Calcium mobilization from the intracellular mitochondrial and nonmitochondrial stores of the rat cerebellum

Changes in the Metabolism of Sphingoid Bases in the Brain and Spinal Cord of Transgenic FUS(1-359) Mice, a Model of Amyotrophic Lateral Sclerosis

The aim of this study was to evaluate changes in the content of sphingoid bases - sphingosine (SPH), sphinganine, and sphingosine-1-phosphate (SPH-1-P) - and in expression of genes encoding enzymes involved in their metabolism in the brain structures (hippocampus, cortex, and cerebellum) and spinal cord of transgenic FUS(1-359) mice. FUS(1-359) mice are characterized by motor impairments and can be used as a model of amyotrophic lateral sclerosis (ALS). Lipids from the mouse brain structures and spinal cord after 2, 3, and 4 months of disease development were analyzed by chromatography/mass spectrometry, while changes in the expression of the SPHK1, SPHK2, SGPP2, SGPL1, ASAH1, and ASAH2 genes were assayed using RNA sequencing. The levels of SPH and sphinganine (i.e., sphingoid bases with pronounced pro-apoptotic properties) were dramatically increased in the spinal cord at the terminal stage of the disease. The ratio of the anti-apoptotic SPH-1-P to SPH and sphinganine sharply reduced, indicating massive apoptosis of spinal cord cells. Significant changes in the content of SPH and SPH-1-P and in the expression of genes related to their metabolism were found at the terminal ALS stage in the spinal cord. Expression of the SGPL gene (SPH-1-P lyase) was strongly activated, while expression of the SGPP2 (SPH-1-P phosphatase) gene was reduced. Elucidation of mechanisms for the regulation of sphingolipid metabolism in ALS will help to identify molecular targets for the new-generation drugs.

Role of Sphingosylphosphorylcholine in Tumor and Tumor Microenvironment

Sphingosylphosphorylcholine (SPC) is a unique type of lysosphingolipid found in some diseases, and has been studied in cardiovascular, neurological, and inflammatory phenomena. In particular, SPC’s studies on cancer have been conducted mainly in terms of effects on cancer cells, and relatively little consideration has been given to aspects of tumor microenvironment. This review summarizes the effects of SPC on cancer and tumor microenvironment, and presents the results and prospects of modulators that regulate the various actions of SPC.

The Many Facets of Sphingolipids in the Specific Phases of Acute Inflammatory Response

This review provides an overview on components of the sphingolipid superfamily, on their localization and metabolism. Information about the sphingolipid biological activity in cell physiopathology is given. Recent studies highlight the role of sphingolipids in inflammatory process. We summarize the emerging data that support the different roles of the sphingolipid members in specific phases of inflammation: (1) migration of immune cells, (2) recognition of exogenous agents, and (3) activation/differentiation of immune cells.

An oxidized metabolite of linoleic acid increases intracellular calcium in rat adrenal glomerulosa cells

EKODE, an epoxy-keto derivative of linoleic acid, was previously shown to stimulate aldosterone secretion in rat adrenal glomerulosa cells. In the present study, we investigated the effect of exogenous EKODE on cytosolic [Ca(2+)] increase and aimed to elucidate the mechanism involved in this process. Through the use of the fluorescent Ca(2+)-sensitive dye Fluo-4, EKODE was shown to rapidly increase intracellular [Ca(2+)] ([Ca(2+)](i)) along a bell-shaped dose-response relationship with a maximum peak at 5 microM. Experiments performed in the presence or absence of Ca(2+) revealed that this increase in [Ca(2+)](i) originated exclusively from intracellular pools. EKODE-induced [Ca(2+)](i) increase was blunted by prior application of angiotensin II, Xestospongin C, and cyclopiazonic acid, indicating that inositol trisphosphate (InsP(3))-sensitive Ca(2+) stores can be mobilized by EKODE despite the absence of InsP(3) production. Accordingly, EKODE response was not sensitive to the phospholipase C inhibitor U-73122. EKODE mobilized a Ca(2+) store included in the thapsigargin (TG)-sensitive stores, although the interaction between EKODE and TG appears complex, since EKODE added at the plateau response of TG induced a rapid drop in [Ca(2+)](i). 9-oxo-octadecadienoic acid, another oxidized derivative of linoleic acid, also increases [Ca(2+)](i), with a dose-response curve similar to EKODE. However, arachidonic and linoleic acids at 10 microM failed to increase [Ca(2+)](i) but did reduce the amplitude of the response to EKODE. It is concluded that EKODE mobilizes Ca(2+) from an InsP(3)-sensitive store and that this [Ca(2+)](i) increase is responsible for aldosterone secretion by glomerulosa cells. Similar bell-shaped dose-response curves for aldosterone and [Ca(2+)](i) increases reinforce this hypothesis.

Heptachlor epoxide induces a non-capacitative type of Ca2+ entry and immediate early gene expression in mouse hepatoma cells

The effects of the organochlorine (OC) liver tumor promoter heptachlor epoxide (HE) and a related non-tumor promoting OC, delta-hexachlorocyclohexane (delta-HCH), on the dynamics of intracellular calcium (Ca2+) were investigated in mouse 1c1c7 hepatoma cells. HE induced a non-capacitative, Ca2+ entry-like phenomenon, which was transient and concentration-dependent with 10 and 50 microM HE. The plasma membrane Ca2+ channel blocker SKF-96365 antagonized this HE-induced Ca2+ entry. delta-HCH failed to induce Ca2+ entry, rather it antagonized the HE-induced Ca2+ entry. Both HE and delta-HCH induced Ca2+ release from endoplasmic reticulum (ER) at treatment concentrations as low as 10 microM; at 50 microM, the former induced 5x as much Ca2+ release as the latter. The HE-induced Ca2+ release from the ER was antagonized using the IP3 receptor/channel blocker xestospongin C, suggesting that HE induces ER Ca2+ release through the IP3 receptor/channel pore. These results show that the effect of HE on cellular Ca2+ mimics that of mitogens such as epidermal and hepatocyte growth factors. They also provide insight into the similarities and differences between tumorigenic and non-tumorigenic OCs, in terms of the mechanisms and the extent of the [Ca2+]i increased by these agents.

Calcium Signals Activated by Arachidonic Acid in Embryonic Chick Ciliary Ganglion Neurons

Arachidonic acid (AA, 20:4) has been reported to modulate a variety of calcium-permeable ionic channels, both in the plasma membrane and in the endoplasmic reticulum. We have studied the effects of AA on calcium signaling in a well-characterized model of developing peripheral neurons, embryonic chick ciliary ganglion neurons in culture. When given at low non-micellar concentrations (5 microM), in the majority of cells AA directly activated a delayed and long-lasting increase in [Ca2+]i, involving both the cytoplasm and the nucleoplasm, that was completely reversed by abolition of extracellular calcium. Other fatty acids (FAs), either saturated like arachidic acid (20:0), or unsaturated like linoleic (18:2) and docosahexaenoic acid (22:6), shared its ability to activate calcium influx. This entry was not suppressed by voltage-dependent calcium channel inhibitors omega-conotoxin and nifedipine, by the voltage-independent calcium channel antagonist LOE-908, by pre-treatment with blockers of AA metabolic pathways or with pertussis toxin. The arachidonate-activated calcium pathway was permeable to Mn2+ and blocked by La3+, Gd3+ and Ni2+. In a neuronal subpopulation, AA at the same concentration was also able to elicit calcium release from thapsigargin-sensitive intracellular stores; we provide evidence that cytochrome P450 epoxygenase is involved in this process.

Extra- and intracellular sphingosylphosphorylcholine promote spontaneous transmitter release from frog motor nerve endings

Similar to phosphatidylinositol bisphosphate, sphingomyelin breakdown generates several lipids, including sphingosylphosphorylcholine (SPC), that are putative signaling molecules. The present study was undertaken to evaluate the involvement of SPC in transmitter release process. Intracellular recordings were made from isolated frog sciatic-sartorius nerve-muscle preparations, and the effects of SPC on neurosecretion in the form of miniature endplate potentials (MEPPs) were assessed. Extracellular application of SPC mixture (D,L-SPC) at 1, 10, and 25 microM increased the MEPP frequency by 68, 96, and 127%, respectively. D-erythro-SPC (dissolved in dimethyl sulfoxide but not coupled to bovine serum albumin), but not L-threo-SPC, was active extracellular; the former (at 10 microM) increased the MEPP frequency by 143%. D-erythro-SPC treatment did not significantly change the median amplitude or frequency-distribution of MEPPs. Intracellular delivery via liposomes, in which 10, 100, or 1000 microM SPC mixture was entrapped in liposomal aqueous phase, induced a concentration-dependent increase in MEPP frequency of 45, 91, and 100%, respectively. D-erythro-SPC and L-threo-SPC at the concentration of 100 microM increased the MEPP frequency by 117 and 67%, respectively, or 91 and 61%, respectively, when coupled to bovine serum albumin. Pretreatment with thapsigargin significantly reduced but did not abolish the effects of extracellular D-erythro-SPC (10 microM) or liposomes containing 100 microM D-erythro-SPC. Liposomes filled with 100 microM D-myo-inositol 1,4,5-trisphosphate (IP3) enhanced the MEPP frequency to the same magnitude as 100 microM D-erythro-SPC entrapped in liposomes. However, administration of 100 microM D-erythro-SPC and IP3 entrapped in the same liposomes enhanced the MEPP frequency by 70%, which was less than that produced by these two compounds alone. The result provides the first electrophysiological evidence that SPC can modulate transmitter release by an extra- or intracellular action at the frog motor nerve ending.

Distinct effects of different calcium-mobilizing agents on cell death in NG108-15 neuroblastoma X glioma cells

The effects of different calcium-mobilizing agents on cell death were characterized in NG108-15 neuroblastoma x glioma hybrid cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and caused cell death. Thapsigargin (TG) not only increased the [Ca(2+)](i) and caused cell death but also induced neurite outgrowth via activation of phospholipase A(2) and cytochrome P450 epoxygenase. In contrast, bradykinin increased the [Ca(2+)](i), but had no effect on cell morphology or cell death. Cell death occurred by two different mechanisms, one of which was caspase-3-dependent and the other caspase-3-independent. Caspase-3 activation was Ca(2+)-dependent, whereas neurite outgrowth was Ca(2+)-independent. TG- or FCCP-induced caspase-3 activation occurred at the same time, but the cell death induced by TG was delayed. TG treatment did not enhance the generation of nitric oxide or cAMP or secretion of glial-derived neurotrophic factor or neurotrophin-3, but activated sphingosine kinase. Furthermore, inhibition of sphingosine kinase accelerated TG-induced cell death, and exogenous sphingosine 1-phosphate (S1P) protected cells from FCCP-induced cell death by about 60%. These results indicate that, in these cells, depletion of intracellular nonmitochondrial or mitochondrial Ca(2+) stores causes cell death, that TG activates phospholipase A(2) and sphingosine kinase, and that arachidonic acid induces neurite outgrowth, whereas S1P delays cell death.

Effects of Ca2+ on flavin-linked complex enzymes in mitochondria isolated from eggs and embryos of sea urchin

Mitochondria isolated from sea urchin embryos in early development show almost the same activities of cytochrome c oxidase and flavin-linked complex enzymes, which are estimated by cytochrome c reductases as in those isolated from unfertilized eggs. The activities of these cytochrome c reductases are inhibited by Ca2+ at above 10-5 M more strongly than cytochrome c oxidase. To investigate the changes in intramitochondrial Ca2+ concentration at fertilization, the activity of pyruvate dehydrogenase, another mitochondrial enzyme, was measured. The activity of this enzyme was controlled by phosphorylation and Ca2+-dependent dephosphorylation of the catalytic unit. The enzyme activity increased for 30 min after fertilization, decreased and became close to zero within ~60 min. Then, the activity appreciably increased again after hatching. This seems to reflect changes in the intramitochondrial Ca2+ concentration. The enzyme activity was enhanced by pre-incubation with Ca2+ at concentrations up to 10-5 M but was made quite low at above 10-4 M Ca2+ and 10-3 M adenosine triphosphate. Although the changes in pyruvate dehydrogenase activity observed at fertilization will reflect the changes in the intramitochondrial calcium concentration, the intramitochondrial Ca2+ concentration of unfertilized eggs cannot be estimated from these results because high (> 10-4 M) or low (10-6 M) Ca2+ can inhibit the enzyme. Measurement of respiration of a single egg showed that injection of ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid released the mitochondrial electron transport in the unfertilized egg. The possibility that changes in intramitochondrial calcium concentration occur at fertilization is discussed in relation to activation of both mitochondrial respiration and pyruvate dehydrogenase.

Sphingosine 1-phosphate signalling in mammalian cells

Sphingosine 1-phosphate is formed in cells in response to diverse stimuli, including growth factors, cytokines, G-protein-coupled receptor agonists, antigen, etc. Its production is catalysed by sphingosine kinase, while degradation is either via cleavage to produce palmitaldehyde and phosphoethanolamine or by dephosphorylation. In this review we discuss the most recent advances in our understanding of the role of the enzymes involved in metabolism of this lysolipid. Sphingosine 1-phosphate can also bind to members of the endothelial differentiation gene (EDG) G-protein-coupled receptor family [namely EDG1, EDG3, EDG5 (also known as H218 or AGR16), EDG6 and EDG8] to elicit biological responses. These receptors are coupled differentially via G(i), G(q), G(12/13) and Rho to multiple effector systems, including adenylate cyclase, phospholipases C and D, extracellular-signal-regulated kinase, c-Jun N-terminal kinase, p38 mitogen-activated protein kinase and non-receptor tyrosine kinases. These signalling pathways are linked to transcription factor activation, cytoskeletal proteins, adhesion molecule expression, caspase activities, etc. Therefore sphingosine 1-phosphate can affect diverse biological responses, including mitogenesis, differentiation, migration and apoptosis, via receptor-dependent mechanisms. Additionally, sphingosine 1-phosphate has been proposed to play an intracellular role, for example in Ca(2+) mobilization, activation of non-receptor tyrosine kinases, inhibition of caspases, etc. We review the evidence for both intracellular and extracellular actions, and extensively discuss future approaches that will ultimately resolve the question of dual action. Certainly, sphingosine 1-phosphate will prove to be unique if it elicits both extra- and intra-cellular actions. Finally, we review the evidence that implicates sphingosine 1-phosphate in pathophysiological disease states, such as cancer, angiogenesis and inflammation. Thus there is a need for the development of new therapeutic compounds, such as receptor antagonists. However, identification of the most suitable targets for drug intervention requires a full understanding of the signalling and action profile of this lysosphingolipid. This article describes where the research field is in relation to achieving this aim.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Caffeine-induced [Ca2+] oscillations in neurones of frog sympathetic ganglia

1. Single cell fluorimetry was used to monitor caffeine-induced oscillations of cytosolic [Ca2+] in frog sympathetic ganglion neurones in 2.0 mM K+ Ringer solution. 2. [Ca2+] oscillations decreased in frequency and exhibited three different amplitude patterns after the first large peak of [Ca2+]: (a) a series of big oscillations (BOs) of constant large amplitude (300-400 nM), (b) a series of much smaller oscillations (SOs) (40-60 nM), or (c) a series of decaying oscillations (DOs) of rapidly decreasing amplitude. 3. A model in which the oscillation amplitude was determined by the Ca2+ content of the endoplasmic reticulum (ER) whereas the oscillation frequency was controlled by how rapidly the cytosolic [Ca2+] reached the threshold for Ca2+-induced Ca2+ release (CICR) was able to simulate each observed pattern by varying the level of activity of the ER Ca2+ pump (SERCA), CICR and release-activated Ca2+ transport (RACT). A cumulative, cytosolic Ca2+-dependent inactivation of the plasma membrane (PM) Ca2+ influx or of the Ca2+-sensitive leak coefficient of the ryanodine receptors caused the oscillation frequency to decrease in the model. 4. Transitions between BOs and SOs and changes in [Ca2+] oscillations caused by ryanodine, thapsigargin, lanthanum and FCCP could also be simulated. 5. We conclude that RACT, SERCA, CICR and Ca2+-dependent PM Ca2+ influx are major mechanisms underlying [Ca2+] oscillations in these neurones.

Sphingosylphosphorylcholine stimulates mitogen-activated protein kinase via a Ca2+-dependent pathway

In cultured porcine aortic smooth muscle cells, sphingosylphosphorylcholine (SPC), ATP, or bradykinin (BK) induced a rapid dose-dependent increase in the cytosolic Ca2+ concentration ([Ca2+]i) and also stimulated inositol 1,4,5-trisphosphate (IP3) generation. Pretreatment of cells with pertussis toxin blocked the SPC-induced IP3 generation and [Ca2+]i increase but had no effect on the action of ATP or BK. In addition, SPC stimulated the mitogen-activated protein kinase (MAPK) and increased DNA synthesis, whereas neither ATP nor BK produced such effects. Both the SPC-induced MAPK activation and DNA synthesis were pertussis toxin sensitive. SPC-induced MAPK activation was blocked by treatment of cells with the phospholipase C inhibitor, U-73122, or the intracellular Ca2+-ATPase inhibitor, thapsigargin, but not by removal of extracellular Ca2+. Lysophosphatidic acid induced cellular responses similar to SPC in a pertussis toxin-sensitive manner in terms of [Ca2+]i increase, IP3 generation, MAPK activation, and DNA synthesis. Platelet-derived growth factor (PDGF) also induced a [Ca2+]i increase, MAPK activation, and DNA synthesis in the same cells; however, the PDGF-induced MAPK activation was not sensitive to pertussis toxin and changes in [Ca2+]i. SPC-induced MAPK activation was inhibited by pretreatment of cells with staurosporine, W-7, or calmidazolium. Our results suggest that, in porcine aortic smooth muscle cells, MAPK is not activated by the increase in [Ca2+]i unless a pertussis toxin-sensitive G protein is simultaneously stimulated, indicating the role of Ca2+ in pertussis toxin-sensitive G protein-mediated MAPK activation.

Ca2+ signaling induced by sphingosylphosphorylcholine and sphingosine 1-phosphate via distinct mechanisms in rat glomerular mesangial cells

BACKGROUND

To elucidate the molecular mechanism underlying sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) mediated signaling, we compared their effects with those of adenosine triphosphate (ATP) and angiotensin II (Ang II) on the cytosolic free Ca2+ concentration ([Ca2+]i), inositol 1,4, 5-trisphosphate (IP3) generation and arachidonic acid release in rat glomerular mesangial cells.

METHODS

The fluorescent Ca2+ indicator, Fura-2, was used to measure the [Ca2+]i changes in cultured rat glomerular mesangial cells either in suspension or attached to the coverslips.

RESULTS

SPC 5 microM, S1P 5 microM, ATP 100 microM and Ang II 90 nM all induced increases in the [Ca2+]i, and the effect showed marked homologous desensitization, while heterologous desensitization was less. After the initial exposure of the cells to SPC, the increase in [Ca2+]i induced by subsequent addition of ATP or Ang II was only reduced by about 14.3% and 4.8%, respectively. After the initial exposure to S1P, a greater reduction was seen (42. 1% and 47.7%, respectively). Both arachidonic acid release and IP3 generation were activated by all four agonists with an identical rank order of effectiveness of SPC >> S1P > ATP = Ang II; both were pertussis toxin-sensitive and cholera toxin-resistant. The arachidonic acid release induced by all four agonists showed identical susceptibility to removal of extracellular Ca2+, whereas IP3 generation displayed differential extracellular Ca2+ dependence. Only SPC-induced IP3 generation was highly sensitive to extracellular Ca2+ level, and this Ca2+ dependence was abolished after pretreatment of cells with arachidonyl trifluoromethyl ketone (AACOCF3), a phospholipase A2 inhibitor. Furthermore, the Mn2+ influx was markedly greater in SPC-stimulated cells than in either control or other agonist-stimulated cells, and was decreased by prior exposure of cells to AACOCF3. After phospholipase A2 was inhibited or in the absence of extracellular Ca2+, SPC displayed identical effectiveness as S1P on desensitizing the action of ATP or Ang II on the increase in [Ca2+]i. Conclusions. Our results indicate that all four agents primarily activate phospholipase C through their receptor occupancies, but that SPC alone also induces further significant Mn2+ influx and IP3 generation attributable to its primary stimulatory effect on arachidonic acid release. Thus, the heterologous desensitization to ATP or Ang II induced by SPC was less profound than that induced by S1P, since SPC induced a Ca2+ influx.

Fatty acids induce release of Ca2+ from acidosomal stores and activate capacitative Ca2+ entry in Dictyostelium discoideum

cAMP-induced Ca2+ fluxes in Dictyostelium discoideum largely depend on phospholipase A2 activity generating non-esterified fatty acids [Schaloske and Malchow (1997) Biochem. J. 327, 233-238]. In the present study the effect of fatty acids on Ca2+ homoeostasis in D. discoideum was investigated. Cytosolic free Ca2+ concentration ([Ca2+]i) was analysed by digital imaging of single fura2-dextran-loaded cells. Arachidonic acid and linoleic acid induced a transient increase in [Ca2+]i. The concentration of arachidonic acid determined the percentage of responding cells, with the mean height of the increase being dose-independent. In nominally Ca2+-free medium or in the presence of bis-(o-aminophenoxy)ethane-N, N,N',N'-tetra-acetic acid (BAPTA), no [Ca2+]i transient was detectable. In spite of this, we found that (1) arachidonic acid induced Ca2+ release from permeabilized cells and from vesicular fractions at concentrations that elicited Ca2+ influx in intact cells and (2) Ca2+ entry was inhibited by inhibitors of Ca2+-transport ATPases and V-type H+-ATPase, indicating that intracellular Ca2+ release precedes Ca2+ entry. Inhibition studies and mutant analysis point to the acidosomal Ca2+ stores as a target of fatty acids. Although fatty acids can substitute fully for cAMP with respect to Ca2+ influx in wild-type cells, experiments with a mutant strain revealed that cAMP also sensitizes the Ca2+-entry mechanism: cAMP-induced Ca2+ influx was normal in a phospholipase C knockout mutant but influx was fairly insensitive to arachidonic acid in this strain. This defect could be overcome by higher doses of arachidonic acid which cause sufficient Ca2+ to be released from the stores to trigger extracellular Ca2+ entry.

Ultrastructural localization of sorcin, a 22 kDa calcium binding protein, in the rat caudate-putamen nucleus: association with ryanodine receptors and intracellular calcium release

Sorcin is a 22 kDa calcium binding protein that is widely distributed in mammalian tissues, including brain, and is associated with the ryanodine receptor (RyR) family of intracellular calcium-release channels in the heart. To determine the cellular sites for potential central functions of sorcin, we examined the electron microscopic immunocytochemical localization of antipeptide antisera against sorcin and against cardiac and brain RyR in the rat caudate-putamen nucleus (CPN), one of the few regions expressing high levels of brain RyR. Sorcin-like immunoreactivity (S-LI) was detected in both neurons and glia by using immunoperoxidase and immunogold methods. Of 1,735 profiles containing immunogold-silver labeling for sorcin, almost 50% were dendrites and many of these dendrites were spiny. The remainder were mainly small axons, axon terminals, and, more rarely, glia. Furthermore, analysis of dually labeled tissue sections showed the presence of sorcin in many of the dendrites and some of the axonal and glial processes containing RyR. In dendrites, gold-silver deposits showing S-LI were prominently localized to saccules of smooth endoplasmic reticulum and mitochondria, both of which are known to store calcium. These labeled structures were located near the plasma membrane at sites postsynaptic to excitatory-type asymmetric junctions, as well as non-synaptic portions of the plasma membrane. In axons, S-LI was also often seen at extrasynaptic sites on, or near, the plasma membrane. We conclude that in the rat CPN, sorcin may act independently or, in conjunction with RyR, to modulate cytoplasmic release of calcium, mainly from smooth endoplasmic reticulum and/or mitochondria in neurons.

Release-activated Ca2+ transport in neurons of frog sympathetic ganglia

Frog sympathetic ganglion neurons exhibit a novel Ca2+ uptake mechanism, release-activated calcium transport or RACT, which is manifest in both cytosolic and store [Ca2+] signals as greatly accelerated Ca2+ uptake after Ca2+ release from internal stores. RACT is activated by Ca2+ release but not by Ca2+ entry and serves to selectively refill Ca2+ stores after release. RACT lowers cytosolic [Ca2+] with a rate constant about 1.6 times that of the SERCA pump with empty ER. RACT is thapsigargin-insensitive, was eliminated by ryanodine, but was not affected by blocking mitochondrial or plasma membrane Ca2+ transport. A Ca2+ flux model with RACT in the ER membrane reproduced the cytosolic and store [Ca2+] responses to all stimuli.