Relation between intracellular Ca2+ signals and Ca(2+)-activated Cl- current in Xenopus oocytes

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Ginseng saponins induce store-operated calcium entry in Xenopus oocytes

1 We investigated the effect of the active ingredients of Panax ginseng, ginsenosides, on store-operated Ca2+ entry (SOCE) using a two-electrode voltage clamp technique in Xenopus oocytes in which SOCE is monitored through Ca(2+)-activated Cl- currents. 2 Under hyperpolarizing voltage clamp conditions, treatment with ginsenosides produced a biphasic Ca(2+)-activated Cl- current consisting of a rapid transient inward current and a slowly developing secondary sustained inward current. The transient inward current was inactivated rapidly, whereas the sustained inward current persisted for nearly 10 min. The effect of ginsenosides on the biphasic current was dose-dependent and reversible. The EC50 was 42.8+/-11.6 and 46.6+/-7.1 microg ml(-1) for the transient and sustained inward current, respectively. 3 In the absence of extracellular Ca2+ ginsenosides induced only a transient inward current but in the presence of extracellular Ca2+ ginsenosides induced the biphasic current. Magnitudes of the sustained currents were dependent on extracellular Ca2+ concentration. Sustained inward current induced by ginsenosides, but not transient inward current, and ginsenoside-induced store-operated Ca2+ (SOC) currents (ISOC) were blocked by La3+, a Ca2+ channel blocker, suggesting that the sustained inward current and ISOC was derived from an influx of extracellular Ca2+. 4 Treatment with 2-APB and heparin, which are IP3 receptor antagonists, inhibited the ginsenoside-induced biphasic current. Treatment with the PLC inhibitor, U73122, also inhibited the ginsenoside-induced biphasic current. Intraoocyte injection of ATP-gammaS, but not adenylyl AMP-PCP, induced a persistent activation of ginsenoside-induced sustained current but did not affect the transient current. 5 In rat hippocampal neurons, ginsenosides inhibited both carbachol-stimulated intracellular Ca2+ release and intracellular Ca2+ depletion-activated SOCE. 6 These results indicate that ginsenoside might act as a differential regulator of intracellular Ca2+ levels in neurons and Xenopus oocytes.

A novel activation of Ca(2+)-activated Cl(-) channel in Xenopus oocytes by Ginseng saponins: evidence for the involvement of phospholipase C and intracellular Ca(2+) mobilization

1. The signal transduction mechanism of ginsenosides, the active ingredients of ginseng, was studied in Xenopus oocytes using two-electrode voltage-clamp technique. Ginseng total saponin (GTS), i.e., an unfractionated mixture of ginsenosides produced a large outward current at membrane potentials more positive than -20 mV when it was applied to the exterior of oocytes, but not when injected intracellularly. The effect of GTS was concentration-dependent (EC(50): 4.4 microg ml(-1)) and reversible. 2. Certain fractionated ginsenosides (Rb(1), Rb(2), Rc, Rf, Rg(2) and Ro) also produced an outward current in a concentration-dependent manner with the order of potency of Rf>Ro>Rb(1)=Rb(2)>Rg(2)>Rc. Other ginsenosides (Rd, Re and Rg(1)) had little or no effect. 3. The GTS effect was completely blocked by bath application of the Ca(2+)-activated Cl(-) channel blocker niflumic acid and by intracellular injection of the calcium chelator BAPTA or the IP(3) receptor antagonist heparin. Also, the effect was partially blocked by bath-applied U-73122, a phospholipase C (PLC) inhibitor and by intracellularly injected GTP gamma S, a non-hydrolyzable GTP analogue. Whereas, it was not altered by pertussin toxin (PTX) pretreatment. 4. These results indicate that: (1) interaction of ginsenosides with membrane component(s) at the extracellular side leads to Ca(2+)-activated Cl(-) channel opening in Xenopus oocyte membrane; and (2) this process involves PLC activation, the release of Ca(2+) from the IP(3)-sensitive intracellular store and PTX-insensitive G protein activation.

Intracellular Ca2+ and Cl- channel activation in secretory cells

Molecular and functional evidence indicates that a variety of Ca(2+)-dependent chloride (Cl(Ca)) channels are involved in fluid secretion from secretory epithelial cells in different tissues and species. Most Cl(Ca) channels so far characterized have an I- permeability greater than Cl-, and most are sensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Whole-cell Cl(Ca) currents show outward rectification. Single-channel current voltage relationships are linear with conductances ranging from 2 to 30 pS. Some Cl(Ca) channels are blocked by Ca(2+)-calmodulin-dependent protein kinase (CAMKII) inhibitors. Others, such as the Cl(Ca) channels of parotid and submandibular acinar cells, appear to be directly regulated by Ca2+. In native cells, the Cl(Ca) channels are located on the apical plasma membrane and activated by localized mechanisms of Ca2+ release. This positioning allows the Cl(Ca) channel to respond specifically to localized Ca2+ signals that do not invade other regions of the cell. The Cl(Ca) follows the rising phase of the Ca2+ signal, but in the falling phase hysteresis occurs where the Cl(Ca) current decays more rapidly than the underlying Ca2+. The future elucidation of the identity and mechanisms of regulation of Cl(Ca) channels will be critical to our understanding of stimulus-secretion coupling.

Ca(2+)-dependent activation of Cl(-) currents in Xenopus oocytes is modulated by voltage

Ca(2+)-activated Cl(-) currents (I(Cl,Ca)) were examined using fluorescence confocal microscopy to monitor intracellular Ca(2+) liberation evoked by flash photolysis of caged inositol 1,4, 5-trisphosphate (InsP(3)) in voltage-clamped Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on InsP(3) concentration ([InsP(3)]), whereas currents at -140 mV exhibited a higher threshold and more graded relationship with [InsP(3)]. Ca(2+) levels required to half-maximally activate I(Cl,Ca) were about 50% larger at -140 mV than at +40 mV, and currents evoked by small Ca(2+) elevations were reduced >25-fold. The half-decay time of Ca(2+) signals shortened at increasingly positive potentials, whereas the decay of I(Cl,Ca) lengthened. The steady-state current-voltage (I-V) relationship for I(Cl,Ca) exhibited outward rectification with weak photolysis flashes but became more linear with stronger stimuli. Instantaneous I-V relationships were linear with both strong and weak stimuli. Current relaxations following voltage steps during activation of I(Cl,Ca) decayed with half-times that shortened from about 100 ms at +10 mV to 20 ms at -160 mV. We conclude that InsP(3)-mediated Ca(2+) liberation activates a single population of Cl(-) channels, which exhibit voltage-dependent Ca(2+) activation and voltage-independent instantaneous conductance.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

The role of Ca2+ feedback in shaping InsP3-evoked Ca2+ signals in mouse pancreatic acinar cells

1. Cytosolic Ca2+ has been proposed to act as both a positive and a negative feedback signal on the inositol trisphosphate (InsP3) receptor. However, it is unclear how this might affect the Ca2+ response in vivo. 2. Mouse pancreatic acinar cells were whole-cell patch clamped to record the Ca2+-dependent chloride (Cl(Ca)) current spikes and imaged to record the cytosolic Ca2+ spikes elicited by the injection of Ins(2,4,5)P3. Increasing concentrations of Ca2+ buffer (up to 200 microM EGTA or BAPTA) were associated with the appearance of steps in the current activation phase and a prevalence of smaller-amplitude Cl(Ca) spikes. Imaging experiments showed that with increased buffer the secretory pole cytosolic Ca2+ signal became fragmented and spatially discrete Ca2+ release events were observed. 3. At higher buffer concentrations (200-500 microM), increasing concentrations of EGTA increased spike frequency and reduced spike amplitude. In contrast, BAPTA decreased spike frequency and maintained large spike amplitudes. 4. We conclude that, during InsP3-evoked spiking, long-range Ca2+ feedback ( approximately 2-4 microm) shapes the rising phase of the Ca2+ signal by acting to co-ordinate discrete Ca2+ release events and short-range ( approximately 40 nm) Ca2+ feedback acts to inhibit further Ca2+ release.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Adenophostin A and inositol 1,4,5-trisphosphate differentially activate Cl- currents in Xenopus oocytes because of disparate Ca2+ release kinetics

Depletion of endoplasmic reticulum Ca2+ stores induces Ca2+ entry from the extracellular space by a process termed "store-operated Ca2+ entry" (SOCE). It has been suggested that the novel fungal metabolite adenophostin-A may be able to stimulate Ca2+ entry without stimulating Ca2+ release from stores. To test this idea further, we compared Ca2+ release, SOCE, and the stimulation of Ca2+-activated Cl- currents in Xenopus oocytes in response to inositol 1,4,5-trisphosphate (IP3) and adenophostin-A injection. IP3 stimulated an outward Cl- current, ICl1-S, in response to Ca2+ release from stores followed by an inward current, ICl2, in response to SOCE. In contrast, low concentrations of adenophostins (AdAs) activated ICl2 without activating ICl1-S, consistent with the suggestion that AdA can activate Ca2+ entry without stimulating Ca2+ release. However, when Ca2+ entry has been stimulated by AdA, Ca2+ stores are largely depleted of Ca2+, as assessed by the inability of ionomycin to release additional Ca2+. The Ca2+ release stimulated by AdA, however, was 7 times slower than the release stimulated by IP3, which could explain the minimal activation of ICl1-S; when Ca2+ is released slowly, the threshold level required for ICl1-S activation is not attained.

Reversible Ca gradients between the subplasmalemma and cytosol differentially activate Ca-dependent Cl currents

Xenopus oocytes express several different Ca-activated Cl currents that have different waveforms and biophysical properties. We compared the stimulation of Ca-activated Cl currents measured by two-microelectrode voltage clamp with the Ca transients measured in the same cell by confocal microscopy and Ca-sensitive fluorophores. The purpose was to determine how the amplitude and/or spatio-temporal features of the Ca signal might explain how these different Cl currents were activated by Ca. Because Ca release from stores was voltage independent, whereas Ca influx depended upon the electrochemical driving force, we were able to separately assess the contribution of Ca from these two sources. We were surprised to find that Ca signals measured with a cytosolic Ca-sensitive dye, dextran-conjugated Ca-green-1, correlated poorly with Cl currents. This suggested that Cl channels located at the plasma membrane and the Ca-sensitive dye located in the bulk cytosol were sensing different [Ca]. This was true despite Ca measurement in a confocal slice very close to the plasma membrane. In contrast, a membrane-targeted Ca-sensitive dye (Ca-green-C18) reported a Ca signal that correlated much more closely with the Cl currents. We hypothesize that very local, transient, reversible Ca gradients develop between the subplasmalemmal space and the bulk cytosol. [Ca] is higher near the plasma membrane when Ca is provided by Ca influx, whereas the gradient is reversed when Ca is released from stores, because Ca efflux across the plasma membrane is faster than diffusion of Ca from the bulk cytosol to the subplasmalemmal space. Because dissipation of the gradients is accelerated by inhibition of Ca sequestration into the endoplasmic reticulum with thapsigargin, we conclude that [Ca] in the bulk cytosol declines slowly partly due to futile recycling of Ca through the endoplasmic reticulum.

Dynamics of calcium regulation of chloride currents in Xenopus oocytes

Ca-activated Cl currents are widely expressed in many cell types and play diverse and important physiological roles. The Xenopus oocyte is a good model system for studying the regulation of these currents. We previously showed that inositol 1,4,5-trisphosphate (IP3) injection into Xenopus oocytes rapidly elicits a noninactivating outward Cl current (ICl1-S) followed several minutes later by the development of slow inward (ICl2) and transient outward (ICl1-T) Cl currents. In this paper, we investigate whether these three currents are mediated by the same or different Cl channels. Outward Cl currents were more sensitive to Ca than inward Cl currents, as shown by injection of different amounts of Ca or by Ca influx through a heterologously expressed ligand-gated Ca channel, the ionotropic glutamate receptor iGluR3. These data could be explained by two channels with different Ca affinities or one channel with a higher Ca affinity at depolarized potentials. To distinguish between these possibilities, we determined the anion selectivity of the three currents. The anion selectivity sequences for the three currents were the same (I > Br > Cl), but ICl1-S had an I-to-Cl permeability ratio more than twofold smaller than the other two currents. The different anion selectivities and instantaneous current-voltage relationships were consistent with at least two different channels mediating these currents. However, after consideration of possible errors, the hypothesis that a single type of Cl channel underlies the complex waveforms of the three different macroscopic Ca-activated Cl currents in Xenopus oocytes remains a viable alternative.

Hemispheric asymmetry of macroscopic and elementary calcium signals mediated by InsP3 in Xenopus oocytes

1. The mechanisms underlying hemispheric asymmetry of the inositol 1, 4,5-trisphosphate (InsP3)-calcium signalling pathway in Xenopus oocytes were examined by fluorescence imaging of calcium signals and recording calcium-activated Cl- currents (ICl,Ca) evoked by intracellular calcium injections and photorelease of InsP3. 2. The maximal ICl,Ca evoked by strong photorelease of InsP3 was 8 times greater in the animal than the vegetal hemisphere, but the average threshold amounts of InsP3 required to evoke detectable currents were similar in each hemisphere. 3. Currents evoked by injections of calcium were about 2.5 times greater near the animal pole than near the vegetal pole, whereas fluorescence signals evoked by injections were similar in each hemisphere. 4. Calcium waves were evoked by photolysis flashes of similar strengths in both hemispheres of albino oocytes, but peak calcium levels evoked by supramaximal stimuli were 70 % greater in the animal hemisphere. 5. Elementary calcium release events (puffs) in the animal hemisphere had amplitudes about double that in the vegetal hemisphere, and more often involved coupled release from adjacent sites. Calcium release sites were more closely packed in the animal hemisphere, with a mean spacing of about 1.5 micro m compared with 2.25 micro m in the vegetal hemisphere. 6. The larger amplitude of currents mediated by InsP3 in the animal hemisphere, therefore, involves an increased flux of calcium at individual release units, a more dense packing of release units and a higher density of Cl- channels.

cDNA cloning and functional characterization of the mouse Ca2+-gated K+ channel, mIK1. Roles in regulatory volume decrease and erythroid differentiation

We have cloned from murine erythroleukemia (MEL) cells, thymus, and stomach the cDNA encoding the Ca2+-gated K+ (KCa) channel, mIK1, the mouse homolog of hIK1 (Ishii, T. M., Silvia, C., Hirschberg, B., Bond, C. T., Adelman, J. P., and Maylie, J. (1997) Proc. Natl. Acad. Sci.(U. S. A. 94, 11651-11656). mIK1 mRNA was detected at varied levels in many tissue types. mIK1 KCa channel activity expressed in Xenopus oocytes closely resembled the Kca of red cells (Gardos channel) and MEL cells in its single channel conductance, lack of voltage-sensitivity of activation, inward rectification, and Ca2+ concentration dependence. mIK1 also resembled the erythroid channel in its pharmacological properties, mediating whole cell and unitary currents sensitive to low nM concentrations of both clotrimazole (CLT) and its des-imidazolyl metabolite, 2-chlorophenyl-bisphenyl-methanol, and to low nM concentrations of iodocharybdotoxin. Whereas control oocytes subjected to hypotonic swelling remained swollen, mIK1 expression conferred on oocytes a novel, Ca2+-dependent, CLT-sensitive regulatory volume decrease response. Hypotonic swelling of voltage-clamped mIK1-expressing oocytes increased outward currents that were Ca2+-dependent, CLT-sensitive, and reversed near the K+ equilibrium potential. mIK1 mRNA levels in ES cells increased steadily during erythroid differentiation in culture, in contrast to other KCa mRNAs examined. Low nanomolar concentrations of CLT inhibited proliferation and erythroid differentiation of peripheral blood stem cells in liquid culture.

Expression of the mammalian calcium signaling response to Trypanosoma cruzi in Xenopus laevis oocytes

Infective stages of the protozoan parasite Trypanosoma cruzi contain a soluble factor that induces elevation in the intracellular free Ca2+ concentration ([Ca2+]i) of mammalian cells. The process is pertussis toxin (PTx)-sensitive, and involves phospholipase C (PLC) activation, inositol 1,4,5-trisphosphate (IP3) formation and Ca2+ release from intracellular stores (Tardieux I, et al. J Exp Med 1994;179:1017-1022; Rodriguez A, et al. J Cell Biol 1995;129:1263-1273). We now report that a molecule exposed on the surface of the target cells is required to trigger the signaling cascade, and that a response with identical characteristics can be induced in Xenopus laevis oocytes injected with mRNA from normal rat kidney (NRK) fibroblasts. Xenopus oocytes do not show an endogenous response to the trypomastigote Ca2+ signaling factor, but a vigorous response in the form of a propagating Ca2+ wave is expressed after injection of NRK cell mRNA. As previously demonstrated for mammalian cells, the response is inhibited when injected oocytes are pretreated with PTx, implicating Galphai or Galphao trimeric G-proteins, and with thapsigargin, which depletes intracellular Ca2+ stores. Moreover, the [Ca2+]i transients triggered by the T. cruzi soluble factor in mRNA-injected oocytes are blocked by the same inhibitors of the parasite oligopeptidase B that abolish the [Ca2+]i response in NRK cells (Burleigh B, Andrews NW. J Biol Chem 1995;270:5172-5180; Burleigh BA et al. J Cell Biol 1997;136:609-620). The NRK mRNA fraction that induces expression of the [Ca2+]i response to the T. cruzi signaling factor contains messages from 1.5 to 2.0 kb, a size range consistent with the family of seven-transmembrane G-protein-coupled receptors.

Asymmetrical distribution of Ca-activated Cl channels in Xenopus oocytes

Xenopus oocytes are a popular model system for studying Ca signaling. They endogenously express two kinds of Ca-activated Cl currents, I(Cl-1), and I(Cl-2). I(Cl-1) is activated by Ca released from internal stores and, with appropriate voltage protocols, by Ca influx. In contrast, I(Cl-2) activation is dependent on Ca influx. We are interested in understanding how these two different Cl channels are activated differently by Ca from different sources. One could hypothesize that these channels are activated differently because they are differentially localized near the corresponding Ca source. As an initial investigation of this hypothesis, we examined the distribution of I(Cl-1) and I(Cl-2) channels in the oocyte. We conclude that both I(Cl-1) and (Cl-2) channels are primarily localized to the animal hemisphere of the oocyte, but that capacitative Ca influx occurs over the entire oocyte membrane. Evidence supporting this view includes the following observations: 1) Injection of IP3 into the animal hemisphere produced larger and faster I(Cl-1) responses than injection into the vegetal hemisphere. 2) Exposure of the animal hemisphere to Cl-free solution almost completely abolished I(Cl-1) produced by IP3-induced release of Ca from internal stores or by capacitative Ca entry. 3) Loose macropatch recording showed that both I(Cl-1) and I(Cl-2) currents were approximately four times and approximately three times, respectively, more dense in the animal than in the vegetal hemisphere. 4) Confocal imaging of oocytes loaded with fluorescent Ca-sensitive dyes showed that the time course of activation of I(Cl-1) corresponded to the appearance of the wave of Ca release at the animal pole. 5) Ca release and Ca influx, although twofold higher in the animal pole, were evident over the entire oocyte.

Calcium dependence and distribution of calcium-activated chloride channels in Xenopus oocytes

1. The Ca(2+)-dependent Cl- current (ICl,Ca), expressed in the plasma membrane of Xenopus oocytes, was examined in excised inside-out macropatches using a rapid perfusion system. 2. Application of Ca(2+)-containing Ringer solution resulted in the activation of a current whose reversal potential shifted to the right by 51 +/- 5.2 mV when Cl- in the pipette solution was lowered from 119.3 to 10 mM. No currents were generated when Ca2+ was omitted from the solution. The current is therefore a Ca(2+)-activated Cl- one. 3. Following exposure to Ca2+, the half-time for activation of ICl,Ca was not voltage dependent, whereas deactivation was strongly so. 4. ICl,Ca was stable in the continuous presence of Ca2+ and showed no sign of inactivation or adaptation. 5. Comparison of the size of the currents (normalized to pipette resistance) from the animal and vegetal poles revealed that ICl,Ca had a highly polarized distribution. The current density was almost 10 times higher in the animal pole. 6. The results suggest that Cl- channels provide a continuous and reliable indication of submembranous Ca2+, at least in an excised patch, and the clustering of the Cl- channels renders it necessary to exert caution in interpreting results involving the kinetics of Ca2+ signalling, when ICl,Ca is used as the sole monitor of calcium.

Calcium current activated by depletion of calcium stores in Xenopus oocytes

Ca(2+) currents activated by depletion of Ca(2+) stores in Xenopus oocytes were studied with a two-electrode voltage clamp. Buffering of cytosolic Ca(2+) with EGTA and MeBAPTA abolished I(Cl(Ca)) and unmasked a current in oocytes that was activated by InsP(3) or ionomycin in minutes and by thapsigargin or the chelators themselves over hours. At -60 mV in 10 mM extracellular CaCl(2), the current was typically around -90 or -160 nA in oocytes loaded with EGTA or MeBAPTA, respectively. This current was judged to be a Ca(2+)-selective current for the following reasons: (a) it was inwardly rectifying and reversed at membrane potentials usually more positive than +40 mV; (b) it was dependent on extracellular [CaCl(2)] with K(m) = 11.5 mM; (c) it was highly selective for Ca(2+) against monovalent cations Na(+) and K(+), because replacing Na(+) and K(+) by N-methyl-d-glucammonium did not reduce the amplitude or voltage dependence of the current significantly; and (d) Ca(2+), Sr(2+), and Ba(2+) currents had similar instantaneous conductances, but Sr(2+) and Ba(2+) currents appeared to inactivate more strongly than Ca(2+). This Ca(2+) current was blocked by metal ions with the following potency sequence: Mg(2+) << Ni(2+) approximately Co(2+) approximately Mn(2+) < Cd(2+) << Zn(2+) << La(3+). It was also inhibited by niflumic acid, which is commonly used to block I(Cl(Ca)). PMA partially inhibited the Ca(2+) current, and this effect was mostly abolished by calphostin C, indicating that the Ca(2+) current is sensitive to protein kinase C. These results are the first detailed electrophysiological characterization of depletion-activated Ca(2+) current in nondialyzed cells. Because exogenous molecules and channels are easy to introduce into oocytes and the distortions in measuring I(Cl(Ca)) can now be bypassed, oocytes are now a superior system in which to analyze the activation mechanisms of capacitative Ca(2+) influx.

Effects of adenophostin-A and inositol-1,4,5-trisphosphate on Cl- currents in Xenopus laevis oocytes

Adenophostin-A, a novel compound isolated from cultures of Penicillium brevicompactum, has been shown to stimulate Ca2+ release from inositol-1,4,5-trisphosphate (IP3)-sensitive Ca2+ stores in microsomal preparations, permeabilized cells, and lipid vesicles containing purified IP3 receptor. The purpose of the current study was to compare the effects of adenophostin-A and IP3 on Ca2+ release from stores and Ca2+ influx in intact Xenopus laevis oocytes. Ca2+ influx though store-operated Ca2+ channels and Ca2+ release from stores were monitored by measuring two Ca2+ -activated Cl- currents that can be used as real-time indicators of Ca2+ release and Ca2+ influx (I(Cl-1) and I(Cl-2), respectively). We find that high concentrations (final intraoocyte concentrations of 5-10 microM) of adenophostin-A and IP3 stimulate a large Ca2+ release from stores (as measured by I(Cl-1)) followed by Ca2+ influx (as measured by I(Cl-2)). Low concentrations (approximately 50 nM) of IP3 stimulate oscillations in Ca2+ release without stimulating Ca2+ influx. In contrast, low concentrations of adenophostin-A can stimulate Ca2+ influx without stimulating a large Ca2+ release. However, Ca2+ influx did not occur in the complete absence of Ca2+ release. Therefore, it is unlikely that adenophostin-A directly stimulates store-operated Ca2+ channels. We hypothesize that adenophostin-A releases Ca2+ from a subpopulation of stores that is tightly coupled to store-operated Ca2+ channels.

Characterization of soluble forms of nonchimeric type V adenylyl cyclases

Type V adenylyl cyclase (ACV) belongs to the family of Ca2+-inhibited cyclases. We have generated two soluble forms of the enzyme containing the C1 or C1a region (which lacks the C-terminal 112 amino acids) linked to the C2 domain and compared their regulation with the full-length ACV. All three forms of ACV were stimulated by the alpha subunit of the stimulatory G protein Gs (G(s alpha)) and forskolin. However, the synergistic stimulation by both these activators was markedly enhanced in the soluble enzymes. Moreover, the alpha subunit of the inhibitory G protein Gi (G(i alpha)) inhibited all forms of the enzyme, indicating that the regions for G(s alpha) and G(i alpha) interaction are preserved in the soluble forms. Ca2+ inhibited forskolin-stimulated adenylyl cyclase (AC) activity of the full-length and C1-C2 forms of ACV but did not alter the activity of the C1a-C2 form. Maximal stimulation of AC activity by combination of G(s alpha) and forskolin obliterated the Ca2+-mediated inhibition of the full-length and C1-C2 forms of ACV. In 45Ca2+ overlay experiments, the C1-C2 but not the C1a-C2 soluble ACV bound Ca2+. Moreover, proteins corresponding to the C1a and C2 domains did not bind calcium. On the other hand, the proteins corresponding to C1 and its C-terminal 112 amino acids (C1b) bound 45Ca2+. To our knowledge, this is the first report of nonchimeric soluble forms of AC in which regulation by G(s alpha) and G(i alpha) is preserved. Moreover, we demonstrate that the 112 amino acid C1b region of ACV is responsible for the binding of Ca2+ and inhibition of enzyme activity.

Involvement of the Ca2+/calmodulin-dependent protein kinase II pathway in the Ca2+-mediated regulation of the capacitative Ca2+ entry in Xenopus oocytes

Activation of the phosphoinositide transduction pathway induces capacitative Ca2+ entry in Xenopus oocytes. This can also be evoked by intracellular injection of Ins(1,4.5)P3, external application of thapsigargin and/or incubation in a Ca2+-free medium. Readmission of Ca2+ to voltage-clamped, thapsigargin-treated Xenopus oocytes triggers Ca2+-dependent Cl- current variations that reflect capacitative Ca2+ entry. Inhibition of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by specific peptides markedly increased the amplitude of the transients, suggesting an involvement of the CaMKII pathway in the regulation of capacitative Ca2+ entry. Biochemical studies provide evidence for the activation of CaMKII in response to the development of capacitative Ca2+ entry. In effect, a CaMKII assay in vivo allows us to postulate that readmission of Ca2+ to thapsigargin-treated oocytes can induce a burst of CaMKII activity. Finally, analysis of the Cl- transient kinetics at high resolution of time suggests that CaMKII inhibition blocks the onset of the inactivation process without affecting the activation rate. We therefore postulate that CaMKII might participate in a negative feedback regulation of store-depletion-evoked Ca2+ entry in Xenopus oocytes.

Micromolar and submicromolar Ca2+ spikes regulating distinct cellular functions in pancreatic acinar cells

Agonists induce Ca2+ spikes, waves and oscillations initiating at a trigger zone in exocrine acinar cells via Ca2+ release from intracellular Ca2+ stores. Using a low affinity ratiometric Ca2+ indicator dye, benzothiazole coumarin (BTC), we found that high concentrations of agonists transiently increased Ca2+ concentrations to the micromolar range (>10 microM) in the trigger zone. Comparison with results obtained with a high affinity Ca2+ indicator dye, fura-2, indicated that fura-2 was in fact saturated with Ca2+ during the agonist-induced Ca2+ spikes in the trigger zone. We further revealed that the micromolar Ca2+ spikes were necessary for inducing exocytosis of zymogen granules investigated using capacitance measurements. In contrast, submicromolar Ca2+ spikes selectively gave rise to sequential activation of luminal and basal ion channels. These results suggest new functional diversity in Ca2+ spikes and a critical role for the micromolar Ca2+ spikes in exocytotic secretion from exocrine acinar cells. Our data also emphasize the value of investigating the Ca2+ signalling using low affinity Ca2+ indicators.

Desensitization of inositol 1,4,5-trisphosphate/Ca2+-induced Cl- currents by prolonged activation of G proteins in Xenopus oocytes

Expression of G protein alpha subunits of the Gq family with various G protein-coupled receptors induces activation of an inositol 1,4, 5-trisphosphate (IP3)/Ca2+-mediated Cl- conductance in Xenopus oocytes. Our present data show that two members of this family, the human Galpha16 subunit and the murine homologue Galpha15, can induce both activation and inhibition of these agonist-induced currents. Although extremely low amounts (10-50 pg) of injected Galpha16 subunit cRNA cause modest ( approximately 2-fold) enhancement of ligand-induced Cl- currents in oocytes co-injected with thyrotropin-releasing hormone (TRH) receptor cRNA 48 h postinjection, larger Galpha16 and Galpha15 cRNA injections cause >10-fold inhibition of TRH or 5HT2c receptor responses. The inhibition is analyzed in this study. The inhibited currents are recovered if various Gbetagamma subunit combinations are also expressed with the Galpha subunits. The constitutively active mutant, Galpha16Q212L, also causes a strong attenuation of the ligand-induced Cl- currents, but this inhibition is not recovered by co-expression of Gbetagamma subunits. These results indicate that the free Galpha subunit is responsible for the inhibitory signal. Although expression of TRH receptor alone produces maximum responses approximately 48 h after injection, co-expression of TRH receptor with Galpha16 results in enhanced responses 6-12 h postinjection, followed by complete attenuation at 36 h. Furthermore, injection of Galpha16 cRNA alone at comparable levels gives rise to spontaneous Cl- currents within 6-12 h postinjection, suggesting that the early spontaneous activation underlies the later suppression. Expression of other G protein alpha subunits of the Gq family, at cRNA levels considerably higher than effective for Galpha16, produces both analogous spontaneous Cl- currents and, later, inhibition of ligand-induced Cl- currents. Experiments with direct injection of IP3 and of Ca2+ suggest that this inhibition is consistent with the down-regulation of IP3 receptors. These data indicate that both enhancement and inhibition of signaling through G protein-coupled receptors can be mediated by the expression level and/or activity of an individual G protein.

Regulation of inositol trisphosphate-induced membrane currents in Xenopus oocytes by a Jurkat cell calcium influx factor

The functional interactions of a Jurkat cell-derived calcium influx factor (CIF) with Ins(1,4,5)P3 were examined by microinjection and voltage-clamp recording of current responses in Xenopus oocytes. CIF, which stimulates Ca2+ entry directly on microinjection, was active at dilutions at which it had no direct effect by augmenting both initial rapid Ins(1,4,5)P3-mediated Ca2+ discharge-activated currents and later sustained Ca2+ entry-activated currents. Augmented initial membrane currents were 3-5-fold greater in peak amplitude than currents evoked by injection of the same dose of Ins(1,4,5)P3 alone. The augmented initial response was not decreased by removal of extracellular Ca2+, suggesting that there is potentiation of Ins(1,4,5)P3-mediated discharge from intracellular Ca2+ stores. However, the augmentation of Ins(1,4,5)P3-mediated discharge cannot be due to an enhanced production of endogenous Ins(1,4,5)P3 because maximal Ins(1,4,5)P3-activated currents saturate (approx. 500 nA) with supramaximal levels of Ins(1,4,5)P3 (10-50 microM). Depletion of Ca2+ stores, by pretreatment with thapsigargin or by prior injection with the Ins(1,4,5)P3 receptor antagonist heparin, abolished membrane currents elicited by Ins(1,4,5)P3/CIF co-injection, further suggesting that the Ins(1,4,5)P3 receptor was the target for the initial-current potentiating actions of CIF. In this regard, CIF also induced augmented initial currents with co-injection of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. The augmentation of Ins(1,4,5)P3-mediated currents by CIF was bell-shaped with regard to Ins(1,4,5)P3 concentration, reminiscent of the regulatory influence of Ca2+ on Ins(1,4,5)P3 responses. Co-injection of Ins(1,4,5)P3 and CIF also augmented (2-3-fold) later current responses arising from sustained Ca2+ entry. The augmented late-current responses were not due to enhanced Ca2+ store depletion because supramaximal levels of Ins(1,4,5)P3 (50 microM) or injection of the poorly metabolized Ins(1,4,5)P3 analogue, Ins(2,4,5)P3, cannot activate the same magnitude of Ca(2+)-entry-dependent currents. These results suggest that CIF at low levels interacts with Ins(1,4,5)P3 to sensitize two pathways of Ca2+ signalling: initial discharge and later Ca2+ entry. Thus under physiological conditions CIF might be more potent as a co-messenger than as a direct Ca2+ entry signal and might provide a novel type of direct feedback regulation between the stores-activated influx pathway and the Ins(1,4,5)P3 receptor. Moreover these results suggest that CIF modulation of the receptor for Ins(1,4,5)P3 may underlie control of both augmentation of discharge and Ca2+ entry, as has been predicted from the conformational coupling model of Ca2+ entry.

Elementary events of InsP3-induced Ca2+ liberation in Xenopus oocytes: hot spots, puffs and blips

Liberation of sequestered Ca2+ ions in Xenopus oocytes by the second messenger inositol 1,4,5-trisphosphate (InP3) occurs from functionally discrete sites, which are spaced at intervals of several microns and probably represent clusterings of InsP3 receptor/channels (InsP3R) in the endoplasmic reticulum. As well as requiring InsP3, opening of release channels is regulated by dual positive and negative feedback by cytosolic Ca2+, leading to regenerative Ca2+ transients. Because the sensitivity of this process is determined by [InsP3], the ability of Ca2+ ions diffusing from one location to activate increasingly distant InsP3R is enhanced by increasing [InsP3]. Together with the spatial distribution of receptors, this results in generation of a hierarchy of Ca2+ release events, which may involve individual InsP3R (Ca2+ 'blips'), concerted activation of several receptors within a single release site (Ca2+ 'puffs'), and recruitment of successive sites by Ca2+ diffusing over micron distances to produce propagating Ca2+ waves. Thus, Ca2+ signalling in the oocyte is organized as at least two sizes of elemental 'building blocks'; highly localized Ca2+ transients that arise autonomously and stochastically from discrete sites at low [InsP3], but which become coordinated at higher [InsP3] to produce global Ca2+ responses.

Interaction between capacitative Ca2+ influx and Ca2+-dependent Cl- currents in Xenopus oocytes

The relationship between capacitative Ca2+ influx and activation of Ca2+-dependent Cl- channels was monitored in intact Xenopus oocytes following stimulation of 5-hydroxytryptamine (5-HT) receptors, through the activity of Ca2+-dependent Cl- channels using the double-electrode voltage-clamp technique. Under voltage-clamp conditions, 5-HT evoked a rapid transient inward current followed by a slowly developing secondary inward current. The secondary current reflected depletion-activated Ca2+ entry. Hyperpolarising pulses evoked sustained Ca2+-dependent Cl- currents when applied during the transient inward current, but evoked hump-like currents which inactivated rapidly when applied during the secondary inward current. Hump currents arose from Ca2+ entering through the depletion-activated pathway. The hump currents inactivated with hyperpolarising pulses at < 5-s intervals, and recovered monoexponentially with a time constant of around 8 s. Currents in response to hyperpolarising pulses during the transient current did not inactivate, suggesting that inactivation was associated with Ca2+ entry. When ca2+ release evoked by inositol 1,4,5-triphosphate [ins(1,4,5)p3] was prevented by heparin injection, hyperpolarising pulses during ca2+ ionophore application also generated hump currents that were dependent on external ca2+, inactivated and recovered from inactivation with a similar time course as the humps following 5-ht treatment. Pretreatment with the Ca2+ adenosine 5'-triphosphatase (Ca2+ATPase) inhibitor thapsigargin reduced the rate of rise of the hump current, increased the time-to-peak of the current and slowed the rate of decay. Pharmacological interventions to disrupt the cytoskeleton reduced the amplitude of the hump current. It is suggested that, following hyperpolarisation in the presence of Ca2+ entry, the ensuing Ca2+ influx interacts with Cl- channels in a way that might reflect both Ca2+ inhibition of Ca2+ entry and clustering of Cl- channels in the plasma membrane.

Contribution of response kinetics to the response pattern: studies of responses to thyrotropin-releasing hormone in Xenopus oocytes

In Xenopus oocytes injected with total rat pituitary GH3 cell RNA, thyrotropin-releasing hormone (TRH) causes the activation of the inositol lipid transduction pathway and the induction of chloride conductance via calcium-activated channels (Oron et al., 1987, Mol. Endocrinol., 1:918-925). This response exhibits characteristic prolonged latency (Oron et al., 1988, Proc. Natl. Acad. Sci. U.S.A., 85:3820-3824; Lipinsky et al., 1993, Pflugers Arch., 425:140-149). We examined the role of agonist diffusion in the extracellular medium in the generation of latency and the determination of response amplitude. An increase in the viscosity of the medium markedly prolonged the latency and decreased the amplitude of the response. Moreover, an increase in the viscosity of the medium in the immediate vicinity of the oocyte had a major effect on both the latency and the amplitude of the response, which appeared to be a result of desensitization rather than restricted diffusion of chloride to the medium. Extrapolation to [TRH] infinity yielded a diffusion-dependent latency value of 0 and a diffusion-independent latency value of 4 seconds. In low viscosity medium, at all TRH concentrations, diffusion contributed less than 2% to the latency of the response. This implied that events distal to ligand binding are responsible for a major part of latency. Analysis of the dependence of latency and amplitude of the response on [TRH] yielded Hill coefficients markedly smaller than unity, suggesting postreceptor negative modulation of the response. Preincubation of cells with a specific inhibitor of protein kinase C, chelerythrine, increased the Hill coefficients to unity and changed the shape of the Hill plot of response amplitudes. Our results suggest that at low agonist concentrations, even in a low viscosity medium, the prolonged latency allows negative effects on both latency and amplitude by a simultaneous activation of a protein kinase C.

Desensitization of the response to thyrotropin-releasing hormone in Xenopus oocytes is an amplified process that precedes calcium mobilization

Consecutive challenges with thyrotropin-releasing hormone (TRH) of oocytes expressing the TRH receptor (TRH-R) resulted in a pronounced desensitization, manifested as a decrease in chloride current amplitude and an increase in response latency. Exposure to low concentrations of TRH resulted in a marked decrease in the amplitude of the subsequent response to a higher concentration of the agonist, even though the second challenge was given before the onset of the response to the first challenge (within 3 - 15 s). Cellular calcium concentration ([Ca]i) did not increase within this interval, suggesting that calcium was not involved in the desensitization process. The latency of the second response, however, was either unchanged or shortened, implying additive effects of processes initiated by the first challenge. A longer interval (30 s) between the two challenges brought about a more pronounced decrease in amplitude and a prolongation of response latency. The calcium mobilization initiated by a second challenge with a high concentration of the agonist exhibited a longer latency, a lower rate of [Ca]i increase and a lower amplitude. Stimulation of co-expressed cholinergic-muscarinic ml receptors with a low concentration of acetylcholine resulted in a pronounced desensitization of the TRH response (heterologous desensitization). Activation of protein kinase C by beta-phorbol 12-myristate, 13-acetate resulted in a dose-dependent inhibition of the response to TRH, suggesting that protein kinase C was involved in desensitization. Chelerythrine, a specific inhibitor of protein kinase C, abolished a large part of the desensitization. A mutant of the TRH-R that lacks protein kinase C consensus phosphorylation sites in the C-terminal region, exhibited desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Autosomal dominant hypocalcaemia caused by a Ca(2+)-sensing receptor gene mutation

Defects in the human Ca(2+)-sensing receptor gene have recently been shown to cause familial hypocalciuric hypercalcaemia and neonatal severe hyperparathyroidism. We now demonstrate that a missense mutation (Glu128Ala) in this gene causes familial hypocalcaemia in affected members of one family. Xenopus oocytes expressing the mutant receptor exhibit a larger increase in inositol 1,4,5-triphosphate in response to Ca2+ than oocytes expressing the wild-type receptor. We conclude that this extracellular domain mutation increases the receptor's activity at low Ca2+ concentrations, causing hypocalcaemia in patients heterozygous for such a mutation.

The initiation of a calcium signal in Xenopus oocytes

Application of acetylcholine to Xenopus oocytes evoked increases in the cytosolic free calcium ion concentration ([Ca2+]i) after latencies of up to several seconds depending on the agonist dose. Higher acetylcholine concentrations evoked responses with larger amplitudes and shorter latencies. The latencies of responses to acetylcholine could be increased by application of caffeine, injection of calcium buffers or depletion of intracellular calcium stores. Acute inhibition of endoplasmic reticulum calcium pumps without substantial reduction of the calcium store content (by application of thapsigargin shortly before agonist stimulation) reduced the latencies of responses to acetylcholine. A schematic and mathematical model are presented to show a possible mechanism by which a calcium signal is initiated following a latent period after the elevation of the inositol trisphosphate concentration. During the latent period, calcium is slowly released from the intracellular stores. The released calcium is rapidly buffered by cytosolic calcium-binding proteins and some is resequestered into the stores by calcium pumps. The [Ca2+]i changes very little until the buffering is locally saturated. The [Ca2+]i then rises above a threshold concentration which evokes an explosive release of calcium due to positive feedback by calcium on the inositol trisphosphate receptor.