Calcium Transport in Sealed Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue : I. Characterization of a Ca-Pumping ATPase Associated with the Endoplasmic Reticulum

Calcium transport was examined in microsomal membrane vesicles from red beet (Beta vulgaris L.) storage tissue using chlorotetracycline as a fluorescent probe. This probe demonstrates an increase in fluorescence corresponding to calcium accumulation within the vesicles which can be collapsed by the addition of the calcium ionophore A23187. Calcium uptake in the microsomal vesicles was ATP dependent and completely inhibited by orthovanadate. Centrifugation of the microsomal membrane fraction on a linear 15 to 45% (w/w) sucrose density gradient revealed the presence of a single peak of calcium uptake which comigrated with the marker for endoplasmic reticulum. The calcium transport system associated with endoplasmic reticulum vesicles was then further characterized in fractions produced by centrifugation on discontinous sucrose density gradients. Calcium transport was insensitive to carbonylcyanide m-chlorophenylhydrazone indicating the presence of a primary transport system directly linked to ATP utilization. The endoplasmic reticulum vesicles contained an ATPase activity that was calcium dependent and further stimulated by A23187 (Ca(2+), A23187 stimulated-ATPase). Both calcium uptake and Ca(2+), A23187 stimulated ATPase demonstrated similar properties with respect to pH optimum, inhibitor sensitivity, substrate specificity, and substrate kinetics. Treatment of the red beet endoplasmic reticulum vesicles with [gamma-(32)P]-ATP over short time intervals revealed the presence of a rapidly turning over 96 kilodalton radioactive peptide possibly representing a phosphorylated intermediate of this endoplasmic reticulum associated ATPase. It is proposed that this ATPase activity may represent the enzymic machinery responsible for mediating primary calcium transport in the endoplasmic reticulum linked to ATP utilization.     

The endoplasmic reticulum retention signal of the E3/19K protein of adenovirus-2 is microtubule binding.

The signal for retention in the endoplasmic reticulum of the E3/19K protein of adenovirus type 2 is located within the carboxyl-terminal cytoplasmic extension. A synthetic peptide corresponding to this sequence showed affinity for beta-tubulin, could promote tubulin polymerization in vitro, and bound to taxol-polymerized microtubules. When compared with the microtubule binding sequences from two microtubule-associated proteins (MAPs; MAP2 and tau), we found similarities suggesting that the cytoplasmic tail might bind to tubulin/microtubules in a MAPs-like fashion. A synthetic peptide corresponding to the cytoplasmic tail of an E3/19K deletion mutant not retained in the endoplasmic reticulum was also tested. It had the same net charge but did not promote tubulin polymerization in vitro nor did it show measurable affinity for tubulin or microtubules. This indicates that binding to microtubules is important for retention of the E3/19K protein in the endoplasmic reticulum.     

Ionophore-induced disassembly of blood platelet microtubules: effect of cyclic AMP and indomethacin

Cytoplasmic calcium levels are believed to be important in blood platelet activation. Upon activation, the discrete marginal microtubule band, which maintains the discoid shape of non-activated platelets, becomes disrupted. Present studies demonstrate that the extent of assembly of the marginal microtubule band is related to cytoplasmic calcium levels. The divalent cationophore, A23187, causes platelet aggregation, secretion, and contraction by promoting calcium transport from intraplatelet storage sites into the cytoplasm. A23187 caused disassembly of platelet microtubules. Quantitation of electron micrographs revealed that numbers of microtubules were reduced by approximately 80% after A23187 treatment. Secondly, assembled microtubules in homogenates of platelets, in which microtubules were stabilized prior to homogenization, were decreased in favor of free tubulin in A23187-treated platelets. Thirdly, A23187 increased 14C-colchicine binding by intact platelets; this also indicated a shift in the microtubule subunit equilibrium to favor free, colchicine-binding tubulin subunits. In control experiments, A23187 did not affect the stability of platelet tubulin, the colchicine binding reaction, or the total tubulin content of platelets. Stimulation of colchicine binding depended on A23187 concentration (0.05-0.5 microM) and did not require extracellular calcium. A23187-stimulation of colchicine binding was blocked by dibutyryl cyclic AMP (0.80 mM) and/or 3-isobutyl-1-methylxanthine (50 microM) and by indomethacin (10 microM). Cyclic AMP or indomethacin also interferes with A23187-induced platelet activation, but indomethacin is not likely to completely inhibit the perturbation of intraplatelet calcium gradients by A23187. It is suggested that A23187-induced microtubule disassembly may be an indirect effect of calcium on microtubules.     

Depletion of cellular calcium accelerates protein degradation in the endoplasmic reticulum.

In this study the effects of A23187 and thapsigargin on the degradation of T-cell antigen receptor-beta (TCR-beta) and CD3-delta in the endoplasmic reticulum have been studied. Preliminary experiments showed that these drugs had different effects on the secretory pathway. Depletion of cellular calcium pools by incubation of cells with A23187 in calcium-free medium blocked transport between the endoplasmic reticulum and the Golgi apparatus whereas thapsigargin caused a modest increase in transport. When added to cells transfected with TCR-beta or CD3-delta the drugs caused an immediate stimulation of proteolysis of presynthesized protein and at maximum effective concentrations caused a 3-fold increase in the rate of degradation. They did not affect the lag period of 1 h which precedes degradation of newly synthesized proteins. Chelation of cytosolic calcium also accelerated degradation, suggesting that depletion of calcium from the endoplasmic reticulum was the main stimulus of proteolysis and that increased degradation was not caused by a transient increase in cytosolic calcium levels. The selectivity of degradation in the endoplasmic reticulum was maintained. A23187 had no effect on the stability of CD3-gamma nor co-transfected epsilon-beta dimers. Calcium depletion increased the overall rate of degradation in the endoplasmic reticulum and increased the rate of proteolysis of an "anchor minus" beta chain. The results suggested that proteolysis within the endoplasmic reticulum may be regulated by the high concentrations of Ca2+ which are stored in the organelle. Ca2+ may be required for protein folding. Calcium depletion may have caused the beta and delta chains to adopt a conformation that was more susceptible to proteolysis. Alternatively, calcium depletion may have disrupted the lumenal content of the endoplasmic reticulum and increased the access of proteases to potential substrates.     

p180 is involved in the interaction between the endoplasmic reticulum and microtubules through a novel microtubule-binding and bundling domain

p180 was originally reported as a ribosome-binding protein on the rough endoplasmic reticulum membrane, although its precise role in animal cells has not yet been elucidated. Here, we characterized a new function of human p180 as a microtubule-binding and -modulating protein. Overexpression of p180 in mammalian cells induced an elongated morphology and enhanced acetylated microtubules. Consistently, electron microscopic analysis clearly revealed microtubule bundles in p180-overexpressing cells. Targeted depletion of endogenous p180 by small interfering RNAs led to aberrant patterns of microtubules and endoplasmic reticulum in mammalian cells, suggesting a specific interaction between p180 and microtubules. In vitro sedimentation assays using recombinant polypeptides revealed that p180 bound to microtubules directly and possessed a novel microtubule-binding domain (designated MTB-1). MTB-1 consists of a predicted coiled-coil region and repeat domain, and strongly promoted bundle formation both in vitro and in vivo when expressed alone. Overexpression of p180 induced acetylated microtubules in cultured cells in an MTB-1-dependent manner. Thus, our data suggest that p180 mediates interactions between the endoplasmic reticulum and microtubules mainly through the novel microtubule-binding and -bundling domain MTB-1.     

Regulation of Ca2+ transport by isolated organelles of a rat insulinoma. Studies with endoplasmic reticulum and secretory granules

The regulation of extramicrosomal Ca2+ concentration maintained by suspensions of rat insulinoma microsomes was studied using Ca2+-selective minielectrodes. The Ca2+-transporting activity was MgATP dependent and correlated with the endoplasmic reticulum marker NADPH-cytochrome c reductase. When incubated in a high KCl medium containing Mg2+ and phosphate, the microsomes lowered [Ca2+] within less than 10 min to around 0.2 microM. They had a high Ca2+-sequestering activity since they were able to take up and retain several small Ca2+ additions. No evidence for a Na+/Ca2+ countertransport was obtained. The accumulated Ca2+ was released by the Ca2+ ionophore A23187 or upon transforming ATP into ADP using glucose plus hexokinase. The addition of ADP, at concentrations present in cells, resulted in a dose-dependent and reversible net Ca2+ efflux from the microsomes until a higher [Ca2+] steady state was reached. This was specific for ADP since GDP, UDP, CDP, IDP, and the nonhydrolyzable analogue methylene-ADP as well as AMP and cAMP did not reproduce the effect. Insulin secretory granules were unable to lower medium [Ca2+] or to take up a pulse addition of Ca2+. However, most of the large granular calcium content was released by A23187. The addition of Na+ and lowering or increasing medium pH by 0.2 pH unit did not induce Ca2+ uptake or efflux from the secretory granules. The results indicate that insulinoma endoplasmic reticulum but not insulin secretory granules may play a critical role in the regulation of cytosolic Ca2+. A variation in cellular ADP content following secretagogue addition might modulate Ca2+ fluxes across the endoplasmic reticulum and contribute in raising cytosolic Ca2+.     

Calcium uptake and release by isolated cortices and microsomes from the unfertilized egg of the sea urchin Strongylocentrotus droebachiensis

Isolated cortices from unfertilized sea urchin eggs sequester calcium in an ATP-dependent manner when incubated in a medium containing free calcium levels characteristic of the resting cell (approximately 0.1 microM). This ATP-dependent calcium uptake activity was measured in the presence of 5 mM Na azide to prevent mitochondrial accumulation, was increased by oxalate, and was blocked by 150 microM quercetin and 50 microM vanadate (known inhibitors of calcium uptake into the sarcoplasmic reticulum). Cortical regions preloaded with 45Ca in the presence of ATP were shown to dramatically increase their rate of calcium efflux upon the addition of (a) the calcium ionophore A23187 (10 microM), (b) trifluoperazine (200 microM), (c) concentrations of free calcium that activated cortical granule exocytosis, and (d) the calcium mobilizing agent inositol trisphosphate. This pool of calcium is most likely sequestered in the portion of the egg's endoplasmic reticulum that remains associated with the cortical region during its isolation. We have developed a method for obtaining a high yield of purified microsomal vesicles from whole eggs. This preparation also demonstrates ATP-dependent calcium sequestering activity which increases in the presence of oxalate and has similar sensitivities to calcium transport inhibitors; however, the isolated microsomal vesicles did not show any detectable release of calcium when exposed to inositol trisphosphate.     

Preservation of the secretory response of peritoneal mast cells in the absence of extracellular calcium

The transfer of rat peritoneal mast cells from balanced salt solution to calcium-free buffer led to a time-dependent decrease in their response to compound 48/80 and to ionophore A23187. The concomittant absence of potassium from the calcium-free buffer enabled the mast cells to retain their secretory response. The increase in potassium level, with a parallel decrease in sodium to maintain osmolarity, led to a slight potentiation of the response to 48/80 and to a large but transient potentiation of the response to A23187. Mast cells can be considered nonexcitable. The apparent dependency upon extracellular calcium of mast cell secretory responses might be related to the presumed tight equilibrium between endoplasmic reticulum calcium stores and extracellular calcium. The control of this equilibrium by transmembrane gradients of monovalent ions is proposed.     

Bax inhibitor-1 overexpression reduces the suppressive effect of calcium mobilizing agent on adipogenesis

Bax inhibitor-1 is a conserved protein which suppresses endoplasmic reticulum stress-induced apoptosis and regulates calcium release from the endoplasmic reticulum. Recent studies have revealed that adipogenesis, the process of adipocyte differentiation, is influenced by a change of intracellular calcium concentration. Here, we examined the effect of endoplasmic reticulum calcium regulation by Bax inhibitor-1 on adipogenesis using 3T3-L1 preadipocytes stably transfected with a pcDNA3-BI-1-HA plasmid. Bax inhibitor-1 functionality was confirmed by inhibiting the thapsigargin-induced increase of endoplasmic reticulum stress markers. Bax inhibitor-1 overexpression did not alter normal process of adipogenesis. Thapsigargin treatment inhibited adipogenesis in control cells, but Bax inhibitor-1 overexpressing 3T3-L1 cells retained their adipogenesis function. Endoplasmic reticulum stress did not seem to be involved in thapsigargin-reduced adipogenesis, since other endoplasmic reticulum stress inducers, such as tunicamycin and dithiothreitol, did not suppress the differentiation of 3T3-L1 cells. Bax inhibitor-1 might affect adipogenesis through regulating cytosolic calcium, because the thapsigargin-induced robust intracellular calcium rise was not observed in Bax inhibitor-1 overexpressing 3T3-L1 cells. A23187, a calcium ionophore, showed the same effect on adipogenesis as thapsigargin. Taken together, Bax inhibitor-1 overexpression in 3T3-L1 preadipocytes inhibits calcium mobilizing agent-induced suppression of adipogenesis. As adipogenesis is dependent on a change of intracellular calcium concentration, endoplasmic reticulum calcium regulation by Bax inhibitor-1 may play an important role in adipogenesis process.     

Active calcium sequestration by intestinal microsomes. Stimulation by increased calcium load

Calcium uptake by an endoplasmic reticulum-enriched membrane fraction isolated from rat small intestine was investigated using a rapid filtration technique. Calcium sequestration was stimulated by the presence of ATP and released by the calcium ionophore A23187. ATP stimulation of calcium uptake was dependent on the presence of magnesium, inhibited by vanadate, and refractory to calmodulin. Kinetic studies revealed a K0.5 for the ATP-stimulated uptake of 62.5 nM Ca and a Jmax of 1.4 nmol of Ca/mg protein X min. A high dietary calcium load stimulated maximal uptake by 80% with no change in affinity. The magnitude of maximal uptake and the high affinity of this transport system suggest that the endoplasmic reticulum may play a significant role in cytosolic calcium sequestration and that extracellular calcium leads to modulation of intracellular endoplasmic reticulum calcium buffering.     

Increases in intracellular calcium dephosphorylate histone H3 at serine 10 in human hepatoma cells: potential role of protein phosphatase 2A-protein kinase CbetaII complex

We present evidence that increases in intracellular calcium, induced by treatment with calcium ionophore A23187 or the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin, dephosphorylated histone H3 at serine10 (histone H3-Ser10) in a dose-dependent manner in human hepatoma HepG2 cells. Inhibition of p42/44MAPK, pp90RSK, or p38MAPK did not affect the ability of A23187 to dephosphorylate histone H3-Ser10. This response is significantly blocked by okadaic acid, indicating a requirement for protein phosphatase 2A (PP2A). A23187 increased the activity of PP2A towards phosphorylated histone H3-Ser10. Furthermore, pretreatment with calphostin C, a selective protein kinase C (PKC) inhibitor, blocked A23187-dependent dephosphorylation of histone H3-Ser10, and coimmunoprecipitation analysis showed PP2A association with the PKCbetaII isoform. Unlike untreated cells, coimmunoprecipitated complex from A23187-treated cells showed greater dephosphorylation of histone H3-Ser10 in a PP2A-dependent manner. Inhibition of PP2A increased phosphorylation at Ser660 that determines calcium sensitivity and activity of PKCbetaII isoform, thus supporting a role for intracomplex regulation. Finally, chromatin immunoprecipitation assays following exposure to A23187 and okadaic acid revealed regulatory role of histone H3-Ser10 phosphorylation in selective gene induction. Altogether, our findings suggest a novel role for calcium in modulating histone H3-Ser10 phosphorylation level and led us to propose a model emphasizing PP2A activation, occurring downstream following perturbations in calcium homeostasis, as key event in dephosphorylating histone H3-Ser10 in mammalian cells.     

Evidence that biosynthesis of platelet-activating factor (paf-acether) by human neutrophils occurs in an intracellular membrane

Human polymorphonuclear leukocytes (PMN) were incubated in the absence or presence of the calcium ionophore A23187 (6 microM) for 10 min at 37 degrees C. They were then lysed by nitrogen cavitation and fractionated using Percoll gradients. Three major fractions of increasing density corresponding to plasma membrane, intracellular membranes and secretory granules were detected using [3H]concanavalin A, NADH-dehydrogenase and beta-D-glucuronidase as respective markers. In both cases, the acetyltransferase activity responsible for biosynthesis of paf-acether (platelet-activating factor of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) was detected in the intermediary fraction, the enzyme activity being increased 3-4-fold in stimulated cells. From the comparison with the distribution of various markers, it is concluded that in human PMN the final step of paf-acether assembly occurs in an intracellular membrane, possibly the endoplasmic reticulum.     

A novel direct interaction of endoplasmic reticulum with microtubules.

The positioning and dynamics of organelles in eukaryotic cells critically depend on membrane-cytoskeleton interactions. Motor proteins play an important role in the directed movement of organelle membranes along microtubules, but the basic mechanism by which membranes stably interact with the microtubule cytoskeleton is largely unknown. Here we report that p63, an integral membrane protein of the reticular subdomain of the rough endoplasmic reticulum (ER), binds microtubules in vivo and in vitro. Overexpression of p63 in cell culture led to a striking rearrangement of the ER and to concomitant bundling of microtubules along the altered ER. Mutational analysis of the cytoplasmic domain of p63 revealed two determinants responsible for these changes: an ER rearrangement determinant near the N-terminus and a central microtubule-binding region. The two determinants function independently of one another as indicated by deletion experiments. A peptide corresponding to the cytoplasmic tail of p63 promoted microtubule polymerization in vitro. p63 is the first identified integral membrane protein that can link a membrane organelle directly to microtubules. By doing so, it may contribute to the positioning of the ER along microtubules.     

Localization of GRP78 to mitochondria under the unfolded protein response

The ubiquitously expressed molecular chaperone GRP78 (78 kDa glucose-regulated protein) generally localizes to the ER (endoplasmic reticulum). GRP78 is specifically induced in cells under the UPR (unfolded protein response), which can be elicited by treatments with calcium ionophore A23187 and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase inhibitor TG (thapsigargin). By using confocal microscopy, we have demonstrated that GRP78 was concentrated in the perinuclear region and co-localized with the ER marker proteins, calnexin and PDI (protein disulphide-isomerase), in cells under normal growth conditions. However, treatments with A23187 and TG led to diminish its ER targeting, resulting in redirection into a cytoplasmic vesicular pattern, and overlapping with the mitochondrial marker MitoTracker. Cellular fractionation and protease digestion of isolated mitochondria from ER-stressed cells suggested that a significant portion of GRP78 is localized to the mitochondria and is protease-resistant. Localizations of GRP78 in ER and mitochondria were confirmed by using immunoelectron microscopy. In ER-stressed cells, GRP78 mainly localized within the mitochondria and decorated the mitochondrial membrane compartment. Submitochondrial fractionation studies indicated further that the mitochondria-resided GRP78 is mainly located in the intermembrane space, inner membrane and matrix, but is not associated with the outer membrane. Furthermore, radioactive labelling followed by subcellular fractionation showed that a significant portion of the newly synthesized GRP78 is localized to the mitochondria in cells under UPR. Taken together, our results indicate that, at least under certain circumstances, the ER-resided chaperone GRP78 can be retargeted to mitochondria and thereby may be involved in correlating UPR signalling between these two organelles.     

Mechanism of the ATP-dependent phosphatidylserine synthesis in liver subcellular fractions

It has been shown that the ATP-dependent incorporation of [14C]serine into phosphatidylserine in rat liver mitochondrial and microsomal fractions is prevented by EGTA. On the other hand, at low (microM) Ca2+ concentrations, serine incorporation is strongly stimulated by ATP and Mg2+. This stimulatory effect is reduced by calcium ionophore A23187. It is therefore suggested that the ATP-dependent process is that of serine base-exchange reaction, stimulated by endogenous Ca2+ accumulated inside the microsomal vesicles by Ca2+,Mg2+-ATPase. The mitochondrial activity can be accounted for by contamination by the endoplasmic reticulum.     

Effects of calpain inhibitor on the apoptosis of hepatic stellate cells induced by calcium ionophore A23187

We previously showed that changes in calcium concentrations were related to cell apoptosis in vitro. The endoplasmic reticulum (ER) is the main component of calcium storage and signal transduction, and disrupting the balance of intracellular Ca²⁺ can cause endoplasmic reticulum stress (ERS). In this process, the ER releases stored Ca ²⁺ into the cytoplasm and activates calpain-2. To further investigate the effect of calpain in hepatic stellate cells (HSCs), in the current study, we examine the effect of N-acetyl-leu-leu-norleucinal (ALLN) on apoptosis resulting from calcium ionophore A23187–induced ERS. Our findings indicate that calpain inhibition reduces calcium ionophore A23187–induced apoptosis of HSCs and decreases the expression of ER stress proteins that may be related to the calpain/caspase signaling pathway.     

The origin, quantitation, and kinetics of intracellular calcium mobilization by vasopressin and phenylephrine in hepatocytes

The addition of phenylephrine or vasopressin to isolated hepatocytes resulted in an efflux of calcium. The intracellular source of this calcium was determined by measuring the calcium released upon the sequential additions of an uncoupling agent and the Ca2+ ionophore A23187 to control and hormone-treated cells. The release promoted by these agents was used as an estimate of the calcium content of the mitochondria and endoplasmic reticulum, respectively. The validity and limitations of this method are critically evaluated. The source of the calcium mobilized by the hormones was found to depend on the intracellular calcium distribution. When the amount of total cell-releasable Ca2+ was low (less than 0.9 nmol/mg cell dry weight), the endoplasmic reticulum represented the major cellular calcium pool and was also the predominant pool mobilized by the hormone. As the cell calcium content was increased, the endoplasmic reticulum attained its maximum capacity and the mitochondria sequestered increasing amounts of calcium. Under these conditions, the hormones mobilized calcium from the mitochondria with minimal effects on the endoplasmic reticulum calcium pool. These results suggest that more than one hormone-induced Ca2+-releasing agent may be formed. Both the total amount and the rate of calcium released from the cell under the influence of hormones was independent of the cell calcium content. The appearance of hormone-releasable Ca2+ in the extracellular medium showed a lag period of 5 to 10 s, during which a rapid increase of phosphorylase activity was observed. In contrast, the mobilization of a comparable amount of calcium by carbonyl cyanide p-trifluoromethoxyphenylhydrazone showed no significant lag, but the activation of phosphorylase was slower. A kinetic analysis of the hormone-releasable Ca2+ indicated a rapid onset with a peak increase of cytosolic free Ca2+ between 5 and 10 s prior to release of Ca2+ from the cell. The results suggest that an early action of the hormone is the inhibition of the plasma membrane Ca2+ efflux pump.     

Characterization of the submandibular gland microsomal calcium transport system

Calcium accumulation by submandibular gland microsomes (first described by Selinger and Naim, ((1970) Biochim. Biophys. Acta 323, 337-341) has been further characterized. Accumulation was concentration dependent, had a Km of 25 microM added calcium and a Vmax of 12 nM calcium/mg protein per min. No accumulation was observed in the presence of either the calcium ionophore A23187, or the detergent Triton X-100 (0.05). The divalent cations Sr2+ and Mn2+ inhibited accumulation competitively with Ki values of 67 microM and 200 microM, respectively. The effect of various enzyme inhibitors were tested on the microsomal calcium transport system and it was found that chlorpromazine, trifluoperazine, and DIDS all inhibited. The mitochondrial transport inhibitors ruthenium red and CCCP had no effect on transport. Experiments directed at clarifying the cellular location of the system are described. It was found that the membrane vesicles responsible for transport show different purification properties than the membrane vesicles which contain the standard enzyme markers for total and rough endoplasmic reticulum, Golgi apparatus, plasma membrane, and lysosomes. These conclusions are based upon experiments using these properties for membrane purification, density, size, and electrophoretic mobility. Three possible explanations of the results are given and it is organelles. The significance of the results in: (1) understanding the biochemical properties of the submandibular gland microsomal calcium transport system, (2) clarifying the cellular location of the system, and (3) clarifying the function of the system in salivary secretion are discussed.     

Xestospongin C empties the ER calcium store but does not inhibit InsP3-induced Ca2+ release in cultured dorsal root ganglia neurones

The action of Xestospongin C (XeC) on calcium concentration in the cytosol ([Ca2+]i) and within the lumen of endoplasmic reticulum (ER) ([Ca2+]L) was studied using cultured dorsal root ganglia (DRG) neurones. Application of 2.5 microM of XeC triggered a slow [Ca2+]i transient as measured by Fura-2 video-imaging. The kinetics and amplitude of XeC-induced [Ca2+]i response was similar to that triggered by 1 microM thapsigargin (TG). The [Ca2+]L was monitored in cells loaded with low-affinity Ca2+ indicator Mag-Fura-2. The cytosolic portion of Mag-Fura-2 was removed by permeabilisation of the plasmalemma with saponin. Application of XeC to these permeabilised neurones resulted in a slow depletion of the ER Ca2+ store. XeC, however, failed to inhibit inositol 1,4,5-trisphosphate (InsP3)-induced [Ca2+]L responses. We conclude that XeC is a potent inhibitor of sarco(endo)plasmic reticulum calcium ATPase, and it cannot be regarded as a specific inhibitor of InsP3 receptors in cultured DRG neurones.     

Alkalinization-Induced Changes in Intracellular Calcium in Rat Spinal Cord Neurons

It is well-known that pH changes can influence a lot of cellular processes. In this work, we have specifically studied the influence of alkalinization, which can be developed in spinal cord neurons during hyperventilation (respiratory alkalosis) and chronic renal failure (metabolic alkalosis) on calcium homeostasis. Application of Tyrode solution with increased pH (pH = 8.8) to secondary sensory neurons isolated from rat spinal dorsal horn induced elevation of intracellular free calcium concentration in the cytosol ([Ca2+]i) if applied after membrane depolarization. Repetitive application of alkaline solution led to disappearance of such elevations. Depletion of endoplasmic reticulum (ER) calcium stores by 30 mM caffeine almost completely blocked the effect of elevated extracellular pH. If caffeine-induced [Ca2+]i transients were evoked during alkalinization, their amplitudes were decreased by 41%. Preapplication of 500 nM ionomycin resulted in disappearance of alkalinization-induced [Ca2+]i transients, whereas prolonged applications (for 20 min) of 200 nM thapsigargin, a blocker of Ca2+ ATPase of the endoplasmic reticulum, resulted in disappearance of the rapid phase of the [Ca2+]i transients induced by alkalinization. Preapplication of the mitochondrial protonophore CCCP (10 microM) also induced changes in the alkalinization-induced calcium response--it lost its peak and was transformed into an irregular wave terminating in several seconds. The data obtained indicate that alkalinization induces an increase of [Ca2+]i level in the investigated neurons via a combined action of both intracellular Ca2+-accumulating structures--the endoplasmic reticulum and mitochondria. This suggestion was supported by morphological data that both structures in these neurons are tightly connected and may interact during release of accumulated calcium ions.