Imaging of inositol 1,4,5-trisphosphate-induced Ca2+ fluxes in single permeabilized hepatocytes. Demonstration of both quantal and nonquantal patterns of Ca2+ release

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.

Photochemical control of drug efficacy - a comparison of uncaging and photoswitching ifenprodil on NMDA receptors

Ifenprodil is an important negative allosteric modulator of the N-methyl-D-aspartate (NMDA) receptors. We have synthesized caged and photoswitchable derivatives of this small molecule drug. Caged ifenprodil was biologically inert before photolysis, UV irradiation efficiently released the drug allowing selective inhibition of GluN2B-containing NMDA receptors. Azobenzene-modified ifenprodil, on the other hand, is inert in both its trans and cis configurations, although in silico modeling predicted the trans form to be able to bind to the receptor. The disparity in effectiveness between the two compounds reflects, in part, the inherent ability of each method in manipulating the binding properties of drugs. With appropriate structure-activity relationship uncaging enables binary control of effector binding, whereas photoswitching using feely diffusable chromophores shifts the dose-response curve of drug-receptor interaction. Our data suggest that the efficacy of pharmacophores having a confined binding site such as ifenprodil can be controlled more easily by uncaging in comparison to photoswitching.

Permeant calcium ion feed-through regulation of single inositol 1,4,5-trisphosphate receptor channel gating

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) Ca(2+) release channel plays a central role in the generation and modulation of intracellular Ca(2+) signals, and is intricately regulated by multiple mechanisms including cytoplasmic ligand (InsP(3), free Ca(2+), free ATP(4-)) binding, posttranslational modifications, and interactions with cytoplasmic and endoplasmic reticulum (ER) luminal proteins. However, regulation of InsP(3)R channel activity by free Ca(2+) in the ER lumen ([Ca(2+)](ER)) remains poorly understood because of limitations of Ca(2+) flux measurements and imaging techniques. Here, we used nuclear patch-clamp experiments in excised luminal-side-out configuration with perfusion solution exchange to study the effects of [Ca(2+)](ER) on homotetrameric rat type 3 InsP(3)R channel activity. In optimal [Ca(2+)](i) and subsaturating [InsP(3)], jumps of [Ca(2+)](ER) from 70 nM to 300 µM reduced channel activity significantly. This inhibition was abrogated by saturating InsP(3) but restored when [Ca(2+)](ER) was raised to 1.1 mM. In suboptimal [Ca(2+)](i), jumps of [Ca(2+)](ER) (70 nM to 300 µM) enhanced channel activity. Thus, [Ca(2+)](ER) effects on channel activity exhibited a biphasic dependence on [Ca(2+)](i). In addition, the effect of high [Ca(2+)](ER) was attenuated when a voltage was applied to oppose Ca(2+) flux through the channel. These observations can be accounted for by Ca(2+) flux driven through the open InsP(3)R channel by [Ca(2+)](ER), raising local [Ca(2+)](i) around the channel to regulate its activity through its cytoplasmic regulatory Ca(2+)-binding sites. Importantly, [Ca(2+)](ER) regulation of InsP(3)R channel activity depended on cytoplasmic Ca(2+)-buffering conditions: it was more pronounced when [Ca(2+)](i) was weakly buffered but completely abolished in strong Ca(2+)-buffering conditions. With strong cytoplasmic buffering and Ca(2+) flux sufficiently reduced by applied voltage, both activation and inhibition of InsP(3)R channel gating by physiological levels of [Ca(2+)](ER) were completely abolished. Collectively, these results rule out Ca(2+) regulation of channel activity by direct binding to the luminal aspect of the channel.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Vesicularization of the endoplasmic reticulum is a fast response to plasma membrane injury

The endoplasmic reticulum of most cell types mainly consists of an extensive network of narrow sheets and tubules. It is well known that an excessive increase of the cytosolic Ca(2+) concentration induces a slow but extensive swelling of the endoplasmic reticulum into a vesicular morphology. We observed that a similar extensive transition to a vesicular morphology may also occur independently of a change of cytosolic Ca(2+) and that the change may occur at a time scale of seconds. Exposure of various types of cultured cells to saponin selectively permeabilized the plasma membrane and resulted in a rapid swelling of the endoplasmic reticulum even before a loss of permeability barrier was detectable with a low-molecular mass dye. The structural alteration was reversible provided the exposure to saponin was not too long. Mechanical damage of the plasma membrane resulted in a large-scale transition of the endoplasmic reticulum from a tubular to a vesicular morphology within seconds, also in Ca(2+)-depleted cells. The rapid onset of the phenomenon suggests that it could perform a physiological function. Various mechanisms are discussed whereby endoplasmic reticulum vesicularization could assist in protection against cytosolic Ca(2+) overload in cellular stress situations like plasma membrane injury.

Secretory pathway stress responses as possible mechanisms of disease involving Golgi Ca2+ pump dysfunction

In mammalian tissues, uptake of Ca(2+) and Mn(2+) by Golgi membranes is mediated by the secretory pathway Ca(2+) -ATPases, SPCA1 and SPCA2, encoded by the ATP2C1 and ATP2C2 genes. Loss of one copy of the ATP2C1 gene, which causes SPCA1 haploinsufficiency, leads to squamous cell tumors of keratinized epithelia in mice and to Hailey-Hailey disease, an acantholytic skin disease, in humans. Although the disease phenotypes resulting from SPCA1 haploinsufficiency in mice and humans are quite different, each species-specific phenotype is remarkably similar to those arising as a result of null mutations in one copy of the ATP2A2 gene, encoding SERCA2, the endoplasmic reticulum (ER) Ca(2+) pump. SERCA2 haploinsufficiency, like SPCA1 haploinsufficiency, causes squamous cell tumors in mice and Darier's disease, also an acantholytic skin disease, in humans. The phenotypic similarities between SPCA1 and SERCA2 haploinsufficiency in the two species, and the general functions of the two pumps in consecutive compartments of the secretory pathway, suggest that the underlying disease mechanisms are similar. In this review, we discuss evidence supporting the view that chronic Golgi stress and/or ER stress resulting from Ca(2+) pump haploinsufficiencies leads to activation of cellular stress responses in keratinocytes, with the predominance of proapoptotic pathways (although not necessarily apoptosis itself) leading to acantholytic skin disease in humans and the predominance of prosurvival pathways leading to tumors in mice.

Investigating cADPR and NAADP in intact and broken cell preparations

The body of literature characterizing cyclic adenosine diphosphoribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca2+-mobilizing second messengers is growing apace. However, their unique properties may, for the uninitiated, make them difficult to work with. This article reviews many of the available techniques (and associated pitfalls) for investigating these nucleotide messengers, predominantly focusing upon optical techniques using fluorescent reporters to measure Ca2+ in the cytosol as well as Ca2+ or pH within the lumen of intracellular organelles.

Manganese cell labeling of murine hepatocytes using manganese(III)-transferrin

Manganese(III)-transferrin [Mn(III)-Tf] was investigated as a way to accomplish manganese-labeling of murine hepatocytes for MRI contrast. It is postulated that Mn(III)-Tf can exploit the same transferrin-receptor-dependent and -independent metabolic pathways used by hepatocytes to transport the iron analog Fe(III)-Tf. More specifically, it was investigated whether manganese delivered by transferrin could give MRI contrast in hepatocytes. Comparison of the T1 and T2 relaxation times of Mn(III)-Tf and Fe(III)-Tf over the same concentration range showed that the r1 relaxivities of the two metalloproteins are the same in vitro, with little contribution from paramagnetic enhancement. The degree of manganese cell labeling following incubation for 2-7 h in 31.5 microm Mn(III)-Tf was comparable to that of hepatocytes incubated in 500 microm Mn2+ for 1 h. The intrinsic manganese tissue relaxivity between Mn(III)-Tf-labeled and Mn2+-labeled cells was found to be the same, consistent with Mn(III) being released from transferrin and reduced to Mn2+. For both treatment regimens, manganese uptake by hepatocytes appeared to saturate in the first 1-2 h of the incubation period and may explain why the efficiency of hepatocyte cell labeling by the two methods appeared to be comparable in spite of the approximately 16-fold difference in effective manganese concentration. Hepatocytes continuously released manganese, as detected by MRI, and this was the same for both Mn2+- and Mn(III)-Tf-labeled cells. Manganese release may be the result of normal hepatocyte function, much in the same way that hepatocytes excrete manganese into the bile in vivo. This approach exploits a biological process-namely receptor binding, endocytosis and endosomal acidification-to initiate the release of an MRI contrast agent, potentially conferring more specificity to the labeling process. The ubiquitous expression of transferrin receptors by eukaryotic cells should make Mn(III)-Tf particularly useful for manganese labeling of a wide variety of cells both in culture and in vivo.

Subcellular organization of calcium signalling in hepatocytes and the intact liver

Hepatocytes respond to inositol 1,4,5-trisphosphate (InsP3)-linked agonists with frequency-modulated oscillations in the intracellular free calcium concentration ([Ca2+]i), that occur as waves propagating from a specific origin within each cell. The subcellular distribution and functional organization of InsP3-sensitive Ca2+ pools has been investigated, in both intact and permeabilized cells, by fluorescence imaging of dyes which can be used to monitor luminal Ca2+ content and InsP3-activated ion permeability in a spatially resolved manner. The Ca2+ stores behave as a luminally continuous system distributed throughout the cytoplasm. The structure of the stores, an important determinant of their function, is controlled by the cytoskeleton and can be modulated in a guanine nucleotide-dependent manner. The nuclear matrix is devoid of Ca2+ stores, but Ca2+ waves in the intact cell propagate through this compartment. The organization of [Ca2+]i signals has also been investigated in the perfused liver. Frequency-modulated [Ca2+]i oscillations are still observed at the single cell level, with similar properties to those in the isolated hepatocyte. The [Ca2+]i oscillations propagate between cells in the intact liver, leading to the synchronization of [Ca2+]i signals across part or all of each hepatic lobule.

Loss of the Atp2c1 secretory pathway Ca(2+)-ATPase (SPCA1) in mice causes Golgi stress, apoptosis, and midgestational death in homozygous embryos and squamous cell tumors in adult heterozygotes

Loss of one copy of the human ATP2C1 gene, encoding SPCA1 (secretory pathway Ca(2+)-ATPase isoform 1), causes Hailey-Hailey disease, a skin disorder. We performed targeted mutagenesis of the Atp2c1 gene in mice to analyze the functions of this Golgi membrane Ca(2+) pump. Breeding of heterozygous mutants yielded a normal Mendelian ratio among embryos on gestation day 9.5; however, null mutant (Spca1(-/-)) embryos exhibited growth retardation and did not survive beyond gestation day 10.5. Spca1(-/-) embryos had an open rostral neural tube, but hematopoiesis and cardiovascular development were ostensibly normal. Golgi membranes of Spca1(-/-) embryos were dilated, had fewer stacked leaflets, and were expanded in amount, consistent with increased Golgi biogenesis. The number of Golgi-associated vesicles was also increased, and rough endoplasmic reticulum had fewer ribosomes. Coated pits, junctional complexes, desmosomes, and basement membranes appeared normal in mutant embryos, indicating that processing and trafficking of proteins in the secretory pathway was not massively impaired. However, apoptosis was increased, possibly the result of secretory pathway stress, and a large increase in cytoplasmic lipid was observed in mutant embryos, consistent with impaired handling of lipid by the Golgi. Adult heterozygous mice appeared normal and exhibited no evidence of Hailey-Hailey disease; however, aged heterozygotes had an increased incidence of squamous cell tumors of keratinized epithelial cells of the skin and esophagus. These data show that loss of the Golgi Ca(2+) pump causes Golgi stress, expansion of the Golgi, increased apoptosis, and embryonic lethality and demonstrates that SPCA1 haploinsufficiency causes a genetic predisposition to cancer.

Cell membrane permeable esters of D-myo-inositol 1,4,5-trisphosphate

d-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3, or IP3) is a ubiquitous second messenger that regulates cytosolic Ca2+ activities ([Ca2+]i). To study this signaling branch in intact cells, we have synthesized a caged and cell permeable derivative of IP3, ci-IP3/PM, from myo-inositol in 9 steps. Ci-IP3/PM is a homologue of cm-IP3/PM, a caged and cell permeable IP3 ester developed earlier. In ci-IP3/PM, 2- and 3-hydroxyl groups of myo-inositiol are protected by an isopropylidene group; whereas in cm-IP3/PM, a methoxymethylene is used. Ci-IP3/PM can be loaded into cells non-invasively to high concentrations without activating IP3 receptors (IP3Rs). UV uncaging of loaded ci-IP3 released i-IP3, a potent agonist of IP3Rs, and evoked Ca2+ release from internal stores. Interestingly, elevations of [Ca2+]i by i-IP3 lasted longer than [Ca2+]i transients by m-IP3, the uncaging product of cm-IP3. To understand this difference, we measured the metabolic stability of i-IP3 and m-IP3. Like natural IP3 which is known to be rapidly metabolized in cells, m-IP3 could only be detected within several seconds after uncaging cm-IP3. In contrast, i-IP3 was metabolized at a much slower rate. By exploiting different metabolic rates of m-IP3 and i-IP3, we developed two procedures for activating IP3Rs in cells without UV uncaging. The first method involves photolyzing ci-IP3/PM in vitro to generate i-IP3/PM. Successive additions of low micromolar i-IP3/PM to NIH 3T3 cells caused graded Ca2+ releases, confirming that "quantal Ca2+ release" occurs in fully intact cells with normal ATP supplies and undisrupted endoplasmic reticulum. The second technique utilizes two photon uncaging. After locally illuminating cells loaded with cm-IP3 with femtosecond-pulsed near-infrared light (730 nm), we observed a burst of Ca2+ activity in the uncaging area. This local Ca2+ rise rapidly propagated across cells and could be repeated many times in different sub-cellular locations to produce artificial Ca2+ oscillations of defined amplitudes and frequencies. The complementary advantages of these IP3 prodrugs should provide new approaches for studying IP3-Ca2+ signaling in intact cell populations with high spatiotemporal resolutions.

The structural integrity of the endoplasmic reticulum, and its possible regulation by inositol 1,3,4,5-tetrakisphosphate

The endoplasmic reticulum (ER) is a dynamic organelle thought to consist of a single interconnected network of membranes. Using fluorescence recovery after photobleaching (FRAP) of HEK-293 cells dually transfected with soluble fluorescent proteins targeted to the ER (GFP) and mitochondria (DsRed), we have confirmed this continuity, which contrasts that of the mitochondria, which behave as a population of discrete organelles. The degree of ER integrity (interconnected versus fragmented) has been suggested to be regulated in some cells by inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). In HEK-293 and freshly isolated murine lacrimal acinar cells, we manipulated ER structure by disrupting cellular Ca(2+) homeostasis with the Ca(2+) ionophore ionomycin, and by permeabilisation of the plasma membrane, protocols known to cause ER fragmentation. However, we were subsequently unable to detect by FRAP any significant effect of Ins(1,3,4,5)P(4) on ER integrity.

Study on Action Mode of Sphingosine 1-Phosphate in Rat Hepatocytes

Sphingosine-1-phosphate (S1P) acts on a set of G protein-coupled receptors in the plasma membrane and also as a second messenger in certain cell types. There are two possible pathways to mobilize intracellular Ca(2+) concentration by S1P. One is through phospholipase C, and the other is through intracellular Ca(2+) channels operated by S1P. The Mn(2+) quenching method was applied to elucidate the action mode of S1P-induced Ca(2+) mobilization in rat hepatocytes. In permeabilized hepatocytes, inositol trisphosphate induced Mn(2+) quenching, and it was blocked by heparin. However, S1P did not induce Mn(2+) quenching. Results suggest that S1P did not mobilize Ca(2+) through intracellular Ca(2+) channels.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Quantification of calcium signal transmission from sarco-endoplasmic reticulum to the mitochondria

Recent studies have shown that ryanodine and IP3 receptor (RyR/IP3R)-mediated cytosolic Ca2+ signals propagate to the mitochondria, initiating chains of events vital in the regulation of different cellular functions. However, the fraction of released Ca2+ utilized by the mitochondria during these processes has not been quantified. To measure the amount of Ca2+ taken up by the mitochondria, we used a novel approach that involves simultaneous fluorescence imaging of mitochondrial and cytosolic [Ca2+] in permeabilized H9c2 myotubes and RBL-2H3 mast cells. Communication between sarco-endoplasmic reticulum (SR/ER) and mitochondria is maintained in these permeabilized cells, as evidenced by the large RyR/IP3R-driven mitochondrial matrix [Ca2+] and NAD(P)H signals and also by preservation of the morphology of the SR/ER-mitochondrial junctions. Ca2+ was released from the SR/ER by addition of saturating caffeine or IP3 and subsequently thapsigargin (Tg), an inhibitor of SR/ER Ca2+ pumps. The amount of Ca2+ transmitted to the mitochondria was determined by measuring increases of global [Ca2+] in the incubation medium (cytosolic [Ca2+] ([Ca2+]c)). Mitochondrial Ca2+ uptake was calculated from the difference between [Ca2+]c responses recorded in the absence and presence of uncoupler or from [Ca2+]c elevations evoked by uncoupler or ionophore applied after complete Ca2+ mobilization from the SR/ER. [Ca2+]c increases were calibrated by adding Ca2+ pulses to the permeabilized cells. In H9c2 cells, caffeine induced partial mobilization of SR Ca2+ and mitochondria accumulated 26% of the released Ca2+. Sequential application of caffeine and Tg elicited complete discharge of SR Ca2+ without further increase in mitochondrial Ca2+ uptake. In RBL-2H3 mast cells, IP3 by itself elicited complete discharge of the ER Ca2+ store and the increase of the ionophore-releasable mitochondrial Ca2+ content reached 50% of the Ca2+ amount mobilized by IP3 + Tg. Thus, RyR/IP3R direct a substantial fraction of released Ca2+ to the mitochondria.

Regulation of ryanodine receptor opening by lumenal Ca(2+) underlies quantal Ca(2+) release in PC12 cells

Graded or "quantal" Ca(2+) release from intracellular stores has been observed in various cell types following activation of either ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (InsP(3)R). The mechanism causing the release of Ca(2+) stores in direct proportion to the strength of stimulation is unresolved. We investigated the properties of quantal Ca(2+) release evoked by activation of RyR in PC12 cells, and in particular whether the sensitivity of RyR to the agonist caffeine was altered by lumenal Ca(2+). Quantal Ca(2+) release was observed in cells stimulated with 1 to 40 mM caffeine, a range of caffeine concentrations giving a >10-fold change in lumenal Ca(2+) content. The Ca(2+) load of the caffeine-sensitive stores was modulated by allowing them to refill for varying times after complete discharge with maximal caffeine, or by depolarizing the cells with K(+) to enhance their normal steady-state loading. The threshold for RyR activation was sensitized approximately 10-fold as the Ca(2+) load increased from a minimal to a maximal loading. In addition, the fraction of Ca(2+) released by low caffeine concentrations increased. Our data suggest that RyR are sensitive to lumenal Ca(2+) over the full range of Ca(2+) loads that can be achieved in an intact PC12 cell, and that changes in RyR sensitivity may be responsible for the termination of Ca(2+) release underlying the quantal effect.

Imaging of intracellular calcium stores in single permeabilized lens cells

Intracellular Ca2+ stores in permeabilized sheep lens cells were imaged with mag-fura 2 to characterize their distribution and sensitivity to Ca2+-releasing agents. Inositol 1,4,5-trisphosphate (IP3) or cyclic ADP-ribose (cADPR) released Ca2+ from intracellular Ca2+ stores that were maintained by an ATP-dependent Ca2+ pump. The IP3 antagonist heparin inhibited IP3- but not cADPR-mediated Ca2+ release, whereas the cADPR antagonist 8-amino-cADPR inhibited cADPR- but not IP3-mediated Ca2+ release, indicating that IP3 and cADPR were operating through separate mechanisms. A Ca2+ store sensitive to IP3, cADPR, and thapsigargin appeared to be distributed throughout all intracellular regions. In some cells a Ca2+ store insensitive to IP3, cADPR, thapsigargin, and 2,4-dinitrophenol, but not ionomycin, was present in a juxtanuclear region. We conclude that lens cells contain intracellular Ca2+ stores that are sensitive to IP3, cADPR, and thapsigargin, as well as a Ca2+ store that appears insensitive to all these agents.

Modulation of rat incisor odontoblast plasma membrane-associated Ca2+ with nifedipine

In addition to the Ca2+ portion freely dissociated in the cytosol, another Ca2+ pool is associated with plasma membranes and intracellular organelle membranes. This Ca2+ portion is of importance for regulation of, among other things, the cell cycle, actin-mediated processes, and cell morphology. In the literature, dihydropyridines have been reported to influence this membrane-associated pool of Ca2+ under certain conditions. The aim of this investigation was to study possible modulations of plasma membrane-associated Ca2+ upon treatment with nifedipine in vitro in a Ca2+-transporting cell, the dentin-forming odontoblast. The membrane-associated portion of Ca2+ in dissected dentinogenically active rat incisor odontoblasts was monitored by fluorescence spectrophotometry using chlortetracycline as a probe. In addition, images of chlortetracycline-Ca2+ binding were obtained by fluorescence microscopy. It was found that membrane-associated Ca2+ decreased by the dihydropyridine nifedipine, whereas this Ca2+ pool was unaffected by the cellular polarization state, which was in contrast to cytosolic free Ca2+ as measured by fura-2. The results show that the odontoblast plasma membrane-associated Ca2+-pool can be modulated by nifedipine, thus being dependent on the conformational state of the L-type Ca2+ channels.

Ca2+ feedback on "quantal" Ca2+ release involving ryanodine receptors

The influence of luminal and cytoplasmic Ca2+ on the ability of ryanodine-sensitive stores to undergo multiple partial ("quantal") releases has been assessed. Increased luminal Ca2+ levels do indeed modulate sarcoplasmic reticulum Ca2+ release by lowering the threshold agonist concentration required to elicit release, but the decrease in luminal Ca2+ that accompanies a partial release is not sufficient by itself to terminate release. Similarly, an increase in cytoplasmic Ca2+ lowers the threshold agonist concentration required to elicit release; thus, the bulk cytoplasmic Ca2+ levels attained during a release would only stimulate further release, not terminate it before it reached completion. Very high cytoplasmic Ca2+ levels (1-3 mM) also triggered release but were unable to terminate release before reaching completion. Thus, even the high local cytoplasmic Ca2+ concentration that might accompany release would also not terminate release. It is concluded that Ca2+ feedback can modulate release through ryanodine receptors but that it does not account for the properties of quantal release. The low affinity inhibitor tetracaine induces a decrease in the extent of release that cannot be explained solely by heterogeneous caffeine sensitivity of the stores. The results are interpreted in terms of a scheme that includes (i) heterogeneous sensitivity of stores, conferred in part by differences in luminal Ca2+ content and (ii) adaptive behavior on the part of individual ryanodine receptors.

Activation of calcium signaling in isolated rat hepatocytes is accompanied by shape changes of microvilli

Preceding studies using the hamster insulinoma cell line, HIT, and isolated rat hepatocytes have shown that two essential components of the Ca2+ signaling pathway, the ATP-dependent Ca2+ store and the store-coupled Ca2+ influx pathway, are both located in microvilli covering the surface of these cells. Microvilli-derived vesicles from both cell types exhibited anion and cation pathways which could be inhibited by anion and cation channel-specific inhibitors. These findings suggested that the microvillar tip compartment forms a space which is freely accessible for external Ca2+, ATP, and IP3. The entry of Ca2+ into the cytoplasm, however, is largely restricted by the microvillar core structure, the dense bundle of actin microfilaments acting as a diffusion barrier between the microvillar tip compartment and the cell body. Moreover, evidence has been presented that F-actin may function as ATP-dependent and IP3-sensitive Ca2+ store that can be emptied by profilin-induced depolymerization or reorganization [K. Lange and U. Brandt (1996) FEBS Lett. 395, 137-142]. Here we demonstrate the tight connection between microvillar shape changes and the activation of the Ca2+ signaling system in isolated rat hepatocytes. Using a combination of scanning electron microscopy (SEM) and fura-2 fluorescence technique, we confirmed a consequence of the "diffusion barrier" concept of Ca2+ signaling: Irrespective of the type of the applied stimulus, activation of the Ca2+ influx pathway is accompanied by changes in the structural organization of microvilli indicative of the loss of their diffusion barrier function. We further show that the cell surfaces of unstimulated hepatocytes isolated by either the collagenase or the EDTA perfusion technique are densely covered with microvilli predominantly of a short and slender type. Beside this rather uniformly shaped type of microvilli, a number of dilated surface protrusions were observed. Under these conditions the cells displayed the well known rather high basal [Ca2+]i of 200-250 nM as repeatedly demonstrated for freshly isolated hepatocytes. However, addition of the serine protease inhibitor, phenylmethanesulfonyl fluoride (PMSF), to the cell suspension immediately after its preparation reduced the basal cytoplasmic Ca2+ level to about 100 nM. Concomitantly, dilated surface protrusions disappeared, and cell surfaces exclusively displayed short, slender microvilli. Activation of the Ca2+ signaling pathway by vasopressin, as well as by the IP3-independent acting Ca2+ store inhibitor, thapsigargin, was accompanied by a conspicuous shortening and dilation of microvilli following the same time courses as the respective increases of [Ca2+]i induced by the effectors. Furthermore, the abundance of the large form of surface protrusions on isolated hepatocytes positively correlated with the size of a cellular Ca2+/Fura-2 compartment which is rapidly depleted from Ca2+ by extracellular EGTA. These findings support the postulated localization of the store-coupled Ca2+ influx pathway in microvilli of HIT cells also for hepatocytes and are in accord with the notion of a cytoskeletal diffusion barrier regulating the flux of external Ca2+ via the microvillar tip region in the cytoplasm.

Intracellular Ca2+ signals induced by ATP and thapsigargin in glioma C6 cells. Calcium pools sensitive to inositol 1,4,5-trisphosphate and thapsigargin

In glioma C6 cells, extracellular ATP generates inositol 1,4,5-trisphosphate (InsP3), indicating the presence of purinergic receptors coupled to phosphoinositide turnover. To identify the effect of ATP (acting via InsP3) and thapsigargin (acting without InsP3 production as a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase) on intracellular Ca2+ pools we used video imaging of Fura-2 loaded into single, intact glioma C6 cells. It has been shown that ATP and thapsigargin initiate Ca2+ response consistent with the capacitative model of Ca2+ influx. When the cells were stimulated by increasing concentrations of ATP (1, 10, 50 and 100 microM) the graded, quantal Ca2+ response was observed. In the absence of extracellular Ca2+ thapsigargin and ionomycin-releasable Ca2+ pools are overlapping, demonstrating that Ca2+ stores are located mainly in the endoplasmic reticulum. After maximal Ca2+ mobilization by ATP, thapsigargin causes further increase in cytosolic Ca2+ concentration, whereas emptying of thapsigargin-sensitive intracellular stores prevents any further Ca2+ release by ATP. Thus, the thapsigargin-sensitive intracellular pool of Ca2+ in glioma C6 cells seems to be larger than that sensitive to InsP3. Two hypothesis to explain this result are proposed. One postulates a presence of two different Ca2+ pools, sensitive and insensitive to InsP3 and both discharged by thapsigargin, and the other, the same intracellular pool of Ca2+ completely emptying by thapsigargin and only partially by InsP3. These results may contribute to understanding the mechanism of Ca2+ signalling mediated by ATP, the most potent intracellular Ca2+ mobilizing agonist in all types of glial cells.

Environmental bioadhesion: themes and applications

Many marine organisms attach to underwater surfaces using protein adhesives. These are basic proteins with high levels of the amino acid 3,4-dihydroxyphenylalanine and an extended flexible conformation. The hydroxylation of tyrosine residues plays a key role in the chemisorption of these polymers to surfaces and in the setting of the adhesive. These unique proteins are attracting biotechnological attention for application in industry and medicine. Recent development on the immobilization of antigens and antibodies, enzymes, cells and tissues, illustrate the great potential use of these adhesives for diagnostics and medicine. The use of these adhesive proteins as anticorrosive coats for metal also suggests important applications for industry.

Inhibition of hepatic inositol 1,4,5-trisphosphate receptor function by ethanol and other short chain alcohols

The ability of alcohols to regulate InsP3-receptor activity was examined in permeabilized hepatocytes. Incubation with 30-300 mM ethanol decreased the sensitivity to InsP3 for Ca2+ release, with little effect on the size of the Ca2+ store that could be released with maximal concentrations of InsP3. Ethanol (300 mM) increased the EC50 for InsP3 from a control value of 134.0 +/- 13.5 nM to 220.0 +/- 25.9 nM. Although ethanol also caused a partial depletion of the total pool of stored Ca2+, the ethanol-induced shift in InsP3 sensitivity was not secondary to this alteration in Ca2+ loading. Partial depletion of the Ca2+ stores with low doses of ionomycin and thapsigargin did not cause a shift in InsP3 sensitivity. Furthermore, measurements of InsP3 receptor channel activity using retrograde flux of Mn2+ to quench the fluorescence of Fura-2 within the Ca2+ stores demonstrated that ethanol inhibited InsP3-activated channel activity in the absence of stored Ca2+. Other short chain alcohols (methanol, 1-propanol and 1-butanol) also decreased the efficacy of InsP3 to release Ca2+. Measurements of [3H]-InsP3 binding demonstrated that ethanol decreased the total number of InsP3 binding sites without changing the KD. The effect of ethanol on InsP3 binding was apparent in the presence or absence of Ca2+ and was observed when the cells were pre-incubated with ethanol at either 37 degrees C or 4 degrees C. The initial rate of InsP3-induced Mn2+ quenching of compartmentalized Fura-2 was reduced by ethanol at all doses of InsP3. These data suggest that ethanol decreases the sensitivity of the intracellular Ca2+ store to release by InsP3, by reducing the number of channels that can be activated by InsP3.

Inositol-phosphoglycan inhibits calcium oscillations in hepatocytes by reducing calcium entry

Inositol-phosphoglycan (IPG) is a putative mediator of insulin action that has been shown to affect numerous biochemical processes. IPG, prepared from liver membranes, promptly inhibited phenylephrine- or vasopressin-induced [Ca2+]i oscillations when perfused over Fura-2-dextran injected rat hepatocytes. An antibody to IPG suppressed the inhibitory effect of insulin on the [Ca2+]i oscillations. Measurement of the rate of quench of cytoplasmic Fura-2 by extracellular Mn2+ showed that Ca2+ entry occurred continuously in the unstimulated cell and was not affected by phenylephrine or vasopressin. IPG, specifically, almost completely abolished the Mn2+ quench rate. Elevated extracellular [Ca2+] reversed the inhibitory effect of IPG on [Ca2+]i oscillations. We conclude that IPG inhibits the hepatocyte Ca2+ oscillatory by reducing the continuous Ca2+ influx that is required to sustain oscillations in [Ca2+]i.

Inositol 1,4,5-trisphosphate and calcium regulate the calcium channel function of the hepatic inositol 1,4,5-trisphosphate receptor

The regulation of the inositol 1,4,5-trisphosphate (IP3) receptor in liver was analyzed using a novel superfusion method. Hepatic microsomes were loaded with 45Ca2+, and superfused at high flow rates to provide precise control over IP3 and Ca2+ concentrations ([Ca2+]) and to isolate 45Ca2+ release from reuptake. 45Ca2+ release was dependent on both [Ca2+] and IP3. The initial rate of 45Ca2+ release was a biphasic function of [Ca2+], increasing as [Ca2+] approached 3 microM but decreasing at higher concentrations, suggesting that the hepatic IP3 receptor is regulated by [Ca2+] at two sites, a high affinity potentiation site and a low affinity inhibitory site. The relationship between initial rates and IP3 concentration was steep (Hill coefficient of 3.4), suggesting that activation of the calcium channel requires binding of at least 3 IP3 molecules. IP3 concentrations above 10 microM produced rapid decay of release rates, suggesting receptor inactivation. Superfusion with 10 microM IP3 under conditions that minimize calcium release ([Ca2+] < 1 nM) inhibited 45Ca2+ release in response to subsequent stimulation (400 nM Ca2+). These data suggest sequential positive and negative regulation of the hepatic IP3 receptor by cytosolic calcium and by IP3, which may underlie hepatocellular propagation of regenerative, oscillatory calcium signals.

Ca2+ flow via tunnels in polarized cells: recharging of apical Ca2+ stores by focal Ca2+ entry through basal membrane patch

Intracellular Ca2+ store depletion induces Ca2+ entry across the plasma membrane, allowing the store to recharge. In our experiments, Ca2+ stores in pancreatic acinar cells were depleted by acetylcholine (ACh) stimulation in Ca2+-free solution. Thereafter, Ca2+ entry was only allowed through a CaCl2-containing pipette attached to the basal membrane. Recharging intracellular Ca2+ stores via a patch pipette occurred without a rise in the cytosolic Ca2+ concentration and depended on the operation of a thapsigargin-sensitive Ca2+ pump. After a period of focal Ca2+ entry, ACh could again evoke a rise in the cytosolic Ca2+ concentration, and this rise always started in the apical secretory pole. Recharging the apical Ca2+ store therefore depends on Ca2+ flow through a tunnel from the basal to the secretory pole, and the endoplasmic reticulum Ca2+ pump is essential for this process.

Inositol 1,4,5-trisphosphate-dependent oscillations of luminal [Ca2+] in permeabilized HSY cells

Oscillations in intracellular Ca2+ concentration ([Ca2+]i) are thought to play an important role in phosphoinositide-linked Ca2+ signaling events. We demonstrate corresponding inositol 1,4, 5-trisphosphate (IP3)-dependent oscillations of Ca2+ concentration within the lumen of the IP3-sensitive stores ([Ca2+]L) of saponin-permeabilized HSY cells by monitoring [Ca2+]L with the fluorescent Ca2+ indicator Mag-fura-2. The associated openings and closings of the IP3-sensitive Ca2+ release channel were detected via quenching of Mag-fura-2 fluorescence due to the entry of Mn2+, a Ca2+ surrogate, into the stores. Evidence for complimentary Ca2+ oscillations at the external surface of the stores was provided by the membrane-bound Ca2+ probe Calcium Green C18. The permeabilization of saponin-treated HSY cells to macromolecules was confirmed by demonstrating that permeabilized cells readily took up and lost (t1/2 approximately 46 s) a fluorescently tagged 70-kDa dextran. Our results impose a number of constraints on possible mechanisms for [Ca2+]i oscillations. In addition, they support recent proposals that [Ca2+]i oscillations arise directly from the (biphasic) effects of Ca2+ itself on IP3-sensitive Ca2+ channel activity.

Quantal Ca2+ release and inactivation in a model of the inositol 1,4,5-trisphosphate receptor involving transformation of the ligand by the receptor

A model explaining quantal Ca2+ release as an intrinsic property of the inositol 1,4,5-triphosphate (IP3) receptor has been put forward. The model is based on the hypothesis that the IP3 receptor can catalyze a transformation of the IP3 molecule differing from its conventional metabolism. A simple kinetic mechanism is considered, in which IP3-induced Ca2+ channel opening is followed by the step of IP3 conversion and channel closure. Examination of the resulting mathematical model shows that it can reproduce well both partial release of stored Ca2+ and the same responsiveness to subsequent IP3 additions. On incorporation of an additional closed state of the channel, the model describes also a time-dependent channel inactivation at a high IP3 dose. Temperature sensitivity of the catalytic step accounts for the reported elimination of quantal responses and inactivation at low temperature. The transformation product is surmised to be a positional or stereo isomer of IP3.

Quantal calcium release in electropermeabilized SH-SY5Y neuroblastoma cells perfused with myo-inositol 1,4,5-trisphosphate

Continuous perfusion of immobilized electropermeabilized SH-SY5Y neuroblastoma cells was utilised as a novel approach to the assessment of incremental activation and inactivation of myo-inositol 1,4,5-trisphosphate (IP3)-induced calcium (Ca2+) mobilisation (IICM). SH-SY5Y cells when stimulated with sub-optimal IP3 exhibited a rapid concentration dependent activation of Ca2+ mobilization followed by a partial inactivation. Although this partial inactivation allowed net Ca2+ mobilized to be stringently returned to basal levels, a concentration-dependent depletion of the store was maintained while ever perfusion with the stimulating IP3 concentration was sustained. This partial inactivation of IP3-induced quantal Ca2+ release (QCR) was only compromised if cells, with replete Ca2+ stores, were perfused with supra-maximally effective concentrations of IP3 (5-10 microM). Thus, at supra-optimal IP3 concentrations, a reproducible plateau of Ca2+ release lying 50-150 nM above the basal Ca2+ concentration was observed. Feedback on IP3R sensitivity by gross cytosolic Ca2+ levels could be eliminated as the sustained and exclusive mediator of incremental activation/inactivation cycle of IICM in SH-SY5Y cells, since released Ca2+ was perfused away from the immobilized cells. Thus, while ever the cells were continuously perfused with IP3, impressive incremental inactivation was apparent. Additionally, IP3R partial agonists were found to exhibit lower intrinsic activity for both activation and inactivation of QCR, suggesting that ligand-induced inactivation of the IP3R was more important than inactivation mechanisms reliant on either Ca2+ flux through the channel and/or calcium store depletion. Therefore, we suggest that, in perfused SH-SY5Y cells, the most parsimonious explanation of our data is that IP3 binding probably activates and then partially inactivates its receptor in a concentration-dependent fashion to produce the QCR phenomenon.

Cyclic GMP induces oscillatory calcium signals in rat hepatocytes

The ability of guanosine-3',5'-cyclic monophosphate (cGMP) to induce increases in the intracellular free calcium ion concentration ([Ca2+]i) was studied at the single cell level in fura-2-loaded rat hepatocytes. Both 8-bromo-cGMP (Br-cGMP) and dibutyryl cGMP (db-cGMP) produced oscillatory [Ca2+]i increases in hepatocytes. In addition, Br-cGMP increased the frequency of agonist-induced spiking or converted [Ca2+]i oscillations into sustained nonoscillatory [Ca2+]i responses. Addition of the nitric oxide donor sodium nitroprusside also produced oscillatory [Ca2+]i increases similar to those generated by cGMP analogues. In the absence of extracellular Ca2+, cGMP-induced [Ca2+]i responses were significantly reduced and mainly appeared as single transient [Ca2+]i increases. The effects of cGMP analogues do not appear to be mediated by a secondary increase in cAMP or activation of cAMP-dependent protein kinase (PKA), since [Ca2+]i responses to cGMP analogues were inhibited by the G-kinase inhibitor 8-bromoguanosine-3',5'-cyclic monophosphorothioate (Rp-Br-cGMP[S]). Both Br-cGMP and db-cGMP also increased [Ca2+]i in the presence of the PKA inhibitor 8-bromoadenosine-3',5'-cyclic monophosphorothioate (Rp-Br-cAMP[S]) and when the cGMP-inhibitable cAMP phosphodiesterase activity was inhibited by pretreatment with siguazodan. Br-cGMP stimulated the Mn2+-induced quench of compartmentalized fura-2 in intact hepatocytes, indicating a site of action at the level of the Ca2+ stores. This locus was further supported by the finding that pretreatment of hepatocytes with Br-cGMP potentiated submaximal inositol 1,4,5-trisphosphate (InsP3)-induced Mn2+ quench in subsequently permeabilized hepatocytes. db-cGMP also decreased PKA-mediated back phosphorylation of the hepatic type-1 InsP3 receptor, indicating that G-kinase phosphorylates the InsP3 receptor at sites targeted by PKA. These data indicate that phosphorylation of the hepatic InsP3 receptor by G-kinase increases the sensitivity to InsP3 for [Ca2+]i release and is associated with the production of [Ca2+]i oscillations in single rat hepatocytes.

Evidence that quantal Ca2+ release in HSG cells is not due to 'all-or-none' release from discrete Ca2+ stores with differing sensitivities to IP3

We demonstrate that the application of the muscarinic agonist carbachol to the human salivary epithelial cell line HSG elicits the now well-known phenomenon of 'quantal' Ca2+ release; namely, that the application of a submaximal concentration of agonist results in the release of only a portion of the agonist-sensitive intracellular Ca2+ pool. One explanation that has been proposed to account for this effect is that there are multiple intracellular Ca2+ stores, each with a different agonist sensitivity, which release Ca2+ in an 'all-or-none' fashion. We test this hypothesis in intact HSG cells with an experimental protocol designed to preferentially load less-agonist-sensitive stores with 40Ca2+ and more-agonist-sensitive stores with 45Ca2+. However, contrary to the expectations of the above explanation, these cells do not preferentially release 45Ca2+ in response to low concentrations of agonist. Thus our data suggest that quantal Ca2+ release must arise from some other property of the stores or their Ca2+ release channels.

Large vesicle formation within cells induced by treatment with a mixed surfactant

MDCK cells have been treated with a mixed surfactant at low concentrations to study the induced morphological changes. The most significant change at the light microscope level was the appearance of multiple large vesicles, which increased in size with time, up to approximately 40 microns in diameter. Vesicle formation was shown to be linked with the uptake of the fluid medium, as judged by the presence of FITC-dextran within the vesicles, but was not a result of pinocytosis because cytochalasin D treatment had no effect on their formation. Furthermore, nile red staining demonstrated that the vesicles did not represent fusion of pre-existing lipid droplets. Transmission electron microscopy (TEM) analysis indicated that the vesicles lacked any obvious structure. It is hypothesised that the vesicles are large mixed structures synthesised as a result of interactions between cell membranes and detergent components after saturation with the surfactants. This effect is contrasted with the diffuse uptake of dyes and fluorescently labelled proteins following simple anionic or ionic detergent treatment. The effect of vesicle formation was reversible if the cells were placed in fresh medium lacking detergent. Other effects of mixed detergent included the loss of rounded compact colonies, an increase in mean cell diameter and the almost complete loss of surface microvilli as seen with scanning electron microscopy (SEM). In the TEM the cell ultrastructure was seen to have changed markedly following detergent treatment, with a loss of rough endoplasmic reticulum and an apparent clumping of the cytoplasmic constituents.

Imaging of intracellular calcium stores in individual permeabilized pancreatic acinar cells. Apparent homogeneous cellular distribution of inositol 1,4,5-trisphosphate-sensitive stores in permeabilized pancreatic acinar cells

Several lines of evidence suggest that the existence of a heterogeneous population of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-sensitive Ca2+ stores underlies the polarized agonist-induced rise in cytosolic Ca2+ concentration ([Ca2+]i) in pancreatic acinar cells (Kasai, H., Li, Y. X., and Miyashita, Y. (1993) Cell 74, 669-677; Thorn, P., Lawrie, A. M., Smith, P. M., Gallacher, D. V., and Petersen, O. H. (1993) Cell 74, 661-668). To investigate whether the apical pole of acinar cells contains Ca2+ stores which are relatively more sensitive to Ins(1,4,5)P3 than those in basolateral areas, we studied Ca2+ handling by Ca2+ stores in individual streptolysin O (SLO) permeabilized cells using the low affinity Ca2+ indicator Magfura-2 and an in situ imaging technique. The uptake of Ca2+ by intracellular Ca2+ stores was ATP-dependent. A steady-state level was reached within 10 min, and the free Ca2+ concentration inside loaded Ca2+ stores was estimated to be 70 microM. Ins(1,4,5)P3 induced Ca2+ release in a dose-dependent, "quantal" fashion. The kinetics of this release were similar to those reported for suspensions of permeabilized pancreatic acinar cells. Interestingly, the permeabilized acinar cells showed no intercellular variation in Ins(1,4,5)P3 sensitivity. Although SLO treatment is known to result in a considerable loss of cytosolic factors, permeabilization did not result in a redistribution of zymogen granules, as judged by electron microscope analysis. These results suggest that Ins(1,4,5)P3-sensitive Ca2+ stores are unlikely to be redistributed as a result of SLO treatment. The effects of Ins(1,4,5)P3 were therefore subsequently studied at the subcellular level. Detailed analysis demonstrated that no regional differences in Ins(1,4,5)P3 sensitivity exist in this permeabilized cell system. Therefore, we propose that additional cytosolic factors and/or the involvement of ryanodine receptors underlie the polarized pattern of agonist-induced Ca2+ signaling in intact pancreatic acinar cells.

The quantal nature of calcium release to caffeine in single smooth muscle cells results from activation of the sarcoplasmic reticulum Ca(2+)-ATPase

Calcium release from intracellular stores occurs in a graded manner in response to increasing concentrations of either inositol 1,4,5-trisphosphate or caffeine. To investigate the mechanism responsible for this quantal release phenomenon, [Ca2+] changes inside intracellular stores in isolated single smooth muscle cells were monitored with mag-fura 2. Following permeabilization with saponin or alpha-toxin the dye, loaded via its acetoxymethyl ester, was predominantly trapped in the sarcoplasmic reticulum (SR). Low caffeine concentrations in the absence of ATP induced only partial Ca2+ release; however, after inhibiting the calcium pump with thapsigargin the same stimulus released twice as much Ca2+. When the SR Ca(2+)-ATPase was rendered non-functional by depleting its "ATP pool," submaximal caffeine doses almost fully emptied the stores of Ca2+. We conclude that quantal release of Ca2+ in response to caffeine in these smooth muscle cells is largely due to the activity of the SR Ca(2+)-ATPase, which appears to return a portion of the released Ca2+ back to the SR, even in the absence of ATP. Apparently the SR Ca(2+)-ATPase is fueled by ATP, which is either compartmentalized or bound to the SR.

Kinetics of calcium release by immunoaffinity-purified inositol 1,4,5-trisphosphate receptor in reconstituted lipid vesicles

The kinetics of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release of the immunoaffinity-purified IP3 receptor (IP3R), reconstituted into lipid vesicles, was investigated using the fluorescent Ca2+ indicator fluo-3. IP3R was purified from mouse cerebellar microsomal fraction by using an immunoaffinity column conjugated with an anti-IP3R type 1 (IP3R1) antibody. The immunoblotting analysis using monoclonal antibodies against each IP3R type showed that the purified IP3R is almost homogeneous, composed of IP3R1. Ca2+ efflux from the proteoliposomes was monitored as fluorescence changes of 10 microM fluo-3, whose concentration was high enough to buffer released Ca2+ and to keep deviations of extravesicular free Ca2+ concentration within 30 nM, excluding the possibility of Ca(2+)-mediated regulation of IP3-induced Ca2+ release. We also examined IP3-induced Ca2+ release using 1 microM fluo-3, where the deviations of free Ca2+ concentration were within 300 nM. At both fluo-3 concentrations, IP3-induced Ca2+ release showed similar kinetic properties, i.e. little Ca2+ regulation of Ca2+ release was observed in this system. IP3-induced Ca2+ release of the purified IP3R exhibited positive cooperativity; the Hill coefficient was 1.8 +/- 0.1. The half-maximal initial rate for Ca2+ release occurred at 100 nM IP3. At the submaximal concentrations of IP3, the purified IP3R showed quantal Ca2+ release, indicating that a single type of IP3R is capable of producing the phenomenon of quantal Ca2+ of release. The profiles of the IP3-induced Ca2+ release of the purified IP3R were found to be biexponential with the fast and slow rate constants (k(fast) = 0.3 approximately 0.7 s-1, k(slow) = 0.03 approximately 0.07 s-1), indicating that IP3R has two states to release CA2+. The amount of released Ca2+ by the slow phase was constant over the range of 10-5000 nM IP3 concentrations, whereas that by the fast phase increased in proportion to added IP3. This provides evidence to support the view that the fast phase of Ca2+ release is mediated by the low affinity state and the slow phase by the high affinity state of the IP3R. This also suggests that the fast component of Ca2+ release is responsible for the process of quantal Ca2+ release.

The effect of mersalyl on inositol trisphosphate receptor binding and ion channel function

A number of thiol-reactive agents induce repetitive Ca2+ spiking in cells by a mechanism thought to involve sensitization of the inositol 1,4,5-trisphosphate receptor (IP3R). To further define the basis of this interaction, we have studied the effect of several thiol-reactive agents on [3H]IP3 binding, IP3-gated channel activity, and conformation of the IP3R in membranes from hepatocytes, cultured WB rat liver epithelial cells, and cerebellum microsomes. At 4 degrees C, the organomercurial thiol-reactive agent mersalyl markedly stimulates (3-4fold) [3H]IP3 binding to permeabilized hepatocytes. The closely related molecule, thimerosal, has only a small stimulatory effect under these conditions, and GSSG or N-ethylmaleimide are without effect. The stimulatory effect of mersalyl was associated with a decrease in Kd of the IP3R with no change in Bmax. Mersalyl was without effect on detergent-solubilized hepatocyte binding sites or on the [3H]IP3 binding activity of cerebellum microsomes. In contrast to thimerosal, which potentiates IP3-mediated Ca2+ release, mersalyl blocked IP3-gated Ca2+ channels. Mersalyl pretreatment of WB membranes altered the pattern of immunoreactive receptor fragments generated upon subsequent cleavage of the receptor with proteinase K. This effect was not reproduced by thimerosal and was also not observed in experiments on cerebellum microsomes. We conclude that the WB cell and brain IP3 receptors are differently regulated by modification of thiol groups. Reaction of the WB cell IP3 receptor with mersalyl alters its conformation and modifies the accessibility of sites on the protein that are cleaved by proteinase K. In the presence of mersalyl, the receptor has high affinity for IP3 but is inactive as a Ca2+ channel. This contrasts with the high affinity receptor/active Ca2+ channel induced by thimerosal, suggesting that even closely related thiol agents may interact at different thiol groups.

Differences in uptake, storage and release properties between inositol trisphosphate-sensitive and -insensitive Ca2+ stores in permeabilized pancreatic acinar cells

Rabbit pancreatic acinar cells, permeabilized by saponin treatment, were used to study the kinetics of ATP-dependent Ca2+ uptake and release in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-sensitive and -insensitive stores. Permeabilized acinar cells rapidly accumulated Ca2+ to steady-state. At steady state, approximately 60% of actively stored Ca2+ resided in the Ins-1,4,5-P3-sensitive store. Kinetic analysis of the Ca2+ uptake process revealed that the initial Ca2+ uptake rate was 1.7 times higher in the Ins-1,4,5-P3-insensitive store as compared to the Ins-1,4,5-P3-sensitive store. On the other hand, the Ca2+ uptake capacity was 1.6 times higher in the Ins-1,4,5-P3-sensitive store as compared to the Ins-1,4,5-P3-insensitive store. The Ca2+ uptake rate in the Ins-1,4,5-P3-sensitive store remained virtually constant for at least 4 min, whereas in the Ins-1,4,5-P3-insensitive Ca2+ store this rate progressively declined with time. These observations are compatible with: (i) an Ins-1,4,5-P3-sensitive store containing relatively few Ca2+ pumps but possessing a relatively high Ca2+ uptake capacity, which may reflect the presence of a substantial amount of Ca2+ binding protein; and (ii) an Ins-1,4,5-P3-insensitive Ca2+ store containing relatively many Ca2+ pumps but possessing a relatively low Ca2+ uptake capacity, which may reflect the presence of little if any Ca2+ binding protein. The data presented are consistent with the idea of a heterogeneous distribution of Ca2+ pumps, Ca2+ binding proteins and Ca2+ release channels between intracellular Ca2+ storage organelles.

Heterogeneity of channel density in inositol-1,4,5-trisphosphate-sensitive Ca2+ stores

Inositol-1,4,5-trisphosphate (InsP3)-induced Ca2+ release is a key mechanism for intracellular Ca2+ mobilization. The rate of Ca2+ release declines progressively with time until a higher concentration of InsP3 is added, which is referred to as the incremental detection mechanism. Two hypotheses have been postulated to explain these complex kinetics: (1) Ca2+ stores consist of multiple compartments (quanta) with different sensitivities to InsP3 (refs 3-7), and (2) the rate of Ca2+ release is modulated by the Ca2+ concentration in the lumen of Ca2+ stores. We studied this phenomenon by real-time measurement of the luminal Ca2+ concentration of Ca2+ stores using a Ca(2+)-sensitive fluorescent dye, but our results were not explained by either of these hypotheses. Here we report that the complex kinetics of Ca2+ release results from the heterogeneous density of equally InsP3-sensitive channels on the Ca2+ stores. This heterogeneity creates Ca2+ stores with apparently different sensitivities to InsP3, which may have different functions in Ca2+ mobilization.

Immunogold localization of inositol 1,4,5-trisphosphate receptors and characterization of ultrastructural features of the sarcoplasmic reticulum in phasic and tonic smooth muscle

Although agonist stimulation leads to an increase in inositol 1,4,5-trisphosphate (InsP3) and decreased calcium in peripherally and centrally located sarcoplasmic reticulum in smooth muscle, the distribution of InsP3 receptors is unknown. InsP3 receptor and the calcium binding protein, calsequestrin were localized by immunolabelling in a tonic and a phasic smooth muscle. InsP3 receptor labelling was predominantly localized at the cell periphery, where most of the sarcoplasmic reticulum is localized in vas deferens (phasic muscle). Elements of central sarcoplasmic reticulum, where present, were also labelled. Distribution of calsequestrin in vas deferens was similar to that of the InsP3 receptor. In aorta (tonic muscle) the InsP3 receptor labelling was proportional to sarcoplasmic reticulum distribution: predominantly central. No labelling of sections or immunoblots was observed with the anti-calsequestrin antibody in aorta. InsP3 and caffeine, but not cyclic ADP-ribose, released intracellular Ca2+ in permeabilized vas deferens and aorta. The ultrastructure of the sarcoplasmic reticulum, investigated in stereo views of semi-thick and thin sections of osmium ferricyanide stained tissue, is shown to have several distinctive features, such as fenestrated sheets (single or in stacks), as well as numerous regions of continuity between central and peripheral sarcoplasmic reticulum, suggesting a single compartment within the smooth muscle cell. Regions of the sarcoplasmic reticulum were closely apposed to and often ensheathed mitochondria. We conclude that InsP3 receptors are present in both the central and the peripheral sarcoplasmic reticulum of tonic and phasic smooth muscle, consistent with electron probe analysis results showing calcium release from both regions.

Two inositol 1,4,5-trisphosphate binding sites in rat basophilic leukemia cells: relationship between receptor occupancy and calcium release

Quantal calcium release is a novel paradigm for second messenger signal transduction which provides spatial and temporal control of calcium release from intracellular stores by inositol 1,4,5-trisphosphate (InsP3). We have proposed a mechanism to account for this phenomenon [Kindman, L. A., & Meyer, T. (1993) Biochemistry 32, 1270-1277], which hypothesized the existence of five channels, each with a different affinity for InsP3. As a direct test of this hypothesis, InsP3 binding to microsomes from RBL cells was examined under conditions similar to those used for calcium release. Scatchard analyses performed under a variety of conditions indicates the presence of high affinity (KD = 0.9 +/- 0.3 nM) and low affinity (KD = 47 +/- 5 nM) InsP3 binding sites. The low affinity sites are more prevalent, constituting 82 +/- 5% of the total. Both sites are identified in the presence and absence of MgATP. Moreover, both sites are selective for InsP3 over InsP4, through high concentrations of InsP4 displace InsP3 from each site (with inhibition constants of 16 and 267 nM InsP4, respectively). The relative abundance of the two InsP3 binding sites is Ca2+ dependent. An increase in Ca2+ from 0.1 to 0.5 microM results in the apparent conversion of a portion of the low affinity sites into high affinity sites into high affinity sites. Ca2+ (0.5 microM) also increased the KD of the low affinity InsP3 binding site. Given the presence of both high and low affinity InsP3 binding sites, two simple mathematical models describing both the kinetics of calcium release and quantal calcium release from RBL cells were developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced calcium signalling events in neuroblastoma x glioma hybrid NG108-15 cells after treatment with dibutyryl cyclic AMP

The effects of dibutyryl cyclic AMP (dbcAMP) treatment on Ca2+ channel activities, Ca2+ accumulation by intracellular Ca2+ pools, and sizes of IP3- and GTP-releasable pools in neuroblastoma x glioma hybrid NG108-15 cells were studied. High extracellular K+ induced a greater rise in intracellular calcium concentration ([Ca2+]i) in dbcAMP-treated cells than in control cells. In dbcAMP-treated cells, the initial phase of the high K(+)-induced [Ca2+]i rise displayed a much higher sensitivity to omega-conotoxin than it did in control cells, whereas the plateau phase of the [Ca2+]i rise was sensitive only to nifedipine. These results indicate that predominantly L-type Ca2+ channels exist in control cells, and that N-type channels develop only after dbcAMP treatment. In dbcAMP-treated cells, mitochondria showed an increased Ca2+ uptake capacity (5.3 nmol Ca2+/mg protein) compared with that in control cells (4.2 nmol Ca2+/mg protein). However, dbcAMP treatment did not cause significant change in the affinity for Ca2+. Dibutyryl cAMP treatment enhanced the Ca2+ accumulation activity by nonmitochondrial pools (from 0.84 to 0.97 nmol Ca2+/mg protein) and increased the affinity for Ca2+ (EC50 for Ca2+ decreased from 0.146 microM to 0.063 microM). Our data also indicate that the pool that is sensitive to both IP3 and GTP was enlarged. The affinities for IP3 and GTP in causing Ca2+ release remained the same before or after dbcAMP treatment.

Propagation of cytosolic calcium waves into the nuclei of hepatocytes

The temporal and spatial organization of [Ca2+] changes within the nucleus of Fura-2 loaded hepatocytes maintained in primary culture has been investigated. Vasopressin stimulation induced oscillatory waves of cytosolic free [Ca2+] increase, which propagated freely through the nuclear region. Based on the amplitude of the Fura-2 signals from this region, the morphology of the hepatocyte nucleus and the rapid penetration of the nucleus by injected Fura-2, it can be concluded that the nuclear Ca2+ responses reflect changes occurring within the nucleoplasm. Intranuclear Ca2+ increases occurred as waves that appear to be directed by the Ca2+ waves passing through the surrounding cytoplasm. The apparent velocity of Ca2+ waves was higher in the nucleoplasm than in the cytoplasm (19.5 +/- 2.9 versus 11.0 +/- 1.1 microns/s). The nucleoplasm does not contain vesicular Ca2+ stores that might be released by Ins(1,4,5)P3. However, the nuclear envelope functions as a Ca2+ store that is sensitive to mobilization by Ins(1,4,5)P3. We conclude that the [Ca2+] in the nucleoplasm of the hepatocyte is close to equilibrium with the cytosolic [Ca2+] and that oscillatory waves of cytosolic [Ca2+] are closely paralleled by similar [Ca2+] changes in the nucleoplasm. The nuclear envelope is a component of the intracellular Ins(1,4,5)P3-sensitive Ca2+ storage pool and may serve as a reservoir for [Ca2+] elevations within the nucleus.

The inositol trisphosphate calcium channel is inactivated by inositol trisphosphate

Activation of intracellular Ca2+ channels by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) represents the initial Ca2+ mobilization step in response to many extracellular signals. Here we report that Ins(1,4,5)P3-induced channel activation in permeabilized hepatocytes is followed by a time-dependent inactivation, which is a direct consequence of ligand binding. The inactivation by Ins(1,4,5)P3 parallels the quantal character of channel opening, giving rise to a unique process of incremental inactivation whereby discrete channel populations are inhibited at each Ins(1,4,5)P3 dose. Ins(1,4,5)P3 can induce inactivation in the absence of stored Ca2+, but the inactivation rate is enhanced by increases of cytosolic Ca2+. The inhibitory effect of Ins(1,4,5)P3 can be reversed by Ins(1,4,5)P3 washout, or by chelation of cytosolic Ca2+. Thus, Ins(1,4,5)P3 and Ca2+ act as coinhibitors of the Ins(1,4,5)P3-sensitive Ca2+ channel. Inactivation is an inherent consequence of Ins(1,4,5)P3-induced channel opening which can terminate increases of cytosolic Ca2+.

Intracellular calcium. Questions about quantal Ca2+ release

Several suggestions have been put forward to explain how the release of Ca(2+) from intracellular stores can be graded, but the mechanism underlying this phenomenon remains elusive.