Calcium influx factor directly activates store-operated cation channels in vascular smooth muscle cells

The Obligatory Role of the Acetylcholine-Induced Endothelium-Dependent Contraction in Hypertension: Can Arachidonic Acid Resolve this Inflammation?

The endothelium produces many substances that can regulate vascular tone. Acetylcholine is a widely used pharmacological tool to assess endothelial function. In general, acetylcholine binds to G-protein coupled muscarinic receptors that mediate a transient elevation in intracellular, free calcium. This intracellular rise in calcium is responsible for triggering several cellular responses, including the synthesis of nitric oxide, endothelium- derived hyperpolarizing factor, and eicosanoids derived from arachidonic acid. Endothelial arachidonic acid metabolism is also an important signaling pathway for mediating inflammation. Therefore, in conditions with sustained and excessive inflammation such as hypertension, arachidonic acid serves as a substrate for the synthesis of several vasoconstrictive metabolites, predominantly via the cyclooxygenase and lipoxygenase enzymes. Cyclooxygenase and lipoxygenase products can then activate G-protein coupled receptors expressed on vascular smooth muscle cells to causes contractile responses. As a result, acetylcholine-induced contraction due to arachidonic acid is a commonly observed feature of endothelial dysfunction and vascular inflammation in hypertension. In this review, we will critically analyze the literature supporting this concept, as well as address the potential underlying mechanisms, including the possibility that arachidonic acid signaling is diverted away from the synthesis of pro-resolving metabolites in conditions such as hypertension.

The Complex Role of Store Operated Calcium Entry Pathways and Related Proteins in the Function of Cardiac, Skeletal and Vascular Smooth Muscle Cells

Cardiac, skeletal, and smooth muscle cells shared the common feature of contraction in response to different stimuli. Agonist-induced muscle's contraction is triggered by a cytosolic free Ca²⁺ concentration increase due to a rapid Ca²⁺ release from intracellular stores and a transmembrane Ca²⁺ influx, mainly through L-type Ca²⁺ channels. Compelling evidences have demonstrated that Ca²⁺ might also enter through other cationic channels such as Store-Operated Ca²⁺ Channels (SOCCs), involved in several physiological functions and pathological conditions. The opening of SOCCs is regulated by the filling state of the intracellular Ca²⁺ store, the sarcoplasmic reticulum, which communicates to the plasma membrane channels through the Stromal Interaction Molecule 1/2 (STIM1/2) protein. In muscle cells, SOCCs can be mainly non-selective cation channels formed by Orai1 and other members of the Transient Receptor Potential-Canonical (TRPC) channels family, as well as highly selective Ca²⁺ Release-Activated Ca²⁺ (CRAC) channels, formed exclusively by subunits of Orai proteins likely organized in macromolecular complexes. This review summarizes the current knowledge of the complex role of Store Operated Calcium Entry (SOCE) pathways and related proteins in the function of cardiac, skeletal, and vascular smooth muscle cells.

Major contribution of the 3/6/7 class of TRPC channels to myocardial ischemia/reperfusion and cellular hypoxia/reoxygenation injuries

The injury phase after myocardial infarcts occurs during reperfusion and is a consequence of calcium release from internal stores combined with calcium entry, leading to cell death by apoptopic and necrotic processes. The mechanism(s) by which calcium enters cells has(ve) not been identified. Here, we identify canonical transient receptor potential channels (TRPC) 3 and 6 as the cation channels through which most of the damaging calcium enters cells to trigger their death, and we describe mechanisms activated during the injury phase. Working in vitro with H9c2 cardiomyoblasts subjected to 9-h hypoxia followed by 6-h reoxygenation (H/R), and analyzing changes occurring in areas-at-risk (AARs) of murine hearts subjected to a 30-min ischemia followed by 24-h reperfusion (I/R) protocol, we found: (i) that blocking TRPC with SKF96365 significantly ameliorated damage induced by H/R, including development of the mitochondrial permeability transition and proapoptotic changes in Bcl2/BAX ratios; and (ii) that AAR tissues had increased TUNEL⁺ cells, augmented Bcl2/BAX ratios, and increased p(S240)NFATc3, p(S473)AKT, p(S9)GSK3β, and TRPC3 and -6 proteins, consistent with activation of a positive-feedback loop in which calcium entering through TRPCs activates calcineurin-mediated NFATc3-directed transcription of TRPC genes, leading to more Ca²⁺ entry. All these changes were markedly reduced in mice lacking TRPC3, -6, and -7. The changes caused by I/R in AAR tissues were matched by those seen after H/R in cardiomyoblasts in all aspects except for p-AKT and p-GSK3β, which were decreased after H/R in cardiomyoblasts instead of increased. TRPC should be promising targets for pharmacologic intervention after cardiac infarcts.

Store-Operated Ca2+ Channel in Renal Microcirculation and Glomeruli

Store-operated Ca2+ channel (SOC) is defined as a channel that opens in response to depletion of the internal Ca2+ stores. During the last decade, many investigators have made a great effort to identify and characterize SOC, and to evaluate its physiologic function and pathophysiologic relevance in a variety of cell lines, primary cultures, and native tissues. To date, accumulating evidence has demonstrated that SOC is an essential Ca2+ entry mechanism in vascular smooth-muscle cells of renal microvasculature and glomerular mesangial cells, both of which tightly control glomerular hemodynamics and filtration. Store-operated Ca2+, combined with other types of Ca2+ entry channels, constitutes a profile of Ca2+ changes in response to physiologic vasoconstrictors and, thereby, regulates renal microcirculation and mesangial function. In addition, SOC is associated with altered Ca2+ signaling occurring in diseased kidneys, such as diabetic nephropathy. Although the gating mechanism and molecular identity of SOC are still enigmatic and may be cell-type and tissue specific, data from several independent groups suggest that protein kinase C plays an important role in SOC activation and that certain isoforms of canonical transient receptor potential (TRPC) proteins are candidates of SOC in renal mlcrovessels and mesangial cells.

Phospholipase A2 as a Molecular Determinant of Store-Operated Calcium Entry

Activation of phospholipases A2 (PLA2) leads to the generation of biologically active lipid products that can affect numerous cellular events. Ca(2+)-independent PLA2 (iPLA2), also called group VI phospholipase A2, is one of the main types forming the superfamily of PLA2. Beside of its role in phospholipid remodeling, iPLA2 has been involved in intracellular Ca(2+) homeostasis regulation. Several studies proposed iPLA2 as an essential molecular player of store operated Ca(2+) entry (SOCE) in a large number of excitable and non-excitable cells. iPLA2 activation releases lysophosphatidyl products, which were suggested as agonists of store operated calcium channels (SOCC) and other TRP channels. Herein, we will review the important role of iPLA2 on the intracellular Ca(2+) handling focusing on its role in SOCE regulation and its implication in physiological and/or pathological processes.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

STIM1 regulates calcium signaling in taste bud cells and preference for fat in mice

Understanding the mechanisms underlying oro-gustatory detection of dietary fat is critical for the prevention and treatment of obesity. The lipid-binding glycoprotein CD36, which is expressed by circumvallate papillae (CVP) of the mouse tongue, has been implicated in oro-gustatory perception of dietary lipids. Here, we demonstrate that stromal interaction molecule 1 (STIM1), a sensor of Ca(2+) depletion in the endoplasmic reticulum, mediates fatty acid-induced Ca(2+) signaling in the mouse tongue and fat preference. We showed that linoleic acid (LA) induced the production of arachidonic acid (AA) and lysophosphatidylcholine (Lyso-PC) by activating multiple phospholipase A2 isoforms via CD36. This activation triggered Ca(2+) influx in CD36-positive taste bud cells (TBCs) purified from mouse CVP. LA also induced the production of Ca(2+) influx factor (CIF). STIM1 was found to regulate LA-induced CIF production and the opening of multiple store-operated Ca(2+) (SOC) channels. Furthermore, CD36-positive TBCs from Stim1-/- mice failed to release serotonin, and Stim1-/- mice lost the spontaneous preference for fat that was observed in wild-type animals. Our results suggest that fatty acid-induced Ca(2+) signaling, regulated by STIM1 via CD36, might be implicated in oro-gustatory perception of dietary lipids and the spontaneous preference for fat.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Orai1 and Ca2+-independent phospholipase A2 are required for store-operated Icat-SOC current, Ca2+ entry, and proliferation of primary vascular smooth muscle cells

Store-operated Ca(2+) entry (SOCE) is important for multiple functions of vascular smooth muscle cells (SMC), which, depending of their phenotype, can resemble excitable and nonexcitable cells. Similar to nonexcitable cells, Orai1 was found to mediate Ca(2+)-selective (CRAC-like) current and SOCE in dedifferentiated cultured SMC and smooth muscle-derived cell lines. However, the role of Orai1 in cation-selective store-operated channels (cat-SOC), which are responsible for SOCE in primary SMC, remains unclear. Here we focus on primary SMC, and assess the role of Orai1 and Ca(2+)-independent phospholipase A(2) (iPLA(2)β, or PLA2G6) in activation of cat-SOC current (I(cat-SOC)), SOCE, and SMC proliferation. Using molecular, electrophysiological, imaging, and functional approaches, we demonstrate that molecular knockdown of either Orai1 or iPLA(2)β leads to similar inhibition of the whole cell cat-SOC current and SOCE in primary aortic SMC and results in significant reduction in DNA synthesis and impairment of SMC proliferation. This is the first demonstration that Orai1 and iPLA(2)β are equally important for cat-SOC, SOCE, and proliferation of primary aortic SMC.

COX-mediated endothelium-dependent contractions: from the past to recent discoveries

Endothelial cells release various substances to control the tone of the underlying vascular smooth muscle. Nitric oxide (NO) is the best defined endothelium-derived relaxing factor (EDRF). Endothelial cells can also increase vascular tone by releasing endothelium-derived contracting factors (EDCF). The over-production of EDCF contributes to the endothelial dysfunctions which accompanies various vascular diseases. The present review summarizes and discusses the mechanisms leading to the release of EDCFs derived from the metabolism of arachidonic acid. This release can be triggered by agonists such as acetylcholine, adenosine nucleotides or by stretch. All these stimuli are able to induce calcium influx into the endothelial cells, an effect which can be mimicked by calcium ionophores. The augmentation in intracellular calcium ion concentration initiates the release of EDCF. Downstream processes include activation of phospholipase A(2) (PLA(2)), cyclooxygenases (COX) and the production of reactive oxygen species (ROS) and vasoconstrictor prostanoids (endoperoxides, prostacyclin, thromboxane A(2) and other prostaglandins) which subsequently diffuse to, and activate thromboxane-prostanoid (TP) receptors on the vascular smooth muscle cells leading to contraction.

Molecular basis of calcium signaling in lymphocytes: STIM and ORAI

Ca(2+) entry into cells of the peripheral immune system occurs through highly Ca(2+)-selective channels known as CRAC (calcium release-activated calcium) channels. CRAC channels are a very well-characterized example of store-operated Ca(2+) channels, so designated because they open when the endoplasmic reticulum (ER) Ca(2+) store becomes depleted. Physiologically, Ca(2+) is released from the ER lumen into the cytoplasm when activated receptors couple to phospholipase C and trigger production of the second messenger inositol 1,4,5-trisphosphate (IP(3)). IP(3) binds to IP(3) receptors in the ER membrane and activates Ca(2+) release. The proteins STIM and ORAI were discovered through limited and genome-wide RNAi screens, respectively, performed in Drosophila cells and focused on identifying modulators of store-operated Ca(2+) entry. STIM1 and STIM2 sense the depletion of ER Ca(2+) stores, whereas ORAI1 is a pore subunit of the CRAC channel. In this review, we discuss selected aspects of Ca(2+) signaling in cells of the immune system, focusing on the roles of STIM and ORAI proteins in store-operated Ca(2+) entry.

The role of protein O-linked beta-N-acetylglucosamine in mediating cardiac stress responses

The modification of serine and threonine residues of nuclear and cytoplasmic proteins by O-linked beta-N-acetylglucosamine (O-GlcNAc) has emerged as a highly dynamic post-translational modification that plays a critical role in regulating numerous biological processes. Much of our understanding of the mechanisms underlying the role of O-GlcNAc on cellular function has been in the context of its adverse effects in mediating a range of chronic disease processes, including diabetes, cancer and neurodegenerative diseases. However, at the cellular level it has been shown that O-GlcNAc levels are increased in response to stress; augmentation of this response improved cell survival while attenuation decreased cell viability. Thus, it has become apparent that strategies that augment O-GlcNAc levels are pro-survival, whereas those that reduce O-GlcNAc levels decrease cell survival. There is a long history demonstrating the effectiveness of acute glucose-insulin-potassium (GIK) treatment and to a lesser extent glutamine in protecting against a range of stresses, including myocardial ischemia. A common feature of these approaches for metabolic cardioprotection is that they both have the potential to stimulate O-GlcNAc synthesis. Consequently, here we examine the links between metabolic cardioprotection with the ischemic cardioprotection associated with acute increases in O-GlcNAc levels. Some of the protective mechanisms associated with activation of O-GlcNAcylation appear to be transcriptionally mediated; however, there is also strong evidence to suggest that transcriptionally independent mechanisms also play a critical role. In this context we discuss the potential link between O-GlcNAcylation and cardiomyocyte calcium homeostasis including the role of non-voltage gated, capacitative calcium entry as a potential mechanism contributing to this protection.

Differences in STIM1 and TRPC expression in proximal and distal pulmonary arterial smooth muscle are associated with differences in Ca2+ responses to hypoxia

Hypoxic pulmonary vasoconstriction (HPV) requires Ca(2+) influx through store-operated Ca(2+) channels (SOCC) in pulmonary arterial smooth muscle cells (PASMC) and is greater in distal than proximal pulmonary arteries (PA). SOCC may be composed of canonical transient receptor potential (TRPC) proteins and activated by stromal interacting molecule 1 (STIM1). To assess the possibility that HPV is greater in distal PA because store-operated Ca(2+) entry (SOCE) is greater in distal PASMC, we measured intracellular Ca(2+) concentration ([Ca(2+)](i)) and SOCE in primary cultures of PASMC using fluorescent microscopy and the Ca(2+)-sensitive dye fura 2. Both hypoxia (4% O(2)) and KCl (60 mM) increased [Ca(2+)](i). Responses to hypoxia, but not KCl, were greater in distal cells. We measured SOCE in PASMC perfused with Ca(2+)-free solutions containing cyclopiazonic acid to deplete Ca(2+) stores in sarcoplasmic reticulum and nifedipine to prevent Ca(2+) entry through L-type voltage-operated Ca(2+) channels. Under these conditions, the increase in [Ca(2+)](i) caused by restoration of extracellular Ca(2+) and the decrease in fura 2 fluorescence caused by Mn(2+) were greater in distal PASMC, indicating greater SOCE. Moreover, the increase in SOCE caused by hypoxia was also greater in distal cells. Real-time quantitative polymerase chain reaction analysis of PASMC and freshly isolated deendothelialized PA tissue demonstrated expression of STIM1 and five of seven known TRPC isoforms (TRPC1 > TRPC6 > TRPC4 >> TRPC3 approximately TRPC5). For both protein, as measured by Western blotting, and mRNA, expression of STIM1, TRPC1, TRPC6, and TRPC4 was greater in distal than proximal PASMC and PA. These results provide further support for the importance of SOCE in HPV and suggest that HPV is greater in distal than proximal PA because greater numbers and activation of SOCC in distal PASMC generate bigger increases in [Ca(2+)](i).

Novel role for STIM1 as a trigger for calcium influx factor production

STIM1 has been recently identified as a Ca(2+) sensor in endoplasmic reticulum (ER) and an initiator of the store-operated Ca(2+) entry (SOCE) pathway, but the mechanism of SOCE activation remains controversial. Here we focus on the early ER-delimited steps of the SOCE pathway and demonstrate that STIM1 is critically involved in initiating of production of calcium influx factor (CIF), a diffusible messenger that can deliver the signal from the stores to plasma membrane and activate SOCE. We discovered that CIF production is tightly coupled with STIM1 expression and requires functional integrity of its intraluminal sterile alpha-motif (SAM) domain. We demonstrate that 1) molecular knockdown or overexpression of STIM1 results in corresponding impairment or amplification of CIF production and 2) inherent deficiency in the ER-delimited CIF production and SOCE activation in some cell types can be a result of their deficiency in STIM1 protein; expression of a wild-type STIM1 in such cells was sufficient to fully rescue their ability to produce CIF and SOCE. We found that glycosylation sites in the ER-resident SAM domain of STIM1 are essential for initiation of CIF production. We propose that after STIM1 loses Ca(2+) from EF hand, its intraluminal SAM domain may change conformation, and via glycosylation sites it can interact with and activate CIF-producing machinery. Thus, CIF production appears to be one of the earliest STIM1-dependent events in the ER lumen, and impairment of this process results in impaired SOCE response.

Overexpression of TRPC3 increases apoptosis but not necrosis in response to ischemia-reperfusion in adult mouse cardiomyocytes

An increase in cytosolic Ca2+ via a capacitative calcium entry (CCE)-mediated pathway, attributed to members of the transient receptor potential (TRP) superfamily, TRPC1 and TRPC3, has been reported to play an important role in regulating cardiomyocyte hypertrophy. Increased cytosolic Ca2+ also plays a critical role in mediating cell death in response to ischemia-reperfusion (I/R). Therefore, we tested the hypothesis that overexpression of TRPC3 in cardiomyocytes will increase sensitivity to I/R injury. Adult cardiomyocytes isolated from wild-type (WT) mice and from mice overexpressing TRPC3 in the heart were subjected to 90 min of ischemia and 3 h of reperfusion. After I/R, viability was 51 +/- 1% in WT mice and 42 +/- 5% in transgenic mice (P < 0.05). Apoptosis assessed by annexin V was significantly increased in the TRPC3 group compared with WT (32 +/- 1% vs. 21 +/- 3%; P < 0.05); however, there was no significant difference in necrosis between groups. Treatment of TRPC3 cells with the CCE inhibitor SKF-96365 (0.5 microM) significantly improved cellular viability (54 +/- 4%) and decreased apoptosis (15 +/- 4%); in contrast, the L-type Ca2+ channel inhibitor verapamil (10 microM) had no effect. Calpain-mediated cleavage of alpha-fodrin was increased approximately threefold in the transgenic group following I/R compared with WT (P < 0.05); this was significantly attenuated by SKF-96365. The calpain inhibitor PD-150606 (25 microM) attenuated the increase in both alpha-fodrin cleavage and apoptosis in the TPRC3 group. Increased TRPC3 expression also increased sensitivity to Ca2+ overload stress, but it did not affect the response to TNF-alpha-induced apoptosis. These results suggest that CCE mediated via TRPC may play a role in cardiomyocyte apoptosis following I/R due, at least in part, to increased calpain activation.

Functional consequences of activating store-operated CRAC channels

Store-operated CRAC channels, which are activated by the emptying of the endoplasmic reticulum Ca(2+) stores, are an important and widespread route for triggering rises in cytoplasmic Ca(2+). The cellular responses that are activated in response to Ca(2+) entry through CRAC channels are being dissected out, and recent evidence has established that CRAC channels can induce both short-term (safeguarding the Ca(2+) content of the endoplasmic reticulum, maintenance of cytoplasmic Ca(2+) oscillations, enzyme activation, secretion) and long-term (gene expression) changes in cells. CRAC channel activation is therefore capable of evoking a range of temporally distinct responses, highlighting the versatility of this ubiquitous Ca(2+) entry pathway.

Role of InsP3 and ryanodine receptors in the activation of capacitative Ca2+ entry by store depletion or hypoxia in canine pulmonary arterial smooth muscle cells

BACKGROUND AND PURPOSE

Experiments were performed to determine if capacitative Ca(2+) entry (CCE) in canine pulmonary arterial smooth muscle cells (PASMCs) is dependent on InsP(3) receptors or ryanodine receptors as induction of CCE is dependent on simultaneous depletion of the functionally separate InsP(3)- and ryanodine-sensitive sarcoplasmic reticulum (SR) Ca(2+) stores in these cells.

EXPERIMENTAL APPROACH

Myocytes were isolated from canine pulmonary arteries using enzymatic procedures and were used within 8 h of preparation. Measurements of cytosolic Ca(2+) were made by imaging fura-2 loaded individual myocytes that were perfused with physiological buffered saline solution with or without Ca(2+).

KEY RESULTS

Treating myocytes with 10 microM cyclopiazonic acid (CPA), removing extracellular Ca(2+), and briefly applying 10 mM caffeine and 10 microM 5-hydroxytryptamine (5-HT) depleted SR Ca(2+) stores. Extracellular Ca(2+) reintroduction caused cytosolic [Ca(2+)] to elevate above baseline signifying CCE. The InsP(3) receptor inhibitors 2-aminobiphenylborate (50-75 microM; 2-APB) and xestospongin-C (20 microM; XeC) abolished CCE. Yet, CCE was unaffected by 10 microM or 300 microM ryanodine or 10 microM dantrolene, which modify ryanodine receptor activity. Higher dantrolene concentrations (50 microM), however, can inhibit both ryanodine receptors and InsP(3) receptors, did reduce CCE. In contrast, CCE activated by hypoxia was unaffected by XeC (20 microM).

CONCLUSIONS AND IMPLICATIONS

The results provide evidence that CCE activated by depletion of both InsP(3) and ryanodine SR Ca(2+) stores in canine PASMCs is dependent on functional InsP(3) receptors, whereas the activation of CCE by hypoxia appears to be independent of functional InsP(3) receptors.

New molecular players in capacitative Ca2+ entry

Capacitative Ca2+ entry links the emptying of intracellular Ca2+ stores to the activation of store-operated Ca2+ channels in the plasma membrane. In the twenty years since the inception of the concept of capacitative Ca2+ entry, a number of activation mechanisms have been proposed, and there has been considerable interest in the possibility that TRP channels function as store-operated channels. However, in the past two years, two major players in both the signaling and permeation mechanisms for store-operated channels have been discovered: Stim1 and the Orai proteins. Stim1 is an endoplasmic reticulum Ca2+ sensor. It appears to act by redistributing within a small component of the endoplasmic reticulum, approaching the plasma membrane, but does not seem to translocate into the plasma membrane. Stim1 signals to plasma membrane Orai proteins, which constitute pore-forming subunits of store-operated channels.

Recent breakthroughs in the molecular mechanism of capacitative calcium entry (with thoughts on how we got here)

Activation of phospholipase C by G-protein-coupled receptors results in release of intracellular Ca(2+) and activation of Ca(2+) channels in the plasma membrane. The intracellular release of Ca(2+) is signaled by the second messenger, inositol 1,4,5-trisphosphate. Ca(2+) entry involves signaling from depleted intracellular stores to plasma membrane Ca(2+) channels, a process referred to as capacitative calcium entry or store-operated calcium entry. The electrophysiological current associated with capacitative calcium entry is the calcium-release-activated calcium current, or I(crac). In the 20 years since the inception of the concept of capacitative calcium entry, a variety of activation mechanisms have been proposed, and there has been considerable interest in the possibility of transient receptor potential channels functioning as store-operated channels. However, in the past 2 years, two major players in both the signaling and permeation mechanisms for store-operated channels have been discovered: Stim1 (and possibly Stim2) and the Orai proteins. Activation of store-operated channels involves an endoplasmic reticulum Ca(2+) sensor called Stim1. Stim1 acts by redistributing within a small component of the endoplasmic reticulum, approaching the plasma membrane, but does not appear to translocate into the plasma membrane. Stim1, either directly or indirectly, signals to plasma membrane Orai proteins which constitute pore-forming subunits of store-operated channels.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

Activation mechanism for CRAC current and store-operated Ca2+ entry: calcium influx factor and Ca2+-independent phospholipase A2beta-mediated pathway

Here we tested the role of calcium influx factor (CIF) and calcium-independent phospholipase A2 (iPLA2) in activation of Ca2+ release-activated Ca2+ (CRAC) channels and store-operated Ca2+ entry in rat basophilic leukemia (RBL-2H3) cells. We demonstrate that 1) endogenous CIF production may be triggered by Ca2+ release (net loss) as well as by simple buffering of free Ca2+ within the stores, 2) a specific 82-kDa variant of iPLA2beta and its corresponding activity are present in membrane fraction of RBL cells, 3) exogenous CIF (extracted from other species) mimics the effects of endogenous CIF and activates iPLA2beta when applied to cell homogenates but not intact cells, 4) activation of ICRAC can be triggered in resting RBL cells by dialysis with exogenous CIF, 5) molecular or functional inhibition of iPLA2beta prevents activation of ICRAC, which could be rescued by cell dialysis with a human recombinant iPLA2beta, 6) dependence of ICRAC on intracellular pH strictly follows pH dependence of iPLA2beta activity, and 7) (S)-BEL, a chiral enantiomer of suicidal substrate specific for iPLA2beta, could be effectively used for pharmacological inhibition of ICRAC and store-operated Ca2+ entry. These findings validate and significantly advance our understanding of the CIF-iPLA2-dependent mechanism of activation of ICRAC and store-operated Ca2+ entry.

Ca2+-independent phospholipase A2 enhances store-operated Ca2+ entry in dystrophic skeletal muscle fibers

Duchenne muscular dystrophy is caused by deficiency of dystrophin and leads to progressive weakness. It has been proposed that the muscle degeneration occurring in this disease is caused by increased Ca2+ influx due to enhanced activity of cationic channels that are activated either by stretch of the plasma membrane (stretch-activated channels) or by Ca2+-store depletion (store-operated channels). Using both cytosolic Ca2+ measurements with Fura-2 and the manganese quench method, we show here that store-operated Ca2+ entry is greatly enhanced in dystrophic skeletal flexor digitorum brevis fibers isolated from mdx5cv mice, a mouse model of Duchenne muscular dystrophy. Moreover, we show for the first time that store-operated Ca2+ entry in these fibers is under the control of the Ca2+-independent phospholipase A2 and that the exaggerated Ca2+ influx can be completely attenuated by inhibitors of this enzyme. Enhanced store-operated Ca2+ entry in dystrophic fibers is likely to be due to a near twofold overexpression of Ca2+-independent phospholipase A2. The Ca2+-independent phospholipase A2 pathway therefore appears as an attractive target to reduce excessive Ca2+ influx and subsequent degeneration occurring in dystrophic fibers.

On the activation mechanism of store-operated calcium channels

The development of the patch clamp technique has revolutionised our understanding of the life sciences. One area in which it has made an enormous contribution is cellular signalling. In many cell types, calcium influx across the plasma membrane is essential for the regulation of a wide range of critical physiological responses including secretion, gene transcription and cell growth. For many years the calcium influx pathways in non-excitable cells remained unknown, despite their importance in physiological and pathophysiological states. Very careful and insightful work by James Putney led to the formulation of the capacitative calcium entry (store-operated calcium influx) model, in which the process of emptying intracellular calcium stores resulted in the activation of calcium entry channels. Unequivocal evidence for this revolutionary model was provided by patch clamp studies carried out by Markus Hoth and Reinhold Penner, who demonstrated that store depletion activated a novel class of calcium channel called the CRAC channel. This review provides a historical perspective on the development of store-operated calcium influx and how patch clamping resolved a long-standing controversy in cell physiology. The review also discusses current ideas relating to how store emptying opens channels in the plasma membrane.

Glucosamine inhibits angiotensin II-induced cytoplasmic Ca2+elevation in neonatal cardiomyocytes via protein-associatedO-linkedN-acetylglucosamine

We previously reported that glucosamine and hyperglycemia attenuate the response of cardiomyocytes to inositol 1,4,5-trisphosphate-generating agonists such as ANG II. This appears to be related to an increase in flux through the hexosamine biosynthesis pathway (HBP) and decreased Ca2+entry into the cells; however, a direct link between HBP and intracellular Ca2+homeostasis has not been established. Therefore, using neonatal rat ventricular myocytes, we investigated the relationship between glucosamine treatment; the concentration of UDP- N-acetylglucosamine (UDP-GlcNAc), an end product of the HBP; and the level of protein O-linked N-acetylglucosamine ( O-GlcNAc) on ANG II-mediated changes in intracellular free Ca2+concentration ([Ca2+]i). We found that glucosamine blocked ANG II-induced [Ca2+]iincrease and that this phenomenon was associated with a significant increase in UDP-GlcNAc and O-GlcNAc levels. O-(2-acetamido-2-deoxy-d-glucopyranosylidene)-amino- N-phenylcarbamate, an inhibitor of O-GlcNAcase that increased O-GlcNAc levels without changing UDP-GlcNAc concentrations, mimicked the effect of glucosamine on the ANG II-induced increase in [Ca2+]i. An inhibitor of O-GlcNAc-transferase, alloxan, prevented the glucosamine-induced increase in O-GlcNAc but not the increase in UDP-GlcNAc; however, alloxan abrogated the inhibition of the ANG II-induced increase in [Ca2+]i. These data support the notion that changes in O-GlcNAc levels mediated via increased HBP flux may be involved in the regulation of [Ca2+]ihomeostasis in the heart.

Inhibition by calyculin A and okadaic acid of the Ca(2+) release-activated Ca(2+) entry pathway in rat basophilic leukemia cells: evidence for regulation by type 1/2A serine/threonine phosphatase activity

Using a combination of fluorescence measurements of intracellular Ca(2+) ion concentration ([Ca(2+)](i)) and membrane potential we have investigated the sensitivity to serine/threonine phosphatase inhibition of Ca(2+) entry stimulated by activation of the Ca(2+) release-activated Ca(2+) (CRAC) entry pathway in rat basophilic leukemia cells. In both suspension and adherent cells, addition of the type 1/2A phosphatase inhibitor calyculin A, during activation of CRAC uptake, resulted in a fall in [Ca(2+)](i) to near preactivation levels. Pre-treatment with calyculin A abolished the component of the Ca(2+) rise associated with activation of CRAC uptake and inhibited Mn(2+) entry, consistent with a requirement of phosphatase activity for activation of the pathway. Depletion of intracellular Ca(2+) stores is accompanied by a large depolarisation which is absolutely dependent upon Ca(2+) entry via the CRAC uptake pathway. Application of calyculin A or okadaic acid, a structurally unrelated phosphatase antagonist inhibits this depolarisation. Taken in concert, these data demonstrate a marked sensitivity of the CRAC entry pathway to inhibition by calyculin A and okadaic acid.

CIF and other mysteries of the store-operated Ca2+-entry pathway

The molecular mechanism of the store-operated Ca2+-entry (SOCE) pathway remains one of the most intriguing and long lasting mysteries of Ca2+ signaling. The elusive calcium influx factor (CIF) that is produced upon depletion of Ca2+ stores has attracted growing attention, triggered by new discoveries that filled the gap in the chain of reactions leading to activation of store-operated channels and Ca2+ entry. Ca2+-independent phospholipase A2 emerged as a target of CIF, and a major determinant of the SOCE mechanism. Here, we present our viewpoint on CIF and conformational-coupling models of SOCE from a historical perspective, trying to resolve some of the problem areas, and summarizing our present knowledge on how depletion of intracellular Ca2+ stores signals to plasma membrane channels to open and provide Ca2+ influx that is required for many important physiological functions.

High glucose-induced apoptosis through store-operated calcium entry and calcineurin in human umbilical vein endothelial cells

Diabetes mellitus causes multiple cardiovascular complications. Previous studies have shown that prolonged exposure (96 h) of human umbilical vein endothelial cells (HUVECs) to hyperglycemia causes a significant increase in apoptosis. We report here that this increase in apoptosis is associated with an increase in Ca(2+) current (whole cell patch-clamp recorded) resulting from Ca(2+) entry mediated by store-operated channels (SOCs). The number of apoptotic cells after prolonged high glucose (HG, 30 mmol/L) exposure was significantly reduced in the presence of the SOC inhibitor 2-APB or of La(3+). A marked increase (approximately 80%) in Ca(2+)-dependent calcineurin (CN-A) phosphatase activity also occurred after prolonged HG exposure. Prolonged HG exposure-induced increase in CN-A activity was prevented by 2-APB, and selective CN-A phosphatase inhibition by FK506 or calmodulin inhibition by calmidazolium decreased HG-induced apoptosis. Blocking hydrogen peroxide production using catalase or inhibiting the tyrosine kinase pp60(src) during prolonged exposure to HG, resulted in a marked decrease in apoptosis and was further associated with a significant reduction in CN-A phosphatase activity. The results demonstrate a significant role for Ca(2+) entry in HG-induced apoptosis in HUVECs, and suggest that this role is mediated via H(2)O(2) generation and the action of the Ca(2+)-activated protein phosphatase calcineurin.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Activation of ryanodine receptors induces calcium influx in a neuroblastoma cell line lacking calcium influx factor activity

We have further characterized the Ca2+ signalling properties of the NG115-401L (or 401L) neuroblastoma cell line, which has served as an important cell line for investigating SOC (store-operated channel) influx pathways. These cells possess an unusual Ca2+ signalling phenotype characterized by the absence of Ca2+ influx when Ca2+ stores are depleted by inhibitors of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). Previous studies found that Ca2+-store depletion does not produce a CIF (Ca2+ influx factor) activity in 401L cells. These observations have prompted the question whether 401L cells possess the signalling machinery that permits non-voltage-gated Ca2+ influx to occur. We tested the hypothesis that ryanodine-sensitive Ca2+ pools and activation of RyRs (ryanodine receptors) constitute a signalling pathway capable of inducing Ca2+ influx in 401L cells. We found that 401L cells express mRNA for RyR1 and RyR2 and that RyR activators induced Ca2+ release. Activation of RyRs robustly couples with Ca2+ influx responses in 401L cells, in sharp contrast with absence of Ca2+ influx when cells are treated with SERCA inhibitors. Thus it is clear that 401L cells, despite lacking depletion-induced Ca2+ influx pathways, express the functional components of a Ca2+ influx pathway under the control of RyR function. These findings further support the importance of the 401L cell line as an important cell phenotype for deciphering Ca2+ influx regulation.

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.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Distinct Ca(2+)-permeable cation currents are activated by internal Ca(2+)-store depletion in RBL-2H3 cells and human salivary gland cells, HSG and HSY

Store-operated Ca(2+) influx, suggested to be mediated via store-operated cation channel (SOC), is present in all cells. The molecular basis of SOC, and possible heterogeneity of these channels, are still a matter of controversy. Here we have compared the properties of SOC currents ( I(SOC)) in human submandibular glands cells (HSG) and human parotid gland cells (HSY) with I(CRAC) (Ca(2+) release-activated Ca(2+) current) in RBL cells. Internal Ca(2+) store-depletion with IP(3) or thapsigargin activated cation channels in all three cell types. 1 muM Gd(3+) blocked channel activity in all cells. Washout of Gd(3+) induced partial recovery in HSY and HSG but not RBL cells. 2-APB reversibly inhibited the channels in all cells. I(CRAC )in RBL cells displayed strong inward rectification with E(rev)(Ca) = >+90 mV and E(rev) (Na) = +60 mV. I(SOC) in HSG cells showed weaker rectification with E(rev)(Ca) = +25 mV and E(rev)(Na) = +10 mV. HSY cells displayed a linear current with E(rev) = +5 mV, which was similar in Ca(2+)- or Na(+)-containing medium. pCa/ pNa was >500, 40, and 4.6 while pCs / pNa was 0.1,1, and 1.3, for RBL, HSG, and HSY cells, respectively. Evidence for anomalous mole fraction behavior of Ca(2+)/Na(+) permeation was obtained with RBL and HSG cells but not HSY cells. Additionally, channel inactivation with Ca(2+) + Na(+) or Na(+) in the bath was different in the three cell types. In aggregate, these data demonstrate that distinct store-dependent cation currents are stimulated in RBL, HSG, and HSY cells. Importantly, these data suggest a molecular heterogeneity, and possibly cell-specific differences in the function, of these channels.

Visualization of localized store-operated calcium entry in mouse astrocytes. Close proximity to the endoplasmic reticulum

Unloading of endoplasmic reticulum (ER) Ca(2+) stores activates influx of extracellular Ca(2+) through 'store-operated' Ca(2+) channels (SOCs) in the plasma membrane (PM) of most cells, including astrocytes. A key unresolved issue concerning SOC function is their spatial relationship to ER Ca(2+) stores. Here, using high resolution imaging with the membrane-associated Ca(2+) indicator, FFP-18, it is shown that store-operated Ca(2+) entry (SOCE) in primary cultured mouse cortical astrocytes occurs at plasma membrane-ER junctions. In the absence of extracellular Ca(2+), depletion of ER Ca(2+) stores using cyclopiazonic acid, an ER Ca(2+)-ATPase inhibitor, and caffeine transiently increases the sub-plasma-membrane Ca(2+) concentration ([Ca(2+)](SPM)) within a restricted space between the plasma membrane and adjacent ER. Restoration of extracellular Ca(2+) causes localized Ca(2+) influx that first increases [Ca(2+)](SPM) in the same restricted regions and then, with a delay, in ER-free regions. Antisense knockdown of the TRPC1 gene, proposed to encode endogenous SOCs, markedly reduces SOCE measured with Fura-2. High resolution immunocytochemistry with anti-TRPC1 antibody reveals that these TRPC-encoded SOCs are confined to the PM microdomains adjacent to the underlying 'junctional' ER. Thus, Ca(2+) entry through TRPC-encoded SOCs is closely linked, not only functionally, but also structurally, to the ER Ca(2+) stores.

The mammalian TRPC cation channels

Transient Receptor Potential-Canonical (TRPC) channels are mammalian homologs of Transient Receptor Potential (TRP), a Ca(2+)-permeable channel involved in the phospholipase C-regulated photoreceptor activation mechanism in Drosophila. The seven mammalian TRPCs constitute a family of channels which have been proposed to function as store-operated as well as second messenger-operated channels in a variety of cell types. TRPC channels, together with other more distantly related channel families, make up the larger TRP channel superfamily. This review summarizes recent findings on the structure, regulation and function of the apparently ubiquitous TRPC cation channels.

Calmodulin mediates norepinephrine-induced receptor-operated calcium entry in preglomerular resistance arteries

Although L-type voltage-dependent calcium channels play a major role in mediating vascular smooth muscle cell contraction in the renal vasculature, non-L-type calcium entry mechanisms represent a significant component of vasoactive agonist-induced calcium entry in these cells as well. To investigate the role of these non-voltage-dependent calcium entry pathways in the regulation of renal microvascular reactivity, we have characterized the function of store- and receptor-operated channels (SOCs and ROCs) in renal cortical interlobular arteries (ILAs) of rats. Using fura 2-loaded, microdissected ILAs, we find that the L-type channel antagonist nifedipine blocks less than half the rise in intracellular calcium concentration ([Ca(2+)](i)) elicited by norepinephrine. SOCs were activated in these vessels using the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors cyclopiazonic acid and thapsigargin and were dose dependently blocked by the SOC antagonists Gd(3+) and 2-aminoethoxydiphenyl borate (2-APB) and the combined SOC/ROC antagonist SKF-96365. Gd(3+) had no effect on the non-L-type Ca(2+) entry activated by 1 microM NE. A low concentration of SKF-96365 that did not affect thapsigargin-induced store-operated Ca(2+) entry blocked 60-70% of the NE-induced Ca(2+) entry. Two different calmodulin inhibitors (W-7 and trifluoperazine) also blocked the NE-induced Ca(2+) entry. These data suggest that in addition to L-type channels, NE primarily activates ROCs rather than SOCs in ILAs and that this receptor-operated Ca(2+) entry mechanism is regulated by calmodulin. Interestingly, 2-APB completely blocked the NE-induced non-L-type Ca(2+) entry, implying that SOCs and ROCs in preglomerular resistance vessels share a common molecular structure.

Regulation of phospholipase C-gamma2 networks in B lymphocytes

The modulation of inositol-1,4,5-trisphosphate (IP3), a product of phospholipase C (PLC) activity, is one of a common signaling mechanism used in many biological systems. B lymphocytes also rely on IP3 and subsequent calcium signaling to ensure appropriate developmental outcomes, as well as antigen-specific responses. In establishing the optimal intensity and duration of the PLC-gamma activity, an important role has emerged for adaptor molecules, which direct the appropriate subcellular localization of PLC-gamma and induce its conformational changes. Generated IP3 binds to IP3 receptors located on the endoplasmic reticulum (ER), which in turn is essential for triggering calcium release from the ER and subsequent entry of extracellular calcium by so-called Ca2+ entry channels. Recent data has begun to shed new light on the connection between the calcium release and the influx of extracellular calcium, and the molecular identity of the Ca2+ entry channels.

Lysophosphatidic acid is a mediator of Trp-Lys-Tyr-Met-Val-d-Met-induced calcium influx

Intracellular calcium (Ca(2+)) homeostasis is very strictly regulated, and the activation of G-protein-coupled receptor (GPCR) can cause two different calcium changes, intracellular calcium release, and calcium influx. In this study, we investigated the possible role of lysophosphatidic acid (LPA) on GPCR-induced Ca(2+) signaling. The addition of exogenous LPA induced dramatic Ca(2+) influx but not intracellular Ca(2+) release in U937 cells. LPA-induced Ca(2+) influx was not affected by pertussis toxin and phospholipase C inhibitor (U73122), ruling out the involvement of pertussis toxin-sensitive G-proteins, and phospholipase C. Stimulation of U937 cells with Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm), which binds to formyl peptide receptor like 1, enhanced phospholipase A(2) and phospholipase D activation, indicating LPA formation. The inhibition of LPA synthesis by phospholipase A(2)-specific inhibitor (MAFP) or n-butanol significantly inhibited WKYMVm-induced Ca(2+) influx, suggesting a crucial role for LPA in the process. Taken together, we suggest that LPA mediates WKYMVm-induced Ca(2+) influx.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Two types of non-selective cation channel opened by muscarinic stimulation with carbachol in bovine ciliary muscle cells

In the ciliary muscle, the tonic contraction requires a sustained influx of Ca2+ through the cell membrane. However, little has hitherto been known about the route(s) of Ca2+ influx in this tissue that lacks voltage-gated Ca2+ channels. To identify ion channels as the Ca2+ entry pathway we studied the effects of carbachol (CCh) on freshly isolated bovine ciliary muscle cells by whole-cell voltage clamp. Experiments were carried out using pipettes filled with K+ -free solution containing 100 mm caesium aspartate, 5 mm BAPTA and 180 microm GTP (pH 7.0; the intracellular free Ca2+ concentration, [Ca2+]i = 70 nm). CCh evoked an inward current showing polarity reversal at a holding potential near 0 mV. Analysis of the current noise distinguished two types of non-selective cation channel (NSCCL and NSCCS) with widely different unitary conductances (35 pS and 100 fS). The ratios of the permeabilities to Li+, Na+, Cs+, Mg2+, Ca2+, Sr2+ and Ba2+, estimated by cation replacement procedures, were 0.9: 1.0: 1.5: 0.2: 0.3: 0.4: 0.5 for NSCCL, and 1.0: 1.0: 1.8: 2.5: 2.6: 3.2: 5.0 for NSCCS. NSCCS, but not NSCCL, was strongly inhibited by elevation of [Ca2+]i. Both NSCCL and NSCCS were dose-dependently inhibited by 1-100 microm SKF96365, La3+ and Gd3+, which also inhibited the tonic component of the contraction produced in muscle bundles by CCh without markedly affecting the initial phasic component. NSCCL and/or NSCCS may serve as a major Ca2+ entry pathway required for sustained contraction of the bovine ciliary muscle. RT-PCR experiments in the bovine ciliary muscle (whole tissue) detected mRNAs of several transient receptor potential (TRP) channel homologues (TRPC1, TRPC3, TRPC4 and TRPC6), which are now regarded as possible molecular candidates for receptor-operated cation channels.

Store-operated channels: diversity and activation mechanisms

This perspective addresses two questions: How many store-operated channels (SOCs) are there, and how many mechanisms can account for SOC activation by depleted stores? Accumulating evidence suggests that the SOC family is not limited to the calcium-selective SOC that is responsible for ICRAC (Ca2+-SOC), but includes poorly selective cation SOCs (cat-SOCs) that may satisfy physiological needs in diverse excitable and nonexcitable cells. A growing number of studies in different cell types support the idea that all the members of SOC family (Ca2+-SOC and cat-SOC) may be activated by depletion of the stores through the same mechanism, which is mediated by calcium influx factor (CIF) and calcium-independent phospholipase A2 (iPLA2). A conformational coupling model is also discussed. To account for the most recent findings, we propose that two distinct classes of calcium-conducting channels may exist in plasma membrane, which respond to different signals: SOCs, which are activated by depletion of calcium stores through the CIF-iPLA2 mechanism [no inositol triphosphate (IP3) needed]; and IP3 receptor-operated channels (IP3ROCs), which are activated by IP3 receptor through a direct coupling mechanism (no store depletion is needed). This model, with two separate mechanisms linked to different channels, may resolve many conflicting findings and interpretations and may give a new perspective on the diversity of calcium influx pathways.

Two types of store-operated Ca2+ channels with different activation modes and molecular origin in LNCaP human prostate cancer epithelial cells

The one or more coupling mechanisms of store-operated channels (SOCs) to endoplasmic reticulum (ER) Ca2+ store depletion as well as the molecular identity of SOCs per se still remain a mystery. Here, we demonstrate the co-existence of two populations of molecular distinct endogenous SOCs in LNCaP prostate cancer epithelial cells, which are preferentially activated by either active inositol 1,4,5-trisphosphate (IP3)-mediated or passive thapsigargin-facilitated store depletion and have different ER store content sensitivity. The first population, called SOC(CC) (for "conformational coupling"), is characterized by preferential IP3 receptor-dependent mode of activation, as judged from sensitivity to cytoskeleton modifications, and dominant contribution of transient receptor potential (TRP) TRPC1 within it. The second one, called SOC(CIF) (for "calcium influx factor"), depends on Ca(2+)-independent phospholipase A2 for activation with probable CIF involvement and is mostly represented by TRPC4. The previously identified SOC constituent in LNCaP cells, TRPV6, seems to play equal role in both SOC populations. These results provide new insight into the nature of SOCs and their representation in the single cell type as well as permit reconciliation of current SOC activation hypotheses.

Hexosamine pathway is responsible for inhibition by diabetes of phenylephrine-induced inotropy

Hyperglycemia diminishes positive inotropic responses to agonists that activate phospholipase C (PLC) and generate inositol trisphosphate (1,4,5). The mechanisms underlying both the inotropic responses and hyperglycemia's effects on them remain undetermined, but data from isolated cardiomyocytes suggest the involvement of capacitative Ca(2+) entry (CCE), the influx of Ca(2+) through plasma membrane channels activated in response to depletion of endoplasmic or sarcoplasmic reticulum Ca(2+) stores. In neonatal rat cardiomyocytes, hyperglycemia decreased CCE induced by PLC-mediated agonists. The attenuation of CCE was also seen with glucosamine, and the inhibition by hyperglycemia was prevented by azaserine, thereby implicating hexosamine biosynthesis as the responsible metabolic pathway. In the current study, the importance of hexosamine metabolites to hyperglycemia's effects on inotropic responses was examined in isolated perfused rat hearts. The inhibition by hyperglycemia of phenylephrine-induced inotropy was reversed with azaserine and mimicked by glucosamine. An independent inhibitor of CCE, SKF96365, was also effective in blunting inotropy. These treatments did not inhibit inotropy induced by activation of adenylate cyclase through beta-adrenergic receptors. These data thus implicate CCE in responses to PLC-mediated agonists in the intact heart and point to the hexosamine pathway's negative effect on CCE as being central to the inhibition seen with hyperglycemia.

A novel mechanism for the store-operated calcium influx pathway

Activation of store-operated channels (SOCs) and capacitative calcium influx are triggered by depletion of intracellular calcium stores. However, the exact molecular mechanism of such communication remains unclear. Recently, we demonstrated that native SOC channels can be activated by calcium influx factor (CIF) that is produced upon depletion of calcium stores, and showed that Ca(2+)-independent phospholipase A(2) (iPLA(2)) has an important role in the store-operated calcium influx pathway. Here, we identify the key plasma-membrane-delimited events that result in activation of SOC channels. We also propose a novel molecular mechanism in which CIF displaces inhibitory calmodulin (CaM) from iPLA(2), resulting in activation of iPLA(2) and generation of lysophospholipids that in turn activate soc channels and capacitative calcium influx. Upon refilling of the stores and termination of CIF production, CaM rebinds to iPLA(2), inhibits it, and the activity of SOC channels and capacitative calcium influx is terminated.

Adult rat cardiomyocytes exhibit capacitative calcium entry

Capacitative Ca(2+) entry (CCE) refers to the influx of Ca(2+) through plasma membrane channels activated on depletion of endoplasmic-sarcoplasmic reticulum Ca(2+) stores. We utilized two Ca(2+)-sensitive dyes (one monitoring cytoplasmic free Ca(2+) and the other free Ca(2+) within the sarcoplasmic reticulum) to determine whether adult rat ventricular myocytes exhibit CCE. Treatments with inhibitors of the sarcoplasmic endoplasmic reticulum Ca(2+)-ATPases were not efficient in releasing Ca(2+) from stores. However, when these inhibitors were coupled with either Ca(2+) ionophores or angiotensin II (an agonist generating inositol 1,4,5 trisphosphate), depletion of stores was observed. This depletion was accompanied by a significant influx of extracellular Ca(2+) characteristic of CCE. CCE was also observed when stores were depleted with caffeine. This influx of Ca(2+) was sensitive to four inhibitors of CCE (glucosamine, lanthanum, gadolinium, and SKF-96365) but not to inhibitors of L-type channels or the Na(+)/Ca(2+) exchanger. In the whole cell configuration, an inward current of approximately 0.7 pA/pF at -90 mV was activated when a Ca(2+) chelator or inositol (1,4,5)-trisphosphate was included in the pipette or when Ca(2+) stores were depleted with a Ca(2+)-ATPase inhibitor and ionophore. The current was maximal at hyperpolarizing voltages and inwardly rectified. The channel was relatively permeant to Ca(2+) and Ba(2+) but only poorly to Mg(2+) or Mn(2+). Taken together, these data support the existence of CCE in adult cardiomyocytes, a finding with likely implications to physiological responses to phospholipase C-generating agonists.