Inositol 1,4,5-trisphosphate receptors in Xenopus laevis oocytes: localization and modulation by Ca2+

IP3 receptors: An "elementary" journey from structure to signals

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are large tetrameric channels which sit mostly in the membrane of the endoplasmic reticulum (ER) and mediate Ca2+ release from intracellular stores in response to extracellular stimuli in almost all cells. Dual regulation of IP3Rs by IP3 and Ca2+ itself, upstream "licensing", and the arrangement of IP3Rs into small clusters in the ER membrane, allow IP3Rs to generate spatially and temporally diverse Ca2+ signals. The characteristic biphasic regulation of IP3Rs by cytosolic Ca2+ concentration underpins regenerative Ca2+ signals by Ca2+-induced Ca2+-release, while also preventing uncontrolled explosive Ca2+ release. In this way, cells can harness a simple ion such as Ca2+ as a near-universal intracellular messenger to regulate diverse cellular functions, including those with conflicting outcomes such as cell survival and cell death. High-resolution structures of the IP3R bound to IP3 and Ca2+ in different combinations have together started to unravel the workings of this giant channel. Here we discuss, in the context of recently published structures, how the tight regulation of IP3Rs and their cellular geography lead to generation of "elementary" local Ca2+ signals known as Ca2+ "puffs", which form the fundamental bottleneck through which all IP3-mediated cytosolic Ca2+ signals must first pass.

Hindered cytoplasmic diffusion of inositol trisphosphate restricts its cellular range of action

The range of action of intracellular messengers is determined by their rates of diffusion and degradation. Previous measurements in oocyte cytoplasmic extracts indicated that the Ca²⁺-liberating second messenger inositol trisphosphate (IP₃) diffuses with a coefficient (~280 μm² s^-1) similar to that in water, corresponding to a range of action of ~25 μm. Consequently, IP₃ is generally considered a "global" cellular messenger. We reexamined this issue by measuring local IP₃-evoked Ca²⁺ puffs to monitor IP₃ diffusing from spot photorelease in neuroblastoma cells. Fitting these data by numerical simulations yielded a diffusion coefficient (≤10 μm² s^-1) about 30-fold slower than that previously reported. We propose that diffusion of IP₃ in mammalian cells is hindered by binding to immobile, functionally inactive receptors that were diluted in oocyte extracts. The predicted range of action of IP₃ (<5 μm) is thus smaller than the size of typical mammalian cells, indicating that IP₃ should better be considered as a local rather than a global cellular messenger.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Mean field strategies induce unrealistic non-linearities in calcium puffs

Mean field models are often useful approximations to biological systems, but sometimes, they can yield misleading results. In this work, we compare mean field approaches with stochastic models of intracellular calcium release. In particular, we concentrate on calcium signals generated by the concerted opening of several clustered channels (calcium puffs). To this end we simulate calcium puffs numerically and then try to reproduce features of the resulting calcium distribution using mean field models were all the channels open and close simultaneously. We show that an unrealistic non-linear relationship between the current and the number of open channels is needed to reproduce the simulated puffs. Furthermore, a single channel current which is five times smaller than the one of the stochastic simulations is also needed. Our study sheds light on the importance of the stochastic kinetics of the calcium release channel activity to estimate the release fluxes.

Stochastic fire-diffuse-fire model with realistic cluster dynamics

Living organisms use waves that propagate through excitable media to transport information. Ca2+ waves are a paradigmatic example of this type of processes. A large hierarchy of Ca2+ signals that range from localized release events to global waves has been observed in Xenopus laevis oocytes. In these cells, Ca2+ release occurs trough inositol 1,4,5-trisphosphate receptors (IP3Rs) which are organized in clusters of channels located on the membrane of the endoplasmic reticulum. In this article we construct a stochastic model for a cluster of IP3R 's that replicates the experimental observations reported in [D. Fraiman, Biophys. J. 90, 3897 (2006)]. We then couple this phenomenological cluster model with a reaction-diffusion equation, so as to have a discrete stochastic model for calcium dynamics. The model we propose describes the transition regimes between isolated release and steadily propagating waves as the IP3 concentration is increased.

Simplified model of cytosolic Ca2+ dynamics in the presence of one or several clusters of Ca2+ -release channels

Calcium release from intracellular stores plays a key role in the regulation of a variety of cellular activities. In various cell types this release occurs through inositol-triphosphate (IP3) receptors which are Ca2+ channels whose open probability is modulated by the cytosolic Ca2+ concentration itself. Thus, the combination of Ca2+ release and Ca2+ diffusion evokes a variety of Ca2+ signals depending on the number and relative location of the channels that participate of them. In fact, a hierarchy of Ca2+ signals has been observed in Xenopus laevis oocytes, ranging from very localized events (puffs and blips) to waves that propagate throughout the cell. In this cell type channels are organized in clusters. The behavior of individual channels within a cluster cannot be resolved with current optical techniques. Therefore, a combination of experiments and mathematical modeling is unavoidable to understand these signals. However, the numerical simulation of a detailed mathematical model of the problem is very hard given the large range of spatial and temporal scales that must be covered. In this paper we present an alternative model in which the cluster region is modeled using a relatively fine grid but where several approximations are made to compute the cytosolic Ca2+ concentration ([Ca;{2+}]) distribution. The inner-cluster [Ca;{2+}] distribution is used to determine the openings and closings of the channels of the cluster. The spatiotemporal [Ca;{2+}] distribution outside the cluster is determined using a coarser grid in which each (active) cluster is represented by a point source whose current is proportional to the number of open channels determined before. A full reaction-diffusion system is solved on this coarser grid.

Ca2+ release dynamics in parotid and pancreatic exocrine acinar cells evoked by spatially limited flash photolysis

Intracellular calcium concentration ([Ca(2+)](i)) signals are central to the mechanisms underlying fluid and protein secretion in pancreatic and parotid acinar cells. Calcium release was studied in natively buffered cells following focal laser photolysis of caged molecules. Focal photolysis of caged-inositol 1,4,5 trisphosphate (InsP(3)) in the apical region resulted in Ca(2+) release from the apical trigger zone and, after a latent period, the initiation of an apical-to-basal Ca(2+) wave. The latency was longer and the wave speed significantly slower in pancreatic compared with parotid cells. Focal photolysis in basal regions evoked only limited Ca(2+) release at the photolysis site and never resulted in a propagating wave. Instead, an apical-to-basal wave was initiated following a latent period. Again, the latent period was significantly longer under all conditions in pancreas than parotid. Although slower in pancreas than parotid, once initiated, the apical-to-basal wave speed was constant in a particular cell type. Photo release of caged-Ca(2+) failed to evoke a propagating Ca(2+) wave in either cell type. However, the kinetics of the Ca(2+) signal evoked following photolysis of caged-InsP(3) were significantly dampened by ryanodine in parotid but not pancreas, indicating a more prominent functional role for ryanodine receptor (RyR) following InsP(3) receptor (InsP(3)R) activation. These data suggest that differing expression levels of InsP(3)R, RyR, and possibly cellular buffering capacity may contribute to the fast kinetics of Ca(2+) signals in parotid compared with pancreas. These properties may represent a specialization of the cell type to effectively stimulate Ca(2+)-dependent effectors important for the differing primary physiological role of each gland.

Calcium puffs in Xenopus oocytes

The second messenger inositol 1,4,5-trisphosphate (InsP3) functions in large part by liberating calcium ions from intracellular stores. This release process is highly non-linear and shows a regenerative characteristic that allows production of all-or-none calcium spikes which propagate as waves. However, at low concentrations of InsP3 an additional mode of calcium liberation is seen in Xenopus oocytes, transient 'puffs' of cytosolic calcium that last for a few hundred milliseconds and are restricted to within a few micrometres. Puffs are generally of similar size and the amount of calcium released (about 3 x 10(-18) mol) suggests that they arise through the concerted opening of several InsP3-gated calcium release channels. Puff sites are present at a density of about one per 30 microns 2 in the animal hemisphere of the oocyte. Each site functions autonomously, producing puffs at largely random intervals. We conclude that calcium puffs represent 'quantal' units of InsP3-evoked calcium liberation, which may result from local regenerative feedback by cytosolic calcium ions at functionally discrete release sites.

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca(2+)/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca(2+) signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca(2+) concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + beta-hydroxybutyrate (beta-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca(2+)](i). The rise of [Ca(2+)](i) was characterized by a transient spike followed by a small sustained plateau of elevated [Ca(2+)](i). In the absence of extracellular Ca(2+) 2-DG caused an increase in [Ca(2+)](i), suggesting that inhibition of glycolysis directly triggered release of Ca(2+) from intracellular endoplasmic reticulum (ER) Ca(2+) stores. The inositol-1,4,5-trisphosphate receptor (IP(3)R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca(2+) response. Ca(2+) release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca(2+)](i) wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca(2+)](i). Propagating [Ca(2+)](i) waves were preceded by [Ca(2+)](i) oscillations and small, highly localized elevations of [Ca(2+)](i) (Ca(2+) puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca(2+) response, and vice versa during inhibition of CaMKII 2-DG-induced Ca(2+) release was attenuated. Similar results were obtained with pyr + beta-HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg(2+) concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca(2+) release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP(3)R.

'Trigger' events precede calcium puffs in Xenopus oocytes

The liberation of calcium ions sequestered in the endoplasmic reticulum through inositol 1,4,5-trisphosphate receptors/channels (IP(3)Rs) results in a spatiotemporal hierarchy of calcium signaling events that range from single-channel openings to local Ca(2+) puffs believed to arise from several to tens of clustered IP(3)Rs to global calcium waves. Using high-resolution confocal linescan imaging and a sensitive Ca(2+) indicator dye (fluo-4-dextran), we show that puffs are often preceded by small, transient Ca(2+) elevations that we christen "trigger events". The magnitude of triggers is consistent with their arising from the opening of a single IP(3) receptor/channel, and we propose that they initiate puffs by recruiting neighboring IP(3)Rs within the cluster by a regenerative process of Ca(2+)-induced Ca(2+) release. Puff amplitudes (fluorescence ratio change) are on average approximately 6 times greater than that of the triggers, suggesting that at least six IP(3)Rs may simultaneously be open during a puff. Trigger events have average durations of approximately 12 ms, as compared to 19 ms for the mean rise time of puffs, and their spatial extent is approximately 3 times smaller than puffs (respective widths at half peak amplitude 0.6 and 1.6 micro m). All these parameters were relatively independent of IP(3) concentration, although the proportion of puffs showing resolved triggers was greatest (approximately 80%) at low [IP(3)]. Because Ca(2+) puffs constitute the building blocks from which cellular IP(3)-mediated Ca(2+) signals are constructed, the events that initiate them are likely to be of fundamental importance for cell signaling. Moreover, the trigger events provide a useful yardstick by which to derive information regarding the number and spatial arrangement of IP(3)Rs within clusters.

The number and spatial distribution of IP3 receptors underlying calcium puffs in Xenopus oocytes

Calcium puffs are local Ca(2+) release events that arise from a cluster of inositol 1,4,5-trisphosphate receptor channels (IP(3)Rs) and serve as a basic "building block" from which global Ca(2+) waves are generated. Important questions remain as to the number of IP(3)Rs that open during a puff, their spatial distribution within a cluster, and how much Ca(2+) current flows through each channel. The recent discovery of "trigger" events-small Ca(2+) signals that immediately precede puffs and are interpreted to arise through opening of single IP(3)R channels-now provides a useful yardstick by which to calibrate the Ca(2+) flux underlying puffs. Here, we describe a deterministic numerical model to simulate puffs and trigger events. Based on confocal linescan imaging in Xenopus oocytes, we simulated Ca(2+) release in two sequential stages; representing the trigger by the opening of a single IP(3)R in the center of a cluster for 12 ms, followed by the concerted opening of some number of IP(3)Rs for 19 ms, representing the rising phase of the puff. The diffusion of Ca(2+) and Ca(2+)-bound indicator dye were modeled in a three-dimensional cytosolic volume in the presence of immobile and mobile Ca(2+) buffers, and were used to predict the observed fluorescence signal after blurring by the microscope point-spread function. Optimal correspondence with experimental measurements of puff spatial width and puff/trigger amplitude ratio was obtained assuming that puffs arise from the synchronous opening of 25-35 IP(3)Rs, each carrying a Ca(2+) current of approximately 0.4 pA, with the channels distributed through a cluster 300-800 nm in diameter.

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.

Cell cycle-dependent regulation of structure of endoplasmic reticulum and inositol 1,4,5-trisphosphate-induced Ca2+ release in mouse oocytes and embryos

The organization of endoplasmic reticulum (ER) was examined in mouse eggs undergoing fertilization and in embryos during the first cell cycle. The ER in meiosis II (MII)-arrested mouse eggs is characterized by accumulations (clusters) that are restricted to the cortex of the vegetal hemisphere of the egg. Monitoring ER structure with DiI18 after egg activation has demonstrated that ER clusters disappear at the completion of meiosis II. The ER clusters can be maintained by inhibiting the decrease in cdk1-cyclin B activity by using the proteasome inhibitor MG132, or by microinjecting excess cyclin B. A role for cdk1-cyclin B in ER organization is further suggested by the finding that the cdk inhibitor roscovitine causes the loss of ER clusters in MII eggs. Cortical clusters are specific to meiosis as they do not return in the first mitotic division; rather, the ER aggregates around the mitotic spindle. Inositol 1,4,5-trisphosphate-induced Ca(2+) release is also regulated in a cell cycle-dependent manner where it is increased in MII and in the first mitosis. The cell cycle dependent effects on ER structure and inositol 1,4,5-trisphosphate-induced Ca(2+) release have implications for understanding meiotic and mitotic control of ER structure and inheritance, and of the mechanisms regulating mitotic Ca(2+) signaling.

Functional screening of intracellular proteins in single cells and in patterned cell arrays using electroporation

A tool for detection and characterization of intracellular enzyme-substrate and receptor-ligand interactions inside the cytoplasm of single targeted cells or small confined groups of cells is presented. Fluorogenic enzyme substrates and receptor ligands were rapidly delivered by electroosmosis and internalized by electroporation in cells using an electrolyte-filled capillary (EFC) biased at a high voltage. Specifically, alkaline phosphatase and proteases were detected in single NG108-15 cells using fluorescein diphosphate and casein BODIPY FL, respectively. The intracellular 1,4,5-inositol triphosphate (IP3) and ryanodine receptors were detected after EFC introduction of the selective receptor agonists IP3 and cyclic adenosine diphosphate ribose (cADPr), respectively. Receptor activation in both cases resulted in increased cytosolic concentrations of free calcium ions that were measured using the calcium-ion-selective probe, fluo-3. The effect of cADPr could be blocked by coadministration of the ryanodine receptor antagonist ruthenium red. Furthermore, electroporation of a plurality of cells grown in microwell structures (100 x 100 x 45 microm) molded in PDMS is demonstrated. The methods and systems described using an EFC for electroporation and delivery of protein markers, ligands, and substrates might be useful in high-throughput screening of intracellular targets, with applications in proteomics and phenotype profiling, as well as in drug discovery.

Xenopus tropicalis oocytes as an advantageous model system for the study of intracellular Ca(2+) signalling

1. The purpose of this study was to compare oocytes from the pipid frogs Xenopus tropicalis and Xenopus laevis, with respect to their utility for studying Ca(2+) signalling mechanisms and for expression of heterologous proteins. 2. We show that X. tropicalis oocytes possess an intracellular Ca(2+) store that is mobilized by inositol (1,4,5) trisphosphate (IP(3)). Ca(2+) signalling is activated by endogenous lysophosphatidic acid receptors and cytosolic Ca(2+) activates a plasma membrane chloride conductance. The spatiotemporal organization of cytosolic Ca(2+) signals, from the microscopic architecture of elementary Ca(2+) 'puffs' to the macroscopic patterns of Ca(2+) spiking are closely similar to the local and global patterns of Ca(2+) release previously characterized in oocytes from X. laevis. 3. By injecting X. tropicalis oocytes with cDNA encoding an ER-targeted fluorescent protein construct, we demonstrate the capacity of the X. tropicalis oocyte to readily express heterologous proteins. The organization of ER is polarized across the oocyte, with the IP(3)-releaseable store targeted within an approximately 8 microm wide band that circumscribes the cell. 4. We conclude that the X. tropicalis oocyte shares many of the characteristics that have made oocytes of X. laevis a favoured system for studying Ca(2+) signalling mechanisms. Moreover, X. tropicalis oocytes display further practical advantages in terms of imaging depth, Ca(2+) signal magnitude and electrical properties. These further enhance the appeal of X. tropicalis as an experimental system, in addition to its greater amenability to transgenic approaches.

Progesterone treatment abolishes exogenously expressed ionic currents in Xenopus oocytes

Fully grown oocytes of Xenopus laevis undergo resumption of the meiotic cycle when treated with the steroid hormone progesterone. Previous studies have shown that meiotic maturation results in profound downregulation of specific endogenous membrane proteins in oocytes. To determine whether the maturation impacts the functional properties of exogenously expressed membrane proteins, we used cut-open recordings from Xenopus oocytes expressing several types of Na(+) and K(+) channels. Treatment of oocytes with progesterone resulted in a downregulation of heterologously expressed Na(+) and K(+) channels without a change in the kinetics of the currents. The time course of progesterone-induced ion channel inhibition was concentration dependent. Complete elimination of Na(+) currents temporally coincided with development of germinal vesicle breakdown, while elimination of K(+) currents was delayed by approximately 2 h. Coexpression of human beta(1)-subunit with rat skeletal muscle alpha-subunit in Xenopus oocytes did not prevent progesterone-induced downregulation of Na(+) channels. Addition of 8-bromo-cAMP to oocytes or injection of heparin before progesterone treatment prevented the loss of expressed currents. Pharmacological studies suggest that the inhibitory effects of progesterone on expressed Na(+) and K(+) channels occur downstream of the activation of cdc2 kinase. The loss of channels is correlated with a reduction in Na(+) channel immunofluorescence, pointing to a disappearance of the ion channel-forming proteins from the surface membrane.

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.

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.

Radial localization of inositol 1,4,5-trisphosphate-sensitive Ca2+ release sites in Xenopus oocytes resolved by axial confocal linescan imaging

The radial localization and properties of elementary calcium release events ("puffs") were studied in Xenopus oocytes using a confocal microscope equipped with a piezoelectric focussing unit to allow rapid (>100 Hz) imaging of calcium signals along a radial line into the cell with a spatial resolution of <0.7 micrometer. Weak photorelease of caged inositol 1,4,5-trisphosphate (InsP3) evoked puffs arising predominantly within a 6-micrometer thick band located within a few micrometers of the cell surface. Approximately 25% of puffs had a restricted radial spread, consistent with calcium release from a single site. Most puffs, however, exhibited a greater radial spread (3.25 micrometer), likely involving recruitment of radially neighboring release sites. Calcium waves evoked by just suprathreshold stimuli exhibited radial calcium distributions consistent with inward diffusion of calcium liberated at puff sites, whereas stronger flashes evoked strong, short-latency signals at depths inward from puff sites, indicating deep InsP3-sensitive stores activated at higher concentrations of InsP3. Immunolocalization of InsP3 receptors showed punctate staining throughout a region corresponding to the localization of puffs and subplasmalemmal endoplasmic reticulum. The radial organization of puff sites a few micrometers inward from the plasma membrane may have important consequences for activation of calcium-dependent ion channels and "capacitative" calcium influx. However, on the macroscopic (hundreds of micrometers) scale of global calcium waves, release can be considered to occur primarily within a thin, essentially two-dimensional subplasmalemmal shell.

A steep dependence of inward-rectifying potassium channels on cytosolic free calcium concentration increase evoked by hyperpolarization in guard cells

Inactivation of inward-rectifying K+ channels (IK,in) by a rise in cytosolic free [Ca2+] ([Ca2+]i) is a key event leading to solute loss from guard cells and stomatal closure. However, [Ca2+]i action on IK,in has never been quantified, nor are its origins well understood. We used membrane voltage to manipulate [Ca2+]i (A. Grabov and M.R. Blatt [1998] Proc Natl Acad Sci USA 95: 4778-4783) while recording IK,in under a voltage clamp and [Ca2+]i by Fura-2 fluorescence ratiophotometry. IK,in inactivation correlated positively with [Ca2+]i and indicated a Ki of 329 +/- 31 nM with cooperative binding of four Ca2+ ions per channel. IK,in was promoted by the Ca2+ channel antagonists Gd3+ and calcicludine, both of which suppressed the [Ca2+]i rise, but the [Ca2+]i rise was unaffected by the K+ channel blocker Cs+. We also found that ryanodine, an antagonist of intracellular Ca2+ channels that mediate Ca2+-induced Ca2+ release, blocked the [Ca2+]i rise, and Mn2+ quenching of Fura-2 fluorescence showed that membrane hyperpolarization triggered divalent release from intracellular stores. These and additional results point to a high signal gain in [Ca2+]i control of IK,in and to roles for discrete Ca2+ flux pathways in feedback control of the K+ channels by membrane voltage.

Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

The three subtypes of inositol trisphosphate (InsP3) receptor expressed in mammalian cells are each capable of forming intracellular Ca2+ channels that are regulated by both InsP3 and cytosolic Ca2+. The InsP3 receptors of many, though perhaps not all, tissues are biphasically regulated by cytosolic Ca2+: a rapid stimulation of the receptors by modest increases in Ca2+ concentration is followed by a slower inhibition at higher Ca2+ concentrations. Despite the widespread occurrence of this form of regulation and the belief that it is an important element of the mechanisms responsible for the complex Ca2+ signals evoked by physiological stimuli, the underlying mechanisms are not understood. Both accessory proteins and Ca2+-binding sites on InsP3 receptors themselves have been proposed to mediate the effects of cytosolic Ca2+ on InsP3 receptor function, but the evidence is equivocal. The effects of cytosolic Ca2+ on InsP3 binding and channel opening, and the possible means whereby the effects are mediated are discussed in this review.

The physiologic concentration of inositol 1,4,5-trisphosphate in the oocytes of Xenopus laevis

To measure the concentration of inositol 1,4,5-trisphosphate ([IP3]) in small regions of single Xenopus oocytes, a biological detector cell was combined with capillary electrophoresis. This method is 10, 000 times more sensitive than all existing assays enabling subcellular measurement of [IP3] in Xenopus oocytes. Upon addition of lysophosphatidic acid to an oocyte, [IP3] increased from 40 to 650 nM within 2 min. IP3 concentrations as high as 1.8 microM were measured after activation with lysophosphatidic acid, suggesting that the physiologic concentration of IP3 ranges from the tens of nanomolar to a few micromolar in Xenopus oocytes. Since the IP3 receptor in Xenopus oocytes is nearly identical to the type I receptor of mammalian cells, the range of [IP3] in most mammalian cells is likely to be similar to that in the oocyte. By selecting or engineering the appropriate detector cell, this strategy should be applicable to cyclic adenosine diphosphate ribose and nicotinic acid adenine dinucleotide phosphate, and to the discovery of new Ca2+-releasing second messengers.

Nuclear calcium release by InsP3-receptor channels plays a role in meiosis reinitiation in the mouse oocyte

Our purpose was to investigate the presence of nuclear specific elements of the phosphoinositide pathway, and the link between nuclear calcium events and the first step of meiosis resumption, i.e. germinal vesicle breakdown (GVB) in mouse immature oocytes. Using confocal laser scanning microscopy, we analyzed the effects of nuclear microinjection of inositol 1,4,5-trisphosphate (InsP3), heparin and anti-InsP3 receptor monoclonal antibodies on both spontaneous nuclear and cytoplasmic calcium oscillations, as well as the effects of these components on the GVB. First we observed that nuclear Ca2+ events were dependent upon both nucleoplasmic and cytoplasmic InsP3 levels, highlighting a cross-talk between the GV and the cytoplasm concerning the Ca2+/InsP3 pathway. We demonstrated also that: 1) type 1 InsP3 receptors were localized at the nuclear membrane level while type 3 were absent from the nucleus; 2) calcium release from nuclear stores was mediated by type 1 rather than type 3 InsP3 receptor associated channels; 3) the anti-InsP3 R-1 mAB microinjected into the nucleus inhibited the GVB. These results demonstrate that reinitiation of meiosis requires an increase in nuclear phosphoinositide dependent Ca2+. Thus, the role of nuclear Ca2+ homeostasis is discussed with particular emphasis on nuclear envelope dynamics.

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.

A novel role for calmodulin: Ca2+-independent inhibition of type-1 inositol trisphosphate receptors

Calmodulin inhibits both inositol 1,4,5-trisphosphate (IP3) binding to, and IP3-evoked Ca2+ release by, cerebellar IP3 receptors [Patel, Morris, Adkins, O'Beirne and Taylor (1997) Proc. Natl. Acad. Sci. U. S.A. 94, 11627-11632]. In the present study, full-length rat type-1 and -3 IP3 receptors were expressed at high levels in insect Spodoptera frugiperda 9 cells and the effects of calmodulin were examined. In the absence of Ca2+, calmodulin caused a concentration-dependent and reversible inhibition of [3H]IP3 binding to type-1 IP3 receptors by decreasing their apparent affinity for IP3. The effect was not reproduced by high concentrations of troponin C, parvalbumin or S-100. Increasing the medium free [Ca2+] ([Ca2+]m) inhibited [3H]IP3 binding to type-1 receptors, but the further inhibition caused by a submaximal concentration of calmodulin was similar at each [Ca2+]m. In the absence of Ca2+, 125I-calmodulin bound to a single site on each type-1 receptor subunit and to an additional site in the presence of Ca2+. There was no detectable binding of 125I-calmodulin to type-3 receptors and binding of [3H]IP3 was insensitive to calmodulin at all [Ca2+]m. Both peptide and conventional Ca2+-calmodulin antagonists affected neither [3H]IP3 binding directly nor the inhibitory effect of calmodulin in the absence of Ca2+, but each caused a [Ca2+]m-dependent reversal of the inhibition of [3H]IP3 binding caused by calmodulin. Camstatin, a peptide that binds to calmodulin equally well in the presence or absence of Ca2+, reversed the inhibitory effects of calmodulin on [3H]IP3 binding at all [Ca2+]m. We conclude that calmodulin specifically inhibits [3H]IP3 binding to type-1 IP3 receptors: the first example of a protein regulated by calmodulin in an entirely Ca2+-independent manner. Inhibition of type-1 IP3 receptors by calmodulin may dynamically regulate their sensitivity to IP3 in response to the changes in cytosolic free calmodulin concentration thought to accompany stimulation of neurones.

Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor

The effects of Mg2+ and Ba2+ on single-channel properties of the inositol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel kinetics and presence of conductance substates were similar over a range (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I-V) relation was linear with conductance of approximately 320 pS. Conductance varied slowly and continuously over a wide range (SD approximately 60 pS) and sometimes fluctuated during single openings. The presence of Mg2+ on either or both sides of the channel reduced the current (blocking constant approximately 0.6 mM in symmetrical Mg2+), as well as the range of conductances observed, and made the I-V relation nonlinear (slope conductance approximately 120 pS near 0 mV and approximately 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effects on channel conductance. Mg2+ and Ba2+ permeated the channel with a ratio of permeability of Ba2+ to Mg2+ to K+ of 3.5:2.6:1. These results indicate that divalent cations induce nonlinearity in the I-V relation and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furthermore, stabilization of conductance by divalent cations reveals a novel interaction between the cations and the IP3R.

Single-channel properties of inositol (1,4,5)-trisphosphate receptor heterologously expressed in HEK-293 cells

The inositol (1,4,5)-trisphosphate receptor (InsP3R) mediates Ca2+ release from intracellular stores in response to generation of second messenger InsP3. InsP3R was biochemically purified and cloned, and functional properties of native InsP3-gated Ca2+ channels were extensively studied. However, further studies of InsP3R are obstructed by the lack of a convenient functional assay of expressed InsP3R activity. To establish a functional assay of recombinant InsP3R activity, transient heterologous expression of neuronal rat InsP3R cDNA (InsP3R-I, SI- SII+ splice variant) in HEK-293 cells was combined with the planar lipid bilayer reconstitution experiments. Recombinant InsP3R retained specific InsP3 binding properties (Kd = 60 nM InsP3) and were specifically recognized by anti-InsP3R-I rabbit polyclonal antibody. Density of expressed InsP3R-I was at least 20-fold above endogenous InsP3R background and only 2-3-fold lower than InsP3R density in rat cerebellar microsomes. When incorporated into planar lipid bilayers, the recombinant InsP3R formed a functional InsP3-gated Ca2+ channel with 80 pS conductance using 50 mM Ba2+ as a current carrier. Mean open time of recombinant InsP3-gated channels was 3.0 ms; closed dwell time distribution was double exponential and characterized by short (18 ms) and long (130 ms) time constants. Overall, gating and conductance properties of recombinant neuronal rat InsP3R-I were very similar to properties of native rat cerebellar InsP3R recorded in identical experimental conditions. Recombinant InsP3R also retained bell-shaped dependence on cytosolic Ca2+ concentration and allosteric modulation by ATP, similar to native cerebellar InsP3R. The following conclusions are drawn from these results. (a) Rat neuronal InsP3R-I cDNA encodes a protein that is either sufficient to produce InsP3-gated channel with functional properties identical to the properties of native rat cerebellar InsP3R, or it is able to form a functional InsP3-gated channel by forming a complex with proteins endogenously expressed in HEK-293 cells. (b) Successful functional expression of InsP3R in a heterologous expression system provides an opportunity for future detailed structure-function characterization of this vital protein.

A continuum of InsP3-mediated elementary Ca2+ signalling events in Xenopus oocytes

1. The elementary release events underlying inositol 1,4, 5-trisphosphate (InsP3)-mediated calcium signalling were investigated in Xenopus oocytes by means of high-resolution confocal linescan imaging together with flash photolysis of caged InsP3. 2. Weak photolysis flashes evoked localized, transient calcium signals that arose at specific sites following random latencies of up to several seconds. The duration, spatial spread and amplitude of these elementary events varied widely. Event durations (at half-maximal amplitude) were distributed exponentially between about 100 and 600 ms. Fluorescence magnitudes (F/F0 of Oregon Green 488 BAPTA-1) showed a skewed distribution with a peak at about 1.5 and a tail extending as high as 3.5. 3. Individual release sites exhibited both small events (blips) and large events (puffs). The spatiotemporal distribution of calcium signals during puffs was consistent with calcium diffusion from a point source (< a few hundred nanometres), rather than with propagation of a microscopic calcium wave. 4. Estimates of the calcium flux associated with individual events were made by integrating fluorescence profiles along the scan line in three dimensions to derive the 'signal mass' at each time point. The smallest resolved events corresponded to liberation of < 2 x 10-20 mol Ca2+, and large events to about 2 x 10-18 mol Ca2+. The rise of signal mass was more prolonged than that of the fluorescence intensity, suggesting that calcium liberation persists even while the fluorescence begins to decline. Rates of rise of signal mass corresponded to Ca2+ currents of 0.4-2.5 pA. 5. Measurements of signal mass from different events showed a continuous, exponential distribution, arising through variability in magnitude and duration of calcium flux. 6. We conclude that localized calcium transients in the oocyte represent a continuum of events involving widely varying amounts of calcium liberation, rather than falling into separate populations of 'fundamental' and 'elementary' events (blips and puffs) involving, respectively, single and multiple InsP3 receptor channels. This variability probably arises through stochastic variation in both the number of channels recruited and the duration of channel opening.

Ca2+ wave dispersion and spiral wave entrainment in Xenopus laevis oocytes overexpressing Ca2+ ATPases

Complex, spatiotemporal patterns of intracellular Ca2+ release in Xenopus oocytes can be accounted for by the operation of two fundamental processes: Ca2+ release from the endoplasmic reticulum (ER) via the inositol 1,4,5-trisphosphate receptor (IP3R) with its inherent dependency on cytosolic Ca2+, and Ca2+ uptake via Ca2+ ATPases. Overexpression of sarco-endoplasmic reticulum Ca2+ ATPases (SERCAs) in Xenopus oocytes, increases IP3-induced Ca2+ wave frequency and amplitude [1-3]. This effect can be attributed to an increased removal of cytoplasmic Ca2+ and more efficient refilling of Ca2+ stores. By overexpressing SERCA isoforms, we report here that Ca2+ waves exhibit dispersion [4-6]. At wavelengths greater than 60 microns, wave velocity is constant. However, wave velocity and amplitude progressively decreases at smaller wavelengths. Below gamma approximately 20 microns, Ca2+ waves disperse and fail to propagate. In oocytes exhibiting both spiral and target patterns of Ca2+ release, spiral waves had higher frequencies and showed entrainment of the surrounding regions. These properties are characteristic of a classical excitable medium [4-6].

Multiple mechanisms of regulation of the inositol 1,4,5-trisphosphate receptor by calcium

Ca2+ mobilisation by inositol 1,4,5-trisphosphate (InsP3) is a complex phenomenon which involves positive and negative feedback regulation by cytosolic Ca2+. It has been shown that Ca2+ increased the affinity of [3H]-InsP3 binding to liver membranes and inhibited [3H]-InsP3 binding to cerebellar membranes. We investigated the effects of Ca2+ on the [3H]-InsP3 binding to receptor solubilised and rapidly purified by immunoprecipitation. The InsP3 binding to the purified liver receptor was insensitive to the addition of Ca2+, indicating that Ca2+ did not interact directly with the receptor. The loss of the Ca2+ effect on liver receptor affinity was reproduced by alkaline treatment of liver membranes, which is known to extract the peripheral membrane proteins. This suggests that Ca2+ regulates the liver InsP3 receptor by interacting with a membrane-associated protein. Ca2+ inhibited the binding of [3H]-InsP3 to purified cerebellar receptors as was found with the membrane fraction. The treatment of the purified cerebellar receptor with media of high ionic strength or at alkaline pH did not abolish the effect of Ca2+ on the receptor. This indicates that the inhibitory effect of Ca2+ on [3H]-InsP3 binding to cerebellar membranes occurs either via direct interaction with the receptor or via an integral protein strongly associated with the receptor. In conclusion, the mechanisms of regulation of InsP3-induced Ca2+ release by Ca2+ involve different molecular support in cerebellum and in liver. This may reflect different regulation dependent on the receptor type.

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.

The effects of free [Ca2+] on the cytosolic face of the inositol (1,4,5)-trisphosphate receptor at the single channel level

Cytosolic free Ca2+ has been shown to have both activating and inhibitory effects upon the inositol (1,4,5) trisphosphate receptor (InsP3R) during intracellular Ca2+ release. The effects of cytosolic free Ca2+ on the InsP3R have already been monitored using cerebellar microsomes (containing InsP3R) incorporated into planar lipid bilayers [Bezprozvanny, Watras and Ehrlich (1991) Nature (London) 351, 751-754]. In these experiments the open probability of the channel exhibited a 'bell-shaped Ca2+ dependence'. However, this has only been seen when the receptor is in the presence of its native membrane (e.g. microsomal vesicles). Using solubilized, purified InsP3R incorporated into planar lipid bilayers using the 'tip-dip' technique, investigations were carried out to see if the same effect was seen in the absence of the native membrane. Channel activity was observed in the presence of 4 microM InsP3 and 200 nM free Ca2+. Mean single channel current was 2.69 pA and more than one population of lifetimes was observed. Two populations had mean open times of approx. 9 and 97 ms. Upon increasing the free [Ca2+] to 2 microM, the mean single channel current decreased slightly to 2.39 pA, and the lifetimes increased to 30 and 230 ms. Elevation of free -Ca2+- to 4 microM resulted in a further decrease in mean single channel current to 1.97 pA as well as a decrease in lifetime to approx. 8 and 194 ms. At 10 microM free [Ca2+] no channel activity was observed. Thus, with purified receptor in artificial bilayers, free [Ca2+] on the cytosolic face of the receptor has major effects on channel behaviour, particularly on channel closure, although inhibition of channel activity is not seen until very high free [Ca2+] is reached.

Hypocalcemia modifies the intracellular calcium response to the alpha 1-adrenergic agent phenylephrine in rat hepatocytes

In vivo, extracellular calcium ([Ca2+]e) homeostasis is maintained within a very narrow range by the calcium regulating hormones. At the cellular level, the response to many agents is transduced by changes in cytosolic Ca2+ ([Ca2+]i) which involves both mobilization of cellular pools and entry of [Ca2+]e through plasma membrane channels. To investigate the cellular effects of chronic hypocalcemia (Ca-) on [Ca2+]i homeostasis, hepatocytes, a cell type well characterized for its [Ca2+]i response, were used. Data indicate that Ca- leads to a significant shift to the left in the basal resting cytosolic Ca2+ concentration distribution curve with half-maximum cumulative frequency of 119 versus 149 nM in Ca- and normal rats (N) respectively (P < 0.0001). The response to the alpha 1-adrenergic agonist phenylephrine (Phe) was also influenced by Ca- with a dampening of the dose-response curve, a significant decrease in the frequency of sustained responses (P < 0.001), and significant changes in the oscillation pattern. Indeed, hepatocytes obtained from Ca- exhibited a higher frequency of large amplitude, low frequency oscillations than N most particularly at the 2 and 5 microM Phe dose while N predominantly exhibited low amplitude, high frequency oscillations on sustained plateaus (P < 0.001). IP3 receptor (IP3R) binding studies and Ca2+ mobilization from IP3-sensitive pools showed that IP3R was highly sensitive to the prevailing Ca2+ with, in the range of resting [Ca2+]i, R affinity significantly lower in Ca- than in N. Upon exposure of permeabilized cells to 25 microM IP3, Ca2+ mobilization from the IP3-sensitive intracellular pool was significantly reduced by Ca- (P < 0.05) suggesting a decrease in the IP3-mobilizable Ca2+ pool in Ca-. Our results indicate that hypocalcemia significantly alters [Ca2+]i signalling by perturbing the initial response to agonist and the [Ca2+]i response pattern. In addition, the decrease in Ca2+ mobilization from IP3-sensitive pools suggests that hypocalcemia may also lead to a decrease in the Ca2+ content of intracellular pools.

Inhibition of hepatic inositol 1,4,5-trisphosphate receptor function by ethanol and other short chain alcohols

The ability of alcohols to regulate InsP3-receptor activity was examined in permeabilized hepatocytes. Incubation with 30-300 mM ethanol decreased the sensitivity to InsP3 for Ca2+ release, with little effect on the size of the Ca2+ store that could be released with maximal concentrations of InsP3. Ethanol (300 mM) increased the EC50 for InsP3 from a control value of 134.0 +/- 13.5 nM to 220.0 +/- 25.9 nM. Although ethanol also caused a partial depletion of the total pool of stored Ca2+, the ethanol-induced shift in InsP3 sensitivity was not secondary to this alteration in Ca2+ loading. Partial depletion of the Ca2+ stores with low doses of ionomycin and thapsigargin did not cause a shift in InsP3 sensitivity. Furthermore, measurements of InsP3 receptor channel activity using retrograde flux of Mn2+ to quench the fluorescence of Fura-2 within the Ca2+ stores demonstrated that ethanol inhibited InsP3-activated channel activity in the absence of stored Ca2+. Other short chain alcohols (methanol, 1-propanol and 1-butanol) also decreased the efficacy of InsP3 to release Ca2+. Measurements of [3H]-InsP3 binding demonstrated that ethanol decreased the total number of InsP3 binding sites without changing the KD. The effect of ethanol on InsP3 binding was apparent in the presence or absence of Ca2+ and was observed when the cells were pre-incubated with ethanol at either 37 degrees C or 4 degrees C. The initial rate of InsP3-induced Mn2+ quenching of compartmentalized Fura-2 was reduced by ethanol at all doses of InsP3. These data suggest that ethanol decreases the sensitivity of the intracellular Ca2+ store to release by InsP3, by reducing the number of channels that can be activated by InsP3.

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

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.

Effects of protein phosphorylation on the regulation of capacitative calcium influx in Xenopus oocytes

The regulation of capacitative Ca2+ influx in Xenopus oocytes was investigated using both the two electrode voltage-clamp (where Ca2+ is monitored through the Ca2+-dependent Cl- current) and patch-clamp techniques. Following stimulation of expressed 5-hydroxytryptamine (5-HT) receptors, capacitative Ca2+ influx deactivated in around 15 min. Following injection of [adenosine 5'-O-(3-Thiotriphosphate)] (ATP [gamma-S]), an ATP analogue that is readily used by protein kinases, capacitative Ca2+ influx activated by 5-HT application either did not deactivate or was prolonged around twofold. However, injection of adenylyl 5'-(beta,gamma-methylene)-diphosphonate (AMP-PCP), another ATP analogue that is not utilised by kinases, did not affect the time-course of Ca2+ influx. When capacitative Ca2+ influx was activated by readmission of Ca2+ to oocytes incubated in thapsigargin/0 Ca2+ solution for several hours, Ca2+ influx occurred and a weakly saturating relationship between external Ca2+ and Ca2+ influx was found. Ca2+ influx in thapsigargin-treated cells was unaffected by ATP [gamma-S]. ATP [gamma-s] and several kinases had no effect on the Ca2+-dependent Cl- current when the latter was activated by elevation of Ca2+ independent of capacitative Ca2+ influx. Protein kinase C slowly and partially inhibited the Cl- current. Outside-out patches taken from thapsigargin-treated cells failed to demonstrated any Ca2+ current or Ca2+-dependent Cl- current on reapplying high Ca2+ to the patch, despite the oocyte showing a large capacitative Ca2+ influx. The results suggest that a kinase, activated on receptor stimulation, prolongs the activation time-course of capacitative Ca2+ influx.

Fast kinetics of calcium liberation induced in Xenopus oocytes by photoreleased inositol trisphosphate

Inositol 1,4,5-trisphosphate (InsP3) acts on intracellular receptors to cause liberation of Ca2+ ions into the cytosol as repetitive spikes and propagating waves. We studied the processes underlying this regenerative release of Ca2+ by monitoring with high resolution the kinetics of Ca2+ flux evoked in Xenopus oocytes by flash photolysis of caged InsP3. Confocal microfluorimetry was used to monitor intracellular free [Ca2+] from femtoliter volumes within the cell, and the underlying Ca2+ flux was then derived from the rate of increase of the fluorescence signals. A threshold amount of InsP3 had to be photoreleased to evoke any appreciable Ca2+ signal, and the amount of liberated Ca2+ then increased only approximately fourfold with maximal stimulation, whereas the peak rate of increase of Ca2+ varied over a range of nearly 20-fold, reaching a maximum of approximately 150 microMs-1. Ca2+ flux increased as a first-order function of [InsP3]. Indicating a lack of cooperativity in channel opening, and was half-maximal with stimuli approximately 10 times threshold. After a brief photolysis flash, Ca2+ efflux began after a quiescent latent period that shortened from several hundred milliseconds with near-threshold stimuli to 25 ms with maximal flashes. This delay could not be explained by an initial "foot" of Ca2+ increasing toward a threshold at which regenerative release was triggered, and the onset of release seemed too abrupt to be accounted for by multiple sequential steps involved in channel opening. Ca2+ efflux increased to a maximum after the latent period in a time that reduced from > 100 ms to approximately 8 ms with increasing [InsP3] and subsequently declined along a two-exponential time course: a rapid fall with a time constant shortening from > 100 ms to approximately 25 ms with increasing [InsP3], followed by a much smaller fail persisting for several seconds. The results are discussed in terms of a model in which InsP3 receptors must undergo a slow transition after binding InsP3 before they can be activated by cytosolic Ca2+ acting as a co-agonist. Positive feedback by liberated Ca2+ ions then leads to a rapid increase in efflux to a maximal rate set by the proportion of receptors binding InsP3. Subsequently, Ca2+ efflux terminates because of a slower inhibitory action of cytosolic Ca2+ on gating of InsP3 receptor-channels.

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Characterization of the co-agonist effects of strontium and calcium on myo-inositol trisphosphate-dependent ion fluxes in cerebellar microsomes

Using sheep cerebellum microsomes previously loaded with 45Ca2+ or 90Sr2+, we measured the dependence of inositol 1,4,5-trisphosphate (InsP3)-induced efflux of these ions on Ca2+ or Sr2+ on the cytosolic side. At a low InsP3 concentration, Ca2+ in the submicromolar range only poorly activated 45Ca2+ or 90Sr2+ efflux, and higher Ca2+ concentrations were inhibitory. In contrast, Sr2+ in the micromolar range activated release efficiently, while only very high Sr2+ concentrations were inhibitory. Experiments were repeated in the presence of a high InsP3 concentration, which allowed increasing free Ca2+ to micromolar concentrations without inducing complete inhibition of the InsP3-dependent efflux. Under these conditions, micromolar Ca2+ was found to activate efflux to a large extent, similar to that previously found with Sr2+. Optimal activation by Ca2+ of the InsP3-dependent channel occurs at micromolar rather than submicromolar free Ca2+ concentrations, but at too low an InsP3 concentration, Ca(2+)-induced activation is counteracted by Ca(2+)-induced inactivation. Separate measurements of [3H]-InsP3 binding at a low concentration showed that Sr2+ and Ca2+ did not enhance the amount of bound [3H]-InsP3, implying that the activating effect of Sr2+ and Ca2+ in cerebellar microsomes is mediated by an increase in the channel opening probability and not by an increase in the receptor's affinity for InsP3. A similar relationship also holds in the case of the activating effect of nucleotides.

The inositol trisphosphate receptor of Xenopus oocytes

Xenopus laevis oocytes (stages V and VI) are a widely used model system for the study of Ca2+ signaling. The properties of the Xenopus oocyte InsP3 receptor (InsP3R) are of paramount importance for our thinking about this system and for our efforts to model Ca2+ dynamics in the oocyte cytosol. The recent data regarding the molecular structure, the regulation and the functional properties of the Xenopus oocyte InsP3R are summarized in this review. The main properties of the Xenopus oocyte InsP3R are compared with the properties of the cerebellar InsP3R and are shown to be remarkably similar. The density of the InsP3R in Xenopus oocyte cytoplasm is estimated to a value between 1.1-4.1 x 10(14) tetrameric InsP3R/l. The use of these numbers in a quantitative model of Ca2+ wave propagation leads to values of Ca2+ wave amplitude (0.8-1.5 microM Ca2+) and velocity of the wave propagation (12-24 microns/s) that are in excellent agreement with the values observed experimentally. The density of InsP3Rs in Purkinje cells of the cerebellum is estimated to be about 20,000-fold higher, but in other types of neurons and in peripheral tissues the InsP3R density is estimated to be of the same order of magnitude as, or up to 20-fold higher than, in Xenopus oocytes. The implications of differences in InsP3R density for Ca2+ signaling are discussed.

Inhibition of nuclear vesicle fusion by antibodies that block activation of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate (IP3) receptors are ligand-gated channels that release intracellular Ca2+ stores in response to the second messenger, IP3. We investigated the potential role of IP3 receptors during nuclear envelope assembly in vitro, using Xenopus egg extracts. Previous work suggested that Ca2+ mobilization is required for nuclear vesicle fusion and implicated IP3 receptor activity. To test the involvement of IP3 receptors using selective reagents, we obtained three distinct polyclonal antibodies to the type 1 IP3 receptor. Pretreatment of membranes with two of the antibodies inhibited IP3-stimulated CA2+ release in vitro and also inhibited nuclear vesicle fusion. One inhibitory serum was directed against 420 residues within the "coupling" domain, which includes several potential regulatory sites. The other inhibitory serum was directed against 95 residues near the C terminus and identifies an inhibitory epitope(s) in this region. The antibodies had no effect on receptor affinity for IP3. Because nuclear vesicle fusion was inhibited by antibodies that block Ca2+ flux, but not by control and preimmune antibodies, we concluded that the activation of IP3 receptors is required for fusion. The signal that activates the channel during fusion is unknown.

Calreticulin inhibits repetitive intracellular Ca2+ waves

Inositol 1,4,5-trisphosphate (IP3)-mediated calcium (Ca2+) signaling is subject to cytosolic and luminal regulatory mechanisms. In Xenopus oocytes, Ca(2+)-sensitive gating of the IP3 receptor (IP3R) produces repetitive waves of Ca2+ release. We examined the role of the luminal Ca(2+)-binding protein calreticulin (CRT) in IP3-mediated Ca2+ signaling by using Ca2+ wave activity as a sensitive Ca2+ release assay. Overexpression of CRT inhibited repetitive IP3-induced Ca2+ waves. Deletion mutagenesis demonstrated that CRT inhibition was mediated by the high affinity-low capacity Ca(2+)-binding domain, which contributes little to Ca2+ storage. This novel function of CRT in intracellular Ca2+ signaling may be regulated by Ca2+ occupancy of the high affinity binding site.

Functional properties of intracellular calcium-release channels

Two major classes of intracellular calcium-release channels have been identified, the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor. These channels are the largest ion channels identified to date. Recent studies have established that approximately 90% of each of these proteins protrudes into the cytoplasm, presumably exposing many regulatory sites on the channel and allowing functional interactions with other cytoplasmic proteins. Current work is aimed at understanding the molecular mechanisms and cellular roles of these regulatory processes.