Calcium-dependent immediate feedback control of inositol 1,4,5-triphosphate-induced Ca2+ release

CD38: T Cell Immuno-Metabolic Modulator

Activation and subsequent differentiation of T cells following antigenic stimulation are triggered by highly coordinated signaling events that lead to instilling cells with a discrete metabolic and transcriptional feature. Compelling studies indicate that intracellular nicotinamide adenine dinucleotide (NAD⁺) levels have profound influence on diverse signaling and metabolic pathways of T cells, and hence dictate their functional fate. CD38, a major mammalian NAD⁺ glycohydrolase (NADase), expresses on T cells following activation and appears to be an essential modulator of intracellular NAD⁺ levels. The enzymatic activity of CD38 in the process of generating the second messenger cADPR utilizes intracellular NAD^+, and thus limits its availability to different NAD⁺ consuming enzymes (PARP, ART, and sirtuins) inside the cells. The present review discusses how the CD38-NAD⁺ axis affects T cell activation and differentiation through interfering with their signaling and metabolic processes. We also describe the pivotal role of the CD38-NAD⁺ axis in influencing the chromatin remodeling and rewiring T cell response. Overall, this review emphasizes the crucial contribution of the CD38^-NAD⁺ axis in altering T cell response in various pathophysiological conditions.

RSK2 contributes to myogenic vasoconstriction of resistance arteries by activating smooth muscle myosin and the Na+/H+ exchanger

Smooth muscle contraction is triggered when Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates the regulatory light chain of myosin (RLC₂₀). However, blood vessels from Mlck-deficient mouse embryos retain the ability to contract, suggesting the existence of additional regulatory mechanisms. We showed that the p90 ribosomal S6 kinase 2 (RSK2) also phosphorylated RLC₂₀ to promote smooth muscle contractility. Active, phosphorylated RSK2 was present in mouse resistance arteries under normal basal tone, and phosphorylation of RSK2 increased with myogenic vasoconstriction or agonist stimulation. Resistance arteries from Rsk2-deficient mice were dilated and showed reduced myogenic tone and RLC₂₀ phosphorylation. RSK2 phosphorylated Ser¹⁹ in RLC in vitro. In addition, RSK2 phosphorylated an activating site in the Na⁺/H⁺ exchanger (NHE-1), resulting in cytosolic alkalinization and an increase in intracellular Ca²⁺ that promotes vasoconstriction. NHE-1 activity increased upon myogenic constriction, and the increase in intracellular pH was suppressed in Rsk2-deficient mice. In pressured arteries, RSK2-dependent activation of NHE-1 was associated with increased intracellular Ca²⁺ transients, which would be expected to increase MLCK activity, thereby contributing to basal tone and myogenic responses. Accordingly, Rsk2-deficient mice had lower blood pressure than normal littermates. Thus, RSK2 mediates a procontractile signaling pathway that contributes to the regulation of basal vascular tone, myogenic vasoconstriction, and blood pressure and may be a potential therapeutic target in smooth muscle contractility disorders.

Mitochondrial Ca2+ transport in the endothelium: regulation by ions, redox signalling and mechanical forces

Calcium (Ca²⁺) transport by mitochondria is an important component of the cell Ca²⁺ homeostasis machinery in metazoans. Ca²⁺ uptake by mitochondria is a major determinant of bioenergetics and cell fate. Mitochondrial Ca²⁺ uptake occurs via the mitochondrial Ca²⁺ uniporter (MCU) complex, an inner mitochondrial membrane protein assembly consisting of the MCU Ca²⁺ channel, as its core component, and the MCU complex regulatory/auxiliary proteins. In this review, we summarize the current knowledge on the molecular nature of the MCU complex and its regulation by intra- and extramitochondrial levels of divalent ions and reactive oxygen species (ROS). Intracellular Ca²⁺ concentration ([Ca²⁺]_i), mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) and mitochondrial ROS (mROS) are intricately coupled in regulating MCU activity. Here, we highlight the contribution of MCU activity to vascular endothelial cell (EC) function. Besides the ionic and oxidant regulation, ECs are continuously exposed to haemodynamic forces (either pulsatile or oscillatory fluid mechanical shear stresses, depending on the precise EC location within the arteries). Thus, we also propose an EC mechanotransduction-mediated regulation of MCU activity in the context of vascular physiology and atherosclerotic vascular disease.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

Endothelial mitochondria regulate the intracellular Ca2+ response to fluid shear stress

Shear stress is known to stimulate an intracellular free calcium concentration ([Ca(2+)]i) response in vascular endothelial cells (ECs). [Ca(2+)]i is a key second messenger for signaling that leads to vasodilation and EC survival. Although it is accepted that the shear-induced [Ca(2+)]i response is, in part, due to Ca(2+) release from the endoplasmic reticulum (ER), the role of mitochondria (second largest Ca(2+) store) is unknown. We hypothesized that the mitochondria play a role in regulating [Ca(2+)]i in sheared ECs. Cultured ECs, loaded with a Ca(2+)-sensitive fluorophore, were exposed to physiological levels of shear stress. Shear stress elicited [Ca(2+)]i transients in a percentage of cells with a fraction of them displaying oscillations. Peak magnitudes, percentage of oscillating ECs, and oscillation frequencies depended on the shear level. [Ca(2+)]i transients/oscillations were present when experiments were conducted in Ca(2+)-free solution (plus lanthanum) but absent when ECs were treated with a phospholipase C inhibitor, suggesting that the ER inositol 1,4,5-trisphosphate receptor is responsible for the [Ca(2+)]i response. Either a mitochondrial uncoupler or an electron transport chain inhibitor, but not a mitochondrial ATP synthase inhibitor, prevented the occurrence of transients and especially inhibited the oscillations. Knockdown of the mitochondrial Ca(2+) uniporter also inhibited the shear-induced [Ca(2+)]i transients/oscillations compared with controls. Hence, EC mitochondria, through Ca(2+) uptake/release, regulate the temporal profile of shear-induced ER Ca(2+) release. [Ca(2+)]i oscillation frequencies detected were within the range for activation of mechanoresponsive kinases and transcription factors, suggesting that dysfunctional EC mitochondria may contribute to cardiovascular disease by deregulating the shear-induced [Ca(2+)]i response.

Ca2+ sparks and puffs are generated and interact in rat hippocampal CA1 pyramidal neuron dendrites

1,4,5-Inositol trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) mediate release of Ca(2+) from internal stores in many neurons. The details of the spatial and temporal characteristics of these signals and their interactions in dendrites remain to be clarified. We found that localized Ca(2+) release events, with no associated change in membrane potential, occurred spontaneously in the dendrites of rat hippocampal CA1 pyramidal neurons. Their rate, but not their amplitude or time course, could be modulated by changes in membrane potential. Together, these results suggest that the spontaneous events are similar to RyR-dependent Ca(2+) "sparks" found in cardiac myocytes. In addition, we found that we could generate another kind of localized Ca(2+) release event by either a synaptic tetanus in the presence of 3-((R)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid and CNQX or by uncaging IP3. These events had slower rise times and decay times than sparks and were more heterogeneous. These properties are similar to Ca(2+) "puffs" found in oocytes. These two localized signals interact. Low-intensity tetanic synaptic stimulation or uncaging of IP3 increased the decay time of spontaneous Ca(2+) events without changing their rise time or amplitude. Pharmacological experiments suggest that this event widening is attributable to a delayed IP3R-mediated release of Ca(2+) triggered by the synergistic action of IP3 and Ca(2+) released by RyRs. The actions of IP3 appear to be confined to the main apical dendrite because uncaging IP3 in the oblique dendrites has no effect on the time course of localized events or backpropagating action potential-evoked Ca(2+) signals in this region.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Understanding calcium waves and sparks in central neurons

All cells use changes in intracellular calcium concentration ([Ca(2+)](i)) to regulate cell signalling events. In neurons, with their elaborate dendritic and axonal arborizations, there are clear examples of both localized and widespread Ca(2+) signals. [Ca(2+)](i) changes that are generated by Ca(2+) entry through voltage- and ligand-gated channels are the best characterized. In addition, the release of Ca(2+) from intracellular stores can result in increased [Ca(2+)](i); the signals that trigger this release have been less well-studied, in part because they are not usually associated with specific changes in membrane potential. However, recent experiments have revealed dramatic widespread Ca(2+) waves and localized spark-like events, particularly in dendrites. Here we review emerging data on the nature of these signals and their functions.

STIM1, but not STIM2, is required for proper agonist-induced Ca2+ signaling

The stromal interaction molecules STIM1 and STIM2 sense a decreasing Ca(2+) concentration in the lumen of the endoplasmic reticulum and activate Ca(2+) channels in the plasma membrane. In addition, at least 2 reports suggested that STIM1 may also interact with the inositol 1,4,5-trisphosphate (IP(3)) receptor. Using embryonic fibroblasts from Stim1(-/-), Stim2(-/-) and wild-type mice, we now tested the hypothesis that STIM1 and STIM2 would also regulate the IP(3) receptor. We investigated whether STIM1 or STIM2 would be the luminal Ca(2+) sensor that controls the loading dependence of the IP(3)-induced Ca(2+) release. Partial emptying of the stores in plasma-membrane permeabilized cells resulted in an increased EC(50) and a decreased Hill coefficient for IP(3)-induced Ca(2+) release. This effect occurred both in the presence and absence of STIM proteins, indicating that these proteins were not the luminal Ca(2+) sensor for the IP(3) receptor. Although Stim1(-/-) cells displayed a normal IP(3)-receptor function, agonist-induced Ca(2+) release was reduced. This finding suggests that the presence of STIM1 is required for proper agonist-induced Ca(2+) signaling. Our data do not provide experimental evidence for the suggestion that STIM proteins would directly control the function of the IP(3) receptor.

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.

TEMPERATURE EFFECTS ON THE STOCHASTIC GATING OF THE IP3R CALCIUM RELEASE CHANNEL: A NUMERICAL SIMULATION STUDY

The importance of the kinetic study of endoplasmatic calcium ion channels in different intracellular processes is known today. Although there are few experimental reports on the temperature dependency of IP3R channel functions, we did not find any detailed theoretical study on this subject. For this purpose, we used a modified Gillespie algorithm to investigate the effect of temperature on the conditions affecting the open state of a single subunit of the De Young-Keizer (DYK) model. Population of the states was considered as the subject of fluctuation. Key features of the channel, such as bell-shaped dependency of open probability to the Calcium concentrations were modeled at different temperatures, too. The range of temperature variation was selected by regarding the experimental data on IP3R channel.

By increasing the temperature, we had the very slow time domains (t: 10-1s ) and the much slower time domains (t: 100s ) in addition to other time domains, which could be seen as new time categories in InsP3R studies, and so the results were reported in these time domains, as well.

We found out that increase in temperature declined the open probability in some concentrations of Ca2+and/or IP3. Also, by introducing the intensity graphs, broadening of the range of fluctuations and lowering of the order of frequency of fluctuations for the population of each state were observed due to the temperature increments.

The temperature effects on the activation and inactivation states of the channel were studied in the framework of the reaction paths. We did not find similar paths at different time domains; several paths observed which were totally different all together. These time-dependent reaction paths are also depending on the Ca2+and/or the IP3 concentrations. So, one can predict the most probable reaction paths at different concentrations and temperatures and also determine which kind of the path it is; a path for closing the channel or a path to open it.

Finally, the temperature effects on the calcium inhibited states were studied. We found out that calcium ion inhibitions were shifted to lower calcium concentration by increasing the temperature. The results suggests that inhibiting role of calcium is not only [ Ca2+] and/or [IP3] dependent, but also temperature dependent.

Ca(2+) transfer from the ER to mitochondria: when, how and why

The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca(2+) concentration ([Ca(2+)]c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca(2+)] in the mitochondrial matrix ([Ca(2+)]m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca(2+) transporters, the close proximity to the endoplasmic reticulum (ER) Ca(2+)-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca(2+) channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca(2+)]m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca(2+) homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca(2+) signaling machinery.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Comparative study of calcium ion dynamics and contractile response in rat middle cerebral and basilar arteries

The objective of this study was to compare intracellular calcium concentration ([Ca(2+)](i)) and contractile responses in isolated rat middle cerebral artery (MCA) with those in basilar artery (BA) employing real-time confocal laser microscopy. KCl elicited transient [Ca(2+)](i) elevation and sustained contraction in both arteries; moreover, nearly equal responses were evident in both arteries. Application of 5-hydroxytryptamine (5-HT), vasopressin (VP), and alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-me ATP) also induced elevation of [Ca(2+)](i) and contraction in both arteries. The maximum response of 5-HT and VP necessary to increase [Ca(2+)](i) and to constrict the MCA was less in comparison to the BA; however, a linear relationship emerged between the maximum response of [Ca(2+)](i) and that of contraction. Additionally, the slope of the correlation regression line of MCA was nearly identical to that of BA. On the other hand, cyclopiazonic acid (CPA)-induced Ca(2+) release from store sites following contraction of MCA was distinct from that of BA. In MCA, velocity of [Ca(2+)](i) elevation in smooth muscle cells and Ca(2+)-wave propagation along smooth muscle cells induced by 5-HT were slower than those in BA. These observations revealed that different regions of arteries along the same cerebral tissue may display distinct [Ca(2+)](i) response; moreover, this difference may be one reason for the distinct contractile response.

Porcine cumulus cell influences ooplasmic mitochondria-lipid distributions, GSH-ATP contents and calcium release pattern after electro-activation

The objective was to explore mechanisms of the influence of porcine cumulus cells (CC) on oocyte maturation. Immature porcine oocytes were matured in groups of denuded oocyte (DOs), cumulus-oocyte complexes (COCs), denuded oocytes co-cultured with CC (DoCC), or with cumulus-oocyte complexes (DoCOCs). Ooplasmic mitochondria-lipid distributions, glutathione (GSH)-adenosine triphosphate (ATP) contents, calcium release pattern, and developmental competence after parthenogenetic activation were assessed after IVM. The portion of matured oocytes after IVM and the developmental competence and GSH content in single oocytes were lower in DOs than in COCs (P<0.05). In contrast, the maturation rate and development in DoCOCs and COCs were higher than in DoCC and DOs (P<0.05). The blastocyst rate in DoCOCs was higher than in DOs (P<0.05), and ATP content in COCs was higher than in all other groups (P<0.01). In addition, the rate of oocytes with damaged oolemma in DOs (35%) was significantly higher than in COCs (3%), DoCOCs (7%), and DoCC (10%). The rate of oocytes with evenly distributed mitochondria was 70% in DOs, which was significantly lower than in COCs and DoCC (89 and 84%, respectively). The percentage of oocytes with normal lipid droplets distributions in COCs (70%) was significantly higher than in three other groups, whereas both percentages in DoCC and DoCOCs were higher than in DOs (P<0.05). The duration of [Ca(2+)] rise in DOs was longer than in three other groups, whereas the duration was shortest in COCs. The amplitude of the [Ca(2+)] rise in DOs was significantly lower than in other groups (P<0.05), but the amplitude did not differ significantly among DoCC, DoCOCs and COCs. In conclusion, the presence of porcine CC during IVM functionally affected ooplasmic mitochondria-lipid distributions and GSH-ATP contents, which may affect the calcium release pattern and developmental competence of oocytes after electro-activation.

Enhanced spontaneous Ca2+ events in endothelial cells reflect signalling through myoendothelial gap junctions in pressurized mesenteric arteries

Increases in global Ca(2+) in the endothelium are a crucial step in releasing relaxing factors to modulate arterial tone. In the present study we investigated spontaneous Ca(2+) events in endothelial cells, and the contribution of smooth muscle cells to these Ca(2+) events, in pressurized rat mesenteric resistance arteries. Spontaneous Ca(2+) events were observed under resting conditions in 34% of cells. These Ca(2+) events were absent in arteries preincubated with either cyclopiazonic acid or U-73122, but were unaffected by ryanodine or nicotinamide. Stimulation of smooth muscle cell depolarization and contraction with either phenylephrine or high concentrations of KCl significantly increased the frequency of endothelial cell Ca(2+) events. The putative gap junction uncouplers carbenoxolone and 18alpha-glycyrrhetinic acid each inhibited spontaneous and evoked Ca(2+) events, and the movement of calcein from endothelial to smooth muscle cells. In addition, spontaneous Ca(2+) events were diminished by nifedipine, lowering extracellular Ca(2+) levels, or by blockers of non-selective Ca(2+) influx pathways. These findings suggest that in pressurized rat mesenteric arteries, spontaneous Ca(2+) events in the endothelial cells appear to originate from endoplasmic reticulum IP(3) receptors, and are subject to regulation by surrounding smooth muscle cells via myoendothelial gap junctions, even under basal conditions.

Role of Ca2+ stores in acetylcholine-induced all-or-none shortening of smooth muscle cells from guinea-pig taenia caecum

1. We have reported previously that isolated single smooth muscle cells from guinea-pig taenia caecum respond to acetylcholine (ACh) in an all-or-none manner. 2. To clarify the roles of intracellular Ca(2+) stores in the all-or-none response of isolated smooth muscle cells from guinea-pig taenia caecum to ACh, we examined the inositol 1,4,5-trisphosphate (IP(3))-induced contractile response in Staphylococcus aureus alpha-toxin-permeabilized smooth muscle cells and the effect of depletion of intracellular Ca(2+) stores on the all-or-none response to ACh in intact smooth muscle cells. 3. alpha-Toxin-permeabilized smooth muscle cells responded to 3-30 nmol/L or 0.3-3 nmol/L IP(3) in the presence of 0.2 micromol/L Ca(2+) with 1 mmol/L EGTA or 0.1 mmol/L EGTA, respectively, in an all-or-none manner. These results suggest that Ca(2+) release induced by IP(3) is Ca(2+) dependent and is evoked in an all-or-none manner. 4. In the presence of the Ca(2+) ionophore A23187 (0.1 micromol/L) or the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (1 micromol/L), the shortening of intact smooth muscle cells induced by increasing concentrations of ACh showed a graded response, but not an all-or-none response. 5. In conclusion, the results suggest that Ca(2+) release from Ca(2+) stores induced by IP(3) plays an important role in the all-or-none response of intact smooth muscle cells to ACh.

`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.

Calcium oscillations

Changes in cellular Ca2+ concentration control a wide range of physiological processes, from the subsecond release of synaptic neurotransmitters, to the regulation of gene expression over months or years. Ca2+ can also trigger cell death through both apoptosis and necrosis, and so the regulation of cellular Ca2+ concentration must be tightly controlled through the concerted action of pumps, channels and buffers that transport Ca2+ into and out of the cell cytoplasm. A hallmark of cellular Ca2+ signalling is its spatiotemporal complexity: stimulation of cells by a hormone or neurotransmitter leads to oscillations in cytoplasmic Ca2+ concentration that can vary markedly in time course, amplitude, frequency, and spatial range. In this chapter we review some of the biological roles of Ca2+, the experimental characterisation of complex dynamic changes in Ca2+ concentration, and attempts to explain this complexity using computational models. We consider the 'toolkit' of cellular proteins which influence Ca2+ concentrarion, describe mechanistic models of key elements of the toolkit, and fit these into the framework of whole cell models of Ca2+ oscillations and waves. Finally, we will touch on recent efforts to use stochastic modelling to elucidate elementary Ca2+ signal events, and how these may evolve into global signals.

Calcium signalling during mammalian fertilization

The fertilized mammalian egg is a nice model system for analysing spatiotemporal Ca2+ signalling in the intact cell. Hamster eggs show repetitive Ca2+ transients, associated in the initial response with Ca2+ waves which begin from the site of sperm attachment and are propagated across the deep cytoplasm to the opposite pole. In unfertilized eggs, a regenerative Ca2+ wave is induced by injection of either inositol 1,4,5-trisphosphate (InsP3) or Ca2+, and Ca2+ oscillations are produced by continuous injection of InsP3. These Ca2+ waves and oscillations in both fertilized and unfertilized eggs are inhibited in a dose-dependent manner by a monoclonal antibody to the type 1 InsP3 receptor. Ryanodine receptors (both skeletal and cardiac types) are not detected by physiological or immunoblot analyses. Positive and negative feedback between cytosolic Ca2+ and Ca2+ release from InsP3-sensitive pools accounts for the spatiotemporal Ca2+ signalling. In addition to intracellular Ca2+ release, Ca2+ entry from outside the egg is necessary to refill the Ca2+ pools and maintain Ca2+ oscillations. Evidence suggests that inositol 1,3,4,5-tetrakisphosphate activates the Ca2+ influx. The signal transduction process leading to the production of InsP3 and the mechanism of egg activation following the Ca2+ response still remain to be elucidated.

Pancreatic calcium waves and secretion

Pancreatic acinar cells display stereotypic Ca2+ waves resulting from Ca2+ release from internal stores during stimulation. The Ca2+ waves are initiated at the luminal pole, and, at high agonist concentrations, spread towards the basal pole. Two key mechanisms behind the generation of Ca2+ waves have been identified. First, the Ca2+ waves are composite, mediated by three distinct Ca2+ release mechanisms with a polarized distribution: high-sensitivity inositol 1,4,5-trisphosphate (InsP3) receptors at a small trigger zone (T zone) in the secretory granule area, Ca(2+)-induced Ca2+ release channels in the granular area and low-sensitivity InsP3 receptors in the basal area. Second, InsP3 can readily diffuse in the cytosol, whereas rises in cytosolic Ca2+ concentration ([Ca2+]i) can be confined through strong buffering and sequestration of Ca2+. InsP3 is thus used as a long-range messenger to transmit agonist signals to the T zone, and [Ca2+]i rises at the T zone are used as a local switch. These mechanisms enable preferential activation of the T zone, irrespective of localization of stimuli and agonist receptors. The secretion of enzymes and fluid is a direct consequence of [Ca2+]i rises at the T zone. The Ca2+ waves and oscillations probably boost the T zone functions.

Spiral calcium waves: implications for signalling

Spiral patterns of intracellular Ca2+ release demonstrate a direct relationship between increasing wavefront curvature and increasing propagation velocity. An equally important phenomenon is the annihilation of colliding Ca2+ waves, which reveals an underlying refractory period during which further Ca2+ release is temporarily inhibited. Treatment of intracellular Ca2+ release as an excitable medium accounts for both observations. This theoretical framework is analogous to the more familiar concept of electrical excitability in neuronal membranes. In this analogy, the inositol 1,4,5-trisphosphate receptor ion channel plays a role analogous to that of Na+ channels while Ca(2+)-induced Ca2+ release provides the mechanism for excitation. Furthermore, Ca(2+)-ATPases play a role similar to that of the K+ channels in neuronal excitation, that is, they return the system to rest. We demonstrated that overexpression of a sarco/endoplasmic reticulum Ca(2+)-ATPase increases the frequency of Ca2+ wave activity. More recent experiments reveal a strong dependence of the propagation velocity on wavelength as predicted by the dispersion relation of excitability. This important result accounts for an observed correlation between wave frequency and spatial dominance of Ca2+ foci and suggests a new mechanism for the encoding of signal information.

Calcium oscillations in neurons

Oscillations in the cytosolic free Ca2+ concentration ([Ca2+]i) have been described in a variety of cells. In some cases, [Ca2+]i oscillations reflect cycles of membrane depolarization and voltage-dependent Ca2+ entry. In others, they are caused by periodic Ca2+ uptake and release by internal stores, with little immediate requirement for external Ca2+. A third type of [Ca2+]i oscillation is typified by caffeine-induced oscillations in sympathetic neurons. Here, the oscillations depend on the interplay between Ca2+ transport across the plasma membrane and transport by a caffeine-sensitive store. These oscillations can occur at a steady membrane potential and are blocked by ryanodine (1 microM), indicating that they do not result from voltage-dependent changes in Ca2+ entry but do require Ca(2+)-induced Ca2+ release. Entry of Ca2+ from the external medium is important during all phases of the oscillatory cycle except the rapid upstroke, which is dominated by Ca2+ release from an internal store. It is proposed that caffeine-induced [Ca2+]i oscillations are cyclic perturbations of [Ca2+]i caused by exchange of Ca2+ between the cytosol and the caffeine-sensitive store: net Ca2+ loss from the store increases [Ca2+]i transiently above its steady-state value ([Ca2+]ss), whereas net accumulation of Ca2+ by the store transiently depresses [Ca2+]i below [Ca2+]ss. The effects of rapid removal of Ca2+ and caffeine on the rate of change of [Ca2+]i (d[Ca2+]i/dt) provide estimates of the rates of net Ca2+ entry and (caffeine-sensitive) Ca2+ release and information on the way these rates vary during the oscillatory cycle.

Calcium and other ion dynamics during gamete maturation and fertilization

Ion currents and cytosolic free calcium ([Ca(2+)](i)) elevations are crucial events in triggering the complex machinery involved in both gamete maturation and fertilization. Oocyte maturation is triggered by hormone signaling which causes ion currents and [Ca(2+)](i) increase. Extracellular calcium seems to be required for meiosis progression since: (i) calcium depletion in the maturation medium severely affects oocyte developmental competence; (ii) the activity of plasma membrane L-type Ca(2+) currents decreases during maturation; (iii) the exposure to verapamil, a specific Ca(2+) channel blocker, decreases in vitro maturation efficiency. In spermatozoa, maturation initiates inside the epididymis and ends in the female genital tract. During their journey through the female reproductive tract, sperm undergo a dramatic selection and capacitation achieving fertilization competence. Adhesion to the tubal epithelium extends sperm life through depression of [Ca(2+)](i) until capacitation signals trigger an [Ca(2+)](i) elevation followed by sperm release. At fertilization, egg-sperm interaction evokes well-described transient and almost simultaneous events: i.e., fertilization current, a change in resting potential, and an increase in free [Ca(2+)](i) concentration. These events, termed oocyte activation, are the direct consequence of sperm interaction via either activation of a receptor or entry of a sperm factor. The latter hypothesis has been recently supported by the discovery of PCLzeta, a sperm-specific isozyme triggering a dramatic [Ca(2+)](i) increase via inositol 1,4,5-trisphosphate (IP(3)) production. The course of ion currents and [Ca(2+)](i) transients during maturation and fertilization plays a pivotal role in correct embryo development.

Dynamics of a three-variable nonlinear model of vasomotion: comparison of theory and experiment

The effects of pharmacological interventions that modulate Ca(2+) homeodynamics and membrane potential in rat isolated cerebral vessels during vasomotion (i.e., rhythmic fluctuations in arterial diameter) were simulated by a third-order system of nonlinear differential equations. Independent control variables employed in the model were [Ca(2+)] in the cytosol, [Ca(2+)] in intracellular stores, and smooth muscle membrane potential. Interactions between ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores and transmembrane ion fluxes via K(+) channels, Cl(-) channels, and voltage-operated Ca(2+) channels were studied by comparing simulations of oscillatory behavior with experimental measurements of membrane potential, intracellular free [Ca(2+)] and vessel diameter during a range of pharmacological interventions. The main conclusion of the study is that a general model of vasomotion that predicts experimental data can be constructed by a low-order system that incorporates nonlinear interactions between dynamical control variables.

Calcium signals for egg activation in mammals

A dramatic increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) occurs in eggs at fertilization common to all animal species examined to date, and this serves as a pivotal signal for egg activation characterized by resumption of meiotic cell division and formation of the pronuclei. In mammalian eggs, repetitive [Ca(2+)](i) rises (Ca(2+) oscillations) each of which accompanies a propagating wave across the egg occur due to release of Ca(2+) from the endoplasmic reticulum mainly through type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor. Ca(2+) oscillations are induced by a cytosolic sperm factor driven into the egg cytoplasm upon sperm-egg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP(3)-producing enzyme), PLCzeta. Recent extensive research has reveled characteristics of PLCzeta such as the Ca(2+) oscillation-inducing activity after injection of PLCzeta-encoding RNA or recombinant PLCzeta into mouse eggs, extremely high Ca(2+)-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycle-dependent regulation of Ca(2+) oscillations. [Ca(2+)](i) rises cause successive activation of calmodulin-dependent kinase II and E3 ubiquitin ligase, lead to proteolysis of ubiquitinated cyclin B1 and inactivation of metaphase-promoting factor (Cdk1/cyclin B1 complex), and result in the release of eggs from meiotic arrest.

Localized Ca2+ uncaging induces Ca2+ release through IP3R in smooth muscle

AIM

Our previous study indicated that there are two types of Ca2+ release events seen in intact mouse bladder tissue. In this study our aim is to investigate the mechanism that underlies the phenomena of Ca2+ release in smooth muscle.

METHODS

Single cells were isolated and tissue segments were prepared by cutting the detrusor into 0.1 cm x 0.5 cm strips running along the axis from the neck to the fundus. Single cells and intact tissue strips were co-loaded with the Ca2+ indicator and caged Ca2+ by incubation with 10 micromol/L Fluo-4 AM and DMNP-EDTA-AM. Fluo-4 AM fluorescence was detected by laser scanning confocal microscopy, and local uncaging of DMNP-EGTA was achieved by brief exposure to the output of a diode-pumped, Ti:sapphire laser tuned to 730 nm.

RESULTS

Local uncaging of caged Ca2+ was able to trigger Ca2+ release events in both single cells and tissue strips from mouse bladder. The Ca2+ release events could not be blocked by ryanodine alone, but the property of the Ca2+ release was markedly altered. Surprisingly, in the presence of ryanodine, Xestospongin C completely inhibited the Ca2+ release events both in single cell and tissue experiments.

CONCLUSION

(1) Two photon flash photolysis (TPFP) triggers Ca2+ induced Ca2+ release. This process involves release through type 2 ryanodine receptor channels; (2) TPFP results in the release of Ca2+ through inositol 1,4,5-trisphosphate receptors in the absence of phospholipase C activation.

Ca2+ -induced Ca2+ release through localized Ca2+ uncaging in smooth muscle

Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) occurs in smooth muscle as spontaneous SR Ca²⁺ release or Ca²⁺ sparks and, in some spiking tissues, as Ca²⁺ release that is triggered by the activation of sarcolemmal Ca²⁺ channels. Both processes display spatial localization in that release occurs at a higher frequency at specific subcellular regions. We have used two-photon flash photolysis (TPFP) of caged Ca²⁺ (DMNP-EDTA) in Fluo-4–loaded urinary bladder smooth muscle cells to determine the extent to which spatially localized increases in Ca²⁺ activate SR release and to further understand the molecular and biophysical processes underlying CICR. TPFP resulted in localized Ca²⁺ release in the form of Ca²⁺ sparks and Ca²⁺ waves that were distinguishable from increases in Ca²⁺ associated with Ca²⁺ uncaging, unequivocally demonstrating that Ca²⁺ release occurs subsequent to a localized rise in [Ca²⁺]_i. TPFP-triggered Ca²⁺ release was not constrained to a few discharge regions but could be activated at all areas of the cell, with release usually occurring at or within several microns of the site of photolysis. As expected, the process of CICR was dominated by ryanodine receptor (RYR) activity, as ryanodine abolished individual Ca²⁺ sparks and evoked release with different threshold and kinetics in FKBP12.6-null cells. However, TPFP CICR was not completely inhibited by ryanodine; Ca²⁺ release with distinct kinetic features occurred with a higher TPFP threshold in the presence of ryanodine. This high threshold release was blocked by xestospongin C, and the pharmacological sensitivity and kinetics were consistent with CICR release at high local [Ca²⁺]_i through inositol trisphosphate (InsP₃) receptors (InsP₃Rs). We conclude that CICR activated by localized Ca²⁺ release bears essential similarities to those observed by the activation of I_Ca (i.e., major dependence on the type 2 RYR), that the release is not spatially constrained to a few specific subcellular regions, and that Ca²⁺ release through InsP₃R can occur at high local [Ca²⁺]_i.

Thirty years of calcium signals at fertilization

It was discovered about 30 years ago that a dramatic increase in intracellular calcium ion concentration ([Ca(2+)](i)) occurs at fertilization and that this increase acts as the pivotal signal for egg activation. Later, the Ca(2+) signal at fertilization turned out to be ubiquitous among animal species. Extensive advance has been brought during these 30 years in research on spatiotemporal aspects and signaling mechanisms of the [Ca(2+)](i) increase, sperm factors that induce the Ca(2+) response, and cell cycle resumption caused by the [Ca(2+)](i) rise. I provide a historical account of these advances in mammals, sea urchins, and a few other models.

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.

Two-step Ca2+ intracellular release underlies excitation-contraction coupling in mouse urinary bladder myocytes

The relative contributions of Ca(2+)-induced Ca(2+) release (CICR) versus Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) to excitation-contraction coupling has not been defined in most smooth muscle cells (SMCs). The present study was undertaken to address this issue in mouse urinary bladder (UB) smooth muscle cells (UBSMCs). Confocal Ca(2+) images were obtained under voltage- or current-clamp conditions. When UBSMCs were activated by a 30-ms depolarization to 0 mV, intracellular Ca(2+) concentration ([Ca(2+)](i)) increased in several small, discrete areas just beneath the cell membrane. These Ca(2+) "hot spots" then spread slowly through the myoplasm as Ca(2+) waves, which continued even after repolarization. Shorter depolarizations (5 ms) elicited only a few Ca(2+) sparks, which declined quickly. The number of Ca(2+) sparks, or hot spots, was closely related to the depolarization duration in the range of approximately 5-20 ms. There was an apparent threshold depolarization duration of approximately 10 ms within which to induce enough Ca(2+) transients to spread globally and then induce a contraction. Application of 100 microM ryanodine to the pipette solution did not change the resting [Ca(2+)](i) or the VDCC current, but it did abolish Ca(2+) hot spots elicited by depolarization. Application of 3 microM xestospongin C reduced ACh-induced Ca(2+) release but did not affect depolarization-induced Ca(2+) events. The addition of 100 microM ryanodine to tissue segments markedly reduced the amplitude of contractions triggered by direct electrical stimulation. In conclusion, global [Ca(2+)](i) rise triggered by a single action potential is not due mainly to Ca(2+) influx through VDCCs but is attributable to the subsequent two-step CICR.

Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of [Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level ([Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in [Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced [Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the [Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced [Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

A model of IP3 receptor with a luminal calcium binding site: stochastic simulations and analysis

We have constructed a stochastic model of the inositol 1,4,5-trisphosphate receptor-Ca2+ channel that is based on quantitative measurements of the channel's properties. It displays the observed dependence of the open probability of the channel with cytosolic [Ca2+] and [IP3] and gives values for the dwell times that agree with the observations. The model includes an explicit dependence of channel gating with luminal calcium. This not only explains several observations reported in the literature, but also provides a possible explanation of why the open probabilities and shapes of the bell-shaped curves reported in [Nature 351 (1991) 751] and in [Proc. Natl. Acad. Sci. U.S.A. 269 (1998) 7238] are so different.

Fluorescent biosensor for quantitative real-time measurements of inositol 1,4,5-trisphosphate in single living cells

The second messenger inositol 1,4,5-trisphosphate (IP(3)) plays a central role in the generation of a variety of spatiotemporally complex intracellular Ca(2+) signals involved in the regulation of many essential physiological processes. Here we describe the development of "LIBRA", a novel ratiometric fluorescent IP(3) biosensor that allows for the quantitative monitoring of intracellular IP(3) concentrations in single living cells in real time. LIBRA consists of the IP(3)-binding domain of the rat type 3 IP(3) receptor fused between the fluorescence resonance energy transfer pair cyan fluorescent protein and yellow fluorescent protein and preceded by a membrane-targeting signal. We show that the LIBRA fluorescent signal is highly selective for IP(3) and unaffected by concentrations of Ca(2+) and ATP in the physiological range. In addition, LIBRA can be calibrated in situ. We demonstrate the utility of LIBRA by monitoring the temporal relationship between the responses intracellular IP(3) and Ca(2+) concentrations in SH-SY5Y cells following acetylcholine stimulation.

A comparison of three 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 compare three mathematical models of the IPR, fitting each of them to the same data set to determine ranges for the parameter values. Each of the fits indicates that fast activation of the receptor, followed by slow inactivation, is an important feature of the model, and also that the speed of inositol trisphosphate IP3 binding cannot necessarily be assumed to be faster than Ca2+ activation. In addition, the model which assumed saturating binding rates of Ca2+ to the IPR demonstrated the best fit. However, lack of convergence in the fitting procedure indicates that responses to step increases of Ca2+ and IP3 provide insufficient data to determine the parameters unambiguously in any of the models.

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic beta-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca2+]c) in pancreatic beta-cells stimulated by extracellular ATP. Extracellular ATP (50 microM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse beta-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat beta-cells than in mouse. In response to extracellular ATP, rat beta-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse beta-cells. Rat and mouse beta-cells both responded to 9 mM glucose by increasing [Ca2+]c. This increase, however, was pronounced only in the rat beta-cells. In 9 mM glucose, extracellular ATP induced a pronounced calcium release above the increased level of [Ca2+]c in rat beta-cells. In mouse beta-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca2+]c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse beta-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse beta-cells but stimulates insulin secretion from rat beta-cells, we suggest that store-operated Ca2+ entry may be related to exocytosis in pancreatic rat beta-cells.

Calcium-induced calcium release in smooth muscle: the case for loose coupling

This article reviews the key experiments demonstrating calcium-induced calcium release (CICR) in smooth muscle and contrasts the biophysical and molecular features of coupling between the sarcolemmal (L-type Ca(2+) channel) and sarcoplasmic reticulum (ryanodine receptor) Ca(2+) channels in smooth and cardiac muscle. Loose coupling refers to the coupling process in smooth muscle in which gating of ryanodine receptors is non-obligate and may occur with a variable delay following opening of the sarcolemmal Ca(2+) channels. These features have been observed in the earliest studies of CICR in smooth muscle and are in marked contrast to cardiac CICR, where a close coupling between T-tubular and SR membranes results in tight coupling between the gating events. The relationship between this "loose coupling" and distinct subcellular release sites within smooth muscle cells, termed frequent discharge sites, is discussed.

A reassessment of the effects of luminal [Ca2+] on inositol 1,4,5-trisphosphate-induced Ca2+ release from internal stores

Inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release from intracellular stores displays complex kinetic behavior. While it well established that cytosolic [Ca2+] can modulate release by acting on the InsP3 receptor directly, the role of the filling state of internal Ca2+stores in modulating Ca2+ release remains unclear. Here we have reevaluated this topic using a technique that permits rapid and reversible changes in free [Ca2+] in internal stores of living intact cells without altering cytoplasmic [Ca2+], InsP3 receptors, or sarcoendoplasmic reticulum Ca2+ ATPases (SERCAs). N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylene diamine (TPEN), a membrane-permeant, low affinity Ca2+ chelator was used to manipulate [Ca2+] in intracellular stores, while [Ca2+] changes within the store were monitored directly with the low-affinity Ca2+ indicator, mag-fura-2, in intact BHK-21 cells. 200 microM TPEN caused a rapid drop in luminal free [Ca2+] and significantly reduced the extent of the response to stimulation with 100 nm bradykinin, a calcium-mobilizing agonist. The same effect was observed when intact cells were pretreated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid(acetoxymethyl ester) (BAPTA-AM) to buffer cytoplasmic [Ca2+] changes. Although inhibition of Ca2+ uptake using the SERCA inhibitor tBHQ permitted significantly larger release of Ca2+ from stores, TPEN still attenuated the release in the presence of tBHQ in BAPTA-AM-loaded cells. These results demonstrate that the filling state of stores modulates the magnitude of InsP3-induced Ca2+release by additional mechanism(s) that are independent of regulation by cytoplasmic [Ca2+] or effects on SERCA pumps.

Modeling the dependence of the period of intracellular Ca2+ waves on SERCA expression

Contrary to intuitive expectations, overexpression of sarco-endoplasmic reticulum (ER) Ca(2+) ATPases (SERCAs) in Xenopus oocytes leads to a decrease in the period and an increase in the amplitude of intracellular Ca(2+) waves. Here we examine these experimental findings by modeling Ca(2+) release using a modified Othmer-Tang-model. An increase in the period and a reduction in the amplitude of Ca(2+) wave activity are obtained when increases in SERCA density are simulated while keeping all other parameters of the model constant. However, Ca(2+) wave period can be reduced and the wave amplitude and velocity can be significantly increased when an increase in the luminal ER Ca(2+) concentration due to SERCA overexpression is incorporated into the model. Increased luminal Ca(2+) occurs because increased SERCA activity lowers cytosolic Ca(2+), which is partially replenished by Ca(2+) influx across the plasma membrane. These simulations are supported by experimental data demonstrating higher luminal Ca(2+) levels, decreased periods, increased amplitude, and increased velocity of Ca(2+) waves in response to increased SERCA density.

The contribution of intracellular calcium stores to mEPSCs recorded in layer II neurones of rat barrel cortex

Loading slices of rat barrel cortex with 50 microM BAPTA-AM while recording from pyramidal cells in layer II induces a marked reduction in both the frequency and amplitudes of mEPSCs. These changes are due to a presynaptic action. Blocking the refilling of Ca(2+) stores with 20 microM cyclopiazonic acid (CPA), a SERCA pump inhibitor, in conjunction with neuronal depolarisation to activate Ca(2+) stores, results in a similar reduction of mEPSCs to that observed with BAPTA-AM, indicating that the source for intracellular Ca(2+) is the endoplasmic reticulum. Block or activation of ryanodine receptors by 20 microM ryanodine or 10 mM caffeine, respectively, shows that a significant proportion of mEPSCs are caused by Ca(2+) release from ryanodine stores. Blocking IP(3) receptors with 14 microM 2-aminoethoxydiphenylborane (2APB) also reduces the frequency and amplitude of mEPSCs, indicating the involvement of IP(3) stores in the generation of mEPSCs. Activation of group I metabotropic receptors with 20 microM (RS)-3,5-dihydroxyphenylglycine (DHPG) results in a significant increase in the frequency of mEPSCs, further supporting the role of IP(3) receptors and indicating a role of group I metabotropic receptors in causing transmitter release. Statistical evidence is presented for Ca(2+)-induced Ca(2+) release (CICR) from ryanodine stores after the spontaneous opening of IP(3) stores.

Sequential opening of IP(3)-sensitive Ca(2+) channels and SOC during alpha-adrenergic activation of rabbit vena cava

Alpha(1)-aderenoceptor-mediated constriction of rabbit inferior vena cava (IVC) is signaled by asynchronous wavelike Ca(2+) oscillations in the in situ smooth muscle. We have shown previously that a putative nonselective cationic channel (NSCC) is required for these oscillations. In this report, we show that the application of 2-aminoethoxyphenyl borate (2-APB) to antagonize inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) release channels (IP(3)R channels) can prevent the initiation and abolish ongoing alpha(1)-aderenoceptor-mediated tonic constriction of the venous smooth muscle by inhibiting the generation of these intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations. The observed effects of 2-APB can only be attributed to its selective inhibition on the IP(3)R channels, not to its slight inhibition of the L-type voltage-gated Ca(2+) channel and the sarco(endo)plasmic reticulum Ca(2+) ATPase. Furthermore, 2-APB had no effect on the ryanodine-sensitive Ca(2+) release channel and the store-operated channel (SOC) in the IVC. These results indicate that the putative NSCC involved in refilling the sarcoplasmic reticulum (SR) and maintaining the tonic contraction is most likely an SOC-type channel because it appears to be activated by IP(3)R-channel-mediated SR Ca(2+) release or store depletion. This is in accordance with its sensitivity to Ni(2+) and La(3+) (SOC blockers). More interestingly, RT-PCR analysis indicates that transient receptor potential (Trp1) mRNA is strongly expressed in the rabbit IVC. The Trp1 gene is known to encode a component of the store-operated NSCC. These new data suggest that the activation of both the IP(3)R channels and the SOC are required for PE-mediated [Ca(2+)](i) oscillations and constriction of the rabbit IVC.

Intracellular calcium mobilization in response to the activation of human wild-type and chimeric gonadotropin receptors

It is well established that LH action is mediated primarily by adenylate cyclase/cAMP. However, the role of inositol phosphate/calcium in LH signaling is less well established. We examined the effects of gonadotropins in primary culture human granulosa-lutein cells and in HEK293 cells transiently transfected with human wild-type or chimeric gonadotropin receptors. The intracellular free calcium concentration was measured using fura-2 microspectrofluorometric techniques. Human (h) LH (2-4 microg/ml) and CG (10 IU/ml) consistently evoked oscillatory calcium signals in HEK293 cells transfected with hLH receptor, whereas hFSH (2-4 microg/ml) failed to elicit any response. Conversely, both hLH and hFSH failed to elicit a calcium response from HEK293 cells transfected with hFSHR, indicating the specificity of the response to the LH receptor. Pretreatment of transfected HEK293 cells with pertussis toxin (100 ng/ml) attenuated all gonadotropin-evoked calcium mobilization. Studies with chimeric LH receptor showed that the sequence of the long extracellular portion of the receptor was not critical for stimulation of PLC activity, but maintained agonist binding specificity. The C-terminal sequence of the receptor was clearly important for the generation of the basal calcium oscillations, but the precise extent of the critical sequence has yet to be identified. Although various subdivisions of this region were capable of stimulating calcium transients, an intact carboxyl-terminal third of the receptor was required for normal and sustained intracellular calcium signaling. Our study unequivocally shows that the hLH receptor is coupled to the inositol phosphate/calcium signaling pathway via a pertussis toxin-sensitive G protein-coupled receptor.

Irregular spiking in free calcium concentration in single, human platelets. Regulation by modulation of the inositol trisphosphate receptors

Fluorescence ratio imaging indicates that immobilized, aspirin-treated platelets, loaded with Fura-2, respond to inositol 1,4,5-trisphosphate- (InsP3)-generating agonists such as thrombin by high-frequency, irregular rises in cytosolic [Ca2+]i with spikes that vary in peak level and peak-to-peak interval. This differs from the regular [Ca2+]i oscillations observed in other, larger cells. We found that the thiol-reactive compounds thimerosal (10 microm) and U73122 (10 microm) evoked similar irregular Ca2+ responses in platelets, but in this case in the absence of InsP3 generation. Thrombin-induced spiking was acutely abolished by inhibiting phospholipase C or elevating intracellular cAMP levels, while spiking with sulfhydryl reagents was only partially blocked by cAMP elevation. Confocal laser scanning microscopy using fluo-3-loaded platelets indicated that, with all agonists or conditions, the irregular spikes were almost instantaneously raised in various regions within a single platelet. When using saponin-permeabilized platelets, we found that InsP3-induced Ca2+ release from stores was stimulated by modest Ca2+ concentrations, pointing to a mechanism of InsP3-dependent Ca2+-induced Ca2+ release (CICR). This process was completely inhibitable by heparin. The Ca2+ release by InsP3, but not the CICR sensor, was negatively regulated by cAMP elevation. Thimerosal treatment did not release Ca2+ from intracellular stores, but markedly potentiated the stimulatory effect of InsP3. In contrast, U73122 caused a heparin/cAMP-insensitive Ca2+ leak from stores that differed from those used by InsP3. Taken together, these results demonstrate that InsP3 receptor channels play a crucial role in the irregular, spiking Ca2+ signal of intact platelets, even when induced by agents such as thimerosal or U73122 which do not stimulate InsP3 formation. The irregular Ca2+ release events appear to be subjected to extensive regulation by: (a) InsP3 level, (b) the potentiating effect of elevated Ca2+ on InsP3 action via CICR, (c) InsP3 channel sensitization by sulfhydryl (thimerosal) modification, (d) InsP3 channel-independent Ca2+ leak with U73122, and (e) down-regulation via cAMP elevation. The observation that individual Ca2+ peaks were generated in various parts of a platelet at similar intervals and amplitudes points to effective cooperation of the various stores in the Ca2+-release process.