Membrane potential dependent modulations of calcium oscillations in insulin-secreting INS-1 cells

This study was undertaken to examine the role of K(+) channels on cytosolic Ca(2+) ([Ca(2+)](i)) in insulin secreting cells. [Ca(2+)](i) was measured in single glucose-responsive INS-1 cells using the fluorescent Ca(2+) indicator Fura-2. Glucose, tolbutamide and forskolin elevated [Ca(2+)](i) and induced [Ca(2+)] oscillations. Whereas the glucose effect was delayed and observed in 60% and 93% of the cells, in a poorly and a highly glucose-responsive INS-1 cell clone, respectively, tolbutamide and forskolin increased [Ca(2+)](i) in all cells tested. In the latter clone, glucose induced [Ca(2+)](i) oscillations in 77% of the cells. In 16% of the cells a sustained rise of [Ca(2+)](i) was observed. The increase in [Ca(2+)](i) was reversed by verapamil, an L-type Ca(2+) channel inhibitor. Adrenaline decreased [Ca(2+)](i) in oscillating cells in the presence of low glucose and in cells stimulated by glucose alone or in combination with tolbutamide and forskolin. Adrenaline did not lower [Ca(2+)](i) in the presence of 30mM extracellular K(+), indicating that adrenaline does not exert a direct effect on Ca(2+) channels but increases K(+) channel activity. As for primary b-cells, [Ca(2+)](i) oscillations persisted in the presence of closed K(ATP) channels; these also persisted in the presence of thapsigargin, which blocks Ca(2+) uptake into Ca(2+) stores. In contrast, in voltage-clamped cells and in the presence of diazoxide (50mM), which hyperpolarizes the cells by opening K(ATP) channels, [Ca(2+)](i) oscillations were abolished. These results support the hypothesis that [Ca(2+)](i) oscillations depend on functional voltage-dependent Ca(2+) and K(+) channels and are interrupted by a hyperpolarization in insulin-secreting cells.     

P2Y purinoceptors are responsible for oscillatory fluid flow-induced intracellular calcium mobilization in osteoblastic cells

We previously found that oscillatory fluid flow activated MC3T3-E1 osteoblastic cell Ca(2+)(i) mobilization via the inositol 1,4,5-trisphosphate pathway in the presence of 2% fetal bovine serum (FBS). However, the molecular mechanism of fluid flow-induced Ca(2+)(i) mobilization is unknown. In this study, we first demonstrated that oscillatory fluid flow in the absence of FBS failed to increase [Ca(2+)](i) in MC3T3-E1 cells. Apyrase (10 units/ml), which rapidly hydrolyzes 5' nucleotide triphosphates to monosphophates, prevented the fluid flow induced increases in [Ca(2+)](i) in the presence of FBS. Adding ATP or UTP to flow medium without FBS restored the ability of fluid flow to increase [Ca(2+)](i), suggesting that ATP or UTP may mediate the effect of fluid flow on [Ca(2+)](i). Furthermore, adenosine, ADP, UDP, or adenosine 5'-O-(3-thiotriphosphate) did not induce Ca(2+)(i) mobilization under oscillatory fluid flow without FBS. Pyridoxal phosphate 6-azophenyl-2,4'-disulfonic acid, an antagonist of P2X purinoceptors, did not alter the effect of fluid flow on the Ca(2+)(i) response, whereas pertussis toxin, a G(i/o)-protein inhibitor, inhibited fluid flow-induced increases in [Ca(2+)](i) in the presence of 2% FBS. Thus, by the process of elimination, our data suggest that P2Y purinoceptors (P2Y2 or P2Y4) are involved in the Ca(2+)(i) response to fluid flow. Finally, a decreased percentage of MC3T3-E1 osteoblastic cells treated with P2Y2 antisense oligodeoxynucleotides responded to fluid flow with an increase in [Ca(2+)](i), and an increased percentage of ROS 17/2.8 cells, which do not normally express P2Y2 purinoceptors, transfected with P2Y2 purinoceptors responded to fluid flow in the presence of 2% FBS, confirming that P2Y2 purinoceptors are responsible for oscillatory fluid flow-induced Ca(2+)(i) mobilization. Our findings shed new light of the molecular mechanisms responsible for oscillatory fluid flow-induced Ca(2+)(i) mobilization in osteoblastic cells.     

Role of Mos/MEK/ERK cascade and Cdk1 in Ca2+ oscillations in fertilized ascidian eggs

Intracellular calcium ion concentration ([Ca(2+)](i)) transients are observed in the fertilized eggs of all species investigated so far, and are critical for initiating several events related to egg activation and cell cycle control. Here, we investigated the role of the Mos/MEK/ERK cascade and Cdk1 on Ca(2+) oscillations in fertilized ascidian eggs. The egg of the ascidian Phallusia nigra shows [Ca(2+)](i) oscillations after fertilization: Ca(2+) waves immediately following fertilization (phase I), and [Ca(2+)](i) oscillations between the first and second polar body extrusions (phase II). Our results show that in P. nigra eggs, ERK activity peaked just before the extrusion of the first polar body, and decreased gradually, eventually disappearing at the extrusion of the second polar body. Cyclin-dependent protein kinase 1(Cdk1) activity decreased to undetectable levels immediately after fertilization, and then periodically increased according to the meiotic and mitotic cell cycle. When the unfertilized eggs were incubated with U0126, an inhibitor of MEK, before insemination, ERK was immediately inactivated, and the phase II [Ca(2+)](i) oscillations disappeared. Alternatively, when the constitutively active Mos protein (GST-Mos) was injected into the unfertilized eggs, ERK activity was preserved for at least 120 min after fertilization, and the phase II [Ca(2+)](i) oscillations lasted for more than 120 min after the second polar body extrusion. These results suggest that ERK activity is necessary for maintaining [Ca(2+)](i) oscillations. GST-ΔN85-cyclin, which maintains Cdk1 activity, caused ERK activity in the eggs to persist for over 120 min after fertilization, and prolonged [Ca(2+)](i) oscillations. Moreover, the effects of GST-ΔN85-cyclin on the egg were abrogated by the application of U0126. Thus, Cdk1-mediated [Ca(2+)](i) oscillations seem to require ERK activity. However, GST-Mos triggered [Ca(2+)](i) oscillations after the second polar body extrusion, whereas GST-ΔN85-cyclin did not, although it prolongs the duration of [Ca(2+)](i) oscillations. Interestingly, GST-ΔN85-cyclin increased the frequency of [Ca(2+)](i) transients in the Mos-induced [Ca(2+)](i) oscillations after the extrusion of the second polar body. Thus, Cdk1 could maintain, but not activate, ERK and [Ca(2+)](i) oscillations. ERK activity and [Ca(2+)](i) oscillations seem to form a negative feedback loop which may be responsible for maintaining the meiotic period.     

Mechanosensitivity of bone cells to oscillating fluid flow induced shear stress may be modulated by chemotransport

Fluid flow has been shown to be a potent physical stimulus in the regulation of bone cell metabolism. In addition to membrane shear stress, loading-induced fluid flow will enhance chemotransport due to convection or mass transport thereby affecting the biochemical environment surrounding the cell. This study investigated the role of oscillating fluid flow induced shear stress and chemotransport in cellular mechanotransduction mechanisms in bone. Intracellular calcium mobilization and prostaglandin E(2) (PGE(2)) production were studied with varying levels of shear stress and chemotransport. In this study MC3T3-E1 cells responded to oscillating fluid flow with both an increase in intracellular calcium concentration ([Ca(2+)](i)) and an increase in PGE(2) production. These fluid flow induced responses were modulated by chemotransport. The percentage of cells responding with an [Ca(2+)](i) oscillation increased with increasing flow rate, as did the production of PGE(2). In addition, depriving the cells of nutrients during fluid flow resulted in an inhibition of both [Ca(2+)](i) mobilization and PGE(2) production. These data suggest that depriving the cells of a yet to be determined biochemical factor in media affects the responsiveness of bone cells even at a constant peak shear stress. Chemotransport alone will not elicit a response, but it appears that sufficient nutrient supply or waste removal is needed for the response to oscillating fluid flow induced shear stress.     

Ca(2+)](i) oscillations regulate type II cell exocytosis in the pulmonary alveolus

Pulmonary surfactant, a critical determinant of alveolar stability, is secreted by alveolar type II cells by exocytosis of lamellar bodies (LBs). To determine exocytosis mechanisms in situ, we imaged single alveolar cells from the isolated blood-perfused rat lung. We quantified cytosolic Ca(2+) concentration ([Ca(2+)](i)) by the fura 2 method and LB exocytosis as the loss of cell fluorescence of LysoTracker Green. We identified alveolar cell type by immunofluorescence in situ. A 15-s lung expansion induced synchronous [Ca(2+)](i) oscillations in all alveolar cells and LB exocytosis in type II cells. The exocytosis rate correlated with the frequency of [Ca(2+)](i) oscillations. Fluorescence of the lipidophilic dye FM1-43 indicated multiple exocytosis sites per cell. Intracellular Ca(2+) chelation and gap junctional inhibition each blocked [Ca(2+)](i) oscillations and exocytosis in type II cells. We demonstrated the feasibility of real-time quantifications in alveolar cells in situ. We conclude that in lung expansion, type II cell exocytosis is modulated by the frequency of intercellularly communicated [Ca(2+)](i) oscillations that are likely to be initiated in type I cells. Thus during lung inflation, type I cells may act as alveolar mechanotransducers that regulate type II cell secretion.     

Glucose-induced metabolic oscillations parallel those of Ca(2+) and insulin release in clonal insulin-secreting cells. A multiwell approach to oscillatory cell behavior

Insulin secretion from glucose-stimulated pancreatic beta-cells is oscillatory, and this is thought to result from oscillations in glucose metabolism. One of the primary metabolic stimulus-secretion coupling factors is the ATP/ADP ratio, which can oscillate as a result of oscillations in glycolysis. Using a novel multiwell culture plate system, we examined oscillations in insulin release and the ATP/ADP ratio in the clonal insulin-secreting cell lines HIT T-15 and INS-1. Insulin secretion from HIT cells grown in multiwell plates oscillated with a period of 4 min, similar to that seen previously in perifusion experiments. Oscillations in the ATP/ADP ratio in cells grown under the same conditions also occurred with a period of 4 min, as did oscillations in [Ca(2+)](i) monitored by fluorescence microscopy. In INS-1 cells oscillations in insulin secretion, the ATP/ADP ratio, and [Ca(2+)](i) were also seen, but with a shorter period of about 1.5 min. These observations of oscillations in the ATP/ADP ratio are consistent with their proposed role in driving the oscillations in [Ca(2+)](i) and insulin secretion. Furthermore, these data show that, at least in the clonal beta-cell lines, cell contact or even circulatory connection is not necessary for synchronous oscillations induced by a rise in glucose.     

Pressure-induced endothelial Ca(2+) oscillations in lung capillaries

Endothelial second messenger responses may contribute to the pathology of high vascular pressure but remain poorly understood because of the lack of direct in situ quantification. In lung venular capillaries, we determined endothelial cytosolic Ca(2+) concentration [Ca(2+)](i) by the fura 2 ratioing method. Pressure elevation increased mean endothelial [Ca(2+)](i) by Ca(2+) influx through gadolinium-inhibitable channels and amplified [Ca(2+)](i) oscillations by Ca(2+) release from intracellular stores. Endothelial [Ca(2+)](i) transients were induced by pressure elevations of as little as 5 cmH(2)O and increased linearly with higher pressures. Heptanol inhibition of [Ca(2+)](i) oscillations in a subset of endothelial cells indicated that oscillations originated from pacemaker endothelial cells and were propagated to adjacent nonpacemaker cells by gap junctional communication. Our findings indicate the presence of a sensitive, active endothelial response to pressure challenge in lung venular capillaries that may be relevant in the pathogenesis of pressure-induced lung microvascular injury.     

Stimulation of human spermatozoa with progesterone gradients to simulate approach to the oocyte. Induction of [Ca(2+)](i) oscillations and cyclical transitions in flagellar beating

Progesterone is present at micromolar concentrations in the cumulus matrix, which surrounds mammalian oocytes. Exposure of human spermatozoa to a concentration gradient of progesterone (0-3 microM) to simulate approach to the oocyte induced a slowly developing increase in [Ca(2+)](i) upon which, in many cells, slow oscillations were superimposed. [Ca(2+)](i) oscillations often started at very low progesterone (<10 nm), and their frequency did not change during the subsequent rise in concentration. Oscillations also occurred, but in a much smaller proportion of cells, in response to stepped application of progesterone (3 microM). When progesterone was removed, [Ca(2+)](i) oscillations often persisted or quickly resumed. Superfusion with low-Ca(2+) bathing medium (no added Ca(2+)) did not prevent [Ca(2+)](i) oscillations, but they could be abolished by addition of EGTA or La(3+). Inhibitors of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases or inositol trisphosphate signaling had no effect on [Ca(2+)](i) oscillations, but pharmacological manipulation of ryanodine receptors affected both their frequency and amplitude. Staining of live spermatozoa with BODIPY FL-X ryanodine showed localization of ryanodine binding primarily to the caudal part of the head and mid-piece. [Ca(2+)](i) oscillations did not induce acrosome reaction, but in cells generating oscillations, the flagellar beat mode alternated in synchrony with the oscillation cycle. Flagellar bending and lateral movement of the sperm head during [Ca(2+)](i) peaks were markedly increased compared with during [Ca(2+)](i) troughs. This alternating pattern of activity is likely to facilitate zona penetration. These observations show that progesterone initiates unusual and complex store-mediated [Ca(2+)](i) signaling in human spermatozoa and identify a previously unrecognized effect of progesterone in regulating sperm "behavior" during fertilization.     

ATP autocrine/paracrine signaling induces calcium oscillations and NFAT activation in human mesenchymal stem cells

Human bone marrow-derived mesenchymal stem cells (hMSCs) have the potential to differentiate into several types of cells. Calcium ions (Ca(2+)) play an important role in the differentiation and proliferation of hMSCs. We have demonstrated that spontaneous [Ca(2+)](i) oscillations occur without agonist stimulation in hMSCs. However, the precise mechanism of its generation remains unclear. In this study, we investigated the mechanism and role of spontaneous [Ca(2+)](i) oscillations in hMSCs and found that IP(3)-induced Ca(2+) release is essential for spontaneous [Ca(2+)](i) oscillations. We also found that an ATP autocrine/paracrine signaling pathway is involved in the oscillations. In this pathway, an ATP is secreted via a hemi-gap-junction channel; it stimulates the P(2)Y(1) receptors, resulting in the activation of PLC-beta to produce IP(3). We were able to pharmacologically block this pathway, and thereby to completely halt the [Ca(2+)](i) oscillations. Furthermore, we found that [Ca(2+)](i) oscillations were associated with NFAT translocation into the nucleus in undifferentiated hMSCs. Once the ATP autocrine/paracrine signaling pathway was blocked, it was not possible to detect the nuclear translocation of NFAT, indicating that the activation of NFAT is closely linked to [Ca(2+)](i) oscillations. As the hMSCs differentiated to adipocytes, the [Ca(2+)](i) oscillations disappeared and the translocation of NFAT ceased. These results provide new insight into the molecular and physiological mechanism of [Ca(2+)](i) oscillations in undifferentiated hMSCs.     

Cycle number and waveform of fluid flow affect bovine articular chondrocytes

Many studies have shown that a loading-induced (bio)physical signal regulates chondrocyte behavior. In a recent study our group has demonstrated the shear stress level- and frequency-dependent effect of sinusoidal oscillatory fluid flow on bovine articular chondrocyte (BAC) cytosolic calcium concentration ([Ca(2+)](i)), neglecting the fact that chondrocytes are not likely to see these ideal waveform in vivo or in vitro. Furthermore, possible overload of articular cartilage or excessive shear stress in chondrocyte cultures are more likely to be of a short nature. Therefore, in this study we choose to investigate a saw-tooth waveform oscillating fluid flow at varying exposure times in comparison to the established sinusoidal oscillatory waveform. [Ca(2+)](i), as an early signaling molecule, was quantified using the fluorescent dye fura-2. BAC were exposed to 1 Hz sinusoidal or saw-tooth waveform oscillating fluid flow at 2.2 Pa flow rates in a parallel plate flow chamber for 8 different loading times. As little as 5 cycles of oscillatory fluid flow were sufficient to increase [Ca(2+)](i) significantly over baseline. The number of responding cells could not be increased any further after a sufficient number of cycles (11), regardless of the waveform. Furthermore, a saw-tooth waveform appeared to be more stimulatory than regular sinusoidal oscillating flow at higher cycle numbers. BAC appear to be able to respond to these biophysical stimuli in a differentiated manner. This ability might give every single chondrocyte the capability to maintain its territory autonomously, since chondrocytes distributed in articular cartilage without the possibility to interact, e.g., via cell processes.     

Synchronization and entrainment of cytoplasmic Ca2+ oscillations in cell clusters prepared from single or multiple mouse pancreatic islets

In contrast to pancreatic islets, isolated beta-cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). Here, clusters of 5-30 cells were prepared from a single mouse islet or from pools of islets, loaded with fura-2, and studied with a camera-based system. [Ca(2+)](i) oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index lambda. During perifusion with 12 mM glucose, the clusters exhibited regular [Ca(2+)](i) oscillations that were quasi-perfectly synchronized (Delta period of 1.4% and index lambda close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized, even when prepared from one single islet. Pairs of clusters neighboring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12 mM glucose by applying external signals. A single pulse of 20 mM glucose, 10 mM amino acids, or 10 microM tolbutamide transiently altered [Ca(2+)](i) oscillations but did not reset the clusters to oscillate synchronously. On a background of 12 mM glucose, repetitive applications (1 min/5 min) of 10 microM tolbutamide, but not of 20 mM glucose, synchronized separate clusters. Our results identify a level of beta-cell heterogeneity intermediate between single beta-cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.     

Dichlorodiphenylchloroethylene elevates cytosolic calcium concentrations and oscillations in primary cultures of human granulosa-lutein cells

1,1-Dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), a metabolite of DDT (1,1-dichlorodiphenyltrichloroethane), is a persistent hormonally active environmental toxicant that has been found in human serum and follicular fluid. The objective of this study was to determine whether DDE can alter free calcium ion concentrations in the cytosol ([Ca(2+)](cyt)) of human granulosa cells. Changes in [Ca(2+)](cyt) in single cells loaded with Fura-2 were studied using a dynamic digital Ca(2+) imaging system. At a concentration of 100 ng/ml, DDE stimulated small elevations of [Ca(2+)](cyt) accompanied by Ca(2+) oscillations. At 1 microg DDE/ml, there was a biphasic Ca(2+) response with marked elevations of [Ca(2+)](cyt) over time. In Ca(2+)-free medium, cells showed an initial small elevation of [Ca(2+)](cyt), which was magnified after addition of Ca(2+) to the medium. Washing the cells after DDE treatment failed to remove the elevated [Ca(2+)](cyt) and oscillations, both of which were eliminated by addition of EGTA. ATP also induced [Ca(2+)](cyt) elevations and oscillations, and these effects were potentiated when DDE was added. FSH induced transient [Ca(2+)](cyt) elevations, whereas hCG caused a prolonged elevation and marked oscillations in [Ca(2+)](cyt). These results suggest that DDE at concentrations normally found in human tissues induces elevations in [Ca(2+)](cyt) in granulosa-lutein cells. Our data therefore highlight a novel mechanism through which DDE can alter endocrine homeostasis and possibly act as an endocrine toxicant.     

Ca2+-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor require negative feedback by protein kinase C

We examined the role of protein kinase C (PKC) in the mechanism and regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations elicited by an increase in the extracellular concentration of Ca(2+) ([Ca(2+)](e)) in human embryonic kidney 293 cells expressing the Ca(2+)-sensing receptor (CaR). Exposure to the PKC inhibitors bisindolylmaleimide I (GF I) or Ro-31-8220 converted oscillatory responses to transient, non-oscillatory responses, significantly reducing the percentage of cells that showed [Ca(2+)](i) oscillations but without decreasing the overall response to increase in [Ca(2+)](e). Exposure to 100 nm phorbol 12,13-dibutyrate, a direct activator of PKC, eliminated [Ca(2+)](i) oscillations. Addition of phorbol 12,13-dibutyrate at lower concentrations (3 and 10 nm) did not eliminate the oscillations but greatly reduced their frequency in a dose-dependent manner. Co-expression of CaR with constitutively active mutants of PKC (either epsilon or beta(1) isoforms) also reduced [Ca(2+)](i) oscillation frequency. Expression of a mutant CaR in which the major PKC phosphorylation site is altered by substitution of alanine for threonine (T888A) eliminated oscillatory behavior, producing [Ca(2+)](i) responses almost identical to those produced by the wild type CaR exposed to PKC inhibitors. These results support a model in which phosphorylation of the CaR at the inhibitory threonine 888 by PKC provides the negative feedback needed to cause [Ca(2+)](i) oscillations mediated by this receptor.     

Arachidonic acid in astrocytes blocks Ca(2+) oscillations by inhibiting store-operated Ca(2+) entry,and causes delayed Ca(2+) influx

ATP-elicited oscillations of the concentration of free intracellular Ca(2+) ([Ca(2+)](i)) in rat brain astrocytes were abolished by simultaneous arachidonic acid (AA) addition, whereas the tetraenoic analogue 5,8,11,14-eicosatetraynoic acid (ETYA) was ineffective. Inhibition of oscillations is due to suppression by AA of intracellular Ca(2+) store refilling. Short-term application of AA, but not ETYA, blocked Ca(2+) influx, which was evoked by depletion of stores with cyclopiazonic acid (CPA) or thapsigargin (Tg). Addition of AA after ATP blocked ongoing [Ca(2+)](i) oscillations. Prolonged AA application without or with agonist could evoke a delayed [Ca(2+)](i) increase. This AA-induced [Ca(2+)](i) rise developed slowly, reached a plateau after 5 min, could be reversed by addition of bovine serum albumin (BSA), that scavenges AA, and was blocked by 1 microM Gd(3+), indicative for the influx of extracellular Ca(2+). Specificity for AA as active agent was demonstrated by ineffectiveness of C16:0, C18:0, C20:0, C18:2, and ETYA. Moreover, the action of AA was not affected by inhibitors of oxidative metabolism of AA (ibuprofen, MK886, SKF525A). Thus, AA exerted a dual effect on astrocytic [Ca(2+)](i), firstly, a rapid reduction of capacitative Ca(2+) entry thereby suppressing [Ca(2+)](i) oscillations, and secondly inducing a delayed activation of Ca(2+) entry, also sensitive to low Gd(3+) concentration.     

Albumin-bound lipids induce free cytoplasmic calcium oscillations in human osteoblast-like cells

[Ca(2+)](i) oscillations were found in human osteoblast-like cells (hOB cells) exposed to high-lipid bovine serum albumin (BSA), but not when exposed to low-lipid BSA. These [Ca(2+)](i) oscillations were inhibited by heptanol and suramin, which implies that gap junctions and purinergic signalling may be important for these [Ca(2+)](i) oscillations. The high-lipid BSA preparation that was used contains arachidonic acid. [Ca(2+)](i) oscillations could be induced by low lipid albumin with arachidonic acid added. The albumin-bound lipids were also important for osteoblast growth since DNA synthesis and the total cell protein content was higher in hOB cells exposed to high-lipid BSA. The effect of arachidonic acid on hOB cell proliferation was bone-donor dependent; both stimulatory and inhibitory effects were observed. The physiological importance of albumin-bound lipids is unclear; given that albumin has only minimal contact with osteoblasts under normal conditions. Only when bone capillaries are disrupted, e.g. during a fracture, would significant amounts of albumin reach osteoblasts. Albumin-bound lipids could therefore contribute to stimulation of osteoblast proliferation during fracture healing.     

Mitochondrial calcium oscillations in C2C12 myotubes

Mitochondrial Ca(2+) concentration ([Ca(2+)](m)) was monitored in C2C12 skeletal muscle cells stably expressing the Ca(2+)-sensitive photoprotein aequorin targeted to mitochondria. In myotubes, KCl-induced depolarization caused a peak of 3.03 +/- 0.14 micrometer [Ca(2+)](m) followed by an oscillatory second phase (5.1 +/- 0.1 per min). Chelation of extracellular Ca(2+) or blockade of the voltage-operated Ca(2+) channel attenuated both phases of the KCl response. The inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, cyclopiazonic acid, reduced the amplitude of the KCl-induced [Ca(2+)](m) peak and prevented the oscillations, suggesting that these were generated intracellularly. No such [Ca(2+)](m) oscillations occurred with the nicotinic agonist carbachol, cyclopiazonic acid alone, or the purinergic agonist ATP. In contrast, caffeine produced an oscillatory behavior, indicating a role of ryanodine receptors as mediators of the oscillations. The [Ca(2+)](m) response was desensitized when cells were exposed to two consecutive challenges with KCl separated by a 5-min wash, whereas a second pulse of carbachol potentiated [Ca(2+)](m), indicating differences in intracellular Ca(2+) redistribution. Cross-desensitization between KCl and carbachol and cross-potentiation between carbachol and KCl were observed. These results suggest that close contacts between mitochondria and sarcoplasmic reticulum exist permitting Ca(2+) exchanges during KCl depolarization. These newly demonstrated dynamic changes in [Ca(2+)](m) in stimulated skeletal muscle cells might contribute to the understanding of physiological and pathological processes in muscular disorders.     

Ca(2+) oscillations regulated by Na(+)-Ca(2+) exchanger and plasma membrane Ca(2+) pump induce fluctuations of membrane currents and potentials in human mesenchymal stem cells

Human bone marrow-derived mesenchymal stem cells (hMSCs) have the potential to differentiate into several types of cells. We have demonstrated spontaneous [Ca(2+)](i) oscillations in hMSCs without agonist stimulation, which result primarily from release of Ca(2+) from intracellular stores via InsP(3) receptors. In this study, we further investigated functions and contributions of Ca(2+) transporters on plasma membrane to generate [Ca(2+)](i) oscillations. In confocal Ca(2+) imaging experiments, spontaneous [Ca(2+)](i) oscillations were observed in 193 of 280 hMSCs. The oscillations did not sustain in the Ca(2+) free solution and were completely blocked by the application of 0.1mM La(3+). When plasma membrane Ca(2+) pumps (PMCAs) were blocked with blockers, carboxyeosin or caloxin, [Ca(2+)](i) oscillations were inhibited. Application of Ni(2+) or KBR7943 to block Na(+)-Ca(2+) exchanger (NCX) also inhibited [Ca(2+)](i) oscillations. Using RT-PCR, mRNAs were detected for PMCA type IV and NCX, but not PMCA type II. In the patch clamp experiments, Ca(2+) activated outward K(+) currents (I(KCa)) with a conductance of 170+/-21.6pS could be recorded. The amplitudes of I(KCa) and membrane potential (V(m)) periodically fluctuated liked to [Ca(2+)](i) oscillations. These results suggest that in undifferentiated hMSCs both Ca(2+) entry through plasma membrane and Ca(2+) extrusion via PMCAs and NCXs play important roles for [Ca(2+)](i) oscillations, which modulate the activities of I(KCa) to produce the fluctuation of V(m).     

Requirement of the TRPC1 cation channel in the generation of transient Ca2+ oscillations by the calcium-sensing receptor

The calcium-sensing receptor (CaR) is an allosteric protein that responds to extracellular Ca(2+) ([Ca(2+)](o)) and aromatic amino acids with the production of different patterns of oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). An increase in [Ca(2+)](o) stimulates phospholipase C-mediated production of inositol 1,4,5-trisphosphate and causes sinusoidal oscillations in [Ca(2+)](i). Conversely, aromatic amino acid-induced CaR activation does not stimulate phospholipase C but engages an unidentified signaling mechanism that promotes transient oscillations in [Ca(2+)](i). We show here that the [Ca(2+)](i) oscillations stimulated by aromatic amino acids were selectively abolished by TRPC1 down-regulation using either a pool of small inhibitory RNAs (siRNAs) or two different individual siRNAs that targeted different coding regions of TRPC1. Furthermore, [Ca(2+)](i) oscillations stimulated by aromatic amino acids were also abolished by inhibition of TRPC1 function with an antibody that binds the pore region of the channel. We also show that aromatic amino acid-stimulated [Ca(2+)](i) oscillations can be prevented by protein kinase C (PKC) inhibitors or siRNA-mediated PKCalpha down-regulation and impaired by either calmodulin antagonists or by the expression of a dominant-negative calmodulin mutant. We propose a model for the generation of CaR-mediated transient [Ca(2+)](i) oscillations that integrates its stimulation by aromatic amino acids with TRPC1 regulation by PKC and calmodulin.     

Modulation of Ca2+ transients in cultured endothelial cells in response to fluid flow through alphav integrin

In order to determine whether integrin dynamics is associated with intracellular Ca(2+) concentration ([Ca(2+)](i)) mobilization in ECs in response to hemodynamic forces, changes in [Ca(2+)](i) in fluo-4-loaded cultured bovine aortic endothelial cells (BAECs) under fluid flow conditions were visualized employing laser scanning confocal microscopy. Following the onset of flow stimulus, transient increases in [Ca(2+)](i) occurred several times in individual BAECs during the 30-min observation period. The frequency of these [Ca(2+)](i) transients was clearly reduced by the application of an integrin antagonist (GRGDSP peptide). Furthermore, treatment of cells with an integrin activator (Mn(2+)) resulted in reduction of peak [Ca(2+)](i) levels and elevated frequency, which was markedly rescued upon GRGDSP administration. In contrast, an actin de-polymerizing agent (cytochalasin D) exerted no inhibitory effects; rather, cytochalasin D more likely facilitated [Ca(2+)](i) transients. Moreover, [Ca(2+)](i) transients, which were suppressed by short interference RNA-induced silencing of alphav integrin, exhibited greater frequently in cells cultured on vitronectin substratum in comparison with those cultured on fibronectin or collagen substratum. Either removal of extracellular Ca(2+), application of an inhibitor of endoplasmic reticulum Ca(2+)-ATPase (thapsigargin) or non-selective cation channel blocker (La(3+)) inhibited the [Ca(2+)](i) transients. Additionally, [Ca(2+)](i) transients were attenuated by extracellular signal-regulated kinase (ERK) kinase inhibitor (U0126); in contrast, [Ca(2+)](i) transients were unaffected by tyrosine kinase inhibitor (genistein) or phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002). Therefore, our findings revealed that alphav integrin dynamics modulates the frequency of flow-induced [Ca(2+)](i) transients in BAECs in an ERK-dependent fashion.     

Synchronization of electrically induced calcium firings in self-assembled cardiac cells

We study the adaptive changes of a population of cells responding to external stimulus. Two-dimensionally distributed cardiac cells were homogeneously subjected to periodic electrical stimulus and intracellular calcium concentration ([Ca(2+)](i)) changes were simultaneously observed. In the absence of stimulation, coupled cells in monolayer formed groups of several cells oscillating in similar phase, while isolated cells showed irregular periodicity. In both systems, [Ca(2+)](i) oscillations were modulated by periodic stimulation, and ascending degrees of synchronization among [Ca(2+)](i) oscillations were shown as stimulation intensity increased. In a population of coupled cells, the cells act like a single robust oscillator. These results are evaluated using statistical calculations, comparing the response manner of isolated cells.