Translational suppression by Ca2+ ionophores: Reversibility and roles of Ca2+ mobilization, Ca2+ influx, and nucleotide depletion

The divalent cation selective ionophores A23187 and ionomycin were compared for their effects on the Ca²⁺ contents, nucleotide contents, and protein synthetic rates of several types of cultured cells. Both ionophores reduced amino acid incorporation by approximately 85% at low concentrations (50–300 nmol/L) in cultured mammalian cells without reducing ATP or GTP contents. At these concentrations A23187 and ionomycin each promoted substantial Ca²⁺ efflux, whereas at higher concentrations a large influx of the cation was observed. Ca²⁺ influx occurred at lower ionophore concentrations and to greater extents in C6 glioma and P3X63Ag8 myeloma than in GH₃ pituitary cells. The ATP and GTP contents of the cells and their ability to adhere to growth surfaces declined sharply at ionophore concentrations producing increased Ca²⁺ influx. Prominent reductions of nucleotide contents occurred in EGTA-containing media that were further accentuated by extracellular Ca²⁺. Ionomycin produced more Ca²⁺ influx and nucleotide decline than comparable concentrations of A23187. The inhibition of amino acid incorporation and mobilization of cell-associated Ca²⁺ by ionomycin were readily reversed in GH₃ cells by fatty acid-free bovine serum albumin, whereas the effects of A23187 were only partially reversed. Amino acid incorporation was further suppressed by ionophore concentrations depleting nucleotide contents. Mitochondrial uncouplers potentiated Ca²⁺ accumulation in response to both ionophores. At cytotoxic concentrations Lubrol PX abolished protein synthesis but did not cause Ca²⁺ influx. Nucleotide depletion at high ionophore concentrations is proposed to result from increased plasmalemmal Ca²⁺-ATPase activity and dissipation of mitochondrial proton gradients and to cause intracellular Ca²⁺ accumulation. Increased Ca²⁺ contents in response to Ca²⁺ ionophores are proposed as an indicator of ionophore-induced cytotoxicity.Abbreviations BSA bovine serum albumin - EGTA [ethylenebis(oxyethylenenitrilo)]tetraacetic acid - PKR double-stranded RNA-regulated protein kinase - ER endoplasmic reticulum - eIF eukaryotic initiation factor     

The thermodynamic efficiency of the Ca2+-Mg2+-ATPase is one hundred percent

The thermodynamic efficiency of the Ca²⁺-Mg²⁺-ATPase of skeletal sarcoplasmic reticulum has been evaluated by comparing the Ca²⁺ gradient established with the ATP/(ADP*P_i) ratio. The evaluation was made at an external Ca²⁺ level (4.7 × 10^–8 M) which is below theK _m value of 7 × 10^–8 M. The Mg-ATP and phosphate concentrations were held constant (0.1 mM) and the ADP concentration was varied. Maximal uptake to an internal free Ca²⁺ concentration of 17 mM was observed at infinite ATP/(ADP*P_i) ratio (absence of ADP). This corresponds to a [Ca²⁺]_i/[Ca²⁺]₀ gradient of 3.6 × 10⁵. A Ca²⁺ gradient one-half as large was observed at an ATP/(ADP*P_i) ratio of 3.5 × 10³ M^–1. The square of the Ca²⁺ gradient is shown to be proportional to the ATP/(ADP*P_i) ratio, for finite values of the latter. The proportionality constant is identical to the equilibrium constant for hydrolysis of ATP (9.02 × 10⁶ M) under these conditions (0.1 mM Mg²⁺, 30°C). The intrinsic thermodynamic efficiency of the pump is shown to be 100%, with a maximal uncertainty of 3%. The efficiency is lower under less optimal conditions, when the pump is inhibited and passive leak processes compete.Dedicated to Prof. Philip George, University of Pennsylvania, whose instruction, research, and example made this contribution possible.     

Gibberellic-acid-stimulated Ca2+ accumulation in endoplasmic reticulum of barley aleurone: Ca2+ transport and steady-state levels

The steady-state levels of Ca²⁺ within the endoplasmic reticulum (ER) and the transport of ⁴⁵Ca²⁺ into isolated ER of barley (Hordeum vulgare L. cv. Himalaya) aleurone layers were studied. The Ca²⁺-sensitive dye indo-1. Endoplasmic reticulum was isolated and purified from indo-1-loaded protoplasts, and the Ca²⁺ level in the ER was measured using the Ca²⁺-sensitive dye indo-1. Endoplasmic reticulum was isolated and purified from indo-1-loaded protoplasts, and the Ca²⁺ level in the lumen of the ER was determined by the fluorescence-ratio method to be at least 3 M. Transport of ⁴⁵Ca²⁺ into the ER was studied in microsomal fractions isolated from aleurone layers incubated in the presence and absence of gibberellic acid (GA₃) and Ca²⁺. Isopycinic sucrose density gradient centrifugation of microsomal fractions isolated from aleurone layers or protoplasts separates ER from tonoplast and plasma membranes but not from the Golgi apparatus. Transport of ⁴⁵Ca²⁺ occurs primarily in the microsomal fraction enriched in ER and Golgi. Using monensin and heat-shock treatments to discriminate between uptake into the ER and Golgi, we established that ⁴⁵Ca²⁺ transport was into the ER. The sensitivity of ⁴⁵Ca²⁺ transport to inhibitors and the K_m of ⁴⁵Ca²⁺ uptake for ATP and Ca²⁺ transport in the microsomal fraction of barley aleurone cells. The rate of ⁴⁵Ca²⁺ transport is stimulated several-fold by treatment with GA₃. This effect of GA₃ is mediated principally by an effect on the activity of the Ca²⁺ transporter rather than on the amount of ER.Abbreviations CCR cytochrome-c reductase - DCCD dicyclohexylcarbodiimide - EGTA ethylene glycol bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid - ER endoplasmic reticulum - FCCP carbonylcyanide p-trifluoromethoxyphenyl hydrazone - GA₃ gibberellic acid - IDPase inosine diphosphatase - Mon monensin     

Ca2+/H+ exchange,lumenal Ca2+ release and Ca2+/ATP coupling ratios in the sarcoplasmic reticulum ATPase

The Ca²⁺ transport ATPase (SERCA) of sarcoplasmic reticulum (SR) plays an important role in muscle cytosolic signaling, as it stores Ca²⁺ in intracellular membrane bound compartments, thereby lowering cytosolic Ca²⁺ to induce relaxation. The stored Ca²⁺ is in turn released upon membrane excitation to trigger muscle contraction. SERCA is activated by high affinity binding of cytosolic Ca²⁺, whereupon ATP is utilized by formation of a phosphoenzyme intermediate, which undergoes protein conformational transitions yielding reduced affinity and vectorial translocation of bound Ca²⁺. We review here biochemical and biophysical evidence demonstrating that release of bound Ca²⁺ into the lumen of SR requires Ca²⁺/H⁺ exchange at the low affinity Ca²⁺ sites. Rise of lumenal Ca²⁺ above its dissociation constant from low affinity sites, or reduction of the H⁺ concentration by high pH, prevent Ca²⁺/H⁺ exchange. Under these conditions Ca²⁺ release into the lumen of SR is bypassed, and hydrolytic cleavage of phosphoenzyme may yield uncoupled ATPase cycles. We clarify how such Ca²⁺pump slippage does not occur within the time length of muscle twitches, but under special conditions and in special cells may contribute to thermogenesis.     

The calmodulin-activated form of the Ca2+-pumping ATPase of the cardiac sarcolemmal membrane produces Ca2+ gradients with a thermodynamic efficiency of 100%

The thermodynamic efficiency of the calmodulin-activated form of the Ca²⁺-pumping ATPase of the bovine cardiac sarcolemma (SL) was evaluated in sealed vesicles under reversible conditions. The free internal Ca²⁺ concentration ([Ca²⁺]_i) established in the SL vesicle lumen by action of the ATPase was determined as a function of the [ATP]/([ADP][P_i]) ratio for the following experimental conditions: 250mM sucrose, 100mM KCI, 0.1mM Mg²⁺, 25mM HEPES, 25mM Tris, pH 7.40, at 37°C, [Ca²⁺]_o=50nM (1mM Ca/EGTA buffer), 0.75mM Mg-ATP, 0.1mM P_i, variable [ADP]. Under these conditions, with the pump working near itsK _m of 64nM, the [Ca²⁺]_i achieved was 18mM, decreasing with increasing [ADP] for [ADP] 0.84mM. A plot of the square of the [Ca²⁺]_i/[Ca²⁺]_o ratio against [ATP]/([ADP][P_i]) gave a straight line with a slope of 1.5×10⁷M. This was in agreement, within the experimental error, with the equilibrium constant for ATP hydrolysis under these conditions (1.09×10⁷M). These results demonstrate (1) tight coupling between Ca²⁺ transport and ATP hydrolysis with a stoichiometry of 2 Ca²⁺ moved per ATP split and (2) a low degree of passive leakage. Analysis at low [ADP] (<0.83mM) showed the unexpected result that ADP increases the rate of theforward reaction of the pump. The maximal effect on the initial rate is a 96±5% increase, with an EC₅₀ of approximately 0.4mM (ADP). Similar but lesser stimulation was observed with CDP. The implications of the above results for the energetics of the pump and for its physiological function in the beating heart are discussed.     

Effect of Subtype-Specific Ca2+-Antagonists and Ca2+-Free Media on the Field Stimulation-Evoked Release of ATP and [3H]Acetylcholine from Rat Habenula Slices

The involvement of different subtypes of voltage-sensitive Ca²⁺ channels in the initiation of field stimulation-induced endogenous adenosine triphosphate (ATP) and [³H]acetylcholine ([³H]ACh) release was investigated in the superfused rat habenula slices. ATP, measured by the luciferin-luciferase assay, and [³H]ACh were released simultaneously from the tissue in response to low frequency electrical stimulation (2 Hz, 2.5 msec, 360 shocks). The N-type Ca²⁺ channel blocker -conotoxin GVIA (-CgTX, 0.01–1 M) reduced the stimulation-evoked release of ATP and [³H]ACh in a dose-dependent manner. Similarly, the P-type Ca²⁺ channel antagonist -agatoxin IVA (-Aga IVA) (0.05 M) and the inorganic Ca²⁺ channel blocker Cd²⁺ (0.2 mM) inhibited the outflow of both transmitters, while Ni²⁺ (0.1 mM) was without significant effect. A high correlation was observed between the percent inhibition of ATP release and percent inhibition of ACh release caused by the different Ca²⁺ antagonists. Long-term perfusion (i.e., 90 min) with Ca²⁺ free solution inhibited the evoked-release of ATP and [³H]ACh. In contrast, perfusion of slices with the same media for a shorter time (i.e., 20 min) did not reduce the release of [³H]ACh and ATP but even increased the evoked-release of ATP about fourfold. The breakdown of extracellular ATP was not blocked under low [Ca²⁺]₀ condition, measured by the creatine phosphokinase assay and HPLC-UV technique. Application of extra- or intracellular Ca²⁺ chelators, and dipyridamole (2 M), the nucleoside transporter inhibitor, did not reduce the excess release of ATP after short-term perfusion with Ca²⁺-free media. Tetrodotoxin (TTX, 1 M), while inhibiting the majority of ATP release under normal conditions, was also unable to reduce release under low [Ca²⁺]₀ conditions. In summary, we showed that both N- and P-type Ca²⁺ channels are involved in the initiation of electrical stimulation-evoked release of ATP and [³H]ACh in the rat habenula under normal extracellular calcium concentration. Under low [Ca²⁺]₀ conditions an additional release of ATP occurs, which is not associated with action potential propagation.     

Na+/K+-ATPase inhibition during cardiac myocyte swelling: Involvement of intracellular pH and Ca2+

Previous studies in chick embryo cardiac myocytes have shown that the inhibition of Na⁺/K⁺-ATPase with ouabain induces cell shrinkage in an isosmotic environment (290 mOsm). The same inhibition produces an enhanced RVD (regulatory volume decrease) in hyposmotic conditions (100 mOsm). It is also known that submitting chick embryo cardiomyocytes to a hyperosmotic solution induces shrinkage and a concurrent intracellular alkalization. The objective of this study was to evaluate the involvement of intracellular pH (pH_i), intracellular Ca²⁺ ([Ca²⁺]_i) and Na⁺/K⁺-ATPase inhibition during hyposmotic swelling. Changes in intracellular pH and Ca²⁺ were monitored using BCECF and fura-2, respectively. The addition of ouabain (100 M) under both isosmotic and hyposmotic stimuli resulted in a large increase in [Ca²⁺]_i (200%). A decrease in pH_i (from 7.3 ± 0.09 to 6.4 ± 0.08, n = 6; p < 0.05) was only observed when ouabain was applied during hyposmotic swelling. This acidification was prevented by the removal of extracellular Ca²⁺. Inhibition of Na⁺/H²⁺ exchange with amiloride (1 mM) had no effect on the ouabain-induced acidification. Preventing the mitochondrial accumulation of Ca²⁺ using CCCP (10 M) resulted in a blockade of the progressive acidification normally induced by ouabain. The inhibition of mitochondrial membrane K⁺/H⁺ exchange with DCCD (1 mM) also completely prevented the acidification. Our results suggest that intracellular acidification upon cell swelling is mediated by an initial Ca²⁺ influx via Na⁺/Ca²⁺ exchange, which under hyposmotic conditions activates the K⁺ and Ca²⁺ mitochondrial exchange systems (K⁺/H⁺ and Ca²⁺/H⁺).Deceased     

Heart sarcolemmal Ca2+ transport in endotoxin shock: I. Impairment of ATP-dependent Ca2+ transport

Effects of endotoxin administration on the ATP-dependent Ca²⁺ transport in canine cardiac sarcolemma were investigated. The results show that the sidedness of the sarcolemmal vesicles was not affected but the ATP-dependent Ca²⁺ transport in cardiac sarcolemma was decreased by 22 to 46% (p < 0.05) at 4 h following endotoxin administration. The kinetic analysis indicates that the V_max for ATP and for Ca²⁺ were decreased by 50% (p < 0.01) and 32% (p < 0.01), respectively, while the Km values for ATP and Ca²⁺ were not significantly affected after endotoxin administration. Magnesium (1–5 mM) stimulated while vanadate (0.25–3.0 M) inhibited the ATP-dependent Ca²⁺ transport, but the Mg²⁺-stimulated and the vanadate-inhibitable activities remained significantly lower in the endotoxin-treated animals. These data demonstrate that endotoxin administration impairs the ATP-dependent Ca²⁺ transport in canine cardiac sarcolemma and that the impairment is associated with a mechanism not affecting the affinity towards ATP and Ca²⁺. Additional experiments show that the Ca²⁺ sensitivity of the Ca²⁺-ATPase activity was indifferent between the control and endotoxic groups suggesting that endotoxic injury impairs Ca²⁺ pumping without affecting Ca²⁺-ATPase activity. Since sarcolemmal ATP-dependent Ca²⁺ transport plays an important role in the regulation of cytosolic Ca²⁺ homeostasis, an impairment in the sarcolemmal ATP-dependent Ca²⁺ transport induced by endotoxin administration may have a pathophysiological significance in contributing to the development of myocardial dysfunction in endotoxin shock.     

ATP increases within the lumen of the endoplasmic reticulum upon intracellular Ca2+ release

Multiple functions of the endoplasmic reticulum (ER) essentially depend on ATP within this organelle. However, little is known about ER ATP dynamics and the regulation of ER ATP import. Here we describe real-time recordings of ER ATP fluxes in single cells using an ER-targeted, genetically encoded ATP sensor. In vitro experiments prove that the ATP sensor is both Ca²⁺ and redox insensitive, which makes it possible to monitor Ca²⁺-coupled ER ATP dynamics specifically. The approach uncovers a cell type–specific regulation of ER ATP homeostasis in different cell types. Moreover, we show that intracellular Ca²⁺ release is coupled to an increase of ATP within the ER. The Ca²⁺-coupled ER ATP increase is independent of the mode of Ca²⁺ mobilization and controlled by the rate of ATP biosynthesis. Furthermore, the energy stress sensor, AMP-activated protein kinase, is essential for the ATP increase that occurs in response to Ca²⁺ depletion of the organelle. Our data highlight a novel Ca²⁺-controlled process that supplies the ER with additional energy upon cell stimulation.     

Ins(1,4,5)P3 receptor-mediated Ca2+ signaling and autophagy induction are interrelated

The role of intracellular Ca²⁺ signaling in starvation-induced autophagy remains unclear. Here, we examined Ca²⁺ dynamics during starvation-induced autophagy and the underlying molecular mechanisms. Tightly correlating with autophagy stimulation, we observed a remodeling of the Ca²⁺ signalosome. First, short periods of starvation (1 to 3 h) caused a prominent increase of the ER Ca²⁺-store content and enhanced agonist-induced Ca²⁺ release. The mechanism involved the upregulation of intralumenal ER Ca²⁺-binding proteins, calreticulin and Grp78/BiP, which increased the ER Ca²⁺-buffering capacity and reduced the ER Ca²⁺ leak. Second, starvation led to Ins(1,4,5)P₃R sensitization. Immunoprecipitation experiments showed that during starvation Beclin 1, released from Bcl-2, first bound with increasing efficiency to Ins(1,4,5)P₃Rs; after reaching a maximal binding after 3 h, binding, however, decreased again. The interaction site of Beclin 1 was determined to be present in the N-terminal Ins(1,4,5)P₃-binding domain of the Ins(1,4,5)P₃R. The starvation-induced Ins(1,4,5)P₃R sensitization was abolished in cells treated with BECN1 siRNA, but not with ATG5 siRNA, pointing toward an essential role of Beclin 1 in this process. Moreover, recombinant Beclin 1 sensitized Ins(1,4,5)P₃Rs in ⁴⁵Ca²⁺-flux assays, indicating a direct regulation of Ins(1,4,5)P₃R activity by Beclin 1. Finally, we found that Ins(1,4,5)P₃R-mediated Ca²⁺ signaling was critical for starvation-induced autophagy stimulation, since the Ca2+ chelator BAPTA-AM as well as the Ins(1,4,5)P₃R inhibitor xestospongin B abolished the increase in LC3 lipidation and GFP-LC3-puncta formation. Hence, our results indicate a tight and essential interrelation between intracellular Ca²⁺ signaling and autophagy stimulation as a proximal event in response to starvation.     

Decreased Ca2+-binding and Ca2+-ATPase activities in heart sarcolemma upon phospholipid methylation

Heart sarcolemma has been shown to possess three catalytic sites (I, II and III) for methyl transferase activity (Panagia V, Ganguly PK and Dhalla NS. Biochim Biophys Acta 792: 245–253, 1984). In this study we examined the effect of phosphatidylethanolamine N-methylation on ATP-independent Ca²⁺ binding and ATPase activities in isolated rat heart sarcolemma. Both low affinity (1.25 mM Ca²⁺) and high affinity (50 µM Ca²⁺) Ca²⁺ binding activities were decreased following incubation of sarcolemmal membranes with AdoMet under optimal conditions for site II and III. Similarly, Ca²⁺ ATPase activities measured at 1.25 mM and 4 mM Ca²⁺ were depressed by phospholipid N-methylation. S-adenosyl homocysteine, a specific inhibitor of phospholipid N-methylation, prevented the depression of low affinity Ca²⁺ binding and Ca²⁺ ATPase activities, whereas the methylation-induced effect on the high affinity Ca²⁺ binding was not influenced by this agent. Pretreatment of sarcolemma with methyl acetimidate hydrochloride, an amino group blocking agent, also prevented the methylation-induced inhibition of both Ca²⁺ binding and Ca²⁺ ATPase. A further decrease in Ca²⁺ binding and Ca²⁺ ATPase activities together with a marked increase in the intramembranal level of PC was seen when membranes were methylated under the site III conditions in the presence of phosphatidyldimethylethanolamine as exogenous substrate. There was no effect of phospholipid methylation on sarcolemmal Na⁺-K⁺ ATPase and Mg²⁺ ATPase activities. These results indicate a role of phospholipid N-methylation in the regulation of sarcolemmal Ca²⁺ ATPase and low affinity ATP-independent Ca²⁺ binding.     

Asymmetric effects of divalent cations and protons on active Ca2+ efflux and Ca2+-ATPase in intact red blood cells

Summary The influence of the asymmetric addition of various divalent cations and protons on the properties of active Ca²⁺ transport have been examined in intact human red blood cells. Active Ca²⁺ efflux was determined from the initial rate of⁴⁵Ca²⁺ loss after CoCl₂ was added to block Ca²⁺ loading via the ionophore A23187. Ca²⁺-ATPase activity was measured as phosphate production over 5 min in cells equilibrated with EGTA-buffered free Ca²⁺ in the presence of A23187. The apparent Ca affinity of active Ca²⁺ efflux (K _0.5=30–40 mol/liter cells) was significantly lower than that measured by the Ca²⁺-ATPase assay (K _0.5=0.4 m). Possible reasons for this apparent difference are considered. Both active Ca²⁺ efflux and Ca²⁺-ATPase activity were reduced to less than 5% of maximal levels (20 mmol/liter cells · hr) in Mg²⁺-depleted cells, and completely restored by reintroduction of intracellular Mg²⁺. Active Ca²⁺ efflux was inhibited almost completely by raising external CaCl₂ (but not MgCl₂) to 20mm, probably by interaction of Ca²⁺ at the externally oriented E₂P conformation of the pump. Cd²⁺ was more potent than Ca²⁺ in this inhibition, while Mn²⁺ was less potent and 10mm Ba²⁺ was without effect. A Ca²⁺: proton exchange mechanism for active Ca²⁺ efflux was supported by the results, as external protons (pH 6–6.5) stimulated active Ca²⁺ efflux at least twofold above the efflux rate at pH 7.8 Ca²⁺ transport was not affected by decreasing the membrane potential across the red cell.     

Uncoupling of Ca2+ transport from ATP hydrolysis activity of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

Summary In reconstituted rabbit skeletal muscle (Ca²⁺ + Mg²⁺)-ATPase proteoliposomes, Ca²⁺-uptake is decreased by more than 90% with T₂ cleavage (Arg-198). However, no difference in the ATP dependence of hydrolysis activity is seen between SR and trypsin-treated SR. A large decrease in E-P formation and hydrolysis activity of the enzyme appear only at T₃ cleavage, which represents the cleavage of A₁ fragment to A_1a + A_1b forms. The disappearance of hydrolysis activity due to digestion is prior to the disappearance of E-P formation. No significant difference is found in the passive Ca²⁺ efflux between control SR and tryptically digested SR in the absence of Mg⁺ ruthenium red or in the presence of ATP. However, the passive Ca²⁺ efflux rate for tryptically digested SR is much larger than control SR in the presence of Mg²⁺ + ruthenium red. These results show that the Ca²⁺ channel cannot be closed after trypsin digestion of SR membranes by the presence of the Ca²⁺ channel inhibitors, Mg²⁺ and ruthenium red. In the reconstituted ATPase proteoliposomes, the Ca²⁺ efflux rates are the same regardless of digestion (T₂); also, efflux is not affected by the presence or absence of Mg²⁺ + ruthenium red. These results indicate that T₂ cleavage causes uncoupling of the Ca²⁺-pump from ATP hydrolytic activity.A theoretical model is developed in order to fit the extent of tryptic digestion of the A fragment of the (Ca²⁺ + Mg²⁺)-ATPase polypeptide with the loss of Ca²⁺-transport. Fits of the theoretical equations to the data are consistent with that Ca²⁺-transport system appears to require a dimer of the polypeptide (Ca²⁺ + Mg²⁺)-ATPase.     

Sensory neurons derived from diabetic rats have diminished internal Ca2+ stores linked to impaired re-uptake by the endoplasmic reticulum

Distal symmetrical sensory neuropathy in diabetes involves the dying back of axons, and the pathology equates with axonal dystrophy generated under conditions of aberrant Ca²⁺ signalling. Previous work has described abnormalities in Ca²⁺ homoeostasis in sensory and dorsal horn neurons acutely isolated from diabetic rodents. We extended this work by testing the hypothesis that sensory neurons exposed to long-term Type 1 diabetes in vivo would exhibit abnormal axonal Ca²⁺ homoeostasis and focused on the role of SERCA (sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase). DRG (dorsal root ganglia) sensory neurons from age-matched normal and 3–5-month-old STZ (streptozotocin)-diabetic rats (an experimental model of Type 1 diabetes) were cultured. At 1–2 days in vitro an array of parameters were measured to investigate Ca²⁺ homoeostasis including (i) axonal levels of intracellular Ca²⁺, (ii) Ca²⁺ uptake by the ER (endoplasmic reticulum), (iii) assessment of Ca²⁺ signalling following a long-term thapsigargin-induced blockade of SERCA and (iv) determination of expression of ER mass and stress markers using immunocytochemistry and Western blotting. KCl- and caffeine-induced Ca²⁺ transients in axons were 2-fold lower in cultures of diabetic neurons compared with normal neurons indicative of reduced ER calcium loading. The rate of uptake of Ca²⁺ into the ER was reduced by 2-fold (P<0.05) in diabetic neurons, while markers for ER mass and ER stress were unchanged. Abnormalities in Ca²⁺ homoeostasis in diabetic neurons could be mimicked via long-term inhibition of SERCA in normal neurons. In summary, axons of neurons from diabetic rats exhibited aberrant Ca²⁺ homoeo<1?show=[fo]?>stasis possibly triggered by sub-optimal SERCA activity that could contribute to the distal axonopathy observed in diabetes.     

NaCl-elicited,vacuolar Ca2+ release facilitates prolonged cytosolic Ca2+ signaling in the salt response of Populus euphratica cells

High environmental salt elicits an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt) in plants, which is generated by extracellular Ca²⁺ influx and Ca²⁺ release from intracellular stores, such as vacuole and endoplasmic reticulum. This study aimed to determine the physiological mechanisms underlying Ca²⁺ release from vacuoles and its role in ionic homeostasis in Populus euphratica. In vivo Ca²⁺ imaging showed that NaCl treatment induced a rapid elevation in [Ca²⁺]_cyt, which was accompanied by a subsequent release of vacuolar Ca²⁺. In cell cultures, NaCl-altered intracellular Ca²⁺ mobilization was abolished by antagonists of inositol (1, 4, 5) trisphosphate (IP₃) and cyclic adenosine diphosphate ribose (cADPR) signaling pathways, but not by slow vacuolar (SV) channel blockers. Furthermore, the NaCl-induced vacuolar Ca²⁺ release was dependent on extracellular ATP, extracellular Ca²⁺ influx, H₂O₂, and NO. In vitro Ca²⁺ flux recordings confirmed that IP₃, cADPR, and Ca²⁺ induced substantial Ca²⁺ efflux from intact vacuoles, but this vacuolar Ca²⁺ flux did not directly respond to ATP, H₂O₂, or NO. Moreover, the IP₃/cADPR-mediated vacuolar Ca²⁺ release enhanced the expression of salt-responsive genes that regulated a wide range of cellular processes required for ion homeostasis, including cytosolic K⁺ maintenance, Na⁺ and Cl⁻ exclusion across the plasma membrane, and Na⁺/H⁺ and Cl⁻/H⁺ exchanges across the vacuolar membrane.     

Endothelin-1 induces intracellular [Ca2+] increase via Ca2+ influx through the L-type Ca2+ channel, Ca2+-induced Ca2+ release and a pathway involving ETA receptors, PKC, PKA and AT1 receptors in cardiomyocytes

Using fura-2-acetoxymethyl ester (AM) fluorescence imaging and patch clamp techniques, we found that endothelin-1 (ET-1) significantly elevated the intracellular calcium level ([Ca²⁺]_i) in a dose-dependent manner and activated the L-type Ca²⁺ channel in cardiomyocytes isolated from rats. The effect of ET-1 on [Ca²⁺]_i elevation was abolished in the presence of the ET_A receptor blocker BQ123, but was not affected by the ET_B receptor blocker BQ788. ET-1-induced an increase in [Ca²⁺]_i, which was inhibited 46.7% by pretreatment with a high concentration of ryanodine (10 μmol/L), a blocker of the ryanodine receptor. The ET-1-induced [Ca²⁺]_i increase was also inhibited by the inhibitors of protein kinase A (PKA), protein kinase C (PKC) and angiotensin type 1 receptor (AT1 receptor). We found that ET-1 induced an enhancement of the amplitude of the whole cell L-type Ca²⁺ channel current and an increase of open-state probability (NPo) of an L-type single Ca²⁺ channel. BQ123 completely blocked the ET-1-induced increase in calcium channel open-state probability. In this study we demonstrated that ET-1 regulates calcium overload through a series of mechanisms that include L-type Ca²⁺ channel activation and Ca²⁺-induced Ca²⁺ release (CICR). ETA receptors, PKC, PKA and AT1 receptors may also contribute to this pathway. Supported by the National Natural Science Foundation of China (Grant No. 200830870910).     

Formyl-peptide receptor like 1: a potent mediator of the Ca2+ release-activated Ca2+ current ICRAC

In electrically non-excitable cells, one major source of Ca²⁺ influx is through the store-operated (or Ca²⁺ release-activated Ca²⁺) channel by which the process of emptying the intracellular Ca²⁺ stores results in the activation of Ca²⁺ channels in the plasma membrane. Using both whole-cell patch-clamp and Ca²⁺ imaging technique, we describe the electrophysiology mechanism underlying formyl-peptide receptor like 1 (FPRL1) linked to intracellular Ca²⁺ mobilization. The FPRL1 agonists induced Ca²⁺ release from the endoplasmic reticulum and subsequently evoked I_CRAC-like currents displaying fast inactivation in K562 erythroleukemia cells which expresses FPRL1, but had almost no effect in K562 cells treated with FPRL1 RNA-interference and HEK293 cells which showed no FPRL1 expression. The currents were impaired after either complete store depletion by the sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin, or after inhibition of PLC by U73122. Our results present the first evidence that FPRL1 is a potent mediator in the activation of CRAC channels.     

IP3 receptors and Ca2+ entry

Inositol 1,4,5-trisphosphate receptors (IP₃R) are the most widely expressed intracellular Ca²⁺ release channels. Their activation by IP₃ and Ca²⁺ allows Ca²⁺ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca²⁺] may directly regulate cytosolic effectors or fuel Ca²⁺ uptake by other organelles, while the decrease in ER luminal [Ca²⁺] stimulates store-operated Ca²⁺ entry (SOCE). We are close to understanding the structural basis of both IP₃R activation, and the interactions between the ER Ca²⁺-sensor, STIM, and the plasma membrane Ca²⁺ channel, Orai, that lead to SOCE. IP₃Rs are the usual means through which extracellular stimuli, through ER Ca²⁺ release, stimulate SOCE. Here, we review evidence that the IP₃Rs most likely to respond to IP₃ are optimally placed to allow regulation of SOCE. We also consider evidence that IP₃Rs may regulate SOCE downstream of their ability to deplete ER Ca²⁺ stores. Finally, we review evidence that IP₃Rs in the plasma membrane can also directly mediate Ca²⁺ entry in some cells.     

The voltage-gated Ca2+ channel is the Ca2+ sensor of fast neurotransmitter release

Previously it demonstrated that in the absence of Ca²⁺ entry, evoked secretion occurs neither by membrane depolarization, induction of [Ca²⁺] _i rise, nor by both combined (Ashery, U., Weiss, C., Sela, D., Spira, M. E., and Atlas, D. (1993). Receptors Channels 1:217–220.). These studies designate Ca²⁺ entry as opposed to [Ca²⁺] _i rise, essential for exocytosis. It led us to propose that the channel acts as the Ca²⁺ sensor and modulates secretion through a physical and functional contact with the synaptic proteins. This view was supported by protein–protein interactions reconstituted in the Xenopus oocytes expression system and release experiments in pancreatic cells (Barg, S., Ma, X., Elliasson, L., Galvanovskis, J., Gopel, S. O., Obermuller, S., Platzer, J., Renstrom, E., Trus, M., Atlas, D., Streissnig, G., and Rorsman, P. (2001). Biophys. J.; Wiser, O., Bennett, M. K., and Atlas, D. (1996). EMBO J. 15:4100–4110; Wiser, O., Trus, M., Hernandez, A., Renström, E., Barg, S., Rorsman, P., and Atlas, D. (1999). Proc. Natl. Acad. Sci. U.S.A. 96:248–253). The kinetics of Ca_v1.2 (Lc-type) and Ca_v2.2 (N-type) Ca²⁺ channels were modified in oocytes injected with cRNA encoding syntaxin 1A and SNAP-25. Conserved cysteines (Cys271, Cys272) within the syntaxin 1A transmembrane domain are essential. Synaptotagmin I, a vesicle-associated protein, accelerated the activation kinetics indicating Ca_v2.2 coupling to the vesicle. The unique modifications of Ca_v1.2 and Ca_v2.2 kinetics by syntaxin 1A, SNAP-25, and synaptotagmin combined implied excitosome formation, a primed fusion complex of the channel with synaptic proteins. The Ca_v1.2 cytosolic domain Lc_753–893, acted as a dominant negative modulator, competitively inhibiting insulin release of channel-associated vesicles (CAV), the readily releasable pool of vesicles (RRP) in islet cells. A molecular mechanism is offered to explain fast secretion of vesicles tethered to SNAREs-associated Ca²⁺ channel. The tight arrangement facilitates the propagation of conformational changes induced during depolarization and Ca²⁺-binding at the channel, to the SNAREs to trigger secretion. The results imply a rapid Ca²⁺-dependent CAV (RRP) release, initiated by the binding of Ca²⁺ to the channel, upstream to intracellular Ca²⁺ sensor thus establishing the Ca²⁺ channel as the Ca²⁺ sensor of neurotransmitter release.