2-Aminoethoxydiphenyl borate (2-APB) antagonises inositol 1,4,5-trisphosphate-induced calcium release, inhibits calcium pumps and has a use-dependent and slowly reversible action on store-operated calcium entry channels

Ca2+ signaling in prothoracicotropic hormone-stimulated prothoracic gland cells of Manduca sexta: evidence for mobilization and entry mechanisms

Prothoracicotropic hormone (PTTH) stimulates ecdysteroidogenesis in lepidopteran prothoracic glands (PGs), thus indirectly controlling molting and metamorphosis. PTTH triggers a signal transduction cascade in PGs that involves an early influx of Ca2+. Although the importance of Ca2+ has been long known, the mechanism(s) of PTTH-stimulated changes in cytoplasmic Ca2+ [Ca2+]i are not yet well understood. PGs from the fifth instar of Manduca sexta were exposed to PTTH in vitro. The resultant changes in [Ca2+]i were measured using ratiometric analysis of a fura-2 fluorescence signal in the presence and absence of inhibitors of specific cellular signaling mechanisms. The phospholipase C (PLC) inhibitor U-73122 nearly abolished the PTTH-stimulated increase in [Ca2+]i, as well as PTTH-stimulated ecdysteroidogenesis and extracellular-signal regulated kinase phosphorylation, thus establishing a role for PLC and implicating inositol trisphosphate (IP3) in PTTH signal transduction. Two antagonists of the IP3 receptor, 2-APB and TMB-8, likewise blocked the [Ca2+]i response by a mean of 92%. We describe for the first time the presence of Ca2+ oscillations in PTTH-stimulated cells in Ca2+-free medium. External Ca2+ entered PG cells via at least two routes: store-operated (capacitative) Ca2+ entry channels and L-type voltage-gated Ca2+ channels. We propose that PTTH initiates a transductory cascade typical of many G-protein coupled receptors, involving both Ca2+ mobilization and entry pathways.

Calcium oscillations in interstitial cells of the rabbit urethra

Measurements were made (using fast confocal microscopy) of intracellular Ca2+ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra. These cells exhibited regular Ca2+ oscillations which were associated with spontaneous transient inward currents recorded under voltage clamp. Interference with D-myo-inositol 1,4,5-trisphosphate (IP3) induced Ca2+ release using 100 microm 2-aminoethoxydiphenyl borate, and the phospholipase C (PLC) inhibitors 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate and U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when ryanodine receptors were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external Ca2+, and frequency was directly proportional to the external Ca2+ concentration. Frequency of Ca2+ oscillation was reduced by SKF-96365, but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. These results suggest that Ca2+ oscillations in isolated rabbit urethral interstitial cells are initiated by Ca2+ release from ryanodine-sensitive intracellular stores, that oscillation frequency is very sensitive to the external Ca2+ concentration and that conversion of the primary oscillation to a propagated Ca2+ wave depends upon IP3-induced Ca2+ release.

Agonist activation of arachidonate-regulated Ca2+-selective (ARC) channels in murine parotid and pancreatic acinar cells

ARC channels (arachidonate-regulated Ca(2+)-selective channels) are a novel type of highly Ca(2+)-selective channel that are specifically activated by low concentrations of agonist-induced arachidonic acid. This activation occurs in the absence of any depletion of internal Ca(2+) stores (i.e. they are 'non-capacitative'). Previous studies in HEK293 cells have shown that these channels provide the predominant pathway for the entry of Ca(2+) seen at low agonist concentrations where oscillatory [Ca(2+)](i) signals are typically produced. In contrast, activation of the more widely studied store-operated Ca(2+) channels (e.g. CRAC channels) is only seen at higher agonist concentrations where sustained 'plateau-type'[Ca(2+)](i) responses are observed. We have now demonstrated the presence of ARC channels in both parotid and pancreatic acinar cells and shown that, again, they are specifically activated by the low concentrations of appropriate agonists (carbachol in the parotid, and both carbachol and cholecystokinin in the pancreas) that are associated with oscillatory [Ca(2+)](i) signals in these cells. Uncoupling the receptor-mediated activation of cytosolic phospholipase A(2) (cPLA(2)) with isotetrandrine reduces the activation of the ARC channels by carbachol and, correspondingly, markedly inhibits the [Ca(2+)](i) signals induced by low carbachol concentrations, whilst those signals seen at high agonist concentrations are essentially unaffected. Interestingly, in the pancreatic acinar cells, activation by cholecystokinin induces a current through the ARC channels that is only approximately 60% of that seen with carbachol. This is consistent with previous reports indicating that carbachol-induced [Ca(2+)](i) signals in these cells are much more dependent on Ca(2+) entry than are the cholecystokinin-induced responses.

Deoxycholic acid activates protein kinase C and phospholipase C via increased Ca2+ entry at plasma membrane

BACKGROUND & AIMS

Secondary bile acids like deoxycholic acid (DCA) are well-established tumor promoters that may exert their pathologic actions by interfering with intracellular signaling cascades.

METHODS

We evaluated the effects of DCA on Ca2+ signaling in BHK-21 fibroblasts using fura-2 and mag-fura-2 to measure cytoplasmic and intraluminal internal stores [Ca2+], respectively. Furthermore, green fluorescent protein (GFP)-based probes were used to monitor time courses of phospholipase C (PLC) activation (pleckstrin-homology [PH]-PLCdelta-GFP), and translocation of protein kinase C (PKC) and a major PKC substrate, myristolated alanine-rich C-kinase substrate (MARCKS).

RESULTS

DCA (50-250 micromol/L) caused profound Ca2+ release from intracellular stores of intact or permeabilized cells. Correspondingly, DCA increased cytoplasmic Ca2+ to levels that were approximately 120% of those stimulated by Ca2+-mobilizing agonists in the presence of external Ca2+, and approximately 60% of control in Ca2+-free solutions. DCA also caused dramatic translocation of PH-PLCdelta-GFP, and conventional, Ca2+/diacylglycerol (DAG)-dependent isoforms of PKC (PKC-betaI and PKC-alpha), and MARCKS-GFP, but only in Ca2+-containing solutions. DCA had no effect on localization of a novel (PKCdelta) or an atypical (PKCzeta) PKC isoform.

CONCLUSIONS

Data are consistent with a model in which DCA directly induces both Ca2+ release from internal stores and persistent Ca2+ entry at the plasma membrane. The resulting microdomains of high Ca2+ levels beneath the plasma membrane appear to directly activate PLC, resulting in modest InsP 3 and DAG production. Furthermore, the increased Ca2+ entry stimulates vigorous recruitment of conventional PKC isoforms to the plasma membrane.

Dopamine release evoked by beta scorpion toxin, tityus gamma, in prefrontal cortical slices is mediated by intracellular calcium stores

1. We have investigated the effect of tityus gamma (TiTX gamma) scorpion toxin on the release of [3H]dopamine in rat brain prefrontal cortical slices. The stimulatory effect of TiTX gamma on the release of [3H]dopamine was dose/time-dependent with an EC50 of 0.01 microM. 2. Tetrodotoxin blocked the TiTX gamma-induced release of [3H]dopamine, indicating the dependency for Na+ channels. 3. EGTA had no effect on the TiTX gamma-induced release of [3H]dopamine, indicating the process is independent of extracellular calcium. Release of [3H]dopamine evoked by TiTX gamma was inhibited by 57% by BAPTA, a chelator of intracellular calcium. 4. Xestospongin and 2-APB, putative blockers of IP3-sensitive release of intracellular calcium stores, caused an equal and significant inhibition of 24% of the TiTX gamma-induced release of [3H]dopamine, while the slight inhibition evoked by dantrolene, a putative blocker of ryanodine-sensitive calcium store was not significant. 5. Nomifensine and ascorbic acid, blockers of dopamine transporter (DAT), caused an inhibition of 27 and 29%, respectively, on the toxin-induced release of [3H]dopamine suggesting that most of the TiTX gamma-induced release of dopamine is not due to the reversal of Na+ gradient. 6. In conclusion the majority of the TiTX gamma-induced release of [3H]dopamine is exocytotic and mobilizes calcium from the intracellular IP3-sensitive calcium stores.

Evidence for signaling via gap junctions from smooth muscle to endothelial cells in rat mesenteric arteries: possible implication of a second messenger

We investigated heterocellular communication in rat mesenteric arterial strips at the cellular level using confocal microscopy. To visualize Ca(2+) changes in different cell populations, smooth muscle cells (SMCs) were loaded with Fluo-4 and endothelial cells (ECs) with Fura red. SMC contraction was stimulated using high K(+) solution and Phenylephrine. Depending on vasoconstrictor concentration, intracellular Ca(2+) concentration ([Ca(2+)](i)) increased in a subpopulation of ECs 5-11s after a [Ca(2+)](i) rise was observed in adjacent SMCs. This time interval suggests chemical coupling between SMCs and ECs via gap junctions. As potential chemical mediators we investigated Ca(2+) or inositol 1,4,5-trisphosphate (IP(3)). First, phospholipase C inhibitor U-73122 was added to prevent IP(3) production in response to the [Ca(2+)](i) increase in SMCs. In high K(+) solution, all SMCs presented global and synchronous [Ca(2+)](i) increase, but no [Ca(2+)](i) variations were detected in ECs. Second, 2-aminoethoxydiphenylborate, an inhibitor of IP(3)-induced Ca(2+) release, reduced the number of flashing ECs by 75+/-3% (n = 6). The number of flashing ECs was similarly reduced by adding the gap junction uncoupler palmitoleic acid. Thus, our results suggest a heterocellular communication through gap junctions from SMCs to ECs by diffusion, probably of IP(3).

Hypoxic modulation of Ca2+ signaling in human venous endothelial cells. Multiple roles for reactive oxygen species

The effects of hypoxia (pO2 approximately 25 mm Hg) on Ca2+ signaling stimulated by extracellular ATP in human saphenous vein endothelial cells were investigated using fluorimetric recordings from Fura-2 loaded cells. In the absence of extracellular Ca2+, ATP-evoked rises of cytosolic Ca2+ concentration ([Ca2+]i) because of mobilization from the endoplasmic reticulum (ER). These responses were reduced by prior exposure to hypoxia but potentiated during hypoxia. Hypoxia itself liberated Ca2+ from the ER, but unlike the effects of ATP this effect was not inhibited by blockade of the inositol trisphosphate receptor. By contrast, ryanodine blocked the effects of hypoxia but not those of ATP. Antioxidants abolished the effects of hypoxia but potentiated the effects of ATP. Inhibition of NADPH oxidase also augmented ATP-evoked responses but was without effect on hypoxia-evoked rises of [Ca2+]i. However, either uncoupling mitochondrial electron transport or inhibiting complex I markedly suppressed the actions of hypoxia yet exerted only small inhibitory effects on ATP-evoked rises of [Ca2+]i. Both hypoxia and ATP were able to activate capacitative Ca2+ entry. Our results indicate that hypoxia regulates intracellular Ca2+ signaling via two distinct pathways. First, it modulates agonist-evoked liberation of Ca2+ from the ER primarily through regulation of reactive oxygen species generation from NADPH oxidase. Second, it liberates Ca2+ from the ER via ryanodine receptors, an effect requiring mitochondrial reactive oxygen species generation. These findings suggest that local O2 tension is a major determinant of Ca2+ signaling in the vascular endothelium, a finding that is likely to be of both physiological and pathophysiological importance.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Mitochondrial Ca2+ dynamics reveals limited intramitochondrial Ca2+ diffusion

To reveal heterogeneity of mitochondrial function on the single-mitochondrion level we have studied the spatiotemporal dynamics of the mitochondrial Ca2+ signaling and the mitochondrial membrane potential using wide-field fluorescence imaging and digital image processing techniques. Here we demonstrate first-time discrete sites--intramitochondrial hotspots--of Ca2+ uptake after Ca2+ release from intracellular stores, and spreading of Ca2+ rise within the mitochondria. The phenomenon was characterized by comparison of observations in intact cells stimulated by ATP and in plasma membrane permeabilized or in ionophore-treated cells exposed to elevated buffer [Ca2+]. The findings indicate that Ca2+ diffuses laterally within the mitochondria, and that the diffusion is limited for shorter segments of the mitochondrial network. These observations were supported by mathematical simulation of buffered diffusion. The mitochondrial membrane potential was investigated using the potentiometric dye TMRM. Irradiation-induced fluctuations (flickering) of TMRM fluorescence showed synchronicity over large regions of the mitochondrial network, indicating that certain parts of this network form electrical syncytia. The spatial extension of these syncytia was decreased by 2-aminoethoxydiphenyl borate (2-APB) or by propranolol (blockers of nonclassical mitochondrial permeabilities). Our data suggest that mitochondria form syncytia of electrical conductance whereas the passage of Ca2+ is restricted to the individual organelle.

Endothelin B receptor Ca2+ signaling in shark vascular smooth muscle: participation of inositol trisphosphate and ryanodine receptors

In mammals, endothelin receptors are sub-classified into ET(A) receptors (ET(A)R), which are purely constrictive in vascular smooth muscle (VSM), and ET(B)R, which may produce constriction in VSM or dilatation by stimulating the production of nitric oxide (NO) from endothelial cells. In contrast, previous studies suggested that shark VSM is stimulated exclusively by ET(B)R. The Ca(2+) signaling pathways utilized by shark VSM in response to stimulation by endothelin-1 (ET-1) have not previously been investigated. We measured cytosolic Ca(2+) concentration ([Ca(2+)](i)) in fura-2-loaded VSM of anterior mesenteric artery of Squalus acanthias and show that the ET(B)R agonists IRL 1620 and sarafotoxin S6c (SRX) increase [Ca(2+)](i) in VSM to the same extent as ET-1 and ET(B)R appears to be the only ETR subtype in sharks. To investigate the participation of the inositol trisphosphate (IP(3)) receptors (IP(3)R), we utilized two inhibitors of the mammalian IP(3)R, TMB-8 and 2-APB. In Ca(2+)-free Ringer, these agents inhibit the response to ET(B)R agonist stimulation by 71%. The ryanodine-sensitive receptor (RyR) may be activated by low concentrations of ryanodine, by abrupt local increases of [Ca(2+)](i), (calcium-induced calcium release) or by cyclic adeninediphosphate ribose (cADPR). We employed three inhibitors of activation of the RyR, Ruthenium Red, 8-Br cADPR and high concentrations of ryanodine; these agents blocked the [Ca(2+)](i) response to ET(B)R agonist stimulation by a mean of 39%. These data show for the first time that in VSM of the shark, ET(B)R activation stimulates both IP(3)R and RyR, and that cADPR is involved in RyR activation.

Angiotensin II Ca2+ signaling in rat afferent arterioles: stimulation of cyclic ADP ribose and IP3 pathways

ANG II induces a rise in cytosolic Ca(2+) ([Ca(2+)](i)) in vascular smooth muscle (VSM) cells via inositol trisphosphate receptor (IP(3)R) activation and release of Ca(2+) from the sarcoplasmic reticulum (SR). The Ca(2+) signal is augmented by calcium-induced calcium release (CICR) and by cyclic adeninediphosphate ribose (cADPR), which sensitizes the ryanodine-sensitive receptor (RyR) to Ca(2+) to further amplify CICR. cADPR is synthesized from beta-nicotinamide adenine dinucleotide (NAD(+)) by a membrane-bound bifunctional enzyme, ADPR cyclase. To investigate the possibility that ANG II activates the ADPR cyclase of afferent arterioles, we used inhibitors of the IP(3)R, RyR, and ADPR cyclase. Afferent arterioles were isolated from rat kidney with the magnetized microsphere and sieving technique and loaded with fura-2 to measure [Ca(2+)](i). In Ca(2+)-containing buffer, ANG II increased [Ca(2+)](i) by 125 +/- 10 nM. In the presence of the IP(3)R antagonists TMB-8 and 2-APB, the peak responses to ANG II were reduced by 74 and 81%, respectively. The specific antagonist of cADPR 8-Br ADPR and a high concentration of ryanodine (100 microM) inhibited the ANG II-induced increases in [Ca(2+)](i) by 75 and 69%, respectively. Nicotinamide and Zn(2+) are known inhibitors of the VSM ADPR cyclase. Nicotinamide diminished the [Ca(2+)](i) response to ANG II by 66%. In calcium-free buffer, Zn(2+) reduced the ANG II response by 68%. Simultaneous blockade of the IP(3) and cADPR pathways diminished the [Ca(2+)](i) response to ANG II by 83%. We conclude that ANG II initiates Ca(2+) mobilization from the SR in afferent arterioles via the classic IP(3)R pathway and that ANG II may lead to activation of the ADPR cyclase to form cADPR, which, via its action on the RyR, substantially augments the Ca(2+) response.

Alteration of cyclopiazonic acid-mediated contracture of mouse diaphragm after denervation

As a major Ca(2+) source for muscle contraction, the sarcoplasmic reticulum (SR) of skeletal muscle maintains its Ca(2+) content by uptake of myoplasmic Ca(2+) and by replenishment with extracellular Ca(2+). Since transection of motor nerve alters the functions of SR Ca(2+) pump and sarcolemma ion channels, this study explored the effect of denervation on the contracture evoked by cyclopiazonic acid (CPA), an inhibitor of SR Ca(2+) pump. In innervated hemidiaphragm, CPA elicited a bimodal elevation of muscle tone, which was dependent on extracellular Ca(2+) and differentially inhibited by pretreatment with 2-aminoethoxydiphenylborane (APB) and U73122. Activation of muscle Na(+) channels to simulate denervation-induced membrane depolarization did not change the contracture profile. After denervation for 5-14 days when the contracture induced by caffeine was not yet depressed, CPA elicited only APB-sensitive monophasic contracture. Stimulation of ATP-regulated K(+) channels with lemakalim hyperpolarized muscle membrane and attenuated CPA contracture in denervated, but not innervated, hemidiaphragm. The effects of lemakalim were antagonized by glybenclamide. It is inferred that the bimodal CPA contracture is resulted from distinct recruitments of Ca(2+) entry and that denervation alters the voltage dependence and down-regulates CPA-mediated Ca(2+) influx.

Calmidazolium and arachidonate activate a calcium entry pathway that is distinct from store-operated calcium influx in HeLa cells

Agonists that deplete intracellular Ca2+ stores also activate Ca2+ entry, although the mechanism by which store release and Ca2+ influx are linked is unclear. A potential mechanism involves 'store-operated channels' that respond to depletion of the intracellular Ca2+ pool. Although SOCE (store-operated Ca2+ entry) has been considered to be the principal route for Ca2+ entry during hormonal stimulation of non-electrically excitable cells, recent evidence has suggested that alternative pathways activated by metabolites such as arachidonic acid are responsible for physiological Ca2+ influx. It is not clear whether such messenger-activated pathways exist in all cells, whether they are truly distinct from SOCE and which metabolites are involved. In the present study, we demonstrate that HeLa cells express two pharmacologically and mechanistically distinct Ca2+ entry pathways. One is the ubiquitous SOCE route and the other is an arachidonate-sensitive non-SOCE. We show that both these Ca2+ entry pathways can provide long-lasting Ca2+ elevations, but that the channels are not the same, based on their differential sensitivity to 2-aminoethoxydiphenyl borate, LOE-908 [(R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate] and gadolinium. In addition, non-SOCE and not SOCE was permeable to strontium. Furthermore, unlike SOCE, the non-SOCE pathway did not require store depletion and was not sensitive to displacement of the endoplasmic reticulum from the plasma membrane using jasplakinolide or ionomycin pretreatment. These pathways did not conduct Ca2+ simultaneously due to the dominant effect of arachidonate, which rapidly curtails SOCE and promotes Ca2+ influx via non-SOCE. Although non-SOCE could be activated by exogenous application of arachidonate, the most robust method for stimulation of this pathway was application of the widely used calmodulin antagonist calmidazolium, due to its ability to activate phospholipase A2.

Ryanodine receptor‐operated activation of TRP‐like channels can trigger critical Ca2+signaling events in pancreatic β‐cells

There is little information available concerning the link between the ryanodine (RY) receptors and the downstream Ca(2+) signaling events in beta-cells. In fura-2 loaded INS-1E cells, activation of RY receptors by 9-methyl 5,7-dibromoeudistomin D (MBED) caused a rapid rise of [Ca(2+)]i followed by a plateau and repetitive [Ca(2+)]i spikes on the plateau. The [Ca(2+)]i plateau was abolished by omission of extracellular Ca(2+) and by SKF 96365. In the presence of SKF 96365, MBED produced a transient increase of [Ca(2+)]i, which was abolished by thapsigargin. Activation of RY receptors caused Ca(2+) entry even when the ER Ca(2+) pool was depleted by thapsigargin. The [Ca(2+)]i plateau was not inhibited by nimodipine or ruthenium red, but was inhibited by membrane depolarization, La(3+), Gd(3+), niflumic acid, and 2-aminoethoxydiphenyl borate, agents that inhibit the transient receptor potential channels. The [Ca(2+)]i spikes were inhibited by nimodipine and ryanodine, indicating that they were due to Ca(2+) influx through the voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR). Activation of RY receptors depolarized membrane potential as measured by patch clamp. Thus, activation of RY receptors leads to coherent changes in Ca(2+) signaling, which includes activation of TRP-like channels, membrane depolarization, activation of the voltage-gated Ca(2+) channels and CICR.

Involvement of Inositol 1,4,5-Trisphosphate in Nicotinic Calcium Responses in Dystrophic Myotubes Assessed by Near-plasma Membrane Calcium Measurement

In skeletal muscle cells, plasma membrane depolarization causes a rapid calcium release from the sarcoplasmic reticulum through ryanodine receptors triggering contraction. In Duchenne muscular dystrophy (DMD), a lethal disease that is caused by the lack of the cytoskeletal protein dystrophin, the cytosolic calcium concentration is known to be increased, and this increase may lead to cell necrosis. Here, we used myotubes derived from control and mdx mice, the murine model of DMD, to study the calcium responses induced by nicotinic acetylcholine receptor stimulation. The photoprotein aequorin was expressed in the cytosol or targeted to the plasma membrane as a fusion protein with the synaptosome-associated protein SNAP-25, thus allowing calcium measurements in a restricted area localized just below the plasma membrane. The carbachol-induced calcium responses were 4.5 times bigger in dystrophic myotubes than in control myotubes. Moreover, in dystrophic myotubes the carbachol-mediated calcium responses measured in the subsarcolemmal area were at least 10 times bigger than in the bulk cytosol. The initial calcium responses were due to calcium influx into the cells followed by a fast refilling/release phase from the sarcoplasmic reticulum. In addition and unexpectedly, the inositol 1,4,5-trisphosphate receptor pathway was involved in these calcium signals only in the dystrophic myotubes. This surprising involvement of this calcium release channel in the excitation-contraction coupling could open new ways for understanding exercise-induced calcium increases and downstream muscle degeneration in mdx mice and, therefore, in DMD.

Immunolocalization of type 2 inositol 1,4,5-trisphosphate receptors in cardiac myocytes from newborn mice

The precise localization and role of inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) in cardiac muscle cells are largely unknown. It is believed that waves and oscillations in cytosolic free calcium triggered by activation of InsP3Rs underlie modifications of cellular responses that lead to changes in gene expression in other cells. However, how changes in cytosolic calcium alter gene expression in cardiac cells is unknown. Moreover, it is unclear how changes in cytosolic calcium that alter gene expression do so independently of effects of calcium on other cellular functions, such as contraction. Here we show that InsP3R type 2 is the only isoform present in cardiac myocytes isolated from neonatal mouse ventricles. We also show that type 2 InsP3Rs are associated with the nucleus and that activation of type 2 InsP3Rs with endothelin-1 or phenylephrine selectively increases transcription of atrial natriuretic factor and skeletal α-actin. Type 2 InsP3Rs are also in striations. Activation of InsP3Rs with adenophostin A in permeabilized cells induced calcium release in the nuclear domain and other regions of the cell away from the nucleus. Agonist-induced increase in gene expression and calcium release were blocked by the InsP3R inhibitors 2-aminoethoxydiphenyl borate and xestospongin C. The spatial separation of type 2 InsP3Rs provides support for the concept that microdomains of calcium discretely alter various cell processes. Our experiments suggest that calcium released by InsP3Rs in the nuclear domain provides a direct mechanism for the control of gene expression, whereas release of calcium in the cytoplasm may modulate other processes, such as contraction.

Stimulation of Human Spermatozoa with Progesterone Gradients to Simulate Approach to the Oocyte

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.

Novel role of phospholipase C-delta1: regulation of liver mitochondrial Ca2+ uptake

Mitochondrial Ca2+ (mCa2+) handling is an important regulator of liver cell function that controls events ranging from cellular respiration and signal transduction to apoptosis. Cytosolic Ca2+ enters mitochondria through the ruthenium red-sensitive mCa2+ uniporter, but the mechanisms governing uniporter activity are unknown. Activation of many Ca2+ channels in the cell membrane requires PLC. This activation commonly occurs through phosphitidylinositol-4,5-biphosphate (PIP2) hydrolysis and the production of the second messengers inositol 1,4,5-trisphosphate [I(1,4,5)P3] and 1,2-diacylglycerol (DAG). PIP2 was recently identified in mitochondria. We hypothesized that PLC exists in liver mitochondria and regulates mCa2+ uptake through the uniporter. Western blot analysis with anti-PLC antibodies demonstrated the presence of PLC-delta1 in pure preparations of mitochondrial membranes isolated from rat liver. In addition, the selective PLC inhibitor U-73122 dose-dependently blocked mCa2+ uptake when whole mitochondria were incubated at 37 degrees C with 45Ca2+. Increasing extra mCa2+ concentration significantly stimulated mCa2+ uptake, and U-73122 inhibited this effect. Spermine, a uniporter agonist, significantly increased mCa2+ uptake, whereas U-73122 dose-dependently blocked this effect. The inactive analog of U-73122, U-73343, did not affect mCa2+ uptake in any experimental condition. Membrane-permeable I(1,4,5)P3 receptor antagonists 2-aminoethoxydiphenylborate and xestospongin C also inhibited mCa2+ uptake. Although extra mitochondrial I(1,4,5)P3 had no effect on mCa2+ uptake, membrane-permeable DAG analogs 1-oleoyl-2-acetyl-sn-glycerol and DAG-lactone, which inhibit PLC activity, dose-dependently inhibited mCa2+ uptake. These data indicate that PLC-delta1 exists in liver mitochondria and is involved in regulating mCa2+ uptake through the uniporter.

2-Aminoethoxydiphenyl borate inhibits agonist-induced Ca2+ signals by blocking inositol trisphosphate formation in acutely dissociated mouse pancreatic acinar cells

Evidence suggests that 2-aminoethoxydiphenyl borate (2-APB) modulates intracellular Ca(2+) signals in a complex manner. 2-APB inhibits or potentiates intracellular Ca(2+) signals in different cell types, perhaps through different mechanisms. Here, we report a novel mechanism underlying 2-APB-induced inhibition of agonist-activated Ca(2+) oscillations in mouse pancreatic acinar cells, using patch-clamp and biochemical techniques. Pre-treatment of the cells with 100 microM 2-APB completely abolished ACh- but not inositol trisphosphate (InsP(3))-induced Ca(2+) oscillations, suggesting that the mechanism of inhibition occurs between cytoplasmic receptors and InsP(3) receptor activation. In addition, 100 microM 2-APB significantly inhibited ACh-induced phospholipase C (PLC) activation. These findings indicate that, in mouse pancreatic acinar cells, in addition to modulating InsP(3) receptors and blocking the store-operated Ca(2+) pathway, high concentrations of 2-APB also inhibit agonist-induced Ca(2+) signals by reducing InsP(3) formation.

Effect of halothane on the release of [Ca2+]i in dorsal root ganglion neurons

We investigated the effect of the volatile anaesthetic halothane on [Ca2+]i of dorsal root ganglion neurons. Halothane was able to increase [Ca2+]i in those neurons in a dose-dependent manner and independent of extracellular calcium. However, halothane action was inhibited by BAPTA-AM, suggesting the involvement of intracellular calcium stores. Dantrolene, an inhibitor of ryanodine-sensitive calcium stores had no effect while 2-APB, an inhibitor of IP3-sensitive calcium store reduced by 78% the halothane-evoked increase on [Ca2+]i. These data suggests that halothane increased [Ca2+]i of ganglion neurons through calcium release from IP3-sensitive calcium store. One possible consequence of the halothane action is to alter presynaptic activity and signaling pathways that influence neurotransmission.

ICC pacing mechanisms in intact mouse intestine differ from those in cultured or dissected intestine

Pacing of mouse intestine is driven by spontaneous activity of a network of interstitial cells of Cajal in the myenteric plexus (ICC-MP). So far, highly dissected circular muscle (CM) strips from control and mutant mice lacking ICC-MP and isolated, cultured ICC from newborn control mice were used to analyze its properties. Using intact circular and longitudinal segments of intestine, we recently reported that there were both significant similarities and differences between pacing studied in segments and from isolated, dissected tissues. Here, we report additional similarities and differences in our model from those in highly reduced systems. Similar to cultured or dissected intestine, blockade of sarcoplasmic-endoplasmic reticulum Ca(2+) pumps with thapsigargin or cyclopiazonic acid reduced pacing frequency, but thapsigargin was less effective than in isolated, cultured ICC. Moreover, inhibition of inositol 1,4,5-trisphosphate (IP(3)) receptors with xestospongin C, a putative inhibitor of IP(3) receptors, failed to affect pacing but successfully blocked increased pacing frequency by phorbol ester. 2-Aminoethoxy-diphenylborate, a putative blocker of IP(3)-mediated calcium release, caused a significant decrease in the amplitude and frequency of contractions. The mitochondrial uncoupler carbonyl cyanide p-trifluormethoxyphenylhydrazone blocked pacing and KCl-induced contractions at a concentration of 1 microM. The cyclic nucleotide agonists sodium nitroprusside (SNP), forskolin, and 8-bromo-cGMP inhibited pacing in CM. In longitudinal muscle (LM), SNP and forskolin had little effect on pacing. Furthermore, dibutyryl-cAMP did not affect pacing in CM or LM. These results suggest that pacing in intact intestine is under partly similar regulatory control as in more reduced systems. However, pacing in intact intestine is not affected by xestospongin C, which abolishes pacing in isolated, cultured ICC and exhibits attenuated responses to thapsigargin. Also, major differences between LM and CM suggest a separate pacemaker may drive LM.

Modification of phospholipase C-gamma-induced Ca2+ signal generation by 2-aminoethoxydiphenyl borate

The mechanisms by which Ca(2+)-store-release channels and Ca(2+)-entry channels are coupled to receptor activation are poorly understood. Modification of Ca(2+) signals by 2-aminoethoxydiphenyl borate (2-APB), suggests the agent may target entry channels or the machinery controlling their activation. In DT40 B-cells and Jurkat T-cells, complete Ca(2+) store release was induced by 2-APB (EC(50) 10-20 microM). At 75 microM, 2-APB emptied stores completely in both lymphocyte lines, but had no such effect on other cells. In DT40 cells, 2-APB mimicked B-cell receptor (BCR) cross-linking, but no effect was observed in mutant DT40 lines devoid of inositol 1,4,5-trisphosphate (InsP(3)) receptors (InsP(3)Rs) or phospholipase C-gamma2 (PLC-gamma2). Like the BCR, 2-APB activated transfected TRPC3 (canonical transient receptor potential) channels, which acted as sensors for PLC-gamma2-generated diacylglycerol in DT40 cells. The action of 2-APB on InsP(3)Rs and TRPC3 channels was prevented by PLC-inhibition, and required PLC-gamma2 catalytic activity. However, unlike BCR activation, no increased InsP(3) level could be measured in response to 2-APB. Also, calyculin A-induced cytoskeletal reorganization prevented 2-APB-induced InsP(3)R and TRPC3-channel activation, but not that induced by the BCR. 2-APB still activated TRPC3 channels in DT40 cells with fully depleted Ca(2+) stores, indicating its action was not via Ca(2+) release. Significantly, 2-APB-induced InsP(3)R and TRPC3 activation was prevented in DT40 knockout cells devoid of the BCR- and PLC-gamma2-coupled adaptor/kinases, Syk, Lyn, Btk or BLNK. The results suggest that 2-APB activates Ca(2+) signals in lymphocytes by initiating and enhancing coupling between components of the BCR-PLC-gamma2 complex and both Ca(2+)-entry and Ca(2+)-release channels.

Potent inhibition of Ca2+ release-activated Ca2+ channels and T-lymphocyte activation by the pyrazole derivative BTP2

Ca2+ entry through store-operated Ca2+release-activated Ca2+ (CRAC) channels is essential for T-cell activation and proliferation. Recently, it has been shown that 3,5-bistrifluoromethyl pyrazole (BTP) derivatives are specific inhibitors of Ca2+-dependent transcriptional activity in T-cells (Trevillyan, J. M., Chiou, X. G., Chen, Y. W., Ballaron, S. J., Sheets, M. P., Smith, M. L., Wiedeman, P. E., Warrior, U., Wilkins, J., Gubbins, E. J., Gagne, G. D., Fagerland, J., Carter, G. W., Luly, J. R., Mollison, K. W., and Djuric, S. W. (2001) J. Biol. Chem. 276, 48118-48126). Whereas inhibition of Ca2+ signals was reported for BTP2 (Ishikawa, J., Ohga, K., Yoshino, T., Takezawa, R., Ichikawa, A., Kubota, H., and Yamada, T. (2003) J. Immunol. 170, 4441-4449), it was not found for BTP3 (Chen, Y., Smith, M. L., Chiou, G. X., Ballaron, S., Sheets, M. P., Gubbins, E., Warrior, U., Wilkins, J., Surowy, C., Nakane, M., Carter, G. W., Trevillyan, J. M., Mollison, K., and Djuric, S. W. (2002) Cell. Immunol. 220, 134-142). We show that BTP2 specifically inhibits CRAC channels in T-cells with an IC(50) of approximately 10 nm. It does not interfere with other mechanisms important for Ca2+ signals in T-cells, including Ca2+ pumps, mitochondrial Ca2+ signaling, endoplasmic reticulum Ca2+ release, and K+ channels. BTP2 inhibits Ca2+ signals in peripheral blood T-lymphocytes (in particular in CD4+ T-cells) and in human Jurkat T-cells. Inhibition of Ca2+ signals is independent of the stimulation method as Ca2+ entry was blocked following stimulation with anti-CD3, which activates the T-cell receptor, and also following stimulation with thapsigargin or inositol 1,4,5-trisphosphate. BTP2 also inhibited Ca2+-dependent gene expression (interleukins 2 and 5 and interferon gamma) and proliferation of T-lymphocytes with similar IC(50) values. BTP2 is the first potent and specific inhibitor of CRAC channels in primary T-lymphocytes. The inhibition of CRAC channels as well as Ca2+-dependent signal transduction with similar IC(50) values in T-lymphocytes emphasizes the importance of CRAC channel activity during T-cell activation. Furthermore, BTP2 could prove to be a tool to finally unmask the molecular identity of CRAC channels.

Tetrapandins, a New Class of Scorpion Toxins That Specifically Inhibit Store-operated Calcium Entry in Human Embryonic Kidney-293 Cells

Venoms from 14 snakes and four scorpions were screened for inhibitory activities toward store-operated Ca2+ entry (SOCE) in human embryonic kidney-293 cells. An inhibitory activity was found in venom from the African scorpion Pandinus imperator. The active agent of this venom was purified by gel filtration and reverse-phase high pressure liquid chromatography methods. Sequence information on the purified fraction, by automatic Edman degradation and mass spectrometry analysis, identified the activity as being contained in two tetrapeptides, which we have named tetrapandins. We demonstrate that synthesized tetrapandins have inhibitory activity for SOCE in human embryonic kidney-293 cells while having no effect on either thapsigargin- or carbachol-stimulated release of Ca2+ stores. These toxins should be extremely useful in future studies to determine downstream events regulated by SOCE as well as to determine whether multiple pathways exist for thapsigargin-stimulated Ca2+ entry.

Regulation of InsP3 receptor activity by neuronal Ca2+-binding proteins

Inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) were recently demonstrated to be activated independently of InsP(3) by a family of calmodulin (CaM)-like neuronal Ca(2+)-binding proteins (CaBPs). We investigated the interaction of both naturally occurring long and short CaBP1 isoforms with InsP(3)Rs, and their functional effects on InsP(3)R-evoked Ca(2+) signals. Using several experimental paradigms, including transient expression in COS cells, acute injection of recombinant protein into Xenopus oocytes and (45)Ca(2+) flux from permeabilised COS cells, we demonstrated that CaBPs decrease the sensitivity of InsP(3)-induced Ca(2+) release (IICR). In addition, we found a Ca(2+)-independent interaction between CaBP1 and the NH(2)-terminal 159 amino acids of the type 1 InsP(3)R. This interaction resulted in decreased InsP(3) binding to the receptor reminiscent of that observed for CaM. Unlike CaM, however, CaBPs do not inhibit ryanodine receptors, have a higher affinity for InsP(3)Rs and more potently inhibited IICR. We also show that phosphorylation of CaBP1 at a casein kinase 2 consensus site regulates its inhibition of IICR. Our data suggest that CaBPs are endogenous regulators of InsP(3)Rs tuning the sensitivity of cells to InsP(3).

Cell signaling microdomain with Na,K-ATPase and inositol 1,4,5-trisphosphate receptor generates calcium oscillations

Recent studies indicate novel roles for the ubiquitous ion pump, Na,K-ATPase, in addition to its function as a key regulator of intracellular sodium and potassium concentration. We have previously demonstrated that ouabain, the endogenous ligand of Na,K-ATPase, can trigger intracellular Ca2+ oscillations, a versatile intracellular signal controlling a diverse range of cellular processes. Here we report that Na,K-ATPase and inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) form a cell signaling microdomain that, in the presence of ouabain, generates slow Ca2+ oscillations in renal cells. Using fluorescent resonance energy transfer (FRET) measurements, we detected a close spatial proximity between Na,K-ATPase and InsP3R. Ouabain significantly enhanced FRET between Na,K-ATPase and InsP3R. The FRET effect and ouabain-induced Ca2+ oscillations were not observed following disruption of the actin cytoskeleton. Partial truncation of the NH2 terminus of Na,K-ATPase catalytic alpha1-subunit abolished Ca2+ oscillations and downstream activation of NF-kappaB. Ouabain-induced Ca2+ oscillations occurred in cells expressing an InsP3 sponge and were hence independent of InsP3 generation. Thus, we present a novel principle for a cell signaling microdomain where an ion pump serves as a receptor.

Role of sarcoplasmic reticulum Ca2+ content in Ca2+ entry of bovine airway smooth muscle cells

Depletion of intracellular Ca(2+) stores induces the opening of an unknown Ca(2+ )entry pathway to the cell. We measured the intracellular free-Ca(2+) concentration ([Ca(2+)]i) at different sarcoplasmic reticulum (SR) Ca(2+) content in fura-2-loaded smooth muscle cells isolated from bovine tracheas. The absence of Ca(2+) in the extracellular medium generated a time-dependent decrement in [Ca(2+)]i which was proportional to the reduction in the SR-Ca(2+) content. This SR-Ca(2+) level was indirectly determined by measuring the amount of Ca(2+) released by caffeine. Ca(2+) restoration at different times after Ca(2+)-free incubation (2, 4, 6 and 10 min) induced an increment of [Ca(2+)]i. This increase in [Ca(2+)]i was considered as Ca(2+) entry to the cell. The rate of this entry was slow (~0.3 nM/s) when SR-Ca(2+) content was higher than 50% (2 and 4 min in Ca(2+)-free medium), and significantly ( p<0.01) accelerated (>1.0 nM/s) when SR-Ca(2+) content was lower than 50% (6 and 10 min in Ca(2+)-free medium). Thapsigargin significantly induced a higher rate of this Ca(2+) entry ( p<0.01). Variations in Ca(2+) influx after SR-Ca(2+) depletion were estimated more directly by a Mn(2+) quench approach. Ca(2+) restoration to the medium 4 min after Ca(2+) removal did not modify the Mn(2+) influx. However, when Ca(2+) was added after 10 min in Ca(2+)-free medium, an increment of Mn(2+) influx was observed, corroborating an increase in Ca(2+) entry. The fast Ca(2+) influx was Ni(2+) sensitive but was not affected by other known capacitative Ca(2+) entry blockers such as La(3+), Mg(2+), SKF 96365 and 2-APB. It was also not affected by the blockage of L-type Ca2(+) channels with methoxyverapamil or by the sustained K(+)-induced depolarisation. The slow Ca(2+) influx was only sensitive to SKF 96365. In conclusion, our results indicate that in bovine airway smooth muscle cells Ca(2+) influx after SR-Ca(2+) depletion has two rates: A) The slow Ca(2+) influx, which occurred in cells with more than 50% of their SR-Ca(2+) content, is sensitive to SKF 96365 and appears to be a non-capacitative Ca(2+) entry; and B) The fast Ca(2+) influx, observed in cells with less than 50% of their SR-Ca(2+) content, is probably a capacitative Ca(2+) entry and was only Ni(2+)-sensitive.