Influence of extracellular Ca2+ on endogenous Cl- channels in Xenopus oocytes

Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are competitive antagonists of the human P2X3 receptor

Introduction: The P2X3 receptor (P2X3R), an ATP-gated non-selective cation channel of the P2X receptor family, is expressed in sensory neurons and involved in nociception. P2X3R inhibition was shown to reduce chronic and neuropathic pain. In a previous screening of 2000 approved drugs, natural products, and bioactive substances, various non-steroidal anti-inflammatory drugs (NSAIDs) were found to inhibit P2X3R-mediated currents.

Methods: To investigate whether the inhibition of P2X receptors contributes to the analgesic effect of NSAIDs, we characterized the potency and selectivity of various NSAIDs at P2X3R and other P2XR subtypes using two-electrode voltage clamp electrophysiology.

Results: We identified diclofenac as a hP2X3R and hP2X2/3R antagonist with micromolar potency (with IC50 values of 138.2 and 76.7 µM, respectively). A weaker inhibition of hP2X1R, hP2X4R, and hP2X7R by diclofenac was determined. Flufenamic acid (FFA) inhibited hP2X3R, rP2X3R, and hP2X7R (IC50 values of 221 µM, 264.1 µM, and ∼900 µM, respectively), calling into question its use as a non-selective ion channel blocker, when P2XR-mediated currents are under study. Inhibition of hP2X3R or hP2X2/3R by diclofenac could be overcome by prolonged ATP application or increasing concentrations of the agonist α,β-meATP, respectively, indicating competition of diclofenac and the agonists. Molecular dynamics simulation showed that diclofenac largely overlaps with ATP bound to the open state of the hP2X3R. Our results suggest a competitive antagonism through which diclofenac, by interacting with residues of the ATP-binding site, left flipper, and dorsal fin domains, inhibits the gating of P2X3R by conformational fixation of the left flipper and dorsal fin domains. In summary, we demonstrate the inhibition of the human P2X3 receptor by various NSAIDs. Diclofenac proved to be the most effective antagonist with a strong inhibition of hP2X3R and hP2X2/3R and a weaker inhibition of hP2X1R, hP2X4R, and hP2X7R.

Discussion: Considering their involvement in nociception, inhibition of hP2X3R and hP2X2/3R by micromolar concentrations of diclofenac, which are rarely reached in the therapeutic range, may play a minor role in analgesia compared to the high-potency cyclooxygenase inhibition but may explain the known side effect of taste disturbances caused by diclofenac.

Characterization of constitutive and acid-induced outwardly rectifying chloride currents in immortalized mouse distal tubular cells

Thiazides block Na⁺ reabsorption while enhancing Ca²⁺ reabsorption in the kidney. As previously demonstrated in immortalized mouse distal convoluted tubule (MDCT) cells, chlorothiazide application induced a robust plasma membrane hyperpolarization, which increased Ca²⁺ uptake. This essential thiazide-induced hyperpolarization was prevented by the Cl⁻ channel inhibitor 5-Nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), implicating NPPB-sensitive Cl⁻ channels, however the nature of these Cl⁻ channels has been rarely described in the literature. Here we show that MDCT cells express a dominant, outwardly rectifying Cl⁻ current at extracellular pH 7.4. This constitutive Cl⁻ current was more permeable to larger anions (Eisenman sequence I; I⁻ > Br⁻ ≥ Cl⁻) and was substantially inhibited by > 100 mM [Ca²⁺]_o, which distinguished it from ClC-K2/barttin. Moreover, the constitutive Cl⁻ current was blocked by NPPB, along with other Cl⁻ channel inhibitors (4,4′-diisothiocyanatostilbene-2,2′-disulfonate, DIDS; flufenamic acid, FFA). Subjecting the MDCT cells to an acidic extracellular solution (pH < 5.5) induced a substantially larger outwardly rectifying NPPB-sensitive Cl⁻ current. This acid-induced Cl⁻ current was also anion permeable (I⁻ > Br⁻ > Cl⁻), but was distinguished from the constitutive Cl⁻ current by its rectification characteristics, ion sensitivities, and response to FFA. In addition, we have identified similar outwardly rectifying and acid-sensitive currents in immortalized cells from the inner medullary collecting duct (mIMCD-3 cells). Expression of an acid-induced Cl⁻ current would be particularly relevant in the acidic IMCD (pH < 5.5). To our knowledge, the properties of these Cl⁻ currents are unique and provide the mechanisms to account for the Cl⁻ efflux previously speculated to be present in MDCT cells.

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MDCT cells express a dominant NPPB-sensitive Cl⁻ current at pH 7.4.

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The constitutive Cl⁻ current (pH 7.4) does not arise from ClC-K2/barttin.

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MDCT cells also express an acid-induced NPPB-sensitive Cl⁻ current (pH < 5.5).

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Both the constitutive and acid-induced Cl⁻ currents are unique.

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mIMCD-3 cells express currents with similar biophysical properties.

Xenopus borealis as an alternative source of oocytes for biophysical and pharmacological studies of neuronal ion channels

For the past 30 years, oocytes from Xenopus laevis have been extensively used to express and characterise ion channels in an easily controlled environment. Here we report the first use of oocytes from the closely related species Xenopus borealis as an alternative expression system for neuronal ion channels. Using the two-electrode voltage-clamp technique, we show that a wide variety of voltage- and ligand-gated ion channels have the same channel properties and pharmacological profiles when expressed in either X. laevis or X. borealis oocytes. Potential advantages of the X. borealis oocytes include a smaller endogenous chloride current and the ability to produce more intense fluorescence signals when studied with voltage-clamp fluorometry. Scanning electron microscopy revealed a difference in vitelline membrane structure between the two species, which may be related to the discrepancy in fluorescence signals observed. We demonstrate that X. borealis oocytes are a viable heterologous system for expression of neuronal ion channels with some potential advantages over X. laevis oocytes for certain applications.

Driving force-dependent block by internal Ba(2+) on the Kir2.1 channel: Mechanistic insight into inward rectification

The Kir2.1 channel is characterized by strong inward rectification; however, the mechanism of the steep voltage dependence near the equilibrium potential remains to be investigated. Here, we studied the internal Ba(2+) block of the Kir2.1 channel expressed in Xenopus oocytes. We showed that the driving force and thus the K(+) ion flux significantly influenced the apparent affinity of the block by internal Ba(2+). Kinetic analysis revealed that the binding rate shifted with the driving force and changed steeply near the equilibrium point, either in the presence or absence of the transmembrane electrical field. The unbinding rate was determined by the intrinsic affinity of the site. Mutagenesis studies revealed that the high-affinity binding site for Ba(2+) was located near T141 at the internal entrance of the selectivity filter. The steep change of the blocking affinity near the equilibrium potential may result from the flux-coupling effect in the single-file, multi-ion cytoplasmic pore.

Convergent evolution of sodium ion selectivity in metazoan neuronal signaling

Ion selectivity of metazoan voltage-gated Na⁺ channels is critical for neuronal signaling and has long been attributed to a ring of four conserved amino acids that constitute the ion selectivity filter (SF) at the channel pore. Yet, in addition to channels with a preference for Ca²⁺ ions, the expression and characterization of Na⁺ channel homologs from the sea anemone Nematostella vectensis, a member of the early-branching metazoan phylum Cnidaria, revealed a sodium-selective channel bearing a noncanonical SF. Mutagenesis and physiological assays suggest that pore elements additional to the SF determine the preference for Na⁺ in this channel. Phylogenetic analysis assigns the Nematostella Na⁺-selective channel to a channel group unique to Cnidaria, which diverged >540 million years ago from Ca²⁺-conducting Na⁺ channel homologs. The identification of Cnidarian Na⁺-selective ion channels distinct from the channels of bilaterian animals indicates that selectivity for Na⁺ in neuronal signaling emerged independently in these two animal lineages.

Channel-like characteristics of the low-affinity barley phosphate transporter PHT1;6 when expressed in Xenopus oocytes

Remobilization of inorganic phosphate (P(i)) within a plant is critical for sustaining growth and seed production under external P(i) fluctuation. The barley (Hordeum vulgare) transporter HvPHT1;6 has been implicated in P(i) remobilization. In this report, we expressed HvPHT1;6 in Xenopus laevis oocytes, allowing detailed characterization of voltage-dependent fluxes and currents induced by HvPHT1;6. HvPHT1;6 increased efflux of P(i) near oocyte resting membrane potentials, dependent on external P(i) concentration. Time-dependent inward currents were observed when membrane potentials were more negative than -160 mV, which was consistent with nH(+):HPO(4)(2-) (n > 2) cotransport, based on simultaneous radiotracer and oocyte voltage clamping, dependent upon P(i) concentration gradient and pH. Time- and voltage-dependent inward currents through HvPHT1;6 were also observed for SO(4)(2-)and to a lesser degree for NO(3)(-)Cl(-)but not for malate. Inward and outward currents showed linear dependence on the concentration of external HPO(4)(2-)similar to low-affinity P(i) transport in plant studies. The electrophysiological properties of HvPHT1;6, which locates to the plasma membrane when expressed in onion (Allium cepa) epidermal cells, are consistent with its suggested role in the remobilization of P(i) in barley plants.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

The A563T variation of the renal epithelial calcium channel TRPV5 among African Americans enhances calcium influx

The transient receptor potential cation channel, subfamily V, member 5 (TRPV5) gene, which encodes the Ca(2+) channel in the apical membrane of distal convoluted tubule and connecting tubule of the kidney, exhibits an unusually high frequency of nonsynonymous single nucleotide polymorphisms (SNPs) among African Americans. To assess the functional impacts of the nonsynonymous SNP variations in TRPV5, these variants were analyzed with radiotracer (45)Ca(2+) influx assay and the voltage-clamp technique using Xenopus laevis oocytes. Among the variations tested, including A8V, R154H, A563T, and L712F, the latter two significantly increased TRPV5-mediated Ca(2+) influx. The A563T variant, which exists in African Americans with relative high frequency, exhibited increased Ca(2+) influx at extracellular Ca(2+) from 0.01 to 2 mM despite a lower expression level at the plasma membrane. This variant also exhibited a reduction in Na(+) current as a result of increased sensitivity to extracellular Mg(2+). By substituting threonine-563 (Thr(563)) with serine or valine residue, the bulky side chain of Thr(563) was shown to facilitate Ca(2+) transport, whereas the hydroxyl group of Thr(563) is likely related to Mg(2+) sensitivity. The A563T variant was capable of increasing TRPV5-mediated Ca(2+) influx, even when it was expressed under conditions mimicking heterozygous or compound state with other variants. In conclusion, the A563T variant of TRPV5 significantly increased Ca(2+) influx by affecting the Ca(2+) permeation pathway. Thus the A563T variation in TRPV5 may contribute to the superior ability of renal Ca(2+) conservation in African Americans.

Kinetic relationship between the voltage sensor and the activation gate in spHCN channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are activated by membrane hyperpolarizations that cause an inward movement of the positive charges in the fourth transmembrane domain (S4), which triggers channel opening. The mechanism of how the motion of S4 charges triggers channel opening is unknown. Here, we used voltage clamp fluorometry (VCF) to detect S4 conformational changes and to correlate these to the different activation steps in spHCN channels. We show that S4 undergoes two distinct conformational changes during voltage activation. Analysis of the fluorescence signals suggests that the N-terminal region of S4 undergoes conformational changes during a previously characterized mode shift in HCN channel voltage dependence, while a more C-terminal region undergoes an additional conformational change during gating charge movements. We fit our fluorescence and ionic current data to a previously proposed 10-state allosteric model for HCN channels. Our results are not compatible with a fast S4 motion and rate-limiting channel opening. Instead, our data and modeling suggest that spHCN channels open after only two S4s have moved and that S4 motion is rate limiting during voltage activation of spHCN channels.

The Alphavirus 6K Protein Activates Endogenous Ionic Conductances when Expressed in Xenopus Oocytes

The Alphavirus Sindbis 6K protein is involved in several functions. It contributes to the processing and membrane insertion of E1 and PE2 viral envelope glycoproteins and to virus budding. It also permeabilizes Escherichia coli and mammalian cells. These viroporin-like properties have been proposed to help virus budding by modifying membrane permeabilities. We expressed Sindbis virus 6K cRNA in Xenopus oocytes to further characterize the effect of 6K on membrane conductances and permeabilization. Although no intrinsic channel properties were seen, cell shrinkage was observed within 24 h. Voltage-clamp experiments showed that 6K upregulated endogenous currents: a hyperpolarization-activated inward current (I (in)) and a calcium-dependent chloride current (I (Cl)). 6K was located at both the plasma and the endoplasmic reticulum membranes. The plasma membrane current upregulation likely results from disruption of the calcium homeostasis of the cell at the endoplasmic reticulum level. Indeed, 6K cRNA expression induced reticular calcium store depletion and capacitative calcium entry activation. By experimental modifications of the incubation medium, we showed that downstream of these events cell shrinkage resulted from a 6K -induced KCl efflux (I (Cl) upregulation leads to chloride efflux, which itself electrically drives potassium efflux), which was responsible for an osmotic water efflux. Our data confirm that 6K specifically triggers a sequential cascade of events that leads to cytoplasmic calcium elevation and cell permeabilization, which likely play a role in the Sindbis virus life cycle.

Endogenous hemichannels play a role in the release of ATP from Xenopus oocytes

ATP is an electrically charged molecule that functions both in the supply of energy necessary for cellular activity and as an intercellular signaling molecule. Although controlled ATP secretion occurs via exocytosis of granules and vesicles, in some cells, and under certain conditions, other mechanisms control ATP release. Gap junctions, intercellular channels formed by connexins that link the cytoplasm of two adjacent cells, control the passage of ions and molecules up to 1 kDa. The channel is formed by two moieties called hemichannels, or connexons, and it has been suggested that these may represent an alternative pathway for ATP release. We have investigated the release of ATP through hemichannels from Xenopus oocytes that are formed by Connexin 38 (Cx38), an endogenous, specific type of connexin. These hemichannels generate an inward current that is reversibly activated by calcium-free solution and inhibited by octanol and flufenamic acid. This calcium-sensitive current depends on Cx38 expression: it is decreased in oocytes injected with an antisense oligonucleotide against Cx38 mRNA (ASCx38) and is increased in oocytes overexpressing Cx38. Moreover, the activation of these endogenous connexons also allows transfer of Lucifer Yellow. We have found that the release of ATP is coincident with the opening of hemichannels: it is calcium-sensitive, is inhibited by octanol and flufenamic acid, is inhibited in ASCx38 injected oocytes, and is increased by overexpression of Cx38. Taken together, our results suggest that ATP is released through activated hemichannels in Xenopus oocytes.

Bisphosphonates stimulate an endogenous nonselective cation channel in Xenopus oocytes: potential mechanism of action

The mechanisms of action of bisphosphonates (BPs) have been poorly determined. Besides their actions on osteoclasts, these agents exhibit gastrointestinal complications. They have also recently been described as affecting various preparations that express an epithelial Na+ channel (ENaC). To understand the effects of BP on ion channels and the ENaC in particular, we used the Xenopus oocyte expression system. Alendronate, and similarly risedronate, two aminobisphosphonates, caused a large stimulation of an endogenous nonselective cation conductance (NSCC). This stimulation averaged 63 ± 12 μS ( n = 18) 60 min after the addition of 2 mM alendronate. The effects on the endogenous NSCC were blocked by extracellular acidification to pH 6.4. On the other hand, alendronate caused a small inhibition of ENaC conductance at pH 7.4 and 6.4, but the effects at pH 6.4 were more readily observed in the absence of changes of the endogenous conductance. The effects on membrane capacitance were also markedly different, with a clear decrease at pH 6.4 and no consistent changes at pH 7.4. The effects on the endogenous channel were further augmented by genistein and were inhibited by a tyrosine phosphatase inhibitor, indicating the involvement of the tyrosine kinase pathway. Stimulation of NSCC with BP is expected to cause membrane depolarization and may explain, in part, its mechanisms of action in inhibiting osteoclasts.

Corymine potentiates NMDA-induced currents in Xenopus oocytes expressing NR1a/NR2B glutamate receptors

Previous studies demonstrated that corymine, an indole alkaloid isolated from the leaves of Hunter zeylanica, dose-dependently inhibited strychnine-sensitive glycine-induced currents. However, it is unclear whether this alkaloid can modulate the function of the N-methyl-D-aspartate (NMDA) receptor on which glycine acts as a co-agonist via strychnine-insensitive glycine binding sites. This study aimed to evaluate the effects of corymine on NR1a/NR2B NMDA receptors expressed in Xenopus oocytes using the two-electrode voltage clamp technique. Corymine significantly potentitated the NMDA-induced currents recorded from oocytes on days 3 and 4 after cRNA injection but it showed no effect when the current was recorded on days 5 and 6. The potentiating effect of corymine on NMDA-induced currents was induced in the presence of a low concentration of glycine (< or =0.1 microM). Spermine significantly enhanced the potentiating effect of corymine observed in the oocytes on days 3 and 4, while the NMDA-receptor antagonist 2-amino-5-phosphonopentanone (AP5) and the NMDA-channel blocker 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) reversed the effect of corymine. On the other hand, the nonspecific chloride channel blocker 4,4-di-isothiocyano stilbene-2,2-disulfonoc acid (DIDS) had no effect on the corymine potentiation of NMDA currents. There was no good correlation between corymine- and spermine-induced potentiation of the NMDA-current response in Xenopus oocytes. These results suggest that corymine potentiates the NMDA-induced currents by interacting with a site different from the spermine binding site.

Novel outwardly rectifying anion conductance in Xenopus oocytes

We describe a novel, strongly outwardly rectifying anion current in Xenopus laevis oocytes, that we have named I(Cl,Or)- The properties of I(Cl,Or) are different from those of any other anion conductance previously described in these cells. Typically, I(Cl,Or) amplitude was small when extracellular Cl- (Cle) was the permeant anion. However, when Cle was replaced by lyotropic anions I(Cl,Or) became evident as a time-independent current. (ICl,Or) was voltage dependent and showed a remarkable outwards rectification with little or no inwards tail current. The relative selectivity sequence determined from current amplitudes was: SCN- > or = ClO4- > I- > Br- > or = NO3- > Cl- x I(Cl,Or) was insensitive to Gd3+ but was blocked by micromolar concentrations of niflumic acid, DIDS or Zn2+. Furthermore, I(Cl,Or) was not affected by buffering intracellular Ca2+ with BAPTA. Low extracellular pH inhibited I(Cl,Or) with a pK of 5.8. We propose that I(Cl,Or) might result from activation of endogenous ClC-5-like Cl- channels present in Xenopus oocytes.

Airway surface liquid calcium modulates chloride permeability in the cystic fibrosis airway

Patients with cystic fibrosis (CF) demonstrate a characteristic defect in epithelial chloride movement, which can be demonstrated in vivo by the nasal potential difference technique. After amiloride pretreatment, the CF airway exhibits only a transient response to perfusion with low-chloride solution, contrasting with the sustained hyperpolarization seen in control subjects. This study further investigated the response to low-chloride solution in the CF airway, examining the interaction between surface divalent ions and the low-chloride response. Sequential perfusion with amiloride, low chloride, and isoproterenol was tested in groups of subjects with CF, with the diluent containing different concentrations of calcium and magnesium, on different days. When the low-chloride response was measured with the nominally calcium-free diluents, the subjects with CF had mean (SEM) responses of 8.0 (0.7), 8.6 (2.4), and 9.6 (1.6) mV in the presence of 0, 1, and 3 mM Mg2+, respectively, significantly different from the response in the presence of divalent ions. However, the subsequent response to isoproterenol was not different in the presence or absence of divalent ions. We hypothesize that perfusion of the CF airway with nominally calcium-free solutions reduces tonic inhibition of chloride secretion.

Effects of ionomycin and thapsigargin on ion currents in oocytes of Bufo arenarum

In this study, two electrode voltage clamp technique was used to assess the ionic current of oocytes of the South American toad Bufo arenarum and to study the dependence of these currents on the extracellular and intracellular Ca2+ concentrations. Ca2+ chelators, ionomycin -a calcium ionophore- and thapsigargin, a blocker of the Ca2+ pump of the sarcoplasmic reticulum, were used. The main results were the following: Most oocytes showed a voltage activated rectifying conductance. Ionomycin (1 microM) increased inward and outward currents in control solution. The effect of ionomycin was blocked partially at negative potentials and was blocked completely at positive potentials in absence of extracellular Ca2+. When the oocytes were treated with thapsigargin (2 microM) or BAPTA-am, a membrane-permeant intracellular chelator in control solution (10 microM), ionomycin did not increased either inward nor outward currents. The conclusion of our experiments is that there are two sources of Ca2+ for activation of the current induced by ionomycin, the cytoplasmic stores and the extracellular space. We believe ionomycin directly translocates Ca2+ from the SER into the cytoplasm but not from the extracellular medium. Ca2+ entry probably occurs through store-operated-Ca-channels.

NF449, a novel picomolar potency antagonist at human P2X1 receptors

The antagonistic effects of the novel suramin analogue 4,4',4",4"'-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid (NF449) were analyzed at homomeric human P2X(1) and P2X(7) receptor subtypes (hP2X(1) and hP2X(7)) heterologously expressed in Xenopus oocytes using the two-microelectrode voltage-clamp technique. At activating ATP concentrations of 1 microM (hP2X(1)) and 100 microM (hP2X(7)), IC(50) values of 0.05 nM and 40 microM were found for hP2X(1) and hP2X(7) receptors, respectively. The Schild analysis revealed a pA(2) of 10.7 at hP2X(1). Wash-in and wash-out of 10 nM NF449 were nearly complete within 16 s and 4 min, respectively, at the hP2X(1) receptor. An increase in the activating ATP concentration to 100 microM shifted the NF449 concentration-inhibition curve rightwards for the hP2X(1) receptor. NF449 decelerated activation as well as desensitization of hP2X(1). It is concluded that NF449 acts as a reversible competitive antagonist at the hP2X(1) with much higher potency at hP2X(1) than at hP2X(7) receptors. NF449 may hence be excellently suited to discriminate between both receptors in native human tissues.

Glu496Ala polymorphism of human P2X7 receptor does not affect its electrophysiological phenotype

A glutamate to alanine exchange at amino acid position 496 of the human P2X(7) receptor was recently shown to be associated with a loss of function in human B lymphocytes in terms of ATP-induced ethidium(+) uptake, Ba(2+) influx, and induction of apoptosis (Gu BJ, Zhang WY, Worthington RA, Sluyter R, Dao-Ung P, Petrou S, Barden JA, and Wiley JS. J Biol Chem 276: 11135-11142, 2001). Here we analyzed the effect of the Glu(496) to Ala exchange on the channel properties of the human P2X(7) receptor expressed in Xenopus oocytes with the two-microelectrode voltage-clamp technique. The amplitudes of ATP-induced whole cell currents characteristic of functional expression, kinetic properties including ATP concentration dependence, and permeation behavior were not altered by this amino acid exchange. Also in HEK293 cells, the Ala(496) mutant mediated typical P2X(7) receptor-dependent currents like the parent Glu(496) hP2X(7) receptor. Because the function of the P2X(7) receptor as an ATP-gated channel for small cations including Ba(2+) remained unaffected by this mutation, we conclude that Glu(496) plays a critical role in pore formation but does not determine the ion channel properties of the human P2X(7) receptor.

Functional evidence of distinct ATP activation sites at the human P2X(7) receptor

1. The effect of the agonist ATP on whole cell currents of Xenopus oocytes expressing either the wild-type human P2X(7) receptor (hP2X(7)), an N-terminally hexahistidyl-tagged hP2X(7) receptor (His-hP2X(7)), or a truncated His-hP2X(7) receptor (His-hP2X(7)DeltaC) lacking the C-terminal 156 amino acids was investigated using the two-microelectrode voltage clamp technique. 2. The activation time course of the wild-type hP2X(7) receptor can be described as the sum of an exponentially growing and an additional almost linearly activating current component. 3. The amplitude of the exponentially activating current component of the wild-type hP2X(7) receptor displayed a biphasic dependence on the agonist concentration, which could be best approximated by a model of two equal high-sensitivity and two equal low-sensitivity non-cooperative activation sites with apparent dissociation constants of about 4 and 200 microM free ATP(4-), respectively. 4. The linearly activating current was monophasically dependent on the agonist concentration with an apparent dissociation constant of about 200 microM. 5. The contribution of the low-sensitivity sites to current kinetics was reduced or almost abolished in oocytes expressing His-hP2X(7) or His-hP2X(7)DeltaC. 6. Our data indicate that the hP2X(7) receptor possesses at least two types of activation sites, which differ in ATP(4-) sensitivity by a factor of 50. The degree of occupation of these two sites influences both activation and deactivation kinetics. Both N- and C-terminal domains appear to be important determinants of the current elicited by activation of the sites with low ATP sensitivity, but not for that mediated by the highly ATP-sensitive sites.

Ion channel activation by SPC3, a peptide derived from the HIV-1 gp120 V3 loop

SPC3 is a multibranched peptide containing eight identical GPGRAF motifs which are derived from the human immunodeficiency virus (HIV)-1 gp120 V3 loop consensus sequence. This molecule was reported to prevent the infection of CD4+ cells by various HIV-1 and HIV-2 strains. However, the molecular mode of action of SPC3 remains unclear. Here, we investigated the possibility that SPC3 could interact with alpha/beta-chemokine receptors following observations that, first, the V3 loop is likely to be involved in alpha/beta-chemokine receptor-dependent HIV entry and, second, natural ligands of these receptors are potent inhibitors of cell infection. To address this point, we examined the effects of SPC3 on Xenopus oocytes either uninjected or expressing exogenous human CXCR4 alpha-chemokine receptors. Extracellular applications of micromolar concentrations of SPC3 onto Xenopus oocytes trigger potent inward chloride currents which can be inhibited by increasing extracellular Ca2+ concentration. This effect can be blocked by chloride channel antagonists and is highly specific to SPC3 as it is not triggered by structural analogs of SPC3. The SPC3-induced chloride conductance in oocytes is alpha/beta-chemokine receptor dependent because: (i) SPC3 alters the sensitivity of this channel to external applications of human recombinant MIP-1alpha, a natural ligand of human CCR5 receptor, and (ii) the amplitude of the inward current could be increased by the expression of exogenous human CXCR4 chemokine receptor. The effect of SPC3 appears to rely on the activation of a phospholipase A2 signaling pathway, but is not affected by changes in cytosolic Ca2+ concentration, or by alterations in Gi/Go protein, adenylate cyclase, phospholipase C or protein kinase C activity. Altogether, the data indicate that SPC3 is capable of activating a surface alpha/beta-chemokine-like receptor-mediated signaling pathway in competent cells, thereby triggering, either directly or indirectly, a Ca2+-inactivated chloride conductance.

Kinetic parameters of calcium currents in maturing acutely isolated CA1 cells

Calcium currents were recorded in CA1 hippocampal cells from immature (P(4-10)) and older (P(22-55)) rats, using whole-cell voltage clamp techniques. Parameters defining the voltage-dependence of activation (tau(m)) and inactivation (tau(h)), steady-state inactivation and activation were determined at both stages of maturation. Current density increased with maturation. A transient low voltage activated (l.v.a.) current was found in P(4-10) cells, but not in the older cells. At voltages less negative than -30 mV, current inactivation was best described by two exponentials (tau(hf), tau(hs)); the ratio of the amplitudes of the two components changed with maturation, with a dominance of the faster component (tau(hf)) in the younger cells. The voltage dependence of tau(hf) followed a simple dependence model, decreased with increasing depolarization, in all cells at both stages of maturation. In P(4-10) cells, tau(hs) was voltage insensitive (range -25 to +30 mV); in P(22-55) cells, the voltage dependence of tau(hs) was found to be complex. Two current components were identified from the voltage dependence of the conductance in both groups. The first, more hyperpolarized component, the l.v.a. current found in P(4-10) cells; this was absent in the older cells, in which we found a component with a different voltage dependence. The voltage dependence of the conductance of the second, more depolarized component did not differ in younger and older cells. In the course of maturation, the steady-state inactivation of the second component underwent a hyperpolarizing shift and a decrease in voltage sensitivity.

Identifying the Ca(++) signalling sources activating chloride currents in Xenopus oocytes using ionomycin and thapsigargin

The calcium ionophore, ionomycin (IM), and the sarcoplasmic/endoplasmic reticulum (SER) calcium pump inhibitor, thapsigargin (TG), were used to study the roles of Ca(++) from different sources in regulating Ca(++)-dependent Cl(-) currents in Xenopus oocytes. The Ca(++)-dependent Cl(-) currents, Ic, were measured in voltage-clamped oocytes (Vc = -60 mV). In the presence of extracellular Ca(++), both TG (0.1 to 10 microM) and IM (0.1 to 10 microM) induce release of Ca(++) from SER and activated capacitative Ca(++) entry (CCE) across the plasma membrane leading to activation of both "fast" and "slow" Cl(-) currents. The fast Ic was produced by Ca(++) release from SER while Ca(++) entry across the plasma membrane activated the slow Ic. Intracellular application of the calcium buffer, BAPTA, blocked activation of the slow Ic due to Ca(++) entry via CCE pathways, but not via IM-mediated movement across the plasma membrane. It is concluded that predominantly Ca(++) release from stores regulates a fast Ic while Ca(++) entry through CCE pathways regulates a slow Ic. Further, the CCE and slow Ic pathways must be located in spatially separated compartments since BAPTA can effectively abolish the effects of Ca(++) entry via the CCE pathway, but not by the IM-mediated entry pathway.

Characteristics of P2X7 receptors from human B lymphocytes expressed in Xenopus oocytes

Human B lymphocytes express an ATP-gated ion channel (P2Z receptor), which shares similarities with the recently identified P2X7 receptor. Using gene specific primers, we have now isolated P2X7 cDNA from the total RNA of human B lymphocytes. This hP2X7 receptor subtype was expressed in Xenopus oocytes and electrophysiologically characterized. The hP2X7 receptor is similar to, but does not completely match, P2Z of human B cells. The hP2X7 receptors resemble the P2Z receptors with regard to the ATP concentration of half maximal activation, reproducibility, permeation characteristics and lack of desensitization of the ATP-evoked currents. However, in contrast to the native lymphocytic P2Z receptor, the time course of activation of hP2X7 displayed an additional linearly increasing current component. Furthermore, a second, small and slowly deactivating current component exists only in hP2X7 expressed in oocytes. The activation and deactivation kinetics as well as permeation characteristics of hP2X7 are different from rat P2X7 recently expressed in oocytes. Unlike in mammalian cells, hP2X7 expressed in Xenopus oocytes is not sufficient to induce large non-selective pores.

Low extracellular Ca(2+) activates a transient Cl(-) current in chicken ovarian granulosa cells

The effects of low Ca(2+) on ion currents in hen ovarian granulosa cells were examined. A fast activating and inactivating transient outward current (TOC) and a slowly activating outward current (SOC) could be observed. In the presence of normal Ca(2+) concentration (2. 5 mM) and with a holding potential of -80 mV, SOC was activated in all cells with command pulses more positive than -20 mV. In 2.5 mM Ca(2+), TOC appeared in 10% of cells at the command pulse of +80 mV and in 60-85% of cells at +100 to +120 mV. In low-Ca(2+) solution and command potential of +80 mV (holding potential of -80 mV), the amplitude of TOC was enhanced in cells that expressed it in normal Ca(2+), and TOC appeared in 43% of the cells that did not express it initially in normal Ca(2+). At both normal and low Ca(2+) levels, TOC decreased as the holding potential became more positive. TOC was reduced in Cl(-)-deficient solution and in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid, a Cl(-) channel blocker. These findings suggest that chicken granulosa cells express a Ca(2+)-inactivated TOC carried by Cl(-). This current may serve as a signal for some of the reduced metabolic functions of granulosa cells associated with Ca(2+) deficiency.

Antagonism by the suramin analogue NF279 on human P2X(1) and P2X(7) receptors

The effect of the suramin analogue 8,8'-(carbonylbis(imino-4, 1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis(1,3 , 5-naphthalenetrisulfonic acid) (NF279) was analyzed on human P2X(1) and P2X(7) receptor subtypes (human P2X(1) and human P2X(7)) heterologously expressed in Xenopus oocytes using the two-microelectrode voltage-clamp technique. At activating ATP concentrations of 1 microM (human P2X(1)) and 10 microM ATP (human P2X(7)), IC(50) values of 0.05 microM and 2.8 microM were found for human P2X(1) and human P2X(7) receptors, respectively. An increase in the activating [ATP] shifted the NF279 concentration-inhibition curve rightwards for both receptors. NF279 slowed the activation of both human P2X(1) and human P2X(7) as well as the desensitization of human P2X(1). The data support a model in which desensitization of P2X(1) is dependent on preceding activation of these P2X receptors. It is concluded that NF279 acts as a competitive antagonist with much higher potency at human P2X(1) than at P2X(7) receptors. NF279 may hence be suited to discriminate between both receptors in native tissues.

Sensing of extracellular calcium by neurones

Transient changes in the intracellular concentration of Ca2+ provide a major signal for the regulation of many ion channels and enzymes in central neurones. In contrast, changes in extracellular Ca2+ are thought to play little or no signaling role. However, concentrations of extracellular calcium in the central nervous system do change dramatically during intense physiological and pathological stimulation, and recent studies have identified a number of membrane proteins that can sense and respond to changes in extracellular Ca2+. These include the recently cloned Ca2+-sensing receptor, hemi-gap-junction channels, and a potential Ca2+-sensing cation channel. Lowering extracellular Ca2+ strongly depolarizes and excites cultured hippocampal neurones. The excitation can be detected with decreases from physiological concentrations of as little as 100 µM. The depolarization results from activation of a nonselective cation current, which is sensitive to block by divalent and polyvalent cations. In outside-out patches, lowering Ca2+ induces a single-channel current with a conductance of 36 pS. Activation of this cation channel, in response to decreases in extracellular Ca2+, likely plays a key role in a positive feedback system of excessive neuronal depolarization, which accompanies intense excitatory activity in the hippocampus.Key words: nonselective cation channel, calcium-sensing receptor, calcium-sensing channel, hemi-gap channels, extracellular calcium.

Characterization of a murine gene homologous to the bovine CaCC chloride channel

The bovine CaCC protein is a putative Ca2+-dependent Cl- channel of airway epithelial cells. Therefore, CaCC proteins could contribute to transepithelial Cl- transport and accordingly modify the phenotype of cystic fibrosis (CF) patients. We have identified a murine EST containing a full-length cDNA coding for a 902-amino-acid protein highly homologous to bovine CaCC. The murine gene (mCaCC) maps to chromosome 3 at the H2-H3 band and is expressed, as indicated by Northern blot analysis, in mouse skin and kidney but not in brain, heart, lung or testis. RT-PCR indicates a low expression in tracheal epithelial cells. Heterologous expression of mCaCC in Xenopus oocytes elicits membrane currents that are anion-selective and inhibited by DIDS and by niflumic acid, a blocker of the endogenous chloride current in oocytes. The identification of genes belonging to the CaCC family will help to evaluate their role as ion channels or channel regulators and their actual contribution to epithelial chloride transport.

Stretch-independent activation of the mechanosensitive cation channel in oocytes of Xenopus laevis

Oocytes of the South African clawed toad Xenopus laevis possess in their plasma membrane a so-called stretch-activated cation channel (SAC) which is activated by gently applying positive or negative pressure (stretch) to the membrane patch containing the channels. We show here that this mechanosensitive channel acted as a spontaneously opening, stretch-independent non-selective cation channel (NSCC) in more than half of the oocytes that we investigated. In 55% of cell-attached patches (total number of patches, 58) on 30 oocytes from several different donors, we found NSCC opening events. These currents were increased by elevating the membrane voltage or raising the temperature. NSCC and SAC currents shared some properties regarding the relative conductances of Na+>Li+>Ca2+, gating behaviour and amiloride sensitivity. Stretch-independent currents could be clearly distinguished from stretch induced SAC currents by their voltage and temperature dependence. Open events of NSCC increased strongly when temperature was raised from 21 to 27 degrees C. NSCC currents could be partly inhibited by high concentrations of extracellular Gd3+ and amiloride (100 and 500 microM, respectively). We further show exemplarily that NSCC can seriously hamper investigations when oocytes are used for the expression of foreign ion channels. In particular, NSCC complicated investigations on cation channels with small conductance as we demonstrate for a 4 pS epithelial Na+ channel (ENaC) from guinea pig distal colon. Our studies on NSCCs suggest the involvement of these channels in oocyte temperature response and ion transport regulation. From our results we suggest that NSCC and SAC currents are carried by one protein operating in different modes.

The ion selectivity of a membrane conductance inactivated by extracellular calcium in Xenopus oocytes

1. The ion selectivity of a membrane ion conductance that is inactivated by extracellular calcium (Ca2+o) in Xenopus oocytes has been studied using the voltage-clamp technique. 2. The reversal potential of the Ca2+o-sensitive current (Ic) was measured using voltage ramps (-80 to +40 mV) as a function of the external concentration (12-240 mM) of NaCl or KCl. The direction and amplitude of the shifts in reversal potentials are consistent with permeability ratios of 1:0.99:0.24 for K+:Na+:Cl-. 3. Current-voltage (I-V ) relations of Ic, determined during either voltage ramps of 0.5 s duration or at steady state, displayed pronounced rectification at both hyperpolarized and depolarized potentials. However, instantaneous I-V relations showed less rectification and could be fitted by the constant field equation assuming the above K+:Na+:Cl- permeability ratios. 4. Ion substitution experiments indicated that relatively large organic monovalent cations and anions are permeant through Ic channels with the permeability ratios K+:NMDG+:TEA+:TPA+:TBA+:Gluc- = 1:0.45:0. 35:0.2:0.2:0.2. 5. External amiloride (200 microM), gentamicin (220 microM), flufenamic acid (40 microM), niflumic acid (100 microM), Gd3+ (0.3 microM) or Ca2+ (200 microM) caused reversible block of Ic without changing its reversal potential. 6. Preinjection of oocytes with antisense oligonucleotide against connexin 38, the Xenopus hemi-gap-junctional protein, inhibited Ic by 80 % without affecting its ion selectivity, thus confirming and extending the recent suggestion of Ebihara that Ic represents current carried through hemi-gap-junctional channels. 7. In vitro and in vivo maturation of oocytes resulted in a significant decrease in Ic conductance to 7 % and 2 % of control values, respectively. This developmental downregulation of Ic minimizes any toxic effect Ic activation would have when the mature egg is released into Ca2+o-free pond water. 8. The results of this study are discussed in relation to other Ca2+o-inactivated conductances seen in a wide variety of cell types and which have previously been interpreted as arising either from Ca2+o-masked channels or from changes in the ion selectivity of voltage-gated Ca2+ or K+ channels.

The GXGXG motif in the pI(Cln) protein is not important for the nucleotide sensitivity of the pI(Cln)-induced Cl- current in Xenopus oocytes

It has been proposed that the pI(Cln) protein forms a nucleotide-sensitive plasma membrane anion channel with a GXGXG motif being an essential component of the extracellular nucleotide-binding site. To evaluate this hypothesis, we have performed voltage-clamp experiments on Xenopus laevis oocytes injected with RNA encoding a rat mutant pI(Cln) in which the three glycines of the putative nucleotide-binding site have been changed into alanines (G54A; G56A; G58A). The injected oocytes displayed outwardly rectifying anion currents, which were voltage-dependently blocked by extracellular cAMP, but which were not affected by removal of extracellular Ca2+. Furthermore, the mutation did not affect the voltage-dependent inactivation. We therefore conclude that there is no evidence in favour of an extracellular nucleotide-binding site in pI(Cln).

Functional expression of a truncated Ca(2+)-activated Cl- channel and activation by phorbol ester

We have isolated a niflumic acid-insensitive, Ca(2+)-activated Cl- channel (CaCC) from bovine trachea that migrates at 38 kDa (p38) on reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, a cloned CaCC isolated from a tracheal cDNA expression library by screening with an antibody raised against p38 has a primary cDNA transcript of 2712 base pairs that codes for a 100-kDa protein and is not susceptible to dithiothreitol reduction. To test the hypothesis that the functional channel may be a much smaller posttranslationally processed form of the 100-kDa protein, we generated a mutant construct (CaCCX, 42.5-kDa protein) truncated at the NH2 and COOH termini. The whole cell currents of wild-type (wt) CaCC and CaCCX expressed in Xenopus oocytes were 10-fold higher than those of water-injected oocytes and were further increased by ionomycin or A-23187 and inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and dithiothreitol. Whole cell currents in wtCaCC- and CaCCX-expressing oocytes could also be activated by phorbol 12-myristate 13-acetate and could be inhibited by chelerythrine chloride, suggesting that the cloned CaCC is regulated by protein kinase C. These results suggest that a smaller form of the full-length CaCC can form a functional channel.

Extracellular divalent cations block a cation non-selective conductance unrelated to calcium channels in rat cardiac muscle

1. The effect of removing extracellular divalent cations on resting potential (Vrest) and background conductance of rat cardiac muscle was studied. Vrest was measured with 3 M KCl-filled microelectrodes in papillary muscles, or with a patch electrode in ventricular myocytes. Whole-cell membrane currents were measured in myocytes using step or ramp voltage commands. 2. In both muscles and single cells, decrease or removal of Ca2+o and Mg2+o caused a nifedipine-resistant depolarization, which was reversed upon readmission of Ca2+o or Mg2+o (half-maximal effect at 0.8 mM Ca2+o or 3 mM Mg2+o in muscles). 3. In single myocytes, removal of Ca2+o and Mg2+o had no effect on the seal resistance in nonruptured cell-attached recordings, but reversibly induced a current with a reversal potential (Vrev) of -8 +/- 3.4 mV (with internal Cs+; mean +/- S.E.M., n = 23) during whole-cell recordings. The current was insensitive to nifedipine (3-100 microM) or amiloride (1 mM). Vrev was insensitive to changes in the equilibrium potential for chloride ions (ECl). 4. The current induced in the absence of extracellular divalent cations was blocked in a concentration-dependent manner by Ca2+o. (At -80 mV, the affinity constant KCa was 60 microM with a Hill coefficient of 0.9) KCa was voltage dependent at positive but not negative potentials. Mg2+o, Ni2+o, Sr2+o, Ba2+o, Cd2+o and Gd3+o also blocked the current. 5. In 0 mM Na+ (145 mM NMDG+), the inward component of the divalent cation-sensitive current was decreased and Vrev shifted to more negative potentials. 6. These results suggest that a novel conductance pathway, permeable to monovalent cations but not to Cl- and blocked by divalent cations, exists in ventricular myocytes.

cAMP-activation of amiloride-sensitive Na+ channels from guinea-pig colon expressed in Xenopus oocytes

Guinea-pig distal colonic mRNA injection into Xenopus laevis oocytes resulted in expression of functional active epithelial Na+ channels in the oocyte plasma membrane. Poly(A)+ RNA was extracted from distal colonic mucosa of animals fed either a high-salt (HS) or a low-salt (LS) diet. The electrophysiological properties of the expressed amiloride-sensitive Na+ conductances were investigated by conventional two-electrode voltage-clamp and patch-clamp measurements. Injection of poly(A)+ RNA from HS-fed animals [from hereon referred to as HS-poly(A)+ RNA] into oocytes induced the expression of amiloride-sensitive Na+ conductances. On the other hand, oocytes injected with poly(A)+ RNA from LS-fed animals [LS-poly(A)+ RNA] expressed a markedly larger amount of amiloride-blockable Na+ conductances. LS-poly(A)+ RNA-induced conductances were completely inhibitable by amiloride with a Ki of 77 nM, and were also blocked by benzamil with a Ki of 1.8 nM. 5-(N-Ethyl-N-isopropyl)-amiloride (EIPA), even in high doses (25 "mu"M), had no detectable effect on the Na+ conductances. Expressed amiloride-sensitive Na+ channels could be further activated by cAMP leading to nearly doubled clamp currents. When Na+ was replaced by K+, amiloride (1 "mu"M) showed no effect on the clamp current. Single-channel analysis revealed slow gating behaviour, open probabilities (Po) between 0.4 and 0.9, and slope conductances of 3. 8 pS for Na+ and 5.6 pS for Li+. The expressed channels showed to be highly selective for Na+ over K+ with a permeability ratio PNa/PK > 20. Amiloride (500 nM) reduced channel Po to values < 0.05. All these features make the guinea-pig distal colon of LS-fed animals an interesting mRNA source for the expression of highly amiloride-sensitive Na+ channels in Xenopus oocytes, which could provide new insights in the regulatory mechanism of these channels.

The Ca(2+)-induced leak current in Xenopus oocytes is indeed mediated through a Cl- channel

Defolliculated oocytes of Xenopus laevis responded to removal of external divalent cations with large depolarizations and, when voltage clamped, with huge currents. Single channel analysis revealed a Cl- channel with a slope conductance of about 90 pS at positive membrane potentials with at least four substates. Single channel amplitudes and mean channel currents had a reversal potential of approximately -15 mV as predicted by the Nernst equation for a channel perfectly selective for Cl-. Readdition of Ca2+ immediately inactivated the channel and restored the former membrane potential or clamp current. The inward currents were mediated by a Ca2+ inactivated Cl- channel (CaIC). The inhibitory potency of Ca2+ was a function of the external Ca2+ concentration with a half maximal blocker concentration of about 20 microM. These channels were inhibited by the Cl- channel blockers flufenamic acid, niflumic acid and diphenylamine-2-carboxylate (DPC). In contrast, 4,4'-acetamido-4'-isothiocyanatostilbene-2, 2'-disulfonicacid (SITS), another Cl- channel blocker, led to activation of this Cl- channel. Like other Cl- channels, the CaIC was activated by cytosolic cAMP. Extracellular ATP inhibited the channel while ADP was without any effect. Injection of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activating phorbol ester, stimulated the Cl- current. Cytochalasin D, an actin filament disrupting compound, reversibly decreased the clamp current demonstrating an influence of the cytoskeleton. The results indicate that removal of divalent cations activates Cl- channels in Xenopus oocytes which share several features with Cl- channels of the CLC family. The former so-called leak current of oocytes under divalent cation-free conditions is nothing else than an activation of Cl- channels.