Effects of the Ca2+ sensitizers EMD 57033 and CGP 48506 on myocardial contractility and Ca2+ transients in human ventricular and atrial myocardium

Zusammenfassung Ca²⁺-Sensitizer, wie z.B. EMD 57033 (EMD) und CGP 48506 (CGP) erhöhen die Kontraktionskraft ohne dabei den intrazellulären Ca²⁺-Transienten zu erhöhen und sind somit möglicherweise für die Behandlung der menschlichen Herzinsuffizienz von Bedeutung. Es ist jedoch unklar, ob sich Ca²⁺-Sensitizer in ihrem pharmakokinetischen Wirkprinzip am menschlichen Myokard unterscheiden. Daher wurde der Einfluss von EMD und CGP auf die Kontraktionskraft und den intrazellulären Ca²⁺-Transienten (Fura 2) an linksventrikulären Papillarmuskelstreifen (PAP) von menschlichem insuffizientem Myokard sowie in rechtsatrialen Trabekeln (RA) von Patienten untersucht, die sich einer aortokoronaren Bypass-Operation unterziehen mussten. In PAP wurde die Kraft effizienter und stärker nach Applikation von EMD (EC₅₀ EMD: 4,7ǃ,0 7mol/l, max. PIE EMD: +12,0DŽ,0 mN/mm²) erhöht als nach CGP (EC₅₀: 16,9lj,6 7mol/l, max. PIE: +6,4DŽ,8 mN/mm²). Ähnliche Ergebnisse wurden an RA erhalten. Carbachol (100 7mol/l) hatte keinen Einfluss auf die positiv inotrope Wirkung von EMD und CGP. Beide Ca²⁺-Sensitizer erhöhten signifikant die Relaxationszeit und die diastolischen Spannung. EMD und CGP veränderten den intrazellulären Ca²⁺-Transienten nicht. Schlussfolgerung Die Ca²⁺-Sensitizer EMD und CGP erhöhen cAMP- und Ca²⁺-unabhängig die Kontraktionskraft am menschlichen Myokard. Da sie die Relaxation beeinträchtigen, ist ihr therapeutischer Nutzen für die Herzinsuffizienz begrenzt. Summary Ca²⁺ sensitizers like EMD 57033 (EMD) and CGP 48506 (CGP) may be advantageous for the treatment of human heart failure, as they increase force of contraction without increasing the intracellular Ca²⁺ transients or energy consumption. However, whether or not Ca²⁺ sensitizers differ in their mode of action in human myocardium is not fully understood. The present study investigates the influence of EMD and CGP on force of contraction (FOC) and the intracellular Ca²⁺ transient (fura-2 ratio method) in left ventricular papillary muscle strips from left ventricular failing human myocardium (DCM, n=28) as well as in right atrial trabeculae (RA, n=21) obtained from patients undergoing cardiac bypass surgery. In isolated trabeculae of DCM, FOC was more efficacious and potently increased after application of EMD (EC₅₀ EMD: 4.7ǃ.0 7mol/l, max. PIE EMD: +12.0DŽ.0 mN/mm²) than CGP (EC₅₀: 16.9lj.6 7mol/l, max. PIE: +6.4DŽ.8 mN/mm²). Similar results were obtained in RA. Application of carbachol (100 7mol/l) had no effect on the positive inotropic effect of EMD or CGP. Both Ca²⁺ sensitizers significantly increased time to half peak relaxation as well as diastolic tension in DCM. EMD (10 7mol/l) and CGP (30 7mol/l) did not affect the Ca²⁺ transients in RA. The Ca²⁺ sensitizers EMD and CGP increase cAMP and Ca²⁺ independently from the force of contraction in the human myocardium. However, their therapeutic use in human heart failure may be limited as they impair relaxation.     

Cellular mechanism of calcification and its prevention in glutaraldehyde treated vascular tissue

Summary Objectives. Prevention of calcification in glutaraldehyde (GA) treated porcine aortic valve fibroblasts and rat aorta with Ca²⁺ channel blockers (Ca²⁺-CBs). Background. GA causes a massive increase in [Ca²⁺]_i and a many fold increase in [P_i]_i followed by calcification of porcine aortic valve fibroblasts. The influx of extracellular Ca²⁺ into[P_i]_i rich cells apparently underlies the mechanism of calcification. Inhibition of Ca²⁺ influx is likely to prevent calcification in GA-treated cells. Methods. [Ca²⁺]_i in GA-treated cells was measured by fluorescence image analysis. [Ca²⁺]_i increase in fibroblasts treated with various Ca²⁺-CBs was compared with the untreated control. To study the role of Ca²⁺ influx in calcification and to find out the portals of Ca²⁺ entry, porcine aortic valve fibroblasts and freshly removed rat aorta were treated with verapamil + ryanodine, or verapamil + econazole, fixed with GA and incubated in Hank's balanced salt solution with 2.5 mmol/L calcium. The progress of calcification was monitored by the rate of Ca and P_i depletions from the supernatant. Calcified cells and tissues were identified by calcein fluorescence. Results. Verapamil + ryanodine or econazole inhibited the GA-induced Ca²⁺ influx and prevented calcification of the cells and rat aorta. The effect of verapamil was additive to that of ryanodine and econazole. Conclusions. Findings further support the influx theory of calcification. Ca²⁺ enters GA-treated cells mainly through the store operated and the L-type Ca²⁺ channels. Ca²⁺-CBs may be useful for prevention of calcification in GA-treated vascular bioprostheses. Cell culture serves as a convenient model for screening drug effects on calcification.     

Ca2+ channels, 'quantized' Ca2+ release, and differentiation of myocytes in the cardiovascular system

The application of confocal microscopy to cardiac and skeletal muscle has resulted in the observation of transient, spatially localized elevations in [Ca2+]i, termed 'Ca2+ sparks'. Ca2+ sparks are thought to represent 'elementary' Ca2+ release events, which arise from one or more ryanodine receptor (RyR) channels in the sarcoplasmic reticulum. In cardiac muscle, Ca2+ sparks appear to be key elements of excitation-contraction coupling, in which the global [Ca2+]i transient is thought to involve the recruitment of Ca2+ sparks, each of which is controlled locally by single coassociated L-type Ca2+ channels. Recently, Ca2+ sparks have been detected in smooth muscle cells of arteries. In this review, we analyse the complex relationship of Ca2+ influx and Ca2+ release with local, subcellular Ca2+ microdomains in light of recent studies on Ca2+ sparks in cardiovascular cells. We performed a comparative analysis of 'elementary' Ca2+ release units in mouse, rat and human arterial smooth muscle cells, using measurements of Ca2+ sparks and plasmalemmal K(Ca) currents activated by Ca2+ sparks (STOCs). Furthermore, the appearance of Ca2+ sparks during ontogeny of arterial smooth muscle is explored. Using intact pressurized arteries, we have investigated whether RyRs causing Ca2+ sparks (but not smaller 'quantized' Ca2+ release events, e.g. hypothetical 'Ca2+ quarks') function as key signals that, through membrane potential and global cytoplasmic [Ca2+], oppose arterial myogenic tone and influence vasorelaxation. We believe that voltage-dependent Ca2+ channels and local RyR-related Ca2+ signals are important in differentiation, proliferation, and gene expression. Our findings suggest that 'elementary' Ca2+ release units may represent novel potent therapeutic targets for regulating function of intact arterial smooth muscle tissue.     

Hyperpolarization induced by K+ channel openers inhibits Ca2+ influx and Ca2+ release in coronary artery

The vasodilating mechanisms of the K⁺ channel openers—cromakalim, pinacidil, nicorandil, KRN2391, and Ki4032—were examined by measurement of the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) using the fura-2 method in canine or porcine coronary arterial smooth muscle. The five K⁺ channel openers all produced a reduction of [Ca²⁺]_i in 5 and 30 mM KCl physiological salt solution (PSS), the effects of which were antagonized by tetrabutylammonium (TBA) or glibenclamide, but failed to affect [Ca²⁺]_i in 45 and 90 mM MCl-PSS. Cromakalim and Ki4032 only partially inhibited the 30 mM KCl-induced contractures, whereas pinacidil, nicorandil, and KRN2391 nearly abolished contractions produced by high KCl-PSS. The increased [Ca²⁺]_i and force produced by a thromboxane A₂ analogue, U46619, were inhibited by K⁺ channel openers and verapamil. In the absence of extracellular Ca²⁺, U46619 induced a transient increase in [Ca²⁺]_i with a contraction, which is effectively inhibited by cromakalim and Ki4032. Their inhibitory effects were blocked by TBA and counteracted by 20 mM KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca²⁺ release. Cromakalim reduced U46619-induced IP₃ production and TBA blocked this inhibitory effect. Thus, cromakalim and Ki4032 are more specific K⁺ channel openers than pinacidil, nicorandil, and KRN2391. The vasodilation related with a reduction of [Ca²⁺]_i produced by K⁺ channel openers is due to the hyperpolarization of the plasma membrane resulting in not only the closure of voltage-dependent Ca²⁺ channels but also inhibition of the production of IP₃ and Ca²⁺ release from intracellular stores related to stimulation of the thromboxane A₂ receptor.     

Calcium sparks in human coronary artery smooth muscle cells resolved by confocal imaging

OBJECTIVE: The observation of local 'elementary' Ca2+ release events (Ca2+ sparks) through ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR) has changed our understanding of excitation-contraction (EC) coupling in cardiac and smooth muscle. In arterial smooth muscle, Ca2+ sparks have been suggested to oppose myogenic vasoconstriction and to influence vasorelaxation by activating co-localized Ca2+ activated K+ (K(Ca)) channels (STOCs). However, all prior studies on Ca2+ sparks have been performed in non-human tissues. METHODS: In order to understand the possible significance of Ca2+ sparks to human cardiovascular function, we used high spatial resolution confocal imaging to record Ca2+ sparks in freshly-isolated, individual myocytes of human coronary arteries loaded with the Ca2+ indicator fluo-3. RESULTS: Local SR Ca2+ release events recorded in human myocytes were similar to 'Ca2- sparks' recorded previously from non-human smooth muscle cells. In human myocytes, the peak [Ca2+]i amplitudes of Ca2+ sparks (measured as F/F0) and width at half-maximal amplitude were 2.3 and 2.27 microm, respectively. The duration of Ca2+ sparks was 62 ms. Ca2+ sparks were completely inhibited by ryanodine (10 micromol/l). Ryanodine-sensitive STOCs could be identified with typical properties of K(Ca) channels activated by Ca2+ sparks. CONCLUSION: Our data implies that modern concepts suggesting an essential role of Ca2+ spark generation in EC coupling recently derived from non-human muscle are applicable to human cardiovascular tissue. Although the basic properties of Ca2+ sparks are similar, our results demonstrate that Ca2+ sparks in coronary arteries in humans, have features distinct from non-arterial smooth muscle cells of other species.     

TRPV4 forms a novel Ca2+ signaling complex with ryanodine receptors and BKCa channels

Vasodilatory factors produced by the endothelium are critical for the maintenance of normal blood pressure and flow. We hypothesized that endothelial signals are transduced to underlying vascular smooth muscle by vanilloid transient receptor potential (TRPV) channels. TRPV4 message was detected in RNA from cerebral artery smooth muscle cells. In patch-clamp experiments using freshly isolated cerebral myocytes, outwardly rectifying whole-cell currents with properties consistent with those of expressed TRPV4 channels were evoked by the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) (5 micromol/L) and the endothelium-derived arachidonic acid metabolite 11,12 epoxyeicosatrienoic acid (11,12 EET) (300 nmol/L). Using high-speed laser-scanning confocal microscopy, we found that 11,12 EET increased the frequency of unitary Ca2+ release events (Ca2+ sparks) via ryanodine receptors located on the sarcoplasmic reticulum of cerebral artery smooth muscle cells. EET-induced Ca2+ sparks activated nearby sarcolemmal large-conductance Ca2+-activated K+ (BKCa) channels, measured as an increase in the frequency of transient K+ currents (referred to as "spontaneous transient outward currents" [STOCs]). 11,12 EET-induced increases in Ca2+ spark and STOC frequency were inhibited by lowering external Ca2+ from 2 mmol/L to 10 micromol/L but not by voltage-dependent Ca2+ channel inhibitors, suggesting that these responses require extracellular Ca2+ influx via channels other than voltage-dependent Ca2+ channels. Antisense-mediated suppression of TRPV4 expression in intact cerebral arteries prevented 11,12 EET-induced smooth muscle hyperpolarization and vasodilation. Thus, we conclude that TRPV4 forms a novel Ca2+ signaling complex with ryanodine receptors and BKCa channels that elicits smooth muscle hyperpolarization and arterial dilation via Ca2+-induced Ca2+ release in response to an endothelial-derived factor.     

Low intravascular pressure activates endothelial cell TRPV4 channels,local Ca2+ events,and IKCa channels,reducing arteriolar tone

Endothelial cell (EC) Ca²⁺-activated K channels (SK_Ca and IK_Ca channels) generate hyperpolarization that passes to the adjacent smooth muscle cells causing vasodilation. IK_Ca channels focused within EC projections toward the smooth muscle cells are activated by spontaneous Ca²⁺ events (Ca²⁺ puffs/pulsars). We now show that transient receptor potential, vanilloid 4 channels (TRPV4 channels) also cluster within this microdomain and are selectively activated at low intravascular pressure. In arterioles pressurized to 80 mmHg, ECs generated low-frequency (∼2 min⁻¹) inositol 1,4,5-trisphosphate receptor-based Ca²⁺ events. Decreasing intraluminal pressure below 50 mmHg increased the frequency of EC Ca²⁺ events twofold to threefold, an effect blocked with the TRPV4 antagonist RN1734. These discrete events represent both TRPV4-sparklet- and nonsparklet-evoked Ca²⁺ increases, which on occasion led to intracellular Ca²⁺ waves. The concurrent vasodilation associated with increases in Ca²⁺ event frequency was inhibited, and basal myogenic tone was increased, by either RN1734 or TRAM-34 (IK_Ca channel blocker), but not by apamin (SK_Ca channel blocker). These data show that intraluminal pressure influences an endothelial microdomain inversely to alter Ca²⁺ event frequency; at low pressures the consequence is activation of EC IK_Ca channels and vasodilation, reducing the myogenic tone that underpins tissue blood-flow autoregulation.     

Membrane depolarization causes a direct activation of G protein-coupled receptors leading to local Ca2+ release in smooth muscle

Membrane depolarization activates voltage-dependent Ca²⁺ channels (VDCCs) inducing Ca²⁺ release via ryanodine receptors (RyRs), which is obligatory for skeletal and cardiac muscle contraction and other physiological responses. However, depolarization-induced Ca²⁺ release and its functional importance as well as underlying signaling mechanisms in smooth muscle cells (SMCs) are largely unknown. Here we report that membrane depolarization can induce RyR-mediated local Ca²⁺ release, leading to a significant increase in the activity of Ca²⁺ sparks and contraction in airway SMCs. The increased Ca²⁺ sparks are independent of VDCCs and the associated extracellular Ca²⁺ influx. This format of local Ca²⁺ release results from a direct activation of G protein-coupled, M₃ muscarinic receptors in the absence of exogenous agonists, which causes activation of Gq proteins and phospholipase C, and generation of inositol 1,4,5-triphosphate (IP₃), inducing initial Ca²⁺ release through IP₃ receptors and then further Ca²⁺ release via RyR2 due to a local Ca²⁺-induced Ca²⁺ release process. These findings demonstrate an important mechanism for Ca²⁺ signaling and attendant physiological function in SMCs.     

Calcium antagonists and vascular smooth muscle cell reactivity

Summary Objectives: To determine the mechanisms whereby calcium channel blockers (CCBs) control the reactivity of vascular smooth muscle cells (VSMCs). Background: Although CCBs are known to play an important role in the calcium homeostasis of VSMCs, they are suspected to exert additional effects in this cell type. Thus, the possibility that CCGs could affect VSMC growth/proliferation through a mechanism distinct from the inhibition of calcium channels was investigated. Methods: VSMCs were isolated from rat aortae and cultured. The influence of nifedipine and amlodipine on basic fibroblast growth factor (bFGF)-stimulated DNA synthesis and proliferation was studied by measuring bFGF-induced BrdU incorporation into VSMCs and cell counts, respectively. The influence of amlodipine (and of isradipine) on the mobilization of intracellular Ca²⁺ stores was determined by studying the fluorescence of thapsigargin -stimulated VSMCs pre-labeled with the fluoroprobe Fura-2. Results: Both nifedipine and amlodipine inhibited bFGF-induced VSMC growth/proliferation. In the case of nifedipine but not in that of amlodipine, this inhibitory effect could be accounted for by the L-type Ca²⁺-channel antagonist property of the drug. On the other hand, amlodipine but not isradipine, diltiazem, and verapamil, did inhibit thapsigargin-induced Ca²⁺ mobilization. Conclusions: These findings suggest that in addition to its L-type Ca²⁺-channel antagonist property, amlodipine also exerts a "thapsigargin-like" activity which, together with its particular antioxidant property, might participate in its antiatherogenic potency.     

Acute hypoxia activates store-operated Ca2+ entry and increases intracellular Ca2+ concentration in rat distal pulmonary venous smooth muscle cells

Rationale

Exposure to acute hypoxia causes vasoconstriction in both pulmonary arteries (PA) and pulmonary veins (PV). The mechanisms on the arterial side have been studied extensively. However, bare attention has been paid to the venous side.

Objectives

To investigate if acute hypoxia caused the increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i), and Ca²⁺ influx through store-operated calcium channels (SOCC) in pulmonary venous smooth muscle cells (PVSMCs).

Methods

Fluorescent microscopy and fura-2 were used to measure effects of 4% O₂ on [Ca²⁺]_i and store-operated Ca²⁺ entry (SOCE) in isolated rat distal PVSMCs.

Measurements and main results

In PVSMCs perfused with Ca²⁺-free Krebs Ringer bicarbonate solution (KRBS) containing cyclopiazonic acid to deplete Ca²⁺ stores in the sarcoplasmic reticulum (SR) and nifedipine to prevent Ca²⁺ entry through L-type voltage-depended Ca²⁺ channels (VDCC), hypoxia markedly enhanced both the increase in [Ca²⁺]_i caused by restoration of extracellular [Ca²⁺] and the rate at which extracellular Mn²⁺ quenched fura-2 fluorescence. Moreover, the increased [Ca²⁺]_i in PVSMCs perfused with normal salt solution was completely blocked by SOCC antagonists SKF-96365 and NiCl₂ at concentrations that SOCE >85% was inhibited but [Ca²⁺]_i responses to 60 mM KCl were not altered. On the contrary, L-type VDCC antagonist nifedipine inhibited increase in [Ca²⁺]_i to hypoxia by only 50% at concentrations that completely blocked responses to KCl. The increased [Ca²⁺]_i caused by hypoxia was completely abolished by perfusion with Ca²⁺-free KRBS.

Conclusions

These results suggest that acute hypoxia enhances SOCE via activating SOCCs, leading to increased [Ca²⁺]_i in distal PVSMCs.KEYWORDS : Calcium signaling, pulmonary venous smooth muscle (PVSM), store-operated Ca²⁺ entry (SOCE), intracellular Ca²⁺ concentration ([Ca²⁺]_i)     

Experimental diabetes enhances Ca2+ mobilization and glutamate exocytosis in cerebral synaptosomes from mice

The present study was conducted to investigate the effects of the diabetic condition on the Ca²⁺ mobilization and glutamate release in cerebral nerve terminals (synaptosomes). Diabetes was induced in male mice by intraperitoneal injection of streptozotocin. Cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and glutamate release in synaptosomes were determined using fura-2 and enzyme-linked fluorometric assay, respectively. Diabetes significantly enhanced the ability of the depolarizing agents K⁺ and 4-aminopyridine (4-AP) to increase [Ca²⁺]_i. In addition, diabetes significantly enhanced K⁺- and 4-AP-evoked Ca²⁺-dependent glutamate release. The pretreatment of synaptosomes with a combination of ω-agatoxin IVA (a P-type Ca²⁺ channel blocker) and ω-conotoxin GVIA (an N-type Ca²⁺ channel blocker) inhibited K⁺- or 4-AP-induced increases in [Ca²⁺]_i and Ca²⁺-dependent glutamate release in synaptosomes from the control and diabetic mice to a similar extent, respectively. These results indicate that diabetes enhances a K⁺- or 4-AP-evoked Ca²⁺-dependent glutamate release by increasing [Ca²⁺]_i via stimulation of Ca²⁺ entry through both P- and N-type Ca²⁺ channels.     

Imaging microdomain Ca2+ in muscle cells

Ca2+ ions passing through a single or a cluster of Ca2+-permeable channels create microscopic, short-lived Ca2+ gradients that constitute the building blocks of cellular Ca2+ signaling. Over the last decade, imaging microdomain Ca2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca2+ dynamics and has reshaped our understanding of Ca2+ signaling mechanisms. Major advances include the visualization of "Ca2+ sparks" as the elementary events of Ca2+ release from the sarcoplasmic reticulum (SR), "Ca2+ sparklets" produced by openings of single Ca2+-permeable channels, miniature Ca2+ transients in single mitochondria ("marks"), and SR luminal Ca2+ depletion transients ("scraps"). As a model system, a cardiac myocyte contains a 3-dimensional grid of 104 spark ignition sites, stochastic activation of which summates into global Ca2+ transients. Tracking intermolecular coupling between single L-type Ca2+ channels and Ca2+ sparks has provided direct evidence validating the local control theory of Ca2+-induced Ca2+ release in the heart. In vascular smooth muscle myocytes, Ca2+ can paradoxically signal both vessel constriction (by global Ca2+ transients) and relaxation (by subsurface Ca2+ sparks). These findings shed new light on the origin of Ca2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca2+ channels in intact cells.     

Calcium ions as extracellular, first messengers

Summary Ca²⁺ concentration in the extracellular fluids ([Ca²⁺]_o) is essential for a number of vital processes from bone formation to blood clotting. For this reason, it is necessary that [Ca²⁺]_o must be strictly controlled. Mammalian species have developed a complex homeostatic system that includes parathyroid glands, kidney and bone. The extracellular Ca²⁺-sensing receptor (CaR) is an essential component of this system, regulating parathyroid hormone secretion, calcium excretion by the kidney and bone remodeling. Initially identified from bovine parathyroid glands (1), within the five years following its identification CaR presence has rapidly been extended to organs where the link with mineral ion metabolism has not been elucidated (i.e., brain, stomach, eye, skin, and many other epithelial cells) (see 2 for review). This review will address the discovery of a novel class of ion-sensing receptors, receptor-effector coupling, and the roles of the CaR inside and outside the Ca²⁺_o homeostatic systems.     

Store-Operated Ca2+ Entry is involved in Transforming Growth Factor-β1 Facilitated Proliferation of Rat Airway Smooth Muscle Cells

Objective. To investigate the role and underlying mechanisms of store-operated Ca²⁺ entry (SOCE) in mediating the promoting effect of transforming growth factor (TGF)-β1 on the proliferation of airway smooth muscle cells (ASMCs). Methods. Rat bronchial smooth muscle cells were cultured as we described previously. The intracellular Ca²⁺ concentration ([Ca²⁺]_i) of ASMCs was measured by laser confocal microscope Ca²⁺ fluorescence imaging with Fluo-3/AM. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and p27 expression assay were used to determine the proliferation rate of ASMCs. Results. We demonstrated that TGF-β1 (10 ng/ml) increased basal (Ca²⁺]_i) level, [Ca²⁺]_i rise induced by thapsigargin-induced Ca²⁺ release and SOCE in rat ASMCs. This effect of TGF-β1 on SOCE was not inhibited by glucocorticoid dexamethasone (DXM, 100 nM), antioxidant α-tocopherol (100 μM), and intermediate-conductance Ca²⁺-activated K⁺ channels (IK_Ca) inhibitor charybdotoxin (100 nM), suggesting that reactive oxygen species and IK_Ca channels might not mediate the effect of TGF-β1. TGF-β1 slightly increased the expression of Orai1 and STIM1, two important molecules involved in the molecule component and regulation of SOC channels, in the presence of 10% fetal bovine serum (FBS). The proliferation of ASMC stimulated with 2.5% FBS was promoted by TGF-β1, and partly inhibited by non-specific Ca²⁺ channel blocker SKF-96365 (10 μM) and Ni²⁺ (100 μM). DXM, α-tocopherol, and charybdotoxin had no effect on the proliferation promoted by TGF-β1. Conclusion. TGF-β1 promotes ASMC proliferation partly through increasing the expression and activity of SOC channels.     

Association Between Ligand-Induced Conformational Changes of Integrin alpha IIbbeta 3 and alpha IIbbeta 3-Mediated Intracellular Ca2+ Signaling

Platelet _IIb3 is a prototypic integrinand plays a critical role in platelet aggregation. Occupancy of_IIb3 with multivalent RGD ligands, suchas fibrinogen, induces both expression of ligand-induced binding sites(LIBS) and _IIb3 clustering, which arethought to be necessary for outside-in signaling. However, theassociation between LIBS expression and outside-in signaling remainselusive. In this study, we used various_IIb3-specific peptidomimetic compounds asa monovalent ligand instead of fibrinogen and examined the associationbetween LIBS expression and outside-in signaling such as_IIb3-mediated intracellularCa²⁺ signaling. Using a set of monoclonal antibodies(MoAbs) against LIBS, we showed that antagonists can be divided intotwo groups. In group I, antagonists can induce LIBS on both_IIb and ₃ subunits. In group II,antagonists can induce LIBS on the _IIb subunit, but noton the ₃ subunit. Inhibition studies suggested thatgroup I and group II antagonists interact with distinct but mutually exclusive sites on _IIb3. Neither group Inor group II antagonist increased intracellular Ca²⁺concentrations ([Ca²⁺]_i) in nonactivatedplatelets. All antagonists at nanomolar concentrations abolished theincrease in [Ca²⁺]_i in 0.03 U/mLthrombin-stimulated platelets, which is dependent on bothfibrinogen-binding to _IIb3 andplatelet-aggregation. However, only group I antagonists at higherconcentrations dose-dependently augmented the[Ca²⁺]_i increase, which is due toaggregation-independent thromboxane A₂ production. Thisincrease in [Ca²⁺]_i was not observed inthrombasthenic platelets, which express no detectable_IIb3. Thus, only the group I antagonists,albeit a monovalent ligand, can initiate_IIb3-mediated intracellular Ca²⁺ signaling in the presence of thrombin stimulation.Our findings strongly suggest the association between ₃LIBS expression and _IIb3-mediatedintracellular Ca²⁺ signaling in platelets.     

Diminished sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) expression contributes to airway remodelling in bronchial asthma

Phenotypic modulation of airway smooth muscle (ASM) is an important feature of airway remodeling in asthma that is characterized by enhanced proliferation and secretion of pro-inflammatory chemokines. These activities are regulated by the concentration of free Ca²⁺ in the cytosol ([Ca²⁺]_i). A rise in [Ca²⁺]_i is normalized by rapid reuptake of Ca²⁺ into sarcoplasmic reticulum (SR) stores by the sarco/endoplasmic reticulum Ca²⁺ (SERCA) pump. We examined whether increased proliferative and secretory responses of ASM from asthmatics result from reduced SERCA expression. ASM cells were cultured from subjects with and without asthma. SERCA expression was evaluated by western blot, immunohistochemistry and real-time PCR. Changes in [Ca²⁺]_i, cell spreading, cellular proliferation, and eotaxin-1 release were measured. Compared with control cells from healthy subjects, SERCA2 mRNA and protein expression was reduced in ASM cells from subjects with moderately severe asthma. SERCA2 expression was similarly reduced in ASM in vivo in subjects with moderate/severe asthma. Rises in [Ca²⁺]_i following cell surface receptor-induced SR activation, or inhibition of SERCA-mediated Ca²⁺ re-uptake, were attenuated in ASM cells from asthmatics. Likewise, the return to baseline of [Ca]_i after stimulation by bradykinin was delayed by approximately 50% in ASM cells from asthmatics. siRNA-mediated knockdown of SERCA2 in ASM from healthy subjects increased cell spreading, eotaxin-1 release and proliferation. Our findings implicate a deficiency in SERCA2 in ASM in asthma that contributes to its secretory and hyperproliferative phenotype in asthma, and which may play a key role in mechanisms of airway remodeling.Asthma is a chronic inflammatory disease which is accompanied by extensive changes in normal airway tissue architecture, termed remodeling (1, 2). Airway remodeling in asthma comprises epithelial dysfunction, hypertrophy of the mucus glands, subepithelial vascularization, and changes in extracellular matrix composition (2). In addition, airway smooth muscle (ASM) from people suffering with asthma exhibits enhanced proliferative (3) and migratory responses (4, 5), as well as increased secretion of a myriad of pro-inflammatory cytokines/chemokines and growth factors (6). The mechanisms that underly the exaggerated function of ASM in asthma are unknown.Smooth muscle responses to diverse stimuli are controlled by changes in the concentration of free cytosolic Ca²⁺ ([Ca²⁺]_i). Elevation of [Ca²⁺]_i results from increased Ca²⁺ influx across the plasma membrane following activation of Ca²⁺-permeable ion channels and the Na⁺-Ca²⁺-exchanger (NCX, 3Na⁺:1Ca²⁺), and by release of stored Ca²⁺ from the sarcoplasmic reticulum (SR), in turn triggered by inositol 1,4,5-triphosphate (IP₃) or ryanodine receptor (RyR) channels (7). Termination of the cytosolic Ca²⁺ signal occurs by extracellular removal of cytosolic Ca²⁺ by the NCX and by its rapid sequestration into SR stores by the sarco/endoplasmic reticulum Ca²⁺ (SERCA) pump (7). Impaired replenishment of SR stores arising from reduced activity of the SERCA pump could impact on a wide range of Ca²⁺-dependent smooth muscle functions (8) and abnormal Ca²⁺ handling by ASM has previously been proposed to be an important determinant of the airway hyperresponsiveness that is characteristically present in asthma (9, 10).There are 3 tissue-specific members of the mammalian SERCA family, SERCA1, SERCA2 and SERCA3, each encoded by a separate gene (ATP2A1, ATP2A2, and ATP2A3) (11), with SERCA2 being the most highly expressed in smooth muscle (12, 13). The function of the different isoforms of SERCA2 is similar (14). We have investigated if the secretory and hyperproliferative phenotype of ASM in asthma is associated with impaired SERCA isoform expression.     

Relationships between dopamine-induced changes in cytosolic free calcium concentration ([Ca2+]i) and rate of prolactin secretion

This study was undertaken to investigate the relationship between dopamine (DA) induced changes in the cytosolic calcium concentration ([Ca²⁺]_i) and the rate of prolactin secretion using GH₄ZR₇, a rat pituitary cell line, which express only one subtype of D₂ receptor. GH₄ZR₇ cells were loaded with Fluo-3, a fluorescent Ca²⁺ indicator, and then perifused with two different doses of DA (10⁻⁷ mol/L and 5×10⁻⁴ mol/L). We monitored changes in [Ca²⁺]_i and rate of prolactin release simultaneously by attaching a spectrofluorometer to a dynamic perifusion system. DA has stimulatory and inhibitory effect on prolactin secretion in GH₄ZR₇ cells; 10⁻⁷ mol/L DA slightly increased [Ca²⁺]_i and stimulated prolactin release, whereas 5×10⁻⁴ mol/L DA decreased [Ca²⁺]_i and inhibited prolactin secretion. When the cells were pretreated with pertussis toxin (PTX), 10⁻⁷ mol/L DA had no significant change in [Ca²⁺]_i while stimulating prolactin release, and 5×10⁻⁴ mol/L DA reduced [Ca²⁺]_i without having any significant effect on the rate of prolactin secretion. The results of this study demonstrate that changes in [Ca²⁺]_i do not always correlate with the rate of prolactin release from lactotrophs. The dissociation between [Ca²⁺]_i and prolactin release is somewhat expected considering the diverse role of [Ca²⁺]_i and post-[Ca²⁺]_i events, which can change the rate of prolactin release.     

Coupled Ca2+/H+ transport by cytoplasmic buffers regulates local Ca2+ and H+ ion signaling

Ca²⁺ signaling regulates cell function. This is subject to modulation by H⁺ ions that are universal end-products of metabolism. Due to slow diffusion and common buffers, changes in cytoplasmic [Ca²⁺] ([Ca²⁺]_i) or [H⁺] ([H⁺]_i) can become compartmentalized, leading potentially to complex spatial Ca²⁺/H⁺ coupling. This was studied by fluorescence imaging of cardiac myocytes. An increase in [H⁺]_i, produced by superfusion of acetate (salt of membrane-permeant weak acid), evoked a [Ca²⁺]_i rise, independent of sarcolemmal Ca²⁺ influx or release from mitochondria, sarcoplasmic reticulum, or acidic stores. Photolytic H⁺ uncaging from 2-nitrobenzaldehyde also raised [Ca²⁺]_i, and the yield was reduced following inhibition of glycolysis or mitochondrial respiration. H⁺ uncaging into buffer mixtures in vitro demonstrated that Ca²⁺ unloading from proteins, histidyl dipeptides (HDPs; e.g., carnosine), and ATP can underlie the H⁺-evoked [Ca²⁺]_i rise. Raising [H⁺]_i tonically at one end of a myocyte evoked a local [Ca²⁺]_i rise in the acidic microdomain, which did not dissipate. The result is consistent with uphill Ca²⁺ transport into the acidic zone via Ca²⁺/H⁺ exchange on diffusible HDPs and ATP molecules, energized by the [H⁺]_i gradient. Ca²⁺ recruitment to a localized acid microdomain was greatly reduced during intracellular Mg²⁺ overload or by ATP depletion, maneuvers that reduce the Ca²⁺-carrying capacity of HDPs. Cytoplasmic HDPs and ATP underlie spatial Ca²⁺/H⁺ coupling in the cardiac myocyte by providing ion exchange and transport on common buffer sites. Given the abundance of cellular HDPs and ATP, spatial Ca²⁺/H⁺ coupling is likely to be of general importance in cell signaling.Most cells are exquisitely responsive to calcium (Ca²⁺) (1) and hydrogen (H⁺) ions (i.e., pH) (2). In cardiac myocytes, Ca²⁺ ions trigger contraction and control growth and development (3), whereas H⁺ ions, which are generated or consumed metabolically, are potent modulators of essentially all biological processes (4). By acting on Ca²⁺-handling proteins directly or via other molecules, H⁺ ions exert both inhibitory and excitatory effects on Ca²⁺ signaling. For example, in the ventricular myocyte, H⁺ ions can reduce Ca²⁺ release from sarcoplasmic reticulum (SR) stores, through inhibition of the SR Ca²⁺ ATPase (SERCA) pump and ryanodine receptor (RyR) Ca²⁺ channels (5, 6). In contrast, H⁺ ions can enhance SR Ca²⁺ release by stimulating sarcolemmal Na⁺/H⁺ exchange (NHE), which raises intracellular [Na⁺] and reduces the driving force for Ca²⁺ extrusion on Na⁺/Ca²⁺ exchange (NCX), leading to cellular retention of Ca²⁺ (7, 8). Ca²⁺ signaling is thus subservient to pH.Cytoplasmic Ca²⁺ and H⁺ ions bind avidly to buffer molecules, such that <1% of all Ca²⁺ ions and <0.001% of all H⁺ ions are free. Some of these buffers bind H⁺ and Ca²⁺ ions competitively, and this has been proposed to be one mechanism underlying cytoplasmic Ca²⁺/H⁺ coupling (9). Reversible binding to buffers greatly reduces the effective mobility of Ca²⁺ and H⁺ ions in cytoplasm (10, 11) and can allow for highly compartmentalized ionic microdomains, and hence a spatially heterogeneous regulation of cell function. In cardiac myocytes under resting (diastolic) conditions, the cytoplasm-averaged concentration of free [Ca²⁺] ([Ca²⁺]_i) and [H⁺] ([H⁺]_i) ions is kept near 10⁻⁷ M by membrane transporter proteins. Thus, [H⁺]_i is regulated by the balance of flux among acid-extruding and acid-loading transporter proteins at the sarcolemma [e.g., NHE and Cl⁻/OH⁻ (CHE) exchangers, respectively] (4). Similarly, the activity of SERCA and NCX proteins returns [Ca²⁺]_i to its diastolic level after evoked signaling events (3, 12). Despite these regulatory mechanisms, cytoplasmic gradients of [H⁺]_i and [Ca²⁺]_i do occur in myocytes and are an important part of their physiology. Gradients arise from local differences in transmembrane fluxes that alter [H⁺]_i or [Ca²⁺]_i. For example, spatial [H⁺]_i gradients are produced when NHE transporters, expressed mainly at the intercalated disk region, are activated (4, 13) or when membrane-permeant weak acids, such as CO₂, are presented locally (14). Similarly, release of Ca²⁺ through a cluster of RyR channels in the SR produces [Ca²⁺]_i nonuniformity in the form of Ca²⁺ sparks (15). Given the propensity of cytoplasm to develop ionic gradients, it is important to understand their underlying mechanism and functional role.The present work demonstrates a distinct form of spatial interaction between Ca²⁺ and H⁺ ions. We show that cytoplasmic [H⁺] gradients can produce stable [Ca²⁺]_i gradients, and vice versa, and that this interaction is mediated by low-molecular-weight (mobile) buffers with affinity for both ions. We demonstrate that the diffusive counterflux of H⁺ and Ca²⁺ bound to these buffers comprises a cytoplasmic Ca²⁺/H⁺ exchanger. This acts like a “pump” without a membrane, which can, for instance, recruit Ca²⁺ to acidic cellular microdomains. Cytoplasmic Ca²⁺/H⁺ exchange adds a spatial paradigm to our understanding of Ca²⁺ and H⁺ ion signaling.