Elevation of intracellular calcium ions is essential for the H2O2-induced activation of SAPK/JNK but not for that of p38 and ERK in Chinese hamster V79 cells

The mitogen-activated protein kinases (MAPK), including stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38, and extracellular signal-related kinase (ERK), are believed to be important biomolecules in cell proliferation, survival, and apoptosis induced by extracellular stimuli. In Chinese hamster V79 cells exposed to hydrogen peroxide (H2O2), we recently demonstrated that SAPK/JNK was activated by tyrosine kinase and intracellular Ca2+ ([Ca2+]i). In this study, we report that [Ca2+]i release from intracellular stores is important in the activation of SAPK/JNK but not p38 and ERK. H2O2-induced elevation of [Ca2+]i was observed in Ca2+-free medium. Pretreatment with thapsigargin, a Ca2+-ATPase inhibition of endoplasmic reticulum (ER), did not influence H2O2-induced elevation of [Ca2+]i in the absence of external Ca2+. An intracellular Ca2+ chelator (BAPTA-AM) inhibited H2O2-induced phosphorylation of SAPK/JNK, but an extracellular Ca2+ chelator (EDTA) or a Ca2+ entry blocker (NiCl2) did not. Activation of p38 and ERK in V79 cells exposed to H2O2 was observed in the presence of these inhibitors. These results suggest that [Ca2+]i release from intracellular stores such as mitochondria or nuclei but not ER, occurred after H2O2 treatment and Ca2+-dependent tyrosine kinase-induced activation of SAPK/JNK, although [Ca2+]i was unnecessary for the H2O2-induced activation of p38 and ERK.     

Electrophysiological properties of gastric pacemaker potentials.

Electrophysiological properties of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) within the guinea-pig gastric antrum are reviewed briefly. Pacemaker potentials consist of two components, a primary component forming a transient depolarization with a rapidly rising initial phase, followed by a secondary component as a plateau with sustained depolarization. The primary component is inhibited by low [Ca2+]o solutions or depolarization of the membrane with high [K+]o solutions. This inhibition could be mimicked by chelating [Ca2+]i with BAPTA-AM, suggesting that this component is produced by activation of voltage-dependent Ca2+ permeable channels. The plateau component is inhibited by low [Cl-]o solution or DIDS, an inhibitor of Ca2+-activated Cl(-)-channels, suggesting that this component is formed by Ca2+-activated Cl(-)-currents. Reduction of Ca2+ release from internal stores by inhibiting the internal Ca-pump with cyclopiazonic acid results in a shortened duration of the plateau component, with no alteration in the rate of rise of the primary component. 2-APB, an inhibitor of the IP3-receptor mediated Ca2+ release from internal stores, abolishes pacemaker potentials, suggesting that the release of Ca2+ from internal IP3-sensitive Ca2+ stores is required for generation of pacemaker potentials. CCCP, a mitochondrial protonophore, depolarizes the membrane and abolishes pacemaker potentials, suggesting that mitochondrial Ca2+ handling functions may be coupled with generation of pacemaker potentials. These results indicate that the two components of pacemaker potentials are generated by different mechanisms; the primary component may be produced by activation of voltage-dependent Ca2+-permeable channels, while the plateau component may be produced by the opening of Ca2+-activated Cl(-)-channels. It is hypothesized that pacemaker potentials are initiated by depolarization of the membrane due to generation of unitary potentials in response to mitochondrial Ca2+ handling. Activation of voltage-dependent Ca2+ influx, IP3-receptor mediated Ca2+ release from the internal stores and Ca2+-activated Cl(-)-channels may be involved as successive steps downstream to the generation of unitary potentials.     

Components of pacemaker potentials recorded from the guinea pig stomach antrum

Pacemaker potentials recorded intracellularly from the guinea pig stomach consisted of initial primary and following plateau components. Inhibition of the internal Ca2+ store pump with cyclopiazonic acid depolarized the membrane and inhibited the plateau component of pacemaker potentials. 2-aminoethoxydiphenyl borate (an inhibitor of IP3-induced Ca2+ release) and carbonyl cyanide m-chlorophenyl-hydrazone (a mitochondrial protonophore) depolarized the membrane and abolished pacemaker potentials. Low [Ca2+]o solution reduced the frequency and rate of rise of pacemaker potentials, and the effects were mimicked by BAPTA-AM (an intracellular Ca2+ chelator). 4,4-diisothiocyanatostilbene-2,2-disulphonic acid and low [Cl-]o solution inhibited the plateau component of pacemaker potentials. Depolarization of the membrane with high [K+]o solutions increased the frequency and reduced the dV/dt(max) of pacemaker potentials. During high-[K+]o-induced depolarization, cyclopiazonic acid abolished pacemaker potentials. Caffeine, forskolin, papaverine, 8-bromo-cGMP and (+/-)S-nitroso-N-acetylpenicillamine (SNAP) inhibited the plateau component, with no alteration of the primary component. It is concluded that the primary and plateau components of pacemaker potentials are related to voltage-gated Ca2+ influx and Ca2+-activated Cl- channels, respectively, and cyclic nucleotides inhibit mainly the latter. Pacemaker potentials may be generated by the release of Ca2+ from internal stores through excitation of inositol 1,4,5-trisphosphate receptors, coupled with Ca2+ uptake into mitochondria.     

Metabotropic receptor-mediated Ca2+ signaling elevates mitochondrial Ca2+ and stimulates oxidative metabolism in hippocampal slice cultures

Metabotropic receptors modulate numerous cellular processes by intracellular Ca2+ signaling, but less is known about their role in regulating mitochondrial metabolic function within the CNS. In this study, we demonstrate in area CA3 of rat organotypic hippocampal slice cultures that glutamatergic, serotonergic, and muscarinic metabotropic receptor ligands, namely trans-azetidine-2,4-dicarboxylic acid, alpha-methyl-5-hydroxytryptamine, and carbachol, transiently increase mitochondrial Ca2+ concentration ([Ca2+]m) as recorded by changes in Rhod-2 fluorescence, stimulate mitochondrial oxidative metabolism as revealed by elevations in NAD(P)H fluorescence, and induce K+ outward currents as monitored by rapid increases in extracellular K+ concentration ([K+]o). Carbachol (1-1,000 microM) elevated NAD(P)H fluorescence by 相似文献   

Glutamate regulates IP3-type and CICR stores in the avian cochlear nucleus

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.     

Ryanodine-, IP3- and NAADP-dependent calcium stores control acetylcholine release

Injections of inositol trisphosphate (IP3) or nicotinamide adenine dinucleotide phosphate (NAADP) into the presynaptic neurone of an identified cholinergic synapse in the buccal ganglion of Aplysia californica increased the amplitude of the inhibitory postsynaptic current evoked by a presynaptic action potential. This suggests that Ca2+ release from various Ca2+ stores can modulate acetylcholine (ACh) release. Specific blockade of the calcium-induced calcium release (CICR) mechanism with ryanodine, or of IP3-induced calcium release with heparin, abolished the effects of IP3, but not the effects of NAADP, suggesting the presence of an intracellular Ca2+ pool independent of those containing ryanodine receptors (RyR) or IP3 receptors. To reinforce electrophysiological observations, intracellular [Ca2+]i changes were measured using the fluorescent dye rhod-2. Injections of cyclic ADP-ribose (an activator of RyR), IP3 or NAADP into the presynaptic neurone induced transient increases in the free intracellular Ca2+ concentration. RyR- and IP3-induced increases were prevented by application of respective selective antagonists but not NAADP-induced increases. Our results show that RyR-dependent, IP3-dependent, and NAADP-dependent Ca2+ stores are present in the same presynaptic terminal but are differently involved in the regulation of the presynaptic Ca2+ concentration that triggers transmitter release.     

Characterization of platelet cytoplasmic Ca2+ mobilization in patients with congenital cyclo-oxygenase deficiency and with defective platelet aggregation to A23187

Agonist-induced platelet cytoplasmic Ca2+ concentrations ([Ca2+]i) in patients with congenital cyclo-oxygenase deficiency (A) and with impaired aggregation to A23187 (B) were measured with aequorin in the presence or absence of extracellular Ca2+. The influence of TMB-8 or ONO3708 on agonist-induced [Ca2+]i in those platelets was also investigated. In Patient 1, there was a single aequorin luminescence peak in response to arachidonate, which was a thromboxane A2(TXA2) independent Ca2+ influx. The luminescence peak due to the formation of TXA2 was not detectable. The A23187-induced [Ca2+] i was decreased in the presence of extracellular Ca2+, but was within normal limits in the absence of extracellular Ca2+. A thrombin or STA2-induced elevation of [Ca2+] i was always within normal limits under any conditions. These results suggest that cyclo-oxygenase activity (CO activity) contributes to the A23187-induced Ca2+ influx, but does not contribute to the Ca2+ release from intracellular stores, and that the thrombin or STA2-induced Ca2+ influx and release do not depend on the CO activity. In Patient 2, the time lag from the addition of A23187 to the aequorin luminescence peak was found both in the presence and absence of extracellular Ca2+, which was more obvious in the latter. This A23187-induced elevation of [Ca2+] i disappeared after treatment of the platelets with TMB-8 in the absence of extracellular Ca2+, which is rarely seen in normal platelets. The most striking finding was that the thrombin-induced rise in [Ca2+] i in the absence of extracellular Ca2+ was not detectable. These findings might be closely related to abnormal platelet function in this patient.     

Synergistic effects of ATP on oxytocin-induced intracellular Ca2+ response in mouse mammary myoepithelial cells

An increase in intracellular Ca2+ ([Ca2+]i) is essential for mammary myoepithelial cells to contract, leading to milk ejection during lactation. In this study, the intracellular signaling leading to the increase in [Ca2+]i in cultured myoepithelial cells from mouse lactating mammary glands was investigated using fura-2 fluorescence ratiometry. [Ca2+]i increased in cultured myoepithelial cells in response to either oxytocin (1 nM) or ATP (10 microM), and the cells then contracted. These [Ca2+]i responses were diminished by treatment with an inhibitor of phospholipase C (> or = 1 microM U73122). Intracellular application of inositol 1,4,5-trisphosphate (IP3: 10 or 100 microM) increased [Ca2+]i. Pretreatment with pertussis toxin (PTX: 0.1 or 1 microgram/ml) inhibited the [Ca2+]i response to ATP, but had less of an effect on the response to oxytocin. These results indicate that oxytocin and purinergic receptors are coupled to PTX-insensitive and PTX-sensitive G proteins, respectively, and that their activation leads to the increase in [Ca2+]i through the release of Ca2+ from IP3-sensitive intracellular stores via the inositol-phospholipid signaling pathway. Furthermore, we found that the [Ca2+]i responses to oxytocin at physiological doses (0.01-0.1 nM) were augmented in the presence of a sub-responsive dose of ATP (1 microM). The activation of purinergic receptors may facilitate myoepithelial cell contraction in milk-ejection responses.     

L-type Ca2+ channels are not involved in coronary endothelial Ca2+ influx mechanism responsible for endothelium-dependent relaxation

Effects of L-type Ca2+ channel blockers on intracellular Ca2+ concentration ([Ca2+]i) changes evoked by the stimulations which cause endothelium-dependent relaxation were examined in freshly isolated pig coronary endothelial cells using fura-2 fluorescent analysis. Substance P and bradykinin produced endothelium-dependent relaxations of pig coronary arteries. The relaxations were inhibited significantly but not completely by N(omega)-nitro-L-arginine (L-NNA) or aspirin, suggesting that nitric oxide (NO), prostacyclin (PGI2) and endothelium-derived hyperpolarizing factor (EDHF) were involved in the responses. Both substance P and bradykinin elevated coronary endothelial [Ca2+]i in a biphasic manner: An initial transient increase was observed within a minute, which was followed by the subsequent sustained increase declining with time. In the medium without Ca2+, substance P-induced elevation of [Ca2+]i was markedly reduced. L-type Ca2+ channel blockers (nicardipine, diltiazem and verapamil) did not affect substance P-induced increase in endothelial [Ca2+]i. In consistent with this finding, Bay k 8644 failed to increase [Ca2+]i in partially depolarized endothelial cells. In contrast, substance P-induced elevation of endothelial [Ca2+]i was suppressed in high K+ solutions. These findings indicate that: (1) Substance P and bradykinin relax pig coronary artery via production/release of NO, PGI2 and EDHF from the endothelium; (2) The synthesis and release of these endothelium-derived factors are accompanied by an increase in endothelial [Ca2+]i; (3) Activation of L-type Ca2+ channels is not involved in coronary endothelial elevation of [Ca2+]i responsible for the production/release of these endothelium-derived factors. L-type Ca2+ channel blockers seem to be advantageous in the application for the disorders of coronary circulation with respect to that they do not prevent endothelial function to produce/release of endogenous vasorelaxants.     

Coexistence of functional IP(3) and ryanodine receptors in vagal sensory neurons and their activation by ATP

Intracellular photorelease of caged D-myo-inositol 1,4,5-trisphosphate (IP(3)), caffeine application, and immunofluorescence confocal microscopy were used to determine that D-myo-inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) coexist in rabbit vagal sensory nodose ganglion neurons (NGNs). ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in cytosolic free Ca(2+) concentration (Ca(2+) transients). ATP applied in Ca(2+)-free physiological saline elicited Ca(2+) transients that averaged approximately 70% of the amplitude of transients evoked in the presence of extracellular Ca(2+). The component of the ATP-evoked Ca(2+) transient that was independent of extracellular Ca(2+) corresponds to Ca(2+) release from intracellular stores. This release component was sensitive to the pharmacological antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), U73122, neomycin, and heparin (13.5-15 kD), indicating that P2 purinoreceptors (P2Y) and the IP(3) signaling pathway are required for ATP-evoked Ca(2+) release. Additionally, a portion of ATP-evoked Ca(2+) release was inhibited by ryanodine, a selective blocker of RyRs. The ryanodine-insensitive component (approximately 70%) of ATP-evoked Ca(2+) release corresponds to IP(3)-induced Ca(2+) release via IP(3)Rs, while the ryanodine-sensitive component (approximately 30%) corresponds to consequent Ca(2+)-induced Ca(2+) release (CICR) via RyRs. These results indicate that functional IP(3)Rs and RyRs coexist in nodose neurons and that both IP(3)-induced Ca(2+) release and CICR can be activated by ATP.     

Ca2+ and electrolyte mobilization following agonist application to the pancreatic β cell line HIT

We have investigated intracellular Ca2+ mobilization in oscillations of cytoplasmic Ca2+ in response to glucagon-like peptide 1 (GLP-1) and glucose in clonal HIT insulinoma cells with a confocal laser-scanning microscope (CLSM). We also used electron probe X-ray microanalysis to determine the GLP-1- and glucose-induced changes in electrolyte levels in the cytoplasm and insulin granules of the cells. GLP-1 produced 10- to 35-s duration oscillations in cytoplasmic Ca2+ concentration ([Ca2+]i), both with and without Ca2+ in the extracellular solution, suggesting that Ca2+ is mobilized from intracellular Ca2+ stores, namely secretory granules. Glucose caused 1- to 3-min duration oscillatory increases in [Ca2+]i when the extracellular solution contained Ca2+. When the cells were cultured without Ca2+ (no Ca2+ added, 1 mM EGTA), an oscillatory [Ca2+]i increase of amplitude and short duration (12-35 s) was produced by 11 mM glucose, and the oscillation was inhibited by ruthenium red. X-ray microanalysis showed that stimulation with glucose increased the total Ca concentration in the cytoplasm and decreased it in the insulin granules with and without Ca2+ in the extracellular solution. The application of glucose significantly decreased K, and increased Na and C1 in the cytoplasm when the extracellular solution contained Ca2+. Our result also suggests that the [Ca2+]i oscillation induced by glucose is involved in the release of Ca2+ from intracellular Ca2+ stores through the ryanodine receptor, which is blocked by ruthenium red, and/or through the inositol trisphosphate receptor that may be present in the membrane of insulin granules.     

Effect of alpha1-adrenergic stimulation of Cl- secretion and signal transduction in exocrine glands (Rana esculenta)

In the present work, the effect of stimulation of alpha-adrenergic receptors on Cl- secretion via exocrine frog skin glands was investigated. The alpha-adrenergic stimulation was performed by addition of the adrenergic agonist noradrenaline in the presence of the beta-adrenergic antagonist propranolol. In the presence of propranolol, noradrenaline had no effect on the cellular cAMP content. The Cl- secretion was measured as the amiloride-insensitive short circuit current (ISC). Addition of noradrenaline induced a biphasic increase in the ISC. The increase in ISC coincided with an increase in the net 36Cl- secretion. The noradrenaline-induced increase in ISC was dose-dependent with an EC50 of 13 +/- 0.3 microM. Epifluorescence microscopic measurements of isolated, fura-2-loaded frog skin gland acini were used to characterize the intracellular calcium ([Ca2+]i) response. Application of noradrenaline induced a biphasic [Ca2+]i response, which was dose-dependent with an EC50 of 11 +/- 6 microM. The Ca2+ plateau unlike the peak-response was sensitive to removal of Ca2+ from the extracellular medium. The noradrenaline-induced increase in the Cl- secretion as well as in [Ca2+]i was sensitive to the alpha1-adrenergic antagonist prazosine. Ryanodine and caffeine had no effect on [Ca2+]i indicating that the release was independent of ryanodine-sensitive Ca2+ stores. Noradrenaline mediated a significant increase in the cellular inositol 1,4,5-trisphosphate (IP3) content suggesting that the signal transduction pathway leading to the noradrenaline-induced increase in Ca2+ involved IP3 and a release of Ca2+ from IP3-sensitive stores.     

The cytosolic free calcium in anti-mu-stimulated human B cells is derived partly from extracellular medium and partly from intracellular stores

The inositol phospholipid metabolism and the increase in cytosolic free Ca2+ concentration ([Ca2+]i) into the cell are recognized as two important events in the anti-mu-induced B cell activation. The anti-mu stimulation caused the [3H]inositol incorporation and also a rapid increase in [Ca2+]i from 85 nM to 285 nM. This signal returned to baseline a few minutes after stimulation. By using the fluorescent indicator quin-2 we demonstrated that this [Ca2+]i uptake was derived part from extracellular medium and part from intracellular stores. Both EGTA (a calcium chelator) and TMB.8 (a drug which interferes with Ca2+ sequestration by smooth endoplasmic reticulum) partially suppressed the intracellular Ca2+ uptake and were fully inhibitory when added together. The role of Ca2+ from intracellular stores may also be evidenced in calcium-free experiments, or in permeabilized experiments using exogenous inositol 1,4,5-trisphosphate (IP3, the putative mobilizer of intracellular Ca2+). Preventing the increase in [Ca2+]i also prevents the apparition of early activation makers. These results are consistent with the hypothesis that the Ca2+ increase in B cells stimulated by anti-mu is caused by the generation of IP3 during the phosphatidyl-inositol metabolism and also by the entry of extracellular Ca2+ through the plasma membrane.     

Evidence for P2Y-type ATP receptors on the serosal membrane of frog skin epithelium

The present study presents the first evidence for P2Y-type adenosine 5'-triphosphate (ATP) receptors on the basolateral membranes of frog skin epithelial cells. Cytosolic calcium ([Ca2+]i) was measured with fura-2 and Calcium-Green-1 using epifluorescence microscopy and confocal laser scanning microscopy respectively. In the presence of Ca2+ in the solutions ATP increased [Ca2+]i. The increase in [Ca2+]i was due to the agonist activity of ATP and not to the activity of the potential products of ATP metabolism, i.e. adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) or adenosine, as shown by a comparison of the magnitude of the increases in [Ca2+]i caused by the various compounds. The rise in [Ca2+]i was predominantly monophasic at low ATP concentrations (below 100 microM). At higher concentrations the initial spike was followed by a plateau phase. In the absence of Ca2+ in the extracellular solution ATP caused Ca2+ release from intracellular stores. This could be inhibited by pre-treatment of the tissue with 1 microM thapsigargin, an inhibitor of the endoplasmic reticulum calcium ATPase. The nucleotide uridine 5'-triphosphate (UTP) had similar effects on [Ca2+]i although the plateau level of the [Ca2+]i response was higher with this P2Y agonist. Confocal laser scanning microscopy showed that all cell layers of the epithelium responded to ATP. Our data indicates that serosal ATP acts on serosal P2Y-type receptors in frog skin epithelium. This is the first evidence of a phospholipase C-coupled receptor in this tissue.     

Discrete influx events refill depleted Ca2+ stores in a chick retinal neuron.

The depletion of ER Ca2+ stores, following the release of Ca2+ during intracellular signalling, triggers the Ca2+ entry across the plasma membrane known as store-operated calcium entry (SOCE). We show here that brief, local [Ca2+]i increases (motes) in the thin dendrites of cultured retinal amacrine cells derived from chick embryos represent the Ca2+ entry events of SOCE and are initiated by sphingosine-1-phosphate (S1P), a sphingolipid with multiple cellular signalling roles. Externally applied S1P elicits motes but not through a G protein-coupled membrane receptor. The endogenous precursor to S1P, sphingosine, also elicits motes but its action is suppressed by dimethylsphingosine (DMS), an inhibitor of sphingosine phosphorylation. DMS also suppresses motes induced by store depletion and retards the refilling of depleted stores. These effects are reversed by exogenously applied S1P. In these neurons formation of S1P is a step in the SOCE pathway that promotes Ca2+ entry in the form of motes.     

Mitochondrial modulation of Ca2+ -induced Ca2+ -release in rat sensory neurons

Ca2+ -induced Ca2+ -release (CICR) from ryanodine-sensitive Ca2+ stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]i oscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+ regulatory mechanisms at the cellular level. Oscillations depended on Ca2+ uptake and release from the endoplasmic reticulum (ER) and Ca2+ influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+ terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+ sequestration by the ER Ca2+ pump. Neither CICR threshold nor Ca2+ clearance by the plasma membrane Ca2+ pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+ exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]i slowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+ signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.     

Mitochondrial Ca2+ flux is a critical determinant of the Ca2+ dependence of mast cell degranulation

An increase in intracellular Ca2+ ([Ca2+]i) is necessary for mast cell exocytosis, but there is controversy over the requirement for Ca2+ in the extracellular medium. Here, we demonstrate that mitochondrial function is a critical determinant of Ca2+ dependence. In the presence of extracellular Ca2+, mitochondrial metabolic inhibitors, including rotenone, antimycin A, and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), significantly reduced degranulation induced by immunoglobulin E (IgE) antigen or by thapsigargin, as measured by beta-hexosaminidase release. In the absence of extracellular Ca2+; however, antimycin A and FCCP, but not rotenone, enhanced, rather than reduced, degranulation to a maximum of 76% of that observed in the presence of extracellular Ca2+. This enhancement of extracellular, Ca2+-independent degranulation was concomitant with a rapid collapse of the mitochondrial transmembrane potential. Mitochondrial depolarization did not enhance degranulation induced by thapsigargin, irrespective of the presence or absence of extracellular Ca2+. IgE antigen was more effective than thapsigargin as an inducer of [Ca2+]i release, and mitochondrial depolarization augmented IgE-mediated but not thapsigargin-induced Ca2+ store release and mitochondrial Ca2+ ([Ca2+]m) release. Finally, atractyloside and bongkrekic acid [an agonist and an antagonist, respectively, of the mitochondrial permeability transition pore (mPTP)], respectively, augmented and reduced IgE-mediated Ca2+ store release, [Ca2+]m release, and/or degranulation, whereas they had no effects on thapsigargin-induced Ca2+ store release. These data suggest that the mPTP is involved in the regulation of Ca2+ signaling, thereby affecting the mode of mast cell degranulation. This finding may shed light on a new role for mitochondria in the regulation of mast cell activation.     

Ca2+ influx in human T lymphocytes is induced independently of inositol phosphate production by mobilization of intracellular Ca2+ stores. A study with the Ca2+ endoplasmic reticulum-ATPase inhibitor thapsigargin.

Thapsigargin (TG), a sesquiterpene lactone and non-phorbol 12-myristate 13-acetate tumor promoter, stimulates a rapid increase in intracellular free Ca2+ [( Ca2+]i) in human T lymphocytes clone P28. The [Ca2+]i response to TG is sustained in the presence of 1 mM extracellular Ca2+, while it becomes transient in Ca2(+)-free medium suggesting that TG activates both the release of Ca2+ from intracellular stores and the entry of Ca2+ from extracellular spaces. TG-induced Ca2+ influx is completely abolished after cell depolarization caused by increased extracellular concentrations of K+. The rise in [Ca2+]i stimulated by TG occurs in the absence of detectable production of inositol phosphates. Moreover, TG does not alter the early biochemical events of T cell activation triggered through the CD2 or the CD3 T cell antigens. Indeed, both inositol phosphate production and intracellular pH increase induced by specific monoclonal antibodies (mAb) remain unchanged after TG treatment. These data suggest that in human T lymphocytes TG releases Ca2+ from an intracellular pool by a mechanism which is independent of the phospholipase C metabolic pathway. Preincubation with TG of T cell clone P28 empties both the CD2 and the CD3-sensitive intracellular Ca2+ pool(s). Conversely, prestimulation of T cell clone P28 by CD3 or CD2-specific mAb inhibits the Ca2(+)-mobilizing effect of TG. Thus it appears that TG and CD2- or CD3-specific mAb mobilize Ca2+ from common Ca2+ pool(s). Taken together, these results demonstrate that Ca2+ influx in human T cells may be linked to mobilization of intracellular Ca2+ pools and by a mechanism independent of phosphoinositide hydrolysis. They further indicate that the release of intracellular Ca2+ pool(s) may play a major role in the opening of cell membrane Ca2+ channels observed during the CD2- or CD3-induced stimulation of human T lymphocytes.     

H2O2 enhances Ca2+ release from osteoblast internal stores

The physiological activity of osteoblasts is known to be closely related to increased intracellular Ca2+ activity ([Ca2+]i) in osteoblasts. The cellular regulation of [Ca2+]i in osteoblasts is mediated by Ca2+ movements associated with Ca2+ release from intracellular Ca2+ stores, and transmembrane Ca2+ influx via Na+-Ca2+ exchanger, and Ca2+ ATPase. Reactive oxygen species, such as H2O2, play an important role in the regulation of cellular functions, and act as signaling molecules or toxins in cells. In this study, we investigated the effects of H2O2 on cellular Ca2+ regulation in osteoblasts by measuring intracellular Ca2+ activities using cellular calcium imaging techniques. Osteoblasts were isolated from the femurs and tibias of neonatal rats, and cultured for 7 days. The cultured osteoblasts were loaded with a Ca2+-sensitive fluorescent dye, Fura-2, and fluorescence images were monitored using a cooled CCD camera, and subsequently analyzed using image analyzing software. The results obtained are as follows: (1) The osteoblasts with lower basal Ca2+ activities yielded a transient Ca2+ increase, a Ca2+ spike, while osteoblasts with higher basal Ca2+ activities showed a continuous increase in [Ca2+]i leading to cell death. (2) Ca2+ spikes, generated after removing Na+ from superfusing solutions, were blocked by H2O2 and this was followed by a sustained increase in Ca2+ activity. (3) ATP- induced Ca2+ spikes were inhibited by pretreating with H2O2 and this was followed by a continuous increase of [Ca2+]i. When cells were pretreated with the exogenous nitric oxide (NO) donor S-Nitroso-N-acetylpenicilance (SNAP, 50 microM), treatments of ATP (1 mM) induced a Ca2+ spike-like increase, but [Ca2+]i did not return to the basal level. (4) The expression of inositol- 1,4,5-triphosphate receptor (IP3R) was enhanced by H2O2. Our results suggest that H2O2 modulates intracellular Ca2+ activity in osteoblasts by increasing Ca2+ release from the intracellular Ca2+ stores.     

Cytoplasmic Ca2+ at low submicromolar concentration stimulates mitochondrial metabolism in rat luteal cells

The cytoplasmic Ca2+ signal is transferred to the mitochondrial matrix and activates mitochondrial dehydrogenases. The requirement for supramicromolar cytoplasmic [Ca2+] ([Ca2+]i) in perimitochondrial microdomains in this response has been suggested. We studied the correlation between [Ca2+]i, mitochondrial [Ca2+] ([Ca2+]m) and mitochondrial formation of reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] in the presence of submicromolar [Ca2+]i in cultured rat "large" luteal cells. [Ca2+]i was monitored fluorimetrically with fura-PE3, [Ca2+]m with rhod-2 and NAD(P)H with autofluorescence. In intact cells, prostaglandin F2alpha, which induces both intracellular Ca2+ release and Ca2+ entry, stimulated mitochondrial NAD(P)H formation. Thapsigargin-induced Ca2+ release and subsequent capacitative Ca2+ entry, both resulting in Ca2+ responses not exceeding 150-200 nM, also enhanced the reduction of pyridine nucleotides. As shown in inhibitor studies, the increased steady-state NAD(P)H level was due to activation of Ca2+-dependent dehydrogenases. [Ca2+]m, measured in permeabilized cells, increased moderately, but significantly, following elevation of [Ca2+]i from 50 to 180 nM, showed a further gradual increase at higher submicromolar [Ca2+]i values and rose steeply at supramicromolar [Ca2+]i. In summary, our results demonstrate that, in a steroid-producing cell type, net mitochondrial Ca2+ uptake and mitochondrial dehydrogenation can be activated even by low submicromolar increases of [Ca2+]i.