Cardiomyogenesis of embryonic stem cells upon purinergic receptor activation by ADP and ATP

Purinergic signaling may be involved in embryonic development of the heart. In the present study, the effects of purinergic receptor stimulation on cardiomyogenesis of mouse embryonic stem (ES) cells were investigated. ADP or ATP increased the number of cardiac clusters and cardiac cells, as well as beating frequency. Cardiac-specific genes showed enhanced expression of α-MHC, MLC2v, α-actinin, connexin 45 (Cx45), and HCN4, on both gene and protein levels upon ADP/ATP treatment, indicating increased cardiomyogenesis and pacemaker cell differentiation. Real-time RT-PCR analysis of purinergic receptor expression demonstrated presence of P2X1, P2X4, P2X6, P2X7, P2Y1, P2Y2, P2Y4, and P2Y6 on differentiating ES cells. ATP and ADP as well as the P2X agonists β,γ-methylenadenosine 5′-triphosphate (β,γ-MetATP) and 8-bromoadenosine 5′-triphosphate (8-Br-ATP) but not UTP or UDP transiently increased the intracellular calcium concentration ([Ca²⁺]_i) as evaluated by the calcium indicator Fluo-4, whereas no changes in membrane potential were observed. [Ca²⁺]_i transients induced by ADP/ATP were abolished by the phospholipase C-β (PLC-β) inhibitor U-73122, suggesting involvement of metabotropic P2Y receptors. Furthermore, partial inhibition of [Ca²⁺]_i transients was achieved in presence of MRS2179, a selective P2Y1 receptor antagonist, whereas PPADS, a non-selective P2 receptor inhibitor, completely abolished the [Ca²⁺]_i response. Consequently, cardiomyocyte differentiation was decreased upon long term co-incubation of cells with ADP and P2 receptor antagonists. In summary, activation of purinoceptors and the subsequent [Ca²⁺]_i transients enhance the differentiation of ES cells toward cardiomyocytes. Purinergic receptor stimulation may be a promising strategy to drive the fate of pluripotent ES cells into a particular population of cardiomyocytes.

Electronic supplementary material

The online version of this article (doi:10.1007/s11302-015-9468-1) contains supplementary material, which is available to authorized users.     

Functional Characteristics of ES Cell–derived Cardiac Precursor Cells Identified by Tissue-specific Expression of the Green Fluorescent Protein

In contrast to terminally differentiated cardiomyocytes, relatively little is known about the characteristics of mammalian cardiac cells before the initiation of spontaneous contractions (precursor cells). Functional studies on these cells have so far been impossible because murine embryos of the corresponding stage are very small, and cardiac precursor cells cannot be identified because of the lack of cross striation and spontaneous contractions.In the present study, we have used the murine embryonic stem (ES, D3 cell line) cell system for the in vitro differentiation of cardiomyocytes. To identify the cardiac precursor cells, we have generated stably transfected ES cells with a vector containing the gene of the green fluorescent protein (GFP) under control of the cardiac α-actin promoter. First, fluorescent areas in ES cell–derived cell aggregates (embryoid bodies [EBs]) were detected 2 d before the initiation of contractions. Since Ca²⁺ homeostasis plays a key role in cardiac function, we investigated how Ca²⁺ channels and Ca²⁺ release sites were built up in these GFP-labeled cardiac precursor cells and early stage cardiomyocytes. Patch clamp and Ca²⁺ imaging experiments proved the functional expression of the L-type Ca²⁺ current (I_Ca) starting from day 7 of EB development. On day 7, using 10 mM Ca²⁺ as charge carrier, I_Ca was expressed at very low densities 4 pA/pF. The biophysical and pharmacological properties of I_Ca proved similar to terminally differentiated cardiomyocytes. In cardiac precursor cells, I_Ca was found to be already under control of cAMP-dependent phosphorylation since intracellular infusion of the catalytic subunit of protein kinase A resulted in a 1.7-fold stimulation. The adenylyl cyclase activator forskolin was without effect. IP₃-sensitive intracellular Ca²⁺ stores and Ca²⁺-ATPases are present during all stages of differentiation in both GFP-positive and GFP-negative cells. Functional ryanodine-sensitive Ca²⁺ stores, detected by caffeine-induced Ca²⁺ release, appeared in most GFP-positive cells 1–2 d after I_Ca. Coexpression of both I_Ca and ryanodine-sensitive Ca²⁺ stores at day 10 of development coincided with the beginning of spontaneous contractions in most EBs.Thus, the functional expression of voltage-dependent L-type Ca²⁺ channel (VDCC) is a hallmark of early cardiomyogenesis, whereas IP₃ receptors and sarcoplasmic Ca²⁺-ATPases are expressed before the initiation of cardiomyogenesis. Interestingly, the functional expression of ryanodine receptors/sensitive stores is delayed as compared with VDCC.     

Extracellular ATP-induced nuclear Ca transient is mediated by inositol 1,4,5-trisphosphate receptors in mouse pancreatic β-cells

Extracellular ATP (eATP) induces an intracellular Ca²⁺ transient by activating phospholipase C (PLC)-associated P2X4 purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent Ca²⁺ release from intracellular stores in mouse pancreatic β-cells. Using laser scanning confocal microscopy, Ca²⁺ indicator fluo-4 AM, and the cell permeable nuclear indicator Hoechst 33342, we examined the properties of eATP-induced Ca²⁺ release in pancreatic β-cell nuclei. eATP induced a higher nuclear Ca²⁺ transient in pancreatic β-cell nuclei than in the cytosol. After pretreatment with thapsigargin (TG), an inhibitor of sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) pumps, the amplitude of eATP-induced Ca²⁺ transients in the nucleus was still much higher than those in the cytosol. This effect of eATP was not altered by inhibition of either the plasma membrane Ca²⁺-ATPase (PMCA) or the plasma membrane Na⁺/Ca²⁺ exchanger (NCX) by LaCl₃ or by replacement of Na⁺ with N-Methyl-Glucosamine. eATP-induced nuclear Ca²⁺ transients were abolished by a cell-permeable IP3R inhibitor, 2-aminoethoxydiphenyl borate (2-APB), but were not blocked by the ryanodine receptor (RyR) antagonist ryanodine. Immunofluorescence studies showed that IP3Rs are expressed on the nuclear envelope of pancreatic β-cells. These results indicate that eATP triggers nuclear Ca²⁺ transients by mobilizing a nuclear Ca²⁺ store via nuclear IP3Rs.     

Effects of carbenoxolone on heart rhythm, contractility and intracellular calcium in streptozotocin-induced diabetic rat

Cardiac dysfunction is a frequently reported complication of clinical and experimental diabetes mellitus. Streptozotocin (STZ) – induced diabetes in rat is associated with a variety of cardiac defects including disturbances to heart rhythm and prolonged time-course of cardiac muscle contraction and/or relaxation. The effects of carbenoxolone (CBX), a selective gap junction inhibitor, on heart rhythm and contractility in STZ-induced diabetic rat have been investigated. Heart rate was significantly (P < 0.05) reduced in Langendorff perfused spontaneously beating diabetic rat heart (171±12 BPM) compared to age-matched controls (229± 9 BPM) and further reduced by 10⁻⁵ M CBX in diabetic (20%) and in control (17%) hearts. Action potential durations (APDs), recorded on the epicardial surface of the left ventricle, were prolonged in paced (6 Hz) diabetic compared to control hearts. Perfusion of hearts with CBX caused further prolongation of APDs and to a greater extent in control compared to diabetic heart. Percentage prolongation at 70% from the peak of the action potential amplitude after CBX was 18% in diabetic compared to 48% in control heart. CBX had no significant effect on resting cell length or amplitude of ventricular myocyte shortening in diabetic or control rats. However, resting fura-2 ratio (indicator for intracellular Ca²⁺ concentration) and amplitude of the Ca²⁺ transient were significantly (P < 0.05) reduced by CBX in diabetic rats but not in controls. In conclusion the larger effects of CBX on APD in control ventricle and the normalizing effects of CBX on intracellular Ca²⁺ in ventricular myocytes from diabetic rat suggest that there may be alterations in gap junction electrophysiology in STZ-induced diabetic rat heart.     

Effect of Chloride Channel Inhibitors on Cytosolic Ca2+ Levels and Ca2+-Activated K+ (Gardos) Channel Activity in Human Red Blood Cells

DIDS, NPPB, tannic acid (TA) and AO1 are widely used inhibitors of Cl^– channels. Some Cl^– channel inhibitors (NPPB, DIDS, niflumic acid) were shown to affect phosphatidylserine (PS) scrambling and, thus, the life span of human red blood cells (hRBCs). Since a number of publications suggest Ca²⁺ dependence of PS scrambling, we explored whether inhibitors of Cl^– channels (DIDS, NPPB) or of Ca²⁺-activated Cl^? channels (DIDS, NPPB, TA, AO1) modified intracellular free Ca²⁺ concentration ([Ca²⁺]_i) and activity of Ca²⁺-activated K⁺ (Gardos) channel in hRBCs. According to Fluo-3 fluorescence in flow cytometry, a short treatment (15 min, +37 °C) with Cl^? channels inhibitors decreased [Ca²⁺]_i in the following order: TA > AO1 > DIDS > NPPB. According to forward scatter, the decrease of [Ca²⁺]_i was accompanied by a slight but significant increase in cell volume following DIDS, NPPB and AO1 treatments. TA treatment resulted in cell shrinkage. According to whole-cell patch-clamp experiments, TA activated and NPPB and AO1 inhibited Gardos channels. The Cl^– channel blockers further modified the alterations of [Ca²⁺]_i following ATP depletion (glucose deprivation, iodoacetic acid, 6-inosine), oxidative stress (1 mM t-BHP) and treatment with Ca²⁺ ionophore ionomycin (1 μM). The ability of the Cl^? channel inhibitors to modulate PS scrambling did not correlate with their influence on [Ca²⁺]_i as TA and AO1 had a particularly strong decreasing effect on [Ca²⁺]_i but at the same time enhanced PS exposure. In conclusion, Cl^– channel inhibitors affect Gardos channels, influence Ca²⁺ homeostasis and induce PS exposure of hRBCs by Ca²⁺-independent mechanisms.     

Apoptotic inducers activate the release of d-aspartate through a hypotonic stimulus-triggered mechanism in PC12 cells

We have characterized release of d-aspartate (d-Asp), a regulator of hormone synthesis and secretion, via a volume-sensitive organic anion channel (VSOC) in PC12 cells by studying its response to apoptotic stimuli. PC12 cells have been demonstrated to endogenously synthesize d-Asp. Apoptotic inducers, including staurosporin (STS), tumor necrosis factor (TNF)-α, H₂O₂, and C2-ceramide, activate the release of d-Asp through a hypotonic stimulus-triggered mechanism. Putative blockers of the anion channel, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and 4,4′-diisothiocyanostilbene-2,2′-sulphonic acid (DIDS), significantly inhibited stress-induced d-Asp release under hypotonic conditions following the application of apoptotic inducers. Hypotonic conditions are essential for activation by apoptotic inducers. Phorbol 12-mirystate 13-acetate and the Ca²⁺ ionophore A23187 increased d-Asp efflux via the VSOC, implying the involvement of intracellular Ca²⁺ in the activation of the d-Asp efflux. However, hypotonic stress and STS had no effect on the concentration of intracellular Ca²⁺ in PC12 cells. Furthermore, an unknown EGTA-sensitive factor(s), other than Ca²⁺, and peroxynitrite may play pivotal roles in STS-enhanced d-Asp release.     

NADPH oxidase and eNOS control cardiomyogenesis in mouse embryonic stem cells on ascorbic acid treatment

Ascorbic acid (AA) increases cardiomyogenesis of embryonic stem (ES) cells. Herein we show that treatment of mouse ES cells with AA enhanced cardiac differentiation accompanied by an upregulation of the NADPH oxidase isoforms NOX2 and NOX4, phosphorylation of endothelial nitric oxide synthase (eNOS), and cyclic GMP (cGMP) formation, indicating that reactive oxygen species (ROS) as well as nitric oxide (NO) may be involved in cardiomyogenesis. In whole mount embryoid bodies as well as isolated Flk-1-positive (Flk-1⁺) cardiovascular progenitor cells ROS elevation by AA was observed in early stages of differentiation (Days 4-7), and absent at Day 10. In contrast NO generation following incubation with AA was absent at Day 4 and increased at Days 7 and 10. AA-mediated cardiomyogenesis was blunted by the NADPH oxidase inhibitors diphenylen iodonium (DPI) and apocynin, the free radical scavengers N-(2-mercaptopropionyl)-glycine (NMPG) and ebselen, and the NOS inhibitor L-NAME. Downregulation of NOX4 by short hairpin RNA (shRNA) resulted in significant inhibition of cardiomyogenesis and abolished the stimulation of MHC-ß and MLC2v gene expression observed on AA treatment. Our data demonstrate that AA stimulates cardiomyocyte differentiation from ES cells by signaling pathways that involve ROS generated at early stages and NO at late stages of cardiomyogenesis.     

Flecainide ameliorates arrhythmogenicity through NCX flux in Andersen-Tawil syndrome-iPS cell-derived cardiomyocytes

Andersen-Tawil syndrome (ATS) is a rare inherited channelopathy. The cardiac phenotype in ATS is typified by a prominent U wave and ventricular arrhythmia. An effective treatment for this disease remains to be established. We reprogrammed somatic cells from three ATS patients to generate induced pluripotent stem cells (iPSCs). Multi-electrode arrays (MEAs) were used to record extracellular electrograms of iPSC-derived cardiomyocytes, revealing strong arrhythmic events in the ATS-iPSC-derived cardiomyocytes. Ca²⁺ imaging of cells loaded with the Ca²⁺ indicator Fluo-4 enabled us to examine intracellular Ca²⁺ handling properties, and we found a significantly higher incidence of irregular Ca²⁺ release in the ATS-iPSC-derived cardiomyocytes than in control-iPSC-derived cardiomyocytes. Drug testing using ATS-iPSC-derived cardiomyocytes further revealed that antiarrhythmic agent, flecainide, but not the sodium channel blocker, pilsicainide, significantly suppressed these irregular Ca²⁺ release and arrhythmic events, suggesting that flecainide's effect in these cardiac cells was not via sodium channels blocking. A reverse-mode Na^+/Ca²⁺exchanger (NCX) inhibitor, KB-R7943, was also found to suppress the irregular Ca²⁺ release, and whole-cell voltage clamping of isolated guinea-pig cardiac ventricular myocytes confirmed that flecainide could directly affect the NCX current (I_NCX). ATS-iPSC-derived cardiomyocytes recapitulate abnormal electrophysiological phenotypes and flecainide suppresses the arrhythmic events through the modulation of I_NCX.     

Effects of bradykinin on Ca2+ mobilization and prostaglandin E2 release in human periodontal ligament cells.

In fura-2-loaded human periodontal ligament (HPDL) cells, bradykinin induced a rapidly transient increase and subsequently sustained increase in cytosolic Ca²⁺ ([Ca²⁺]_i). When external Ca²⁺ was chelated by EGTA, the transient peak of [Ca²⁺]_i was reduced and the sustained level was abolished, implying the Ca²⁺ mobilization consists of intracellular Ca²⁺ release and Ca²⁺ influx. Thapsigargin, a specific Ca²⁺-ATPase inhibitor for inositol 1,4,5-trisphosphate (1,4,5-1P₃)-sensitive Ca²⁺ pool, induced an increase in [Ca²⁺]_i in the absence of external Ca²⁺. After depletion of the intracellular Ca²⁺ pool by thapsigargin, the increase in [Ca²⁺]_i induced by bradykinin was obviously reduced. Bradykinin also stimulated formation of inositol polyphosphates including 1,4,5-IP₃. These results suggest that bradykinin stimulates intracellular Ca²⁺ release from the 1,4,5-1P₃-sensitive Ca²⁺ pool. Bradykinin stimulated prostaglandin E₂ (PGE₂) release in the presence of external Ca²⁺, but not in the absence of external Ca²⁺. Ca²⁺ ionophore A23187 and thapsigargin evoked the release of PGE₂ in the presence of external Ca²⁺ despite no activation of bradykinin receptors. These results indicate that bradykinin induces Ca²⁺ mobilization via activation of phospholipase C and PGE₂ release caused by the Ca²⁺ influx in HPDL cells.     

Requirements for different Ca2+ pools in the activation of rabbit platelets: I. Release reaction and protein phosphorylation

We examined the role of Ca²⁺, both extracellular and intracellular in origin, in the release reaction and protein phosphorylation in rabbit platelets stimulated with platelet activating factor (acetylglyceryl ether phosphorylcholine), thrombin, or ionophore A23187. In the presence of extracellular Ca²⁺, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), a putative antagonist of intracellular Ca²⁺ transport, blocked platelet activating factor-initiated serotonin release at a half-maximal inhibitor concentration of 40 μM, compared to 350 μM for thrombin-induced release and greater than 500 μM, for A23187-induced release. Platelet activating factor-induced phosphorylation of two platelet proteins of M_r=41 000 (P7P) and 20 000 (P9P) was inhibited by TMB-8, an effect which was additive to that caused by removing extracellular Ca²⁺. TMB-8 demonstrated only minor to non-existant inhibitory effects on phosphorylation in thrombin- or A23187-stimulated platelets. In contrast to P9P phosphorylation, phosphorylation of P7P caused by platelet activating factor was more dependent on a TMB-8 sensitive step than on the availability of extracellular Ca²⁺. Experiments with buffers containing fixed concentrations of free Ca²⁺ revealed that both processes (release and phosphorylation), when stimulated by platelet activating factor and thrombin, had the same threshold requirement (1–3 μM) for extracellular free Ca²⁺. These studies provide evidence that stimulation of rabbit platelets by platelet activating factor is more dependent on a TMB-8-sensitive intracellular Ca²⁺ source than is stimulation caused by thrombin. Furthermore, our data indicate that activation of different intracellular processes involved in platelet secretion (such as P7P and P9P phosphorylation) may require Ca²⁺ from different pools.     

Intracellular mobilization of Ca by the insect steroid hormone 20-hydroxyecdysone during programmed cell death in silkworm anterior silk glands

20-Hydroxyecdysone (20E) triggers programmed cell death (PCD) and regulates de novo gene expression in the anterior silk glands (ASGs) of the silkworm Bombyx mori. PCD is mediated via a nongenomic pathway that includes Ca²⁺ as a second messenger and the activation of protein kinase C/caspase-3-like protease; however, the steps leading to a concomitant buildup of intracellular Ca²⁺ are unknown. We employed pharmacological tools to identify the components of this pathway. ASGs were cultured in the presence of 1 μM 20E and one of the following inhibitors: a G-protein-coupled receptor (GPCR) inhibitor, a phospholipase C (PLC) inhibitor, an inositol 1,4,5-trisphosphate receptor (IP₃R) antagonist, and an L- or T-type Ca²⁺ channel blocker. The T-type Ca²⁺ channel blocker inhibited 20E-induced nuclear and DNA fragmentation; in contrast, PCD was induced by 20E in Ca²⁺-free medium, indicating that the source of Ca²⁺ is an intracellular reservoir. The IP₃R antagonist inhibited nuclear and DNA fragmentation, suggesting that the endoplasmic reticulum may be the Ca²⁺ source. Finally, the GPCR and PLC inhibitors effectively blocked nuclear and DNA fragmentation. Our results indicate that 20E increases the intracellular level of Ca²⁺ by activating IP₃R, and that this effect may be brought about by the serial activation of GPCR, PLC, and IP₃.     

Signal transduction in red bloodcells of the lizards Ameiva ameiva and Tupinambis merianae (Squamata, Teiidae)

The fluorescent calcium probe, Fluo-3, AM was used to measure the intracellular calcium concentration in red blood cells (RBCs) of the teiid lizards Ameiva ameiva and Tupinambis merianae. The cytosolic [Ca²⁺] is maintained around 20nM and the cells contain membrane-bound Ca²⁺pools. One pool appears to be identifiable with the endoplasmic reticulum (ER) inasmuch as addition of the sarco-endoplasmic reticulum Ca²⁺ATPase, SERCA, inhibitor thapsigargin induces an increase in cytosolic [Ca²⁺both in the presence and in the absence of extracellular Ca²⁺. In addition to the ER, an acidic compartment appears to be involved in Ca²⁺storage, as collapse of intracellular pHgradients by monensin, a Na⁺–H⁺exchanger, and nigericin, a K⁺–H⁺exchanger, induce the release of Ca²⁺from internal pools. A vacuolar H⁺pump, sensitive to NBD-Cl and bafilomycin appears to be necessary to load the acidic Ca²⁺pools. Finally, the purinergic agonist ATP triggers a rapid and transient increase of [Ca²⁺]_cin the cells from both lizard species, mostly by mobilization of the cation from internal stores.     

Pituitary Adenylate Cyclase-Activating Polypeptide Causes Ca2+ Release from Ryanodine/Caffeine Stores Through a Novel Pathway Independent of Both Inositol Trisphosphates and Cyclic AMP in Bovine Adrenal Medullary Cells

Abstract: Pituitary adenylate cyclase-activating polypeptide (PACAP) causes both Ca²⁺ release and Ca²⁺ influx in bovine adrenal chromaffin cells. To elucidate the mechanisms of PACAP-induced Ca²⁺ release, we investigated expression of PACAP receptors and measured inositol trisphosphates (IP₃), cyclic AMP, and the intracellular Ca²⁺ concentration in bovine adrenal medullary cells maintained in primary culture. RT-PCR analysis revealed that bovine adrenal medullary cells express the PACAP receptor hop, which is known to couple with both IP₃ and cyclic AMP pathways. The two naturally occurring forms of PACAP, PACAP38 and PACAP27, both increased cyclic AMP and IP₃, and PACAP38 was more potent than PACAP27 in both effects. Despite the effects of PACAP on IP₃ production, the Ca²⁺ release induced by PACAP38 or by PACAP27 was unaffected by cinnarizine, a blocker of IP₃ channels. The potencies of the peptides to cause Ca²⁺ release in the presence of cinnarizine were similar. The Ca²⁺ release induced by PACAP38 or by PACAP27 was strongly inhibited by ryanodine and caffeine. In the presence of ryanodine and caffeine, PACAP38 was more potent than PACAP27. PACAP-induced Ca²⁺ release was unaffected by Rp-adenosine 3′,5′-cyclic monophosphothioate, an inhibitor of protein kinase A. Ca²⁺ release induced by bradykinin and angiotensin II was also inhibited by ryanodine and caffeine, but unaffected by cinnarizine. Although IP₃ production stimulated by PACAP38 or bradykinin was abolished by the phospholipase C inhibitor, U-73122, Ca²⁺ release in response to the peptides was unaffected by U-73122. These results suggest that PACAP induces Ca²⁺ release from ryanodine/caffeine stores through a novel intracellular mechanism independent of both IP₃ and cyclic AMP and that the mechanism may be the common pathway through which peptides release Ca²⁺ in adrenal chromaffin cells.     

RyR2 Modulates a Ca2+-Activated K+ Current in Mouse Cardiac Myocytes

In cardiomyocytes, Ca²⁺ entry through voltage-dependent Ca²⁺ channels (VDCCs) binds to and activates RyR2 channels, resulting in subsequent Ca²⁺ release from the sarcoplasmic reticulum (SR) and cardiac contraction. Previous research has documented the molecular coupling of small-conductance Ca²⁺-activated K⁺ channels (SK channels) to VDCCs in mouse cardiac muscle. Little is known regarding the role of RyRs-sensitive Ca²⁺ release in the SK channels in cardiac muscle. In this study, using whole-cell patch clamp techniques, we observed that a Ca²⁺-activated K+ current (I_K,Ca) recorded from isolated adult C57B/L mouse atrial myocytes was significantly decreased by ryanodine, an inhibitor of ryanodine receptor type 2 (RyR2), or by the co-application of ryanodine and thapsigargin, an inhibitor of the sarcoplasmic reticulum calcium ATPase (SERCA) (p<0.05, p<0.01, respectively). The activation of RyR2 by caffeine increased the I_K,Ca in the cardiac cells (p<0.05, p<0.01, respectively). We further analyzed the effect of RyR2 knockdown on I_K,Ca and Ca²⁺ in isolated adult mouse cardiomyocytes using a whole-cell patch clamp technique and confocal imaging. RyR2 knockdown in mouse atrial cells transduced with lentivirus-mediated small hairpin interference RNA (shRNA) exhibited a significant decrease in I_K,Ca (p<0.05) and [Ca²⁺]i fluorescence intensity (p<0.01). An immunoprecipitated complex of SK2 and RyR2 was identified in native cardiac tissue by co-immunoprecipitation assays. Our findings indicate that RyR2-mediated Ca²⁺ release is responsible for the activation and modulation of SK channels in cardiac myocytes.     

Modulation of Intracellular Calcium Waves and Triggered Activities by Mitochondrial Ca Flux in Mouse Cardiomyocytes

Recent studies have suggested that mitochondria may play important roles in the Ca²⁺ homeostasis of cardiac myocytes. However, it is still unclear if mitochondrial Ca²⁺ flux can regulate the generation of Ca²⁺ waves (CaWs) and triggered activities in cardiac myocytes. In the present study, intracellular/cytosolic Ca²⁺ (Ca_i ²⁺) was imaged in Fluo-4-AM loaded mouse ventricular myocytes. Spontaneous sarcoplasmic reticulum (SR) Ca²⁺ release and CaWs were induced in the presence of high (4 mM) external Ca²⁺ (Ca_o ²⁺). The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) reversibly raised basal Ca_i ²⁺ levels even after depletion of SR Ca²⁺ in the absence of Ca_o ²⁺ , suggesting Ca²⁺ release from mitochondria. FCCP at 0.01 - 0.1 µM partially depolarized the mitochondrial membrane potential (Δψ _m) and increased the frequency and amplitude of CaWs in a dose-dependent manner. Simultaneous recording of cell membrane potentials showed the augmentation of delayed afterdepolarization amplitudes and frequencies, and induction of triggered action potentials. The effect of FCCP on CaWs was mimicked by antimycin A (an electron transport chain inhibitor disrupting Δψ _m) or Ru360 (a mitochondrial Ca²⁺ uniporter inhibitor), but not by oligomycin (an ATP synthase inhibitor) or iodoacetic acid (a glycolytic inhibitor), excluding the contribution of intracellular ATP levels. The effects of FCCP on CaWs were counteracted by the mitochondrial permeability transition pore blocker cyclosporine A, or the mitochondrial Ca²⁺ uniporter activator kaempferol. Our results suggest that mitochondrial Ca²⁺ release and uptake exquisitely control the local Ca²⁺ level in the micro-domain near SR ryanodine receptors and play an important role in regulation of intracellular CaWs and arrhythmogenesis.     

Ca influx and ATP release mediated by mechanical stretch in human lung fibroblasts

One cause of progressive pulmonary fibrosis is dysregulated wound healing after lung inflammation or damage in patients with idiopathic pulmonary fibrosis and severe acute respiratory distress syndrome. The mechanical forces are considered to regulate pulmonary fibrosis via activation of lung fibroblasts. In this study, the effects of mechanical stretch on the intracellular Ca²⁺ concentration ([Ca²⁺]_i) and ATP release were investigated in primary human lung fibroblasts. Uniaxial stretch (10–30% in strain) was applied to fibroblasts cultured in a silicone chamber coated with type I collagen using a stretching apparatus. Following stretching and subsequent unloading, [Ca²⁺]_i transiently increased in a strain-dependent manner. Hypotonic stress, which causes plasma membrane stretching, also transiently increased the [Ca²⁺]_i. The stretch-induced [Ca²⁺]_i elevation was attenuated in Ca²⁺-free solution. In contrast, the increase of [Ca²⁺]_i by a 20% stretch was not inhibited by the inhibitor of stretch-activated channels GsMTx-4, Gd³⁺, ruthenium red, or cytochalasin D. Cyclic stretching induced significant ATP releases from fibroblasts. However, the stretch-induced [Ca²⁺]_i elevation was not inhibited by ATP diphosphohydrolase apyrase or a purinergic receptor antagonist suramin. Taken together, mechanical stretch induces Ca²⁺ influx independently of conventional stretch-sensitive ion channels, the actin cytoskeleton, and released ATP.     

The imidazoline derivative calmidazolium inhibits voltage-gated Ca2+-channels and insulin release but has no effect on the phospholipase C system in insulin producing RINm5F-cells

The present study shows that the calmodulin antagonist calmidazolium inhibited influx of Ca²⁺ through voltage-gated Ca²⁺-channels in clonal insulin producing RINm5F-cells. The mechanism of inhibition may involve both Ca²⁺-calmodulin-dependent protein kinases and direct binding of calmidazolium to the Ca²⁺-channel. Calmidazolium did not affect uptake of Ca²⁺ into intracellular Ca²⁺-pools, inositol 1,4,5-trisphosphate (InsP₃) formation or action on intracellular Ca²⁺-pools. The calmodulin inhibitor also did not affect glucose utilization or oxidation in RINm5F-cells, speaking against an unspecific toxic effect of the compound. KCl-and ATP-stimulated insulin release from RINm5F-cells was attenuated by calmidazolium, whereas basal hormone secretion was unaffected.     

Evidence for the possible involvement of the P2Y<Subscript>6</Subscript> receptor in Ca<Superscript>2+</Superscript> mobilization and insulin secretion in mouse pancreatic islets

Subtypes of purinergic receptors involved in modulation of cytoplasmic calcium ion concentration ([Ca²⁺]_i) and insulin release in mouse pancreatic β-cells were examined in two systems, pancreatic islets in primary culture and beta-TC6 insulinoma cells. Both systems exhibited some physiological responses such as acetylcholine-stimulated [Ca²⁺]_i rise via cytoplasmic Ca²⁺ mobilization. Addition of ATP, ADP, and 2-MeSADP (each 100 μM) transiently increased [Ca²⁺]_i in single islets cultured in the presence of 5.5 mM (normal) glucose. The potent P2Y₁ receptor agonist 2-MeSADP reduced insulin secretion significantly in islets cultured in the presence of high glucose (16.7 mM), whereas a slight stimulation occurred at 5.5 mM glucose. The selective P2Y₆ receptor agonist UDP (200 μM) transiently increased [Ca²⁺]_i and reduced insulin secretion at high glucose, whereas the P2Y_2/4 receptor agonist UTP and adenosine receptor agonist NECA were inactive. [Ca²⁺]_i transients induced by 2-MeSADP and UDP were antagonized by suramin (100 μM), U73122 (2 μM, PLC inhibitor), and 2-APB (10 or 30 μM, IP₃ receptor antagonist), but neither by staurosporine (1 μM, PKC inhibitor) nor depletion of extracellular Ca²⁺. The effect of 2-MeSADP on [Ca²⁺]_i was also significantly inhibited by MRS2500, a P2Y₁ receptor antagonist. These results suggested that P2Y₁ and P2Y₆ receptor subtypes are involved in Ca²⁺ mobilization from intracellular stores and insulin release in mouse islets. In beta-TC6 cells, ATP, ADP, 2-MeSADP, and UDP transiently elevated [Ca²⁺]_i and slightly decreased insulin secretion at normal glucose, while UTP and NECA were inactive. RT-PCR analysis detected mRNAs of P2Y₁ and P2Y₆, but not P2Y₂ and P2Y₄ receptors.