stimulation of inositol phosphates accumulation. Whereas this latter effect was significantly inhibited by treatment with pertussis toxin or by short-term incubation with phorbol 12-myristate 13-acetate, Ca2+ mobilization was completely insensitive to both treatments. Experiments with permeabilized cells showed that sphingosine or the sphingosine metabolites sphingosine-1-phosphate and sphingosylphosphorylcholine were unable to directly release Ca2+ from internal stores, whereas phosphatidic acid, but not arachidonic acid, was effective. Phosphatidic acid formation was markedly enhanced (2.9-fold over control) by sphingosine, this effect being significantly reduced by preincubation with the diacylglycerol kinase inhibitor R59022. Ca2+ mobilization by sphingosine was also cut down by preincubation with R59022. In conclusion, the results suggest that sphingosine activates phospholipase C through a mechanism functionally coupled through a G protein and under control of PKC. Mobilization of [Ca2+]i by sphingosine is independent of phospholipase C stimulation and likely due to elevation of phosphatidic acid generated by stimulation of diacylglycerol kinase activity.

Capacitative Ca2+ entry (CCE) induced by luminal and basolateral ATP in polarised MDCK-C7 cells is restricted to the basolateral membrane

In previous studies we have characterised various properties of capacitative Ca2+ entry (CCE) in different epithelia. After Ca2+ store depletion with PLC/InsP3-coupled agonists or by inhibition of store Ca2+ uptake, with for example thapsigargin, Ca2+ influx is activated. This leads to a sustained cellular response (e.g. NaCl secretion). In the present study, we have investigated CCE in polarised MDCK-C7 cells grown on permeable supports in a chamber allowing for separate luminal and basolateral perfusion. The transepithelial resistance (Rte) and voltage (Vte) were measured simultaneously to verify the tightness of the epithelial monolayers. MDCK-C7 cells grew to very tight monolayers (Rto > 3000 omega.cm2). Apical ATP (100 mumol/l) led to a biphasic [Ca2+]i increase. Removal of apical Ca2+ in the continuous presence of ATP did not reduce the stimulated plateau. However, removal of Ca2+ from the basolateral side rapidly and completely interrupted the [Ca2+]i plateau to below basal values ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 213 +/- 15 nmol/l, n = 9). Furthermore, MDCK-C7 responded to basolateral ATP (100 mumol/l) with a biphasic [Ca2+]i transient. Again the plateau phase of the ATP-induced [Ca2+]i effect was fully dependent on the presence of basolateral but not apical Ca2+ ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 196 +/- 5 nmol/l, n = 10). Receptor-independent depletion of cytosolic Ca2+ stores with thapsigargin from both sides led to a rise in [Ca2+]i, which was also exclusively dependent on the presence of basolateral Ca2+ (n = 8). These data indicate that MDCK-C7 cells express luminal and basolateral P2-receptors coupled to PLC/InsP3/Ca2+. ATP applied from both sides induced a sustained [Ca2+]i plateau which was due to transmembrane Ca2+ influx. The ATP- and thapsigargin-induced Ca2+ influx pathway was exclusively located in the basolateral membrane.

Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium

1. Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium was studied using the short-circuit current (Isc) technique. 2. Thapsigargin induced an increase in Isc that could be inhibited when external Ca2+ was chelated by EGTA. 3. The inhibition of the thapsigargin-induced Isc could be reversed by re-addition of Ca2+ to apical or basolateral solutions. The magnitude of the reactivated Isc depended predominantly on basolateral Ca2+ concentration. 4. The magnitude of the reactivated Isc upon basolateral Ca2+ addition increased with the thapsigargin concentration, indicating its dependence on the emptied state of the Ca2+ store induced by thapsigargin. 5. The thapsigargin-induced Isc, as well as the Ca(2+)-dependent reactivation of Isc in EGTA-treated epithelia, was inhibitable by apical, but not basolateral, addition of flufenamate, and by basolateral addition of La3+. Other Ca2+ channel blockers, verapamil and nifedipine, had no effect when applied to either membrane. 6. The results suggest that thapsigargin-induced anion secretion by the equine sweat gland epithelial cells is crucially dependent upon the Ca2+ influx occurring primarily through the basolateral membrane, and that apical and basolateral membranes may possess different pathways for Ca2+ entry.

Sphingosine-induced inhibition of capacitative calcium influx in CFPAC-1 cells

Sphingosine (10 microM) induced mobilization of intracellular Ca2+ stores in the pancreatic duct adenocarcinoma cell line CFPAC-1. The effect was specific for sphingosine, since the sphingosine analog C2-ceramide had no effect. Sphingosine did not cause Ca2+ entry from extracellular medium, as also shown by following Mn2+ quenching of Fura-2 fluorescence. Furthermore, sphingosine, similarly to the mitochondrial inhibitors rotenone and oligomycin, strongly inhibited the rate of Mn2+ entry triggered by both thapsigargin- and agonist-induced depletion of intracellular stores. The uptake of rhodamine 123, a lipophilic cation which estimates mitochondrial energy level, was reduced by sphingosine to an extent similar to that observed in the presence of mitochondrial inhibitors. It is suggested that impairment of mitochondrial function might be responsible for inhibition of capacitative Ca2+ entry caused by sphingosine.

Attenuation of stimulated Ca2+ influx in colonic epithelial (HT29) cells by cAMP

In HT29 colonic epithelial cells agonists such as carbachol (CCH) or ATP increase cytosolic Ca2+ activity ([Ca2+]i) in a biphasic manner. The first phase is caused by inositol 1,4,5-trisphophate-(Ins P3-) mediated Ca2+ release from their respective stores and the second plateau phase is mainly due to stimulated transmembraneous Ca2+ influx. The present study was undertaken to examine the effect of increased adenosine 3',5'-cyclic monophosphate (cAMP) (forskolin 10 micromol/l = FOR) on the Ca2+ transient in the presence of CCH (100 micromol/l). In unpaired experiments it was found that FOR induced a depolarization and reduced cytosolic Ca2+ ([Ca2+]i, measured as the fura-2 fluorescence ratio 340/380 nm) significantly. Dideoxyforskolin had no such effect. The effect of FOR was abolished when the cells were depolarized by a high-K+ solution. In further paired experiments utilizing video imaging in conjunction with whole-cell patch-clamp, [Ca2+]i was monitored separately for the patch-clamped cell and three to seven neighbouring cells. In the presence of CCH, FOR reduced [Ca2+]i uniformly from a fluorescence ratio (345/380) of 2.9 +/- 0.12 to 1.8 +/- 0.07 in the patch-clamped cell and its neighbours (n = 48) and depolarized the membrane voltage (Vm) of the patch-clamped cells significantly and reversibly from -54 +/- 7.4 to -27 +/- 5.9 mV (n = 6). In additional experiments Vm was depolarized by 15-54 mV by various increments in the bath K+ concentration. This led to corresponding reductions in [Ca2+]i. Irrespective of the cause of depolarization (high K+ or FOR) there was a significant correlation between the change in Vm and change in [Ca2+]i. These data indicate that the cAMP-mediated attenuation of Ca2+ influx is caused by the depolarization produced by this second messenger.

Calcium influx pathways in rat pancreatic ducts

A number of agonists increase intracellular Ca2+ activity, [Ca2+]i, in pancreatic ducts, but the influx/efflux pathways and intracellular Ca2+ stores in this epithelium are unknown. The aim of the present study was to characterise the Ca2+ influx pathways, especially their pH sensitivity, in native pancreatic ducts stimulated by ATP and carbachol, CCH. Under control conditions both agonists led to similar changes in [Ca2+]i. However, these Ca2+ transients, consisting of peak and plateau phases, showed different sensitivities to various experimental manoeuvres. In extracellular Ca2+-free solutions, the ATP-induced [Ca2+]i peak decreased by 25%, but the CCH-induced peak was unaffected; both plateaus were inhibited by 90%. Flufenamate inhibited the ATP-induced peak by 35%, but not the CCH-evoked peak; the plateaus were inhibited by 75-80%. La3+ inhibited the ATP-induced plateau fully, but that induced by CCH by 55%. In resting ducts, an increase in extracellular pH, pHe, by means of HEPES and HCO3-/CO2 buffers, increased [Ca2+]i; a decrease in pHe had the opposite effect. In stimulated ducts the pH-evoked effects on Ca2+ influx were more pronounced and depended on the agonist used. At pHe 6.5 both ATP- and CCH-evoked plateaus were inhibited by about 50%. At pH 8.0 the ATP-stimulated plateau was inhibited by 27%, but that stimulated by CCH was increased by 72%. Taken together, we show that CCH stimulates Ca2+ release followed by Ca2+ influx that is moderately sensitive to flufenamate, La3+, depolarisation, it is inhibited by low pH, but stimulated by high pH. ATP stimulates Ca2+ release and probably an early Ca2+ influx, which is more markedly sensitive to flufenamate and La3+, and is both inhibited by low and high pH. Thus our study indicates that there are at least two separate Ca2+ influx pathways in pancreatic ducts cells.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

Characterization of a whole-cell Ca2+-blockable monovalent cation current in isolated ectodermal cells of chick embryo

The presence of a Ca2+-blockable monovalent cation current is demonstrated in isolated ectodermal cells of the chick embryo using the whole-cell patch-clamp method. In the absence of any stimulation, the whole-cell current is time independent and rectifies outwardly at membrane potentials higher than +40 mV. The outward current is neither carried by Cl- channels nor by K+ channels. Application of a Ca2+-free solution containing 1 mmol/l ethylenediaminetetraacetic acid (EDTA) elicits a large inward current and increases the outward current. The inward current can be carried by extracellular Li+, Na+, K+ and Cs+, but not N-methyl-D-glucamine. The Ca2+-blockable monovalent cation channel discriminates very poorly among these cations. The estimated number of channels per cell is around 2000. Extracellular protons block the inward Na+ current in the absence of extracellular Ca2+. The apparent negative logarithm of the dissociation constant for proton (pKH) at -100 mV is 5.8. Among 12 potential channel modulators, including verapamil and nifedipine, only quinine decreases the current. Quinine blocks this current with a dissociation constant, Kd, equal to 0.18 mmol/l, independent of the membrane potential. This study demonstrates the presence of a whole-cell Ca2+-blockable monovalent cation current in dissociated chick ectodermal cells with permeation properties similar to those observed at the single-channel level. Contrary to studies made of other tissues, we did not observe any blocking effect of verapamil and nifedipine on the Ca2+-blockable monovalent cation current.

Chloride-channel block inhibits T lymphocyte activation and signalling

Both large- and small-conductance chloride (Cl-) channels have been found in human T lymphocytes; however, apart from possible roles in mediating regulatory volume decrease, their functions are not understood. We have used patch-clamp electrophysiology, Ca2+ spectrofluorometry, and Western blot assay for phosphotyrosine to investigate the effects of blocking Cl- channels on proliferation and on specific events in the activation of normal human T cells. Four chemically distinct Cl- channel blockers inhibited both the small-conductance Cl- channels and phytohemagglutinin (PHA)-induced lymphocyte proliferation in a similar dose-dependent manner; their order of potency was 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) > 4,4'-diisothiocyano-2,2'-disulfonic acid (DIDS) > flufenamic acid >> IAA-94. The Cl- channel blockers inhibited both the PHA-induced mobilization of Ca2+ and the rapid tyrosine phosphorylation of several polypeptides. Cell proliferation was not rescued by the Ca+ ionophore ionomycin or by addition of exogenous interleukin-2 (IL-2). Moreover, the blockers also inhibited phosphotyrosine expression in IL-2-treated, activated lymphoblasts. Thus, our results support a role for Cl- channels in early, PHA-evoked signalling and in later, II-2-dependent stages of lymphocyte activation and proliferation.

Oxidant stress and endothelial membrane transport

The endothelium modulates vascular tone, vasoreactivity, and permeability in response to agonist-stimulation. Much of the pathophysiology of oxidant-induced vascular injury can be attributed to endothelial cell dysfunction. In the past several years, the effects of oxidant stress on agonist-stimulated Ca(2+)-channels have been described. More recently, the effects of oxidant stress on several other endothelial membrane-transport systems have been elucidated. It now appears that inhibition of the agonist-stimulated Ca2+ channel is due at least in part to membrane depolarization via oxidant-activation of a Na(+)-permeable, nonselective cation channel. In this review, the effects of oxidant stress on ion transport through the agonist-stimulated Ca2+ influx channel, Na+ and K+ channels, Na+/K(+)-ATPase, Ca(2+)-ATPase, and the Na+/K+/2Cl- cotransporter are discussed. The interrelated effects of oxidant stress on these endothelial membrane transport pathways are considered, and the net effect on Ca2+ signaling is described.

Ca2+ influx induced by store release and cytosolic Ca2+ chelation in Ht29 colonic carcinoma cells

Cl- secretion in HT29 cells is regulated by agonists such as carbachol, neurotensin and adenosine 5'-triphosphate (ATP). These agonists induce Ca2+ store release as well as Ca2+ influx from the extracellular space. The increase in cytosolic Ca2+ enhances the Cl- and K+ conductances of these cells. Removal of extracellular Ca2+ strongly attenuates the secretory response to the above-mentioned agonists. The present study utilises patch-clamp methods to characterise the Ca2+ influx pathway. Inhibitors which have been shown previously to inhibit non-selective cation channels, such as flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n=6) inhibited ATP (0.1 mmol.l-1) induced increases in whole-cell conductance (Gm). When Cl- and K+ currents were inhibited by the presence of Cs2SO4 in the patch pipette and gluconate in the bath, ATP (0.1 mmol.l-1) still induced a significant increase in Gm from 1.2 +/- 0.3 nS to 4.7 +/- 1 nS (n = 24). This suggests that ATP induces a cation influx with a conductance of approximately 3-4 nS. This cation influx was inhibited by flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n = 9). When Ba2+ (5 mmol.l-1) and 4,4'-diisothiocyanato-stilbene-2-2'-disulphonic acid (DIDS, 0.1 mmol.l-1) were added to the KCl/K-gluconate pipette solution to inhibit K+ and Cl- currents and the cells were clamped to depolarised voltages, ATP (0.1 mmol.l-1) reduced the membrane current (Im) significantly from 86 +/- 14 pA to 54 +/- 11 pA (n = 13), unmasking a cation inward current. In another series, the cation inward current was activated by dialysing the cell with a KCl/K-gluconate solution containing 5-10 mmol.l-1 1,2-bis-(2-aminoethoxy)ethane-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The zero-current membrane voltage (Vm) and Im (at a clamp voltage of +10 mV) were monitored as a function of time. A new steady-state was reached 30-120 s after membrane rupture. Vm depolarised significantly from -33 +/- 2 mV to -12 +/- 1 mV, and Im fell significantly from 17 +/- 2 pA to 8.9 +/- 1.0 pA (n = 71). This negative current, representing a cation inward current, was activated when Ca2+ stores were emptied and was reduced significantly ( Im) when Ca2+ and/or Na+ were removed from the bathing solution: removal of Ca2+ in the absence of Na+ caused a Im of 5.0 +/- 1.2 pA (n = 12); removal of Na+ in the absence of Ca2+ caused a Im of 12.8 +/- 3.5 pA (n = 4). The cation inward current was also reduced significantly by La3+, Gd3+, and flufenamate. We conclude that store depletion induces a Ca2+/Na+ influx current in these cells. With 145 mmol.l-1 Na+ and 1 mmol.l-1 Ca2+, both ions contribute to this cation inward current. This current is an important component in the agonist-regulated secretory response.