Cooking with calcium: the recipes for composing global signals from elementary events

Entropically modified spiking ability and periodicity in clustered channels

It has been shown that, in models of excitable clusters of voltage-gated sodium channels, the spontaneous rate of action potential and signal encoding ability exhibit multiple peaks at different cluster sizes due to an entropy effect in small system. In this paper, we show that similar results can be found in excitable cluster of ligand-gated calcium channels. Furthermore, we demonstrate that the periodicity of spontaneous Ca2+ spikes elicited by the cluster reveals multiple maxima at small discrete cluster sizes.

Puff-wave transition in an inhomogeneous model for calcium signals

In many cell types, calcium ion channels on the endoplasmic reticulum membrane occur in a clustered distribution. The channels generate either localized puffs, each comprising channels of only one cluster, or global calcium waves. In this work we model the calcium system as a two-dimensional lattice of active elements distributed regularly in an otherwise passive space. We address an important feature of the puff-wave transition, which is the difference in lifetime of puffs at a few hundred milliseconds and long-lived global waves with periods of several seconds. We show that such a lifetime difference between puffs and waves can be understood with strongly reduced ordinary differential equations modified by a time-scale factor that takes into account the coupling strength of active and passive regions determined by the Ca2+ diffusion coefficient. Furthermore, we show that the point model can also describe very well the dependence of Ca2+ oscillation characteristics on the cluster-cluster distance in the case of large diffusivity.

Intra-cluster percolation of calcium signals

Calcium signals are involved in a large variety of physiological processes. Their versatility relies on the diversity of spatio-temporal behaviors that the calcium concentration can display. Calcium entry through inositol 1,4,5-trisphosphate (IP) receptors (IPR's) is a key component that participates in both local signals such as “puffs” and in global waves. IPR's are usually organized in clusters on the membrane of the endoplasmic reticulum and their spatial distribution has important effects on the resulting signal. Recent high resolution observations [1] of Ca puffs offer a window to study intra-cluster organization. The experiments give the distribution of the number of IPR's that open during each puff without much processing. Here we present a simple model with which we interpret the experimental distribution in terms of two stochastic processes: IP binding and unbinding and Ca-mediated inter-channel coupling. Depending on the parameters of the system, the distribution may be dominated by one or the other process. The transition between both extreme cases is similar to a percolation process. We show how, from an analysis of the experimental distribution, information can be obtained on the relative weight of the two processes. The largest distance over which Ca-mediated coupling acts and the density of IP-bound IPR's of the cluster can also be estimated. The approach allows us to infer properties of the interactions among the channels of the cluster from statistical information on their emergent collective behavior.

Cell surface topology creates high Ca2+ signalling microdomains

It has long been speculated that cellular microdomains are important for many cellular processes, especially those involving Ca²⁺ signalling. Measurements of cytosolic Ca²⁺ report maximum concentrations of less than few micromolar, yet several cytosolic enzymes require concentrations of more than 20 μM Ca²⁺ to be activated. In this paper, we have resolved this apparent paradox by showing that the surface topology of cells represents an important and hitherto unrecognized feature for generating microdomains of high Ca²⁺ in cells. We show that whereas the standard modeling assumption of a smooth cell surface predicts only moderate localized effects, the more realistic “wrinkled” surface topology predicts that Ca²⁺ concentrations up to 80 μM can persist within the folds of membranes for significant times. This intra-wrinkle location may account for 5% of the total cell volume. Using different geometries of wrinkles, our simulations show that high Ca²⁺ microdomains will be generated most effectively by long narrow membrane wrinkles of similar dimensions to those found experimentally. This is a new concept which has not previously been considered, but which has ramifications as the intra-wrinkle location is also a strategic location at which Ca²⁺ acts as a regulator of the cortical cytoskeleton and plasma membrane expansion.

Importance of cell variability for calcium signaling in rat airway myocytes

Calcium signaling controls several essential physiological functions in different cell types. Hence, it is not surprising that different aspects of Ca(2+) dynamics are in the focus of in-depth and extensive investigations. Efforts concentrate on the development of proper theoretical models that would provide a unified description of Ca(2+) signaling. Remarkably, experimentally recorded Ca(2+) signals exhibit a rather large diversity, which can be observed irrespective of the cell type, measuring techniques, or the nature of the signal. Our goal in the present study therefore is to present a theoretical explanation for the variability observed in experiments, whereby we focus on caffeine-induced Ca(2+) responses in isolated airway myocytes. By employing a stochastic model, we first test whether the observed variability can be attributed to intrinsic fluctuations that are a common feature of biochemical reactions that govern Ca(2+) signalization. We find that stochastic effects, within ranges that correspond to actual conditions in the cell, are far too modest to explain the large diversity observed in experimental data. Foremost, we reveal that only cell variability in theoretical modeling can appropriately describe the observed diversity in single-cell responses.

Ca(2+) puffs originate from preestablished stable clusters of inositol trisphosphate receptors

Intracellular calcium ion (Ca(2+)) signaling crucially depends on the clustered organization of inositol trisphosphate receptors (IP(3)Rs) in the endoplasmic reticulum (ER) membrane. These ligand-gated ion channels liberate Ca(2+) to generate local signals known as Ca(2+) puffs. We tested the hypothesis that IP(3) itself elicits rapid clustering of IP(3)Rs by using flash photolysis of caged IP(3) in conjunction with high-resolution Ca(2+) imaging to monitor the activity and localization of individual IP(3)Rs within intact mammalian cells. Our results indicate that Ca(2+) puffs arising with latencies as short as 100 to 200 ms after photorelease of IP(3) already involve at least four IP(3)R channels, and that this number does not subsequently grow. Moreover, single active IP(3)Rs show limited mobility, and stochastic simulations suggest that aggregation of IP(3)Rs at puff sites by a diffusional trapping mechanism would require many seconds. We thus conclude that puff sites represent preestablished, stable clusters of IP(3)Rs and that functional IP(3)Rs are not readily diffusible within the ER membrane.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

Mitochondrial Ca2+ uptake increases Ca2+ release from inositol 1,4,5-trisphosphate receptor clusters in smooth muscle cells

Smooth muscle activities are regulated by inositol 1,4,5-trisphosphate (InsP(3))-mediated increases in cytosolic Ca2+ concentration ([Ca2+](c)). Local Ca2+ release from an InsP(3) receptor (InsP(3)R) cluster present on the sarcoplasmic reticulum is termed a Ca2+ puff. Ca2+ released via InsP(3)R may diffuse to adjacent clusters to trigger further release and generate a cell-wide (global) Ca2+ rise. In smooth muscle, mitochondrial Ca2+ uptake maintains global InsP(3)-mediated Ca2+ release by preventing a negative feedback effect of high [Ca2+] on InsP(3)R. Mitochondria may regulate InsP(3)-mediated Ca2+ signals by operating between or within InsP(3)R clusters. In the former mitochondria could regulate only global Ca2+ signals, whereas in the latter both local and global signals would be affected. Here whether mitochondria maintain InsP(3)-mediated Ca2+ release by operating within (local) or between (global) InsP(3)R clusters has been addressed. Ca2+ puffs evoked by localized photolysis of InsP(3) in single voltage-clamped colonic smooth muscle cells had amplitudes of 0.5-4.0 F/F(0), durations of approximately 112 ms at half-maximum amplitude, and were abolished by the InsP(3)R inhibitor 2-aminoethoxydiphenyl borate. The protonophore carbonyl cyanide 3-chloropheylhydrazone and complex I inhibitor rotenone each depolarized DeltaPsi(M) to prevent mitochondrial Ca2+ uptake and attenuated Ca2+ puffs by approximately 66 or approximately 60%, respectively. The mitochondrial uniporter inhibitor, RU360, attenuated Ca2+ puffs by approximately 62%. The "fast" Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acted like mitochondria to prolong InsP(3)-mediated Ca2+ release suggesting that mitochondrial influence is via their Ca2+ uptake facility. These results indicate Ca2+ uptake occurs quickly enough to influence InsP(3)R communication at the intra-cluster level and that mitochondria regulate both local and global InsP(3)-mediated Ca2+ signals.

Ca2+ waves initiate antigen-stimulated Ca2+ responses in mast cells

Ca(2+) mobilization is central to many cellular processes, including stimulated exocytosis and cytokine production in mast cells. Using single cell stimulation by IgE-specific Ag and high-speed imaging of conventional or genetically encoded Ca(2+) sensors in rat basophilic leukemia and bone marrow-derived rat mast cells, we observe Ca(2+) waves that originate most frequently from the tips of extended cell protrusions, as well as Ca(2+) oscillations throughout the cell that usually follow the initiating Ca(2+) wave. In contrast, Ag conjugated to the tip of a micropipette stimulates local, repetitive Ca(2+) puffs at the region of cell contact. Initiating Ca(2+) waves are observed in most rat basophilic leukemia cells stimulated with soluble Ag and are sensitive to inhibitors of Ca(2+) release from endoplasmic reticulum stores and to extracellular Ca(2+), but they do not depend on store-operated Ca(2+) entry. Knockdown of transient receptor potential channel (TRPC)1 and TRPC3 channel proteins by short hairpin RNA reduces the sensitivity of these cells to Ag and shifts the wave initiation site from protrusions to the cell body. Our results reveal spatially encoded Ca(2+) signaling in response to immunoreceptor activation that utilizes TRPC channels to specify the initiation site of the Ca(2+) response.

Localization and socialization: experimental insights into the functional architecture of IP3 receptors

Inositol 1,4,5-trisphosphate (IP(3))-evoked Ca(2+) signals display great spatiotemporal malleability. This malleability depends on diversity in both the cellular organization and in situ functionality of IP(3) receptors (IP(3)Rs) that regulate Ca(2+) release from the endoplasmic reticulum (ER). Recent experimental data imply that these considerations are not independent, such that-as with other ion channels-the local organization of IP(3)Rs impacts their functionality, and reciprocally IP(3)R activity impacts their organization within native ER membranes. Here, we (i) review experimental data that lead to our understanding of the "functional architecture" of IP(3)Rs within the ER, (ii) propose an updated terminology to span the organizational hierarchy of IP(3)Rs observed in intact cells, and (iii) speculate on the physiological significance of IP(3)R socialization in Ca(2+) dynamics, and consequently the emerging need for modeling studies to move beyond gridded, planar, and static simulations of IP(3)R clustering even over short experimental timescales.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

Protein kinase A increases type-2 inositol 1,4,5-trisphosphate receptor activity by phosphorylation of serine 937

Protein kinase A (PKA) phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) represents a mechanism for shaping intracellular Ca(2+) signals following a concomitant elevation in cAMP. Activation of PKA results in enhanced Ca(2+) release in cells that express predominantly InsP(3)R2. PKA is known to phosphorylate InsP(3)R2, but the molecular determinants of this effect are not known. We have expressed mouse InsP(3)R2 in DT40-3KO cells that are devoid of endogenous InsP(3)R and examined the effects of PKA phosphorylation on this isoform in unambiguous isolation. Activation of PKA increased Ca(2+) signals and augmented the single channel open probability of InsP(3)R2. A PKA phosphorylation site unique to the InsP(3)R2 was identified at Ser(937). The enhancing effects of PKA activation on this isoform required the phosphorylation of Ser(937), since replacing this residue with alanine eliminated the positive effects of PKA activation. These results provide a mechanism responsible for the enhanced Ca(2+) signaling following PKA activation in cells that express predominantly InsP(3)R2.

Clustering of InsP3 receptors by InsP3 retunes their regulation by InsP3 and Ca2+

The versatility of Ca2+ signals derives from their spatio-temporal organization. For Ca2+ signals initiated by inositol-1,4,5-trisphosphate (InsP3), this requires local interactions between InsP3 receptors (InsP3Rs) mediated by their rapid stimulation and slower inhibition\ by cytosolic Ca2+. This allows hierarchical recruitment of Ca2+ release events as the InsP3 concentration increases. Single InsP3Rs respond first, then clustered InsP3Rs open together giving a local 'Ca2+ puff', and as puffs become more frequent they ignite regenerative Ca2+ waves. Using nuclear patch-clamp recording, here we demonstrate that InsP3Rs are initially randomly distributed with an estimated separation of 1 m. Low concentrations of InsP3 cause InsP3Rs to aggregate rapidly and reversibly into small clusters of about four closely associated InsP3Rs. At resting cytosolic [Ca2+], clustered InsP3Rs open independently, but with lower open probability, shorter open time, and less InsP3 sensitivity than lone InsP3Rs. Increasing cytosolic [Ca2+] reverses the inhibition caused by clustering, InsP3R gating becomes coupled, and the duration of multiple openings is prolonged. Clustering both exposes InsP3Rs to local Ca2+ rises and increases the effects of Ca2+. Dynamic regulation of clustering by InsP3 retunes InsP3R sensitivity to InsP3 and Ca2+, facilitating hierarchical recruitment of the elementary events that underlie all InsP3-evoked Ca2+ signals.

Imaging the quantal substructure of single IP3R channel activity during Ca2+ puffs in intact mammalian cells

The spatiotemporal patterning of Ca(2+) signals regulates numerous cellular functions, and is determined by the functional properties and spatial clustering of inositol trisphosphate receptor (IP(3)R) Ca(2+) release channels in the endoplasmic reticulum membrane. However, studies at the single-channel level have been hampered because IP(3)Rs are inaccessible to patch-clamp recording in intact cells, and because excised organelle and bilayer reconstitution systems disrupt the Ca(2+)-induced Ca(2+) release (CICR) process that mediates channel-channel coordination. We introduce here the use of total internal reflection fluorescence microscopy to image single-channel Ca(2+) flux through individual and clustered IP(3)Rs in intact mammalian cells. This enables a quantal dissection of the local calcium puffs that constitute building blocks of cellular Ca(2+) signals, revealing stochastic recruitment of, on average, approximately 6 active IP(3)Rs clustered within <500 nm. Channel openings are rapidly ( approximately 10 ms) recruited by opening of an initial trigger channel, and a similarly rapid inhibitory process terminates puffs despite local [Ca(2+)] elevation that would otherwise sustain Ca(2+)-induced Ca(2+) release indefinitely. Minimally invasive, nano-scale Ca(2+) imaging provides a powerful tool for the functional study of intracellular Ca(2+) release channels while maintaining the native architecture and dynamic interactions essential for discrete and selective cell signaling.

Compartmentalized signalling: Ca2+ compartments, microdomains and the many facets of Ca2+ signalling

Ca(2+) regulates a multitude of cellular processes and does so by partitioning its actions in space and time. In this review, we discuss how Ca(2+) responses are constructed from small quantal (elementary) events that have the potential to propagate to produce large pan-cellular responses. We review how Ca(2+) is compartmentalized both physically and functionally, and describe how each organelle has its own distinct Ca(2+)-handling properties. We explain how coordination of the movement of Ca(2+) between organelles is used to shape and hone Ca(2+) signals. Finally, we provide a number of specific examples of where compartmentation and localization of Ca(2+) are crucial to cell function.

Localization of puff sites adjacent to the plasma membrane: functional and spatial characterization of Ca2+ signaling in SH-SY5Y cells utilizing membrane-permeant caged IP3

The Xenopus oocyte has been a favored model system in which to study spatio-temporal mechanisms of intracellular Ca2+ dynamics, in large part because this giant cell facilitates intracellular injections of Ca2+ indicator dyes, buffers and caged compounds. However, the recent commercial availability of membrane-permeant ester forms of caged IP3 (ci-IP3) and EGTA, now allows for facile loading of these compounds into smaller mammalian cells, permitting control of [IP3]i and cytosolic Ca2+ buffering. Here, we establish the human neuroblastoma SH-SY5Y cell line as an advantageous experimental system for imaging Ca2+ signaling, and characterize IP3-mediated Ca2+ signaling mechanisms in these cells. Flash photo-release of increasing amounts of i-IP3 evokes Ca2+ puffs that transition to waves, but intracellular loading of EGTA decouples release sites, allowing discrete puffs to be studied over a wide range of [IP3]. Puff activity persists for minutes following a single photo-release, pointing to a slow rate of i-IP3 turnover in these cells and suggesting that repetitive Ca2+ spikes with periods of 20-30s are not driven by oscillations in [IP3]. Puff amplitudes are independent of [IP3], whereas their frequencies increase with increasing photo-release. Puff sites in SH-SY5Y cells are not preferentially localized near the nucleus, but instead are concentrated close to the plasma membrane where they can be visualized by total internal reflection microscopy, offering the potential for unprecedented spatio-temporal resolution of Ca2+ puff kinetics.

Effect of Celecoxib on Ca2+Fluxes and Proliferation in MDCK Renal Tubular Cells

The effect of celecoxib on renal tubular cells is largely unexplored. In Madin Darby canine kidney (MDCK) cells, the effect of celecoxib on intracellular CaCa2+ concentration ([Ca2+]i) and proliferation was examined by using the Ca(2 +)-sensitive fluorescent dye fura-2 and the viability detecting fluorescent dye tetrazolium, respectively. Celecoxib (> or =1 micro M) caused an increase of [CaCa2+]i in a concentration-dependent manner. Celecoxib-induced [CaCa2+]i increase was partly reduced by removal of extracellular CaCa2+. Celecoxib-induced CaCa2+ influx was independently suggested by MnCa2+ influx-induced fura-2 fluorescence quench. In Ca(2 +)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2 +)-ATPase, caused a monophasic [CaCa2+]i increase, after which celecoxib only induced a tiny [CaCa2+]i increase; conversely, pretreatment with celecoxib completely inhibited thapsigargin-induced [CaCa2+]i increases. U73122, an inhibitor of phospholipase C, abolished ATP (but not celecoxib)-induced [CaCa2+]i increases. Overnight incubation with 1 or 10 micro M celecoxib decreased cell viability by 80% and 100%, respectively. These data indicate that celecoxib evokes a [CaCa2+]i increase in renal tubular cells by stimulating both extracellular CaCa2+ influx and intracellular CaCa2+ release and is highly toxic to renal tubular cells in vitro.

Timing in cellular Ca2+ signaling

Calcium (Ca2+) signals are generated across a broad time range. Kinetic considerations impact how information is processed to encode and decode Ca2+ signals, the choreography of responses that ensure specific and efficient signaling and the overall temporal amplification such that ephemeral Ca2+ signals have lasting physiological value. The reciprocal importance of timing for Ca2+ signaling, and Ca2+ signaling for timing is exemplified by the altered kinetic profiles of Ca2+ signals in certain diseases and the likely role of basal Ca2+ fluctuations in the perception of time itself.

Traction force microscopy on-chip: shear deformation of fibroblast cells

We develop here a microfabrication compatible force measurement technique termed as ultrasoft polydimethylsiloxane-based traction force microscopy (UPTFM). This technique is devised for mapping the cellular traction forces imparted on the adhering substrate, so as to depict the physiological state of the cells surviving in the micro-confinement. We subsequently integrate the technique with a microfluidic platform for evaluating different states of stress in adherent mouse skin fibroblast L929 cells. Utilizing this technique, we monitor the spatio-temporal evolution of cellular traction forces for static incubation periods with no media replenishment as well as for dynamic flow conditions that inherently induce cell deformation and detachment. While the studies conducted on a quiescent fluid medium enable us to obtain an optimal static cell incubation period, those executed under dynamic flow conditions provide us with the minuscule details of the cellular response, deformation and detachment processes. We elucidate the correlation between shear activated cytosolic calcium ion release profile and the local traction forces as an attempt to apply UPTFM in the domain of functional biological purposes. Pertinently, we map the centroidal displacement and the maximum traction stress in characterizing the critical shear rate conditions for the onset of the cell peeling-off process, and demonstrate their contrasting features in comparison to the vesicle lift off processes in a shear flow. Theoretically, these deviations can only be explained by taking physiologically relevant cell adhesion models into consideration, which, while retaining the intrinsic simplicity, are able to reproduce the key experimental outcomes at least with qualitative agreement. We execute further theoretical investigations with variable magnitudes of membrane stiffness, viscosity and adhesion strength, so as to come up with interesting biophysical confluences.

Modeling Ca2+ feedback on a single inositol 1,4,5-trisphosphate receptor and its modulation by Ca2+ buffers

The inositol 1,4,5-trisphosphate receptor/channel (IP(3)R) is a major regulator of intracellular Ca(2+) signaling, and liberates Ca(2+) ions from the endoplasmic reticulum in response to binding at cytosolic sites for both IP(3) and Ca(2+). Although the steady-state gating properties of the IP(3)R have been extensively studied and modeled under conditions of fixed [IP(3)] and [Ca(2+)], little is known about how Ca(2+) flux through a channel may modulate the gating of that same channel by feedback onto activating and inhibitory Ca(2+) binding sites. We thus simulated the dynamics of Ca(2+) self-feedback on monomeric and tetrameric IP(3)R models. A major conclusion is that self-activation depends crucially on stationary cytosolic Ca(2+) buffers that slow the collapse of the local [Ca(2+)] microdomain after closure. This promotes burst-like reopenings by the rebinding of Ca(2+) to the activating site; whereas inhibitory actions are substantially independent of stationary buffers but are strongly dependent on the location of the inhibitory Ca(2+) binding site on the IP(3)R in relation to the channel pore.

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

How does intracellular Ca2+ oscillate: by chance or by the clock?

Ca2+ oscillations have been considered to obey deterministic dynamics for almost two decades. We show for four cell types that Ca2+ oscillations are instead a sequence of random spikes. The standard deviation of the interspike intervals (ISIs) of individual spike trains is similar to the average ISI; it increases approximately linearly with the average ISI; and consecutive ISIs are uncorrelated. Decreasing the effective diffusion coefficient of free Ca2+ using Ca2+ buffers increases the average ISI and the standard deviation in agreement with the idea that individual spikes are caused by random wave nucleation. Array-enhanced coherence resonance leads to regular Ca2+ oscillations with small standard deviation of ISIs.

Ischemia/reperfusion induce renal tubule apoptosis by inositol 1,4,5-trisphosphate receptor and L-type Ca2+ channel opening

Recent studies suggest that besides the L-type calcium channel, two calcium channels on the endoplasmic reticulum (ER), the inositol 1,4,5-trisphosphate receptor (InsP3R) and ryanodine receptor (RyR), may play a role in the apoptotic process of renal tubular cells induced by ischemia/reperfusion (I/R) injury. We used antimycin A to induce cell I/R injury in vitro and found an elevation of the cytosolic calcium concentration and consequently apoptosis. Blocking either the L-type calcium channel with nicardipine or the InsP3R with TMB-8 can inhibit cytochrome c release, activate caspase 3 and decrease the apoptotic cell number. However, blocking the RyR with dantrolene had no effect. We further found that Ca(2+) influx through the L-type channel is needed for the opening of the InsP3R which activates a cascade of Ca(2+) release from the ER store. To test these blockers in vivo, in a rat renal I/R model, pretreatment with nicardipine and TMB-8, but not dantrolene, can protect renal function. Taken together, our results suggest that after I/R injury, Ca(2+) influx through the L-type calcium channel triggers the Ca(2+) release from the InsP3R and finally induces apoptosis. The InsP3R could be a new target for the treatment of renal I/R injury.

`Quantal' Ca2+ release at the cytoplasmic aspect of the Ins(1,4,5)P3R channel in smooth muscle

Smooth muscle responds to activation of the inositol (1,4,5)-trisphosphate receptor [Ins(1,4,5)P3R] with a graded concentration-dependent (`quantal') Ca2+ release from the sarcoplasmic reticulum (SR) store. Graded release seems incompatible both with the finite capacity of the store and the Ca2+-induced Ca2+ release (CICR)-like facility, at Ins(1,4,5)P3Rs, that, once activated, should release the entire content of SR Ca2+. The structural organization of the SR and the regulation of Ins(1,4,5)P3R activity by inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] and Ca2+ have each been proposed to explain `quantal' Ca2+ release. Here, we propose that regulation of Ins(1,4,5)P3R activity by lumenal Ca2+ acting at the cytoplasmic aspect of the receptor might explain `quantal' Ca2+ release in smooth muscle. The entire SR store was found to be lumenally continuous and Ca2+ could diffuse freely throughout: peculiarities of SR structure are unlikely to account for `quantal' release. While Ca2+ release was regulated by [Ca2+] within the SR, the velocity of release increased (accelerated) during the release process. The extent of acceleration of release determined the peak cytoplasmic [Ca2+] and was attenuated by a reduction in SR [Ca2+] or an increase in cytoplasmic Ca2+ buffering. Positive feedback by released Ca2+ acting at the cytoplasmic aspect of Ins(1,4,5)P3Rs (i.e. CICR-like) might (a) account for the acceleration, (b) provide the regulation of release by SR [Ca2+] and (c) explain the `quantal' release process itself. During Ca2+ release, SR [Ca2+] and thus unitary Ins(1,4,5)P3R currents decline, CICR reduces and stops. With increasing [Ins(1,4,5)P3], coincidental activation of several neighbouring Ins(1,4,5)P3Rs offsets the reduced Ins(1,4,5)P3R current to renew CICR and Ca2+ release.

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.

Simulations of intracellular calcium release dynamics in response to a high-intensity, ultrashort electric pulse

Numerical simulations for electrically induced, intracellular calcium release from the endoplasmic reticulum are reported. A two-step model is used for self-consistency. Distributed electrical circuit representation coupled with the Smoluchowski equation yields the ER membrane nanoporation for calcium outflow based on a numerical simulation. This is combined with the continuum Li-Rinzel model and drift diffusion for calcium dynamics. Our results are shown to be in agreement with reported calcium release data. A modest increase (rough doubling) of the cellular calcium is predicted in the absence of extra-cellular calcium. In particular, the applied field of 15 kV/cm with 60 ns pulse duration makes for a strong comparison. No oscillations are predicted and the net recovery period of about 5 min are both in agreement with published experimental results. A quantitative explanation for the lack of such oscillatory behavior, based on the density dependent calcium fluxes, is also provided.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Effect of riluzole on Ca2+ movement and cytotoxicity in Madin-Darby canine kidney cells

Riluzole is a drug used in the treatment of amyotrophic lateral sclerosis; however, its in vitro action is unclear. In this study, the effect of riluzole on intracellular Ca2+ concentration ([Ca2+]i) in Madin-Darby canine kidney (MDCK) cells was investigated using the Ca2+ -sensitive fluorescent dye, fura-2. Riluzole (100-500 microM) caused a rapid and sustained increase of [Ca2+]i in a concentration-dependent manner (EC50 = 150 microM). Some 40 and 50% of this [Ca2+]i increase was prevented by the removal of extracellular Ca2+ and the addition of La3+, respectively, but was unchanged by dihydropyridines, verapamil and diltiazem. In Ca2+ -free medium, thapsigargin - an inhibitor of the endoplasmic reticulum (ER) Caz+ -ATPase--caused a monophasic [Ca2+]i increase, after which the increasing effect of riluzole on [Ca2+]i was attenuated by 70%; in addition, pre-treatment with riluzole abolished thapsigargin-induced [Ca2+]i increases. U73122, an inhibitor of phospholipase C (PLC), abolished ATP (but not riluzole)-induced [Ca2+]i increases. At concentrations of 250 and 500 microM, riluzole killed 40 and 95% cells, respectively. The cytotoxic effect of riluzole (250 microM) was unaltered by pre-chelating cytosolic Ca2+ with BAPTA. Collectively, in MDCK cells, riluzole rapidly increased [Ca2+]i by stimulating extracellular Ca2+ influx via an La3+ -sensitive pathway and intracellular Ca2+ release from the ER via, as yet, unidentified mechanisms. Furthermore, riluzole caused Ca2+ -unrelated cytotoxicity in a concentration-dependent manner.

Imaging single-channel calcium microdomains

The Ca(2+) microdomains generated around the mouth of open ion channels represent the basic building blocks from which cytosolic Ca(2+) signals are constructed. Recent improvements in optical imaging techniques now allow these microdomains to be visualized as single channel calcium fluorescence transients (SCCaFTs), providing information about channel properties that was previously accessible only by electrophysiological patch-clamp recordings. We review recent advances in single channel Ca(2+) imaging methodologies, with emphasis on total internal reflection fluorescence microscopy (TIRFM) as the technique of choice for recording SCCaFTs from voltage- and ligand-gated plasmalemmal ion channels. This technique of 'optical patch-clamp recording' is massively parallel, permitting simultaneous imaging of hundreds of channels; provides millisecond resolution of gating kinetics together with sub-micron spatial resolution of channel locations; and is applicable to diverse families of membrane channels that display partial permeability to Ca(2+) ions.

Calcium microdomains: organization and function

Microdomains of Ca(2+), which are formed at sites where Ca(2+) enters the cytoplasm either at the cell surface or at the internal stores, are a key element of Ca(2+) signalling. The term microdomain includes the elementary events that are the basic building blocks of Ca(2+) signals. As Ca(2+) enters the cytoplasm, it produces a local plume of Ca(2+) that has been given different names (sparks, puffs, sparklets and syntillas). These elementary events can combine to produce larger microdomains. The significance of these localized domains of Ca(2+) is that they can regulate specific cellular processes in different regions of the cell. Such microdomains are particularly evident in neurons where both pre- and postsynaptic events are controlled by highly localized pulses of Ca(2+). The ability of single neurons to process enormous amounts of information depends upon such miniaturization of the Ca(2+) signalling system. Control of cardiac cell contraction and gene transcription provides another example of how the parallel processing of Ca(2+) signalling can occur through microdomains of intracellular Ca(2+).

PKCalpha: a versatile key for decoding the cellular calcium toolkit

Conventional protein kinases C (cPKCs) play an essential role in signal transduction and are believed to integrate both global Ca²⁺ transients and diacylglycerol signals. We provide evidence that PKCα is a ubiquitous readout sensor for the cellular Ca²⁺ toolkit, including highly restricted elementary Ca²⁺ release.

Threshold stimulations of cells with Ca²⁺-mobilizing agonists resulted in PKCα translocation events with limited spatial spreads (<4 μm) comprising two groups of lifetimes; brief events (400–1,500 ms) exclusively mediated by Ca²⁺–C2 domain membrane interactions and long-lasting events (>4 s) resulting from longer DAG-C1a domain–mediated membrane interactions.

Although upon uncaging NP-EGTA, which is a caged Ca²⁺ compound, WT-PKCα displayed rapid membrane translocations within <250 ms, PKCα constructs with C2 domains mutated in their Ca²⁺-binding region lacked any Ca²⁺-dependent translocation. Flash photolysis of diazo-2, a photosensitive caged Ca²⁺ buffer, revealed a biphasic membrane dissociation (slow and fast period) of WT-PKCα. The slow phase was absent in cells expressing PKCα-constructs containing mutated C1a-domains with largely reduced DAG binding. Thus, two groups of PKCα membrane interactions coexist; C2- and C1a-mediated interactions with different lifetimes but rapid interconversion.

We conclude that PKCα can readout very fast and, spatially and temporally, very complex cellular Ca²⁺ signals. Therefore, cPKCs are important transducers for the ubiquitous cellular Ca²⁺ signaling toolkit.

Effect of capsazepine on cytosolic Ca2+ levels and proliferation of human prostate cancer cells

Capsazepine has been widely used as a selective antagonist of vanilloid type 1 receptors; however, its other in vitro effect on most cell types is unknown. In human PC3 prostate cancer cells, the effect of capsazepine on intracellular Ca(2+) concentrations ([Ca(2+)](i)) and cytotoxicity was investigated by using fura-2 and tetrazolium, respectively. Capsazepine caused a rapid rise in [Ca(2+)](i) in a concentration-dependent manner with an EC(50) value of 75 microM. Capsazepine-induced [Ca(2+)](i) rise was reduced by 60% by removal of extracellular Ca(2+), suggesting that the capsazepine-induced [Ca(2+)](i) rise was contributed by extracellular Ca(2+) influx and intracellular Ca(2+). Consistently, the capsazepine (200 microM)-induced [Ca(2+)](i) rise was decreased by La(3+) by half. In Ca(2+)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a monophasic [Ca(2+)](i) rise, after which the effect of capsazepine on [Ca(2+)](i) was inhibited by 80%. Conversely, pretreatment with capsazepine partly reduced thapsigargin-induced [Ca(2+)](i) rise. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca(2+) mobilizer)-induced, but not capsazepine-induced, [Ca(2+)](i) rise. These findings suggest that in human PC3 prostate cancer cells, capsazepine increases [Ca(2+)](i) by evoking Ca(2+) influx and releasing Ca(2+) from the endoplasmic reticulum via a phospholiase C-independent manner. Overnight incubation with capsazepine (200 microM) killed 37% of cells, which could not be prevented by chelating intracellular Ca(2+) with BAPTA.

Dual effect of flurbiprofen on cell proliferation and agonist-induced Ca(2+) movement in human osteosarcoma cells

In human MG63 osteosarcoma cells, the effect of flurbiprofen on intracellular Ca(2+) concentrations ([Ca(2+)](i)) and proliferation was explored. The proliferation was enhanced by 20-120 microM flurbiprofen, and was decreased by 140-200 microM flurbiprofen. The effect of flurbiprofen on the increases in cytosolic free Ca(2+) levels ([Ca(2+)](i)) induced by ATP, bradykinin, histamine and thapsigargin (an inhibitor of the endoplasmic reticulum Ca(2+) ATPase), was examined. In cell preincubated with 20 or 80 microM flurbiprofen, the [Ca(2+)](i) increases induced by all agonists were attenuated. In the presence of 20 microM flurbiprofen, the decreased [Ca(2+)](i) responses with the agonists were attributed to a defective Ca(2+) influx because this decrease was unobserved in agonists-induced [Ca(2+)](i) increases in the absence of extracellular Ca(2+). In the presence of 80 microM flurbiprofen, both the Ca(2+) influx component and the Ca(2+) releasing (from organelles) component were defective. These results suggest that flurbiprofen could alter proliferation and inhibit [Ca(2+)](i) increases.

Essential role of Ca2+ release channels in angiotensin II-induced Ca2+ oscillations and mesangial cell contraction

The increased resistance of the glomerulus as a result of contractile dysfunction of mesangial cells (MCs) is associated with reduction of glomerular filtration rate and development of glomerulosclerosis. Evidences show MCs contraction changes with intracellular Ca(2+) concentration ([Ca(2+)](i)). Here, we explore the mechanism of angiotensin II (AngII)-induced Ca(2+) oscillations and MCs contraction. Primary MCs from 3-month-old and 28-month-old rats were used for detection of Ca(2+) oscillations and MC planar area with confocal microscopy. AngII could induce typical Ca(2+) oscillations and contraction of MCs. This process was abolished by thapsigargin, 2-aminoethoxydiphenyl borate, or 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine, and partially inhibited by ryanodine, but could not be inhibited in the absence of extracellular Ca(2+). Ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate (InsP(3)) receptors displayed a strong colocalization, which may contribute to the amplification of Ca(2+) response. MLC(20) phosphorylation and MC planar area were associated with AngII-induced Ca(2+) oscillations. The frequency of Ca(2+) oscillations was dependent on the AngII concentration and correlated with the MCs' contractive extent, which could be attenuated by KN-93. The amplitude reduction of oscillations correlated with the decrease in aging-related contraction. In conclusion, [Ca(2+)](i) response of MCs to AngII is characterized by repetitive spikes through the following repetitive cycles: Ca(2+) release by phospholipase C -InsP(3) pathway, Ca(2+) amplification by Ca(2+)-activated RyRs and Ca(2+) reuptake by the endoplasmic reticulum. MCs contraction can be modulated by oscillations not only in an AngII-induced frequency-dependent mode but also in an aging-related, amplitude-dependent mode.

NPC-14686 (Fmoc-l-homophenylalanine)-induced CaCa2+ increases and death in human prostate cancer cells

The effect of NPC-14686, a potential anti-inflammatory drug, on cytosolic free Ca2+ levels ([Ca2+]i) and growth in PC3 human prostate cancer cells was examined by using fura-2 as a fluorescent Ca2+ indicator and WST-1 as a fluorescent growth dye. NPC-14686 at concentrations above 10 microM increased [Ca2+]i in a concentration-dependent manner with an EC50 value of 100 microM. NPC-14686-induced Ca2+ influx was confirmed by Mn2+ quench of fura-2 fluorescence. The Ca2+ signal was also reduced by removing extracellular Ca2+. Pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor) to deplete the endoplasmic reticulum Ca2+ nearly abolished 200 microM NPC-14686-induced Ca2+ release; and conversely pretreatment with NPC-14686 completely inhibited thapsigargin-induced Ca2+ release. The Ca2+ release induced by 200 microM NPC-14686 was not affected by inhibiting phospholipase C with 2 microM U73122. Overnight treatment with 1-500 microM NPC-14686 decreased cell viability in a concentration-dependent manner. These findings suggest that in human PC3 prostate cancer cells, NPC-14686 increases [Ca2+]i by evoking extracellular Ca2+ influx and releasing intracellular Ca2+ from the endoplasmic reticulum via a phospholiase C-independent manner. NPC-14686 may be cytotoxic to prostate cancer cells.

Ca2+ shuttling between endoplasmic reticulum and mitochondria underlying Ca2+ oscillations

Although many cell functions are regulated by Ca(2+) oscillations induced by a cyclic release of Ca(2+) from intracellular Ca(2+) stores, the pacemaker mechanism of Ca(2+) oscillations remains to be explained. Using green fluorescent protein-based Ca(2+) indicators that are targeted to intracellular Ca(2+) stores, the endoplasmic reticulum (ER) and mitochondria, we found that Ca(2+) shuttles between the ER and mitochondria in phase with Ca(2+) oscillations. Following agonist stimulation, Ca(2+) release from the ER generated the first Ca(2+) oscillation and loaded mitochondria with Ca(2+). Before the second Ca(2+) oscillation, Ca(2+) release from the mitochondria by means of the Na(+)/Ca(2+) exchanger caused a gradual increase in cytoplasmic Ca(2+) concentration, inducing a regenerative ER Ca(2+) release, which generated the peak of Ca(2+) oscillation and partially reloaded the mitochondria. This sequence of events was repeated until mitochondrial Ca(2+) was depleted. Thus, Ca(2+) shuttling between the ER and mitochondria may have a pacemaker role in the generation of Ca(2+) oscillations.

Induction of calcium influx from extracellular fluid by beauvericin in human leukemia cells

Beauvericin, a cyclic hexadepsipeptide, is a mycotoxin that can induce cell death in human lymphoblastic leukemia CCRF-CEM cells. Our previous data have shown that beauvericin induces cell death in CCRF-CEM cells in a dose- and time-dependent manner, and that this beauvericin-induced cell death can be prevented by administration of intracellular calcium chelator-BAPTA. Therefore, the intracellular Ca2+ concentration ([Ca2+]i) may play an important role in beauvericin-induced cell death in CCRF-CEM cells. In this study, the effect of beauvericin on [Ca2+]i and the possible mechanism responsible for the changes of [Ca2+]i in CCRF-CEM cells were investigated. Beauvericin caused a rapid and sustained [Ca2+]i rise in a dose-dependent manner. Excess extracellular Ca2+ facilitated beauvericin-induced [Ca2+]i rise by adding 1 mM CaCl2 in the bathing medium. On the other hand, beauvericin-induced [Ca2+]i rise was prevented in Ca2+-free Tyrode's solution by 200 microM EGTA. In addition, beauvericin-induced [Ca2+]i rise was also attenuated by intracellular Ca2+ chelator-BAPTA/AM. It is worthy to note that neither the voltage-dependent Ca2+ channel blocker, nimodipine, nor depletion of intracellular Ca2+ with thapsigargin, an endoplasmic reticulum Ca2+ pump inhibitor, has any effect on beauvericin-induced [Ca2+]i rise. The data from present study indicate that beauvericin acts as a potent Ca2+ mobilizer by stimulating extracellular Ca2+ influx CCRF-CEM cells.

Cooperativity can reduce stochasticity in intracellular calcium dynamics

The opening of inositol (1,4,5)-triphosphate (IP(3)) receptors, clustered at discrete sites on the endoplasmic reticulum, can lead to large-scale intracellular calcium waves. Recent experiments in Xenopus oocytes have shown that the inter-wave intervals for these waves have a standard deviation that is much smaller than their mean and that the background calcium concentration exhibits a slow rise during the interwave interval. Using a simple mathematical model, we examine the possibility that this slow rise increases the cooperativity between the openings of the clusters. We find that our model, coupled to the usual assumption that the pumps on the endoplasmic reticulum are activated instantaneously, is unable to explain the observed data: the clusters are found to fire independently and the inter-wave interval distribution is a Poisson distribution with a standard deviation that is approximately equal to its mean. On the other hand, we find that incorporating pumps that slowly activate leads to a slow increase in the background calcium concentration which makes global events progressively more likely to occur. We show that this cooperativity results in much smaller standard deviations and inter-wave interval distributions that are clearly not Poisson distributions.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Direct stochastic simulation of Ca2+ motion in Xenopus eggs

The release of important intracellular ions has been widely modeled using two approaches, namely, (1) Fickian diffusion, in which sometimes tensorial diffusion coefficients are used to fit observed temporally varying concentrations of calcium, and (2) cellular automata, which produce a set of localized finite difference equations that result in complex global behavior. Here, we take a different approach, employing some assumed, a priori, distribution of ion-binding proteins in the cell, and some assumed biochemical capture and release characteristics to explain ionic motion, and ultimately, distribution. We study several scenarios for ion distribution, based on differences in binder action and distribution. The numbers and strengths of ion binders, spatial variation in inositol 1,4,5-triphosphate concentration, together with the escalating distribution of ionic diffusion speed, are found to be key factors leading to concavity in the Ca2+ wave shape. We also offer an explanation for geometrical effects on previously observed ion diffusion speeds in the cellular cortex of the Xenopus laevis egg during fertilization, based on an angle-of-view correction.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Effect of carvedilol on Ca2+ movement and cytotoxicity in human MG63 osteosarcoma cells

Carvedilol is a useful cardiovascular drug for treating heart failure, however, the in vitro effect on many cell types is unclear. In human MG63 osteosarcoma cells, the effect of carvedilol on intracellular Ca2+ concentrations ([Ca2+]i) and cytotoxicity was explored by using fura-2 and tetrazolium, respectively. Carvedilol at concentrations greater than 1 microM caused a rapid rise in [Ca2+]i in a concentration-dependent manner (EC50=15 microM). Carvedilol-induced [Ca2+]i rise was reduced by 60% by removal of extracellular Ca2+. Carvedilol-induced Mn2+-associated quench of intracellular fura-2 fluorescence also suggests that carvedilol induced extracellular Ca2+ influx. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which the increasing effect of carvedilol on [Ca2+]i was inhibited by 50%. Conversely, pretreatment with carvedilol to deplete intracellular Ca2+ stores totally prevented thapsigargin from releasing more Ca2+. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca2+ mobilizer)-induced, but not carvedilol-induced, [Ca2+]i rise. Pretreatment with phorbol 12-myristate 13-acetate and forskolin to activate protein kinase C and adenylate cyclase, respectively, did not alter carvedilol-induced [Ca2+]i rise. Separately, overnight treatment with 0.1-30 microM carvedilol inhibited cell proliferation in a concentration-dependent manner. These findings suggest that in human MG63 osteosarcoma cells, carvedilol increases [Ca2+]i by stimulating extracellular Ca2+ influx and also by causing intracellular Ca2+ release from the endoplasmic reticulum and other stores via a phospholipase C-independent manner. Carvedilol may be cytotoxic to osteoblasts.