Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Determining the Roles of Inositol Trisphosphate Receptors in Neurodegeneration: Interdisciplinary Perspectives on a Complex Topic

It is well known that calcium (Ca²⁺) is involved in the triggering of neuronal death. Ca²⁺ cytosolic levels are regulated by Ca²⁺ release from internal stores located in organelles, such as the endoplasmic reticulum. Indeed, Ca²⁺ transit from distinct cell compartments follows complex dynamics that are mediated by specific receptors, notably inositol trisphosphate receptors (IP3Rs). Ca²⁺ release by IP3Rs plays essential roles in several neurological disorders; however, details of these processes are poorly understood. Moreover, recent studies have shown that subcellular location, molecular identity, and density of IP3Rs profoundly affect Ca²⁺ transit in neurons. Therefore, regulation of IP3R gene products in specific cellular vicinities seems to be crucial in a wide range of cellular processes from neuroprotection to neurodegeneration. In this regard, microRNAs seem to govern not only IP3Rs translation levels but also subcellular accumulation. Combining new data from molecular cell biology with mathematical modelling, we were able to summarize the state of the art on this topic. In addition to presenting how Ca²⁺ dynamics mediated by IP3R activation follow a stochastic regimen, we integrated a theoretical approach in an easy-to-apply, cell biology-coherent fashion. Following the presented premises and in contrast to previously tested hypotheses, Ca²⁺ released by IP3Rs may play different roles in specific neurological diseases, including Alzheimer's disease and Parkinson's disease.

Cell-cell communication in the vessel wall

Intercellular communication between cells within the blood vessel wall plays an important role in the control of artery diameter. The endothelial cells lining the lumen of arteries can evoke smooth muscle hyperpolarization both by the release of a factor (EDHF) and by direct cell-cell coupling through gap junctions. Hyperpolarizing current can spread rapidly to cause widespread vasodilatation, and thus increase blood flow to that segment. In addition to the spread of current, small molecules, such as Ca2+, can also pass between cells, but at a much reduced rate. Instead of co-ordinating changes in diameter, intercellular Ca2+ signalling acts to amplify and, in special cases, modulate vascular responses. Together, direct cell-cell communication enables the blood vessel wall to act as a functional syncytium, which is influenced by surrounding tissues and nerves, and blood constituents.

Genetic architecture dissection by genome-wide association analysis reveals avian eggshell ultrastructure traits

The ultrastructure of an eggshell is considered the major determinant of eggshell quality, which has biological and economic significance for the avian and poultry industries. However, the interrelationships and genome-wide architecture of eggshell ultrastructure remain to be elucidated. Herein, we measured eggshell thickness (EST), effective layer thickness (ET), mammillary layer thickness (MT), and mammillary density (MD) and conducted genome-wide association studies in 927 F₂ hens. The SNP-based heritabilities of eggshell ultrastructure traits were estimated to be 0.39, 0.36, 0.17 and 0.19 for EST, ET, MT and MD, respectively, and a total of 719, 784, 1 and 10 genome-wide significant SNPs were associated with EST, ET, MT and MD, respectively. ABCC9, ITPR2, KCNJ8 and WNK1, which are involved in ion transport, were suggested to be the key genes regulating EST and ET. ITM2C and KNDC1 likely affect MT and MD, respectively. Additionally, there were linear relationships between the chromosome lengths and the variance explained per chromosome for EST (R² = 0.57) and ET (R² = 0.67). In conclusion, the interrelationships and genetic architecture of eggshell ultrastructure traits revealed in this study are valuable for our understanding of the avian eggshell and contribute to research on a variety of other calcified shells.

Do calcium buffers always slow down the propagation of calcium waves?

Calcium buffers are large proteins that act as binding sites for free cytosolic calcium. Since a large fraction of cytosolic calcium is bound to calcium buffers, calcium waves are widely observed under the condition that free cytosolic calcium is heavily buffered. In addition, all physiological buffered excitable systems contain multiple buffers with different affinities. It is thus important to understand the properties of waves in excitable systems with the inclusion of buffers. There is an ongoing controversy about whether or not the addition of calcium buffers into the system always slows down the propagation of calcium waves. To solve this controversy, we incorporate the buffering effect into the generic excitable system, the FitzHugh-Nagumo model, to get the buffered FitzHugh-Nagumo model, and then to study the effect of the added buffer with large diffusivity on traveling waves of such a model in one spatial dimension. We can find a critical dissociation constant (K = K(a)) characterized by system excitability parameter a such that calcium buffers can be classified into two types: weak buffers (K ∈ (K(a), ∞)) and strong buffers (K ∈ (0, K(a))). We analytically show that the addition of weak buffers or strong buffers but with its total concentration b(0)(1) below some critical total concentration b(0,c)(1) into the system can generate a traveling wave of the resulting system which propagates faster than that of the origin system, provided that the diffusivity D1 of the added buffers is sufficiently large. Further, the magnitude of the wave speed of traveling waves of the resulting system is proportional to √D1 as D1 --> ∞. In contrast, the addition of strong buffers with the total concentration b(0)(1) > b(0,c)(1) into the system may not be able to support the formation of a biologically acceptable wave provided that the diffusivity D1 of the added buffers is sufficiently large.

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.

The activation state of the inositol 1,4,5-trisphosphate receptor regulates the velocity of intracellular Ca2+ waves in bovine aortic endothelial cells

Ca(2+) is a highly versatile second messenger that plays a key role in the regulation of many cell processes. This versatility resides in the fact that different signals can be encoded spatio-temporally by varying the frequency and amplitude of the Ca(2+) response. A typical example of an organized Ca(2+) signal is a Ca(2+) wave initiated in a given area of a cell that propagates throughout the entire cell or within a specific subcellular region. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3) R) is responsible for the release of Ca(2+) from the endoplasmic reticulum. IP(3) R activity can be directly modulated in many ways, including by interacting molecules, proteins, and kinases such as PKA, PKC, and mTOR. In the present study, we used a videomicroscopic approach to measure the velocity of Ca(2+) waves in bovine aortic endothelial cells under various conditions that affect IP(3) R function. The velocity of the Ca(2+) waves increased with the intensity of the stimulus while extracellular Ca(2+) had no significant impact on wave velocity. Forskolin increased the velocity of IP(3) R-dependent Ca(2+) waves whereas PMA and rapamycin decreased the velocity. We used scatter plots and Pearson's correlation test to visualize and quantify the relationship between the Ca(2+) peak amplitude and the velocity of Ca(2+) waves. The velocity of IP(3) R-dependent Ca(2+) waves poorly correlated with the amplitude of the Ca(2+) response elicited by agonists in all the conditions evaluated, indicating that the velocity depended on the activation state of IP(3) R, which can be modulated in many ways.

Ca(2+) transfer from the ER to mitochondria: when, how and why

The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca(2+) concentration ([Ca(2+)]c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca(2+)] in the mitochondrial matrix ([Ca(2+)]m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca(2+) transporters, the close proximity to the endoplasmic reticulum (ER) Ca(2+)-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca(2+) channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca(2+)]m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca(2+) homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca(2+) signaling machinery.

Arachidonic acid, ARC channels, and Orai proteins

A critical role for arachidonic acid in the regulation of calcium entry during agonist activation of calcium signals has become increasingly apparent in numerous studies over the past 10 years or so. In particular, low concentrations of this fatty acid, generated as a result of physiologically relevant activation of appropriate receptors, induces the activation of a unique, highly calcium-selective conductance now known as the ARC channel. Activation of this channel is specifically dependent on arachidonic acid acting at the intracellular surface of the membrane, and is entirely independent of any depletion of internal calcium stores. Importantly, a specific role of this channel in modulating the frequency of oscillatory calcium signals in various cell types has been described. Recent studies, subsequent to the discovery of STIM1 and the Orai proteins and their role in the store-operated CRAC channels, have revealed that these same proteins are also integral components of the ARC channels and their activation. However, unlike the CRAC channels, activation of the ARC channels depends on the pool of STIM1 that is constitutively resident in the plasma membrane, and the pore of these channels is comprised of both Orai1 and Orai3 subunits. The clear implication is that CRAC channels and ARC channels are closely related, but have evolved to play unique roles in the modulation of calcium signals-largely as a result of their entirely distinct modes of activation. Given this, although the precise details of how arachidonic acid acts to activate the channels remain unclear, it seems likely that the specific molecular features of these channels that distinguish them from the CRAC channels--namely Orai3 and/or plasma membrane STIM1--will be involved.

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.

Calcium puffs in Xenopus oocytes

The second messenger inositol 1,4,5-trisphosphate (InsP3) functions in large part by liberating calcium ions from intracellular stores. This release process is highly non-linear and shows a regenerative characteristic that allows production of all-or-none calcium spikes which propagate as waves. However, at low concentrations of InsP3 an additional mode of calcium liberation is seen in Xenopus oocytes, transient 'puffs' of cytosolic calcium that last for a few hundred milliseconds and are restricted to within a few micrometres. Puffs are generally of similar size and the amount of calcium released (about 3 x 10(-18) mol) suggests that they arise through the concerted opening of several InsP3-gated calcium release channels. Puff sites are present at a density of about one per 30 microns 2 in the animal hemisphere of the oocyte. Each site functions autonomously, producing puffs at largely random intervals. We conclude that calcium puffs represent 'quantal' units of InsP3-evoked calcium liberation, which may result from local regenerative feedback by cytosolic calcium ions at functionally discrete release sites.

Are buffers boring? Uniqueness and asymptotical stability of traveling wave fronts in the buffered bistable system

Traveling waves of calcium are widely observed under the condition that the free cytosolic calcium is buffered. Thus it is of physiological interest to determine how buffers affect the properties of calcium waves. Here we summarise and extend previous results on the existence, uniqueness and stability of traveling wave solutions of the buffered bistable equation, which is the simplest possible model of the upstroke of a calcium wave. Taken together, the results show that immobile buffers do not change the existence, uniqueness or stability of the traveling wave, while mobile buffers can eliminate a traveling wave. However, if a wave exists in the latter case, it remains unique and stable.

Comparison of SIVmac239(352-382) and SIVsmmPBj41(360-390) enterotoxic synthetic peptides

To characterize the active domain of the simian immunodeficiency virus (SIV) surface unit (SU) enterotoxin, peptides corresponding to the V3 loop of SIVmac239 (SIVmac) and SIVsmmPBj41 (SIVpbj) were synthesized and examined for enterotoxic activity, alpha-helical structure, and interaction(s) with model membranes. SIVmac and SIVpbj induced a dose-dependent diarrhea in 6-8-day-old mouse pups similar to full-length SU. The peptides mobilized [Ca(2+)](i) in HT-29 cells with distinct oscillations and elevated inositol triphosphate levels. Circular dichroism analyses showed the peptides were predominantly random coil in buffer, but increased in alpha-helical content when placed in a hydrophobic environment or with cholesterol-containing membrane vesicles that are rich in anionic phospholipids. None of the peptides underwent significant secondary structural changes in the presence of neutral vesicles indicating ionic interactions were important. These data show that the SIV SU enterotoxic domain localizes in part to the V3 loop region and interacts with anionic membrane domains on the host cell surface.

Signalling specificity in GPCR-dependent Ca2+ signalling

Cells use signalling networks to translate with high fidelity extracellular signals into specific cellular functions. Signalling networks are often composed of multiple signalling pathways that act in concert to regulate a particular cellular function. In the centre of the networks are the receptors that receive and transduce the signals. A versatile family of receptors that detect a remarkable variety of signals are the G protein-coupled receptors (GPCRs). Virtually all cells express several GPCRs that use the same biochemical machinery to transduce their signals. Considering the specificity and fidelity of signal transduction, a central question in cell signalling is how signalling specificity is achieved, in particular among GPCRs that use the same biochemical machinery. Ca(2+) signalling is particularly suitable to address such questions, since [Ca(2+)](i) can be recorded with excellent spatial and temporal resolutions in living cells and tissues and now in living animals. Ca(2+) is a unique second messenger in that both biochemical and biophysical components form the Ca(2+) signalling complex to regulate its concentration. Both components act in concert to generate repetitive [Ca(2+)](i) oscillations that can be either localized or in the form of global, propagating Ca(2+) waves. Most of the key proteins that form Ca(2+) signalling complexes are known and their activities are reasonably well understood on the biochemical and biophysical levels. We review here the information gained from studying Ca(2+) signalling by GPCRs to gain further understanding of the mechanisms used to generate cellular signalling specificity.

Reciprocal regulation of capacitative and arachidonate-regulated noncapacitative Ca2+ entry pathways

Receptor-activated Ca(2+) entry is usually thought to occur via capacitative or store-operated Ca(2+) channels. However, at physiological levels of stimulation, where Ca(2+) store depletion is only transient and/or partial, evidence has suggested that an arachidonic acid-dependent noncapacitative Ca(2+) entry is responsible. Recently, we have described a novel arachidonate-regulated Ca(2+)-selective (ARC) conductance that is entirely distinct from store-operated conductances in the same cell. We now show that these ARC channels are indeed specifically activated by low agonist concentrations and provide the predominant route of Ca(2+) entry under these conditions. We further demonstrate that sustained elevations in cytosolic Ca(2+), such as those resulting from activation of store-operated Ca(2+) entry by high agonist concentrations, inhibit the ARC channels. This explains earlier failures to detect the presence of this noncapacitative pathway in experiments where store-operated entry had already been fully activated. The result is that the respective activities of ARC and store-operated Ca(2+) channels display a unique reciprocal regulation that is related to the specific nature of the [Ca(2+)](i) signals generated at different agonist concentrations. Importantly, these data show that at physiologically relevant levels of stimulation, it is the noncapacitative ARC channels that provide the predominant route for the agonist-activated entry of Ca(2+).

Calcium waves in agarose gel with cell organelles: implications of the velocity curvature relationship

Calcium oscillations and waves have been observed not only in several types of living cells but also in less complex systems of isolated cell organelles. Here we report the determination of apparent Ca2+ diffusion coefficients in a novel excitable medium of agarose gel with homogeneously distributed vesicles of skeletal sarcoplasmic reticulum. Spatiotemporal calcium patterns were visualized by confocal laser scanning fluorescence microscopy. To obtain characteristic parameters of the velocity curvature relationship, namely, apparent diffusion coefficient, velocity of plane calcium waves, and critical radius, positively and negatively curved wave fronts were analyzed. It is demonstrated that gel-immobilized cell organelles reveal features of an excitable medium. Apparent Ca2+ diffusion coefficients of the in vitro system, both in the absence or in the presence of mitochondria, were found to be higher than in cardiac myocytes and lower than in unbuffered agarose gel. Plane calcium waves propagated markedly slower in the in vitro system than in rat cardiac myocytes. Whereas mitochondria significantly reduced the apparent Ca2+ diffusion coefficient of the in vitro system, propagation velocity and critical size of calcium waves were found to be nearly unchanged. These results suggest that calcium wave propagation depends on the kinetics of calcium release rather than on diffusion.

Regulation by Ca2+ and inositol 1,4,5-trisphosphate (InsP3) of single recombinant type 3 InsP3 receptor channels. Ca2+ activation uniquely distinguishes types 1 and 3 insp3 receptors

The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP3R) is an endoplasmic reticulum-localized Ca2+ -release channel that controls complex cytoplasmic Ca(2+) signaling in many cell types. At least three InsP3Rs encoded by different genes have been identified in mammalian cells, with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer formation. To examine regulation of channel gating of the type 3 isoform, recombinant rat type 3 InsP3R (r-InsP3R-3) was expressed in Xenopus oocytes, and single-channel recordings were obtained by patch-clamp electrophysiology of the outer nuclear membrane. Gating of the r-InsP3R-3 exhibited a biphasic dependence on cytoplasmic free Ca2+ concentration ([Ca2+]i). In the presence of 0.5 mM cytoplasmic free ATP, r-InsP3R-3 gating was inhibited by high [Ca2+]i with features similar to those of the endogenous Xenopus type 1 Ins3R (X-InsP3R-1). Ca2+ inhibition of channel gating had an inhibitory Hill coefficient of approximately 3 and half-maximal inhibiting [Ca2+]i (Kinh) = 39 microM under saturating (10 microM) cytoplasmic InsP3 concentrations ([InsP3]). At [InsP3] < 100 nM, the r-InsP3R-3 became more sensitive to Ca2+ inhibition, with the InsP(3) concentration dependence of Kinh described by a half-maximal [InsP3] of 55 nM and a Hill coefficient of approximately 4. InsP(3) activated the type 3 channel by tuning the efficacy of Ca2+ to inhibit it, by a mechanism similar to that observed for the type 1 isoform. In contrast, the r-InsP3R-3 channel was uniquely distinguished from the X-InsP3R-1 channel by its enhanced Ca2+ sensitivity of activation (half-maximal activating [Ca2+]i of 77 nM instead of 190 nM) and lack of cooperativity between Ca2+ activation sites (activating Hill coefficient of 1 instead of 2). These differences endow the InsP3R-3 with high gain InsP3-induced Ca2+ release and low gain Ca2+ -induced Ca2+ release properties complementary to those of InsP3R-1. Thus, distinct Ca2+ signals may be conferred by complementary Ca2+ activation properties of different InsP3R isoforms.

Intercellular Ca2+ signalling: the artery wall

The fact that many cells in the cardiovascular system are coupled via gap junctions enables direct intercellular Ca2+ signalling. Ca2+ ions and/or inositol trisphosphate (IP3) can pass through these aqueous pores. Cell-cell coupling can occur between cells of the same type, or via special junctions between adjacent cells of different types. Homocellular coupling acts to amplify and prolong Ca2+ signals, whereas heterocellular coupling allows complex interactions between the cells, including the modulation of their respective responses. This review will focus on the interactions between cells that form the blood vessel wall, illustrating how cell-cell communication defines important physiological functions.

Molecular and binding characteristics of IP3 receptors in bovine spermatozoa

We have shown the presence of inositol 1,4,5-triphosphate (IP3) receptors in bovine spermatozoa. These receptors are mainly localized and functionally associated with the acrosome region. Molecular characterization of these bovine IP3 receptors has shown that the functional size of the IP3 binding domain is a protein of 66+/-2 kDa, in agreement with the size of both bovine adrenal cortex and bovine adrenal medullar chromaffin cells IP3 receptors. In contrast, bovine cerebellum IP3 receptor displays molecular weight of 220+/-5 kDa, a value in agreement with data in the literature. Bovine IP3 receptors have a one-affinity state characterized by a low affinity (Kd 750 nM) and a relatively high density (7.5 pmol/mg protein). They are functional and release internal calcium upon the binding of the second messenger. Moreover, the finding that the specific A1 adenosine receptor agonist R-PIA elicits almost the same effect as IP3 might be of some help in understanding the physiological role of these inhibitory adenosine receptors in mammalian spermatozoa.

Discrimination of histamine H1 and muscarinic receptor-mediated signalling pathways by phorbol ester in human astrocytoma cells

1. Histamine H1 receptor-mediated signalling was compared with muscarinic receptor-mediated signalling in 1321N1 human astrocytoma cells. 2. Short-term (2 min) treatment of cells with phorbol 12-myristate 13-acetate (PMA) resulted in a reduction of increases in intracellular Ca2+ ([Ca2+]i) induced by carbachol or histamine. 3. Carbachol-induced increases in [Ca2+]i were 10-fold more sensitive to PMA than the histamine-induced increases. 4. When cells were treated with PMA for 48 or 72 h (long-term treatment), protein kinase C (PKC) was down-regulated and PMA did not inhibit carbachol-induced increases in [Ca2+]i. 5. Histamine-induced increases in [Ca2+]i were significantly reduced by long-term treatment with PMA. 6. These findings suggest that the signalling pathways mediated by histamine H1 and muscarinic receptors can be distinguished by using PKC in 1321N1 human astrocytoma cells.

Single-channel properties in endoplasmic reticulum membrane of recombinant type 3 inositol trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is an intracellular Ca(2+)-release channel localized in endoplasmic reticulum (ER) with a central role in complex Ca(2+) signaling in most cell types. A family of InsP(3)Rs encoded by several genes has been identified with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer formation. This diversity suggests that cells require distinct InsP(3)Rs, but the functional correlates of this diversity are largely unknown. Lacking are single-channel recordings of the recombinant type 3 receptor (InsP(3)R-3), a widely expressed isoform also implicated in plasma membrane Ca(2+) influx and apoptosis. Here, we describe functional expression and single-channel recording of recombinant rat InsP(3)R-3 in its native membrane environment. The approach we describe suggests a novel strategy for expression and recording of recombinant ER-localized ion channels in the ER membrane. Ion permeation and channel gating properties of the rat InsP(3)R-3 are strikingly similar to those of Xenopus type 1 InsP(3)R in the same membrane. Using two different two-electrode voltage clamp protocols to examine calcium store-operated calcium influx, no difference in the magnitude of calcium influx was observed in oocytes injected with rat InsP(3)R-3 cRNA compared with control oocytes. Our results suggest that if cellular expression of multiple InsP(3)R isoforms is a mechanism to modify the temporal and spatial features of [Ca(2+)](i) signals, then it must be achieved by isoform-specific regulation or localization of various types of InsP(3)Rs that have relatively similar Ca(2+) permeation properties.

CNS-induced deficits of heavy particle irradiation in space: the aging connection

Our research over the last several years has suggested that young (3 mo) rats exposed to whole-body 56Fe irradiation show neuronal signal transduction alterations and accompanying motor behavioral changes that are similar to those seen in aged (22-24 mo) rats. Since it has been postulated that 1-2% of the composition of cosmic rays contain 56Fe particles of heavy particle irradiation, there may be significant CNS effects on astronauts on long-term space flights which could produce behavioral changes that could be expressed during the mission or at some time after the return. These, when combined with other effects such as weightlessness and exposure to proton irradiations may even supercede mutagenic effects. It is suggested that by determining mechanistic relationships that might exist between aging and irradiation it may be possible to determine the common factor(s) involved in both perturbations and develop procedures to offset their deleterious effects. For example, one method that has been effective is nutritional modification.

An indirect influence of phenylephrine on the release of endothelium-derived vasodilators in rat small mesenteric artery

1. The possibility that stimulation of smooth muscle alpha(1)-adrenoceptors modulates contraction via the endothelium was examined in rat small mesenteric arteries. 2. N(omega)-nitro-L-arginine methyl ester, (L-NAME, 100 microM to inhibit NO synthase) increased contraction to single concentrations of phenylephrine (1 - 3 microM) by approximately 2 fold (from a control level of 14.2+/-3.0 to 34. 1+/-4.2% of the maximum contraction of the artery, n=20). The action of L-NAME was abolished by disrupting the endothelium. 3. The subsequent addition of apamin (to inhibit small conductance Ca(2+)-activated K(+) channels, 50 nM) further augmented phenylephrine contractions, in an endothelium-dependent manner, to more than 3 fold above control (50.4+/-5.3% of the maximum contraction, n=11). 4.Charybdotoxin (non-selective inhibitor of large conductance Ca(2+)-activated K(+) channels, BK(Ca), 50 nM) plus L-NAME augmented the level of phenylephrine contraction to 4 - 5-fold above control (64.1+/-3.1%, n=5), but this effect was independent of the endothelium. The potentiation of contraction by charybdotoxin could be mimicked with the selective BK(Ca) inhibitor, iberiotoxin,. 5. Apamin together with L-NAME and charybdotoxin further significantly increased the phenylephrine contraction by 5 - 6-fold, to 79.9+/-3.5% of the maximum contraction of the artery (n=13). 6. Phenylephrine failed directly to increase the intracellular Ca(2+) concentration in endothelial cells freshly isolated from the small mesenteric artery. 7. Stimulation of smooth muscle alpha(1)-adrenoceptors in the mesenteric artery induces contraction that is markedly suppressed by the endothelium. The attenuation of contraction appears to reflect both the release of NO from the endothelium and the efflux of K(+) from both endothelial and smooth muscle cells. This suggests that the release of NO and endothelium-derived hyperpolarizing factor can be evoked indirectly by agents which act only on the smooth muscle cells.

Spatiotemporal dynamics of inositol 1,4,5-trisphosphate that underlies complex Ca2+ mobilization patterns

Inositol 1,4,5-trisphosphate (IP3) is a second messenger that elicits complex spatiotemporal patterns of calcium ion (Ca2+) mobilization and has essential roles in the regulation of many cellular functions. In Madin-Darby canine kidney epithelial cells, green fluorescent protein-tagged pleckstrin homology domain translocated from the plasma membrane to the cytoplasm in response to increased concentration of IP3. The detection of translocation enabled monitoring of IP3 concentration changes within single cells and revealed spatiotemporal dynamics in the concentration of IP3 synchronous with Ca2+ oscillations and intracellular and intercellular IP3 waves that accompanied Ca2+ waves. Such changes in IP3 concentration may be fundamental to Ca2+ signaling.

Intercellular Ca2+ waves in rat heart muscle

1. Confocal laser scanning microscopy was used to visualize intercellular transmission of Ca2+ waves in intact rat ventricular trabeculae micro-injected with the calcium indicator fluo-3. 2. Ca2+ waves usually failed to be transmitted from cell to cell. At identified individual end-to-end cell contacts, successful transmission interspersed with failure, which sometimes occurred despite an apparent small spritz of Ca2+ between cells. The probability of cell to cell transmission (Ptran) was 0.13. 3. Ca2+ waves arose preferentially near junctions of connected cells, where connexin-43 was found, but randomly in enzymatically disconnected heart cells. 4. beta-Adrenergic stimulation significantly increased Ptran (to 0.22) and heptanol, an uncoupler of gap junction channels, significantly decreased it (to 0.045). 5. In regions of high [Ca2+]i due to damage, wave frequency decreased markedly with each cell-cell junction passed. 6. The Ca2+ permeability of cardiac gap junctions may be regulated, and the low ability of cardiac gap junctions to transmit Ca2+ may help control the spread of Ca2+ from damaged regions.

Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C

BACKGROUND

Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.

METHODS

Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.

RESULTS

VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.

CONCLUSIONS

Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.

Differential modulation of SERCA2 isoforms by calreticulin

In Xenopus laevis oocytes, overexpression of calreticulin suppresses inositol 1,4,5-trisphosphate-induced Ca2+ oscillations in a manner consistent with inhibition of Ca2+ uptake into the endoplasmic reticulum. Here we report that the alternatively spliced isoforms of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2 gene display differential Ca2+ wave properties and sensitivity to modulation by calreticulin. We demonstrate by glucosidase inhibition and site-directed mutagenesis that a putative glycosylated residue (N1036) in SERCA2b is critical in determining both the selective targeting of calreticulin to SERCA2b and isoform functional differences. Calreticulin belongs to a novel class of lectin ER chaperones that modulate immature protein folding. In addition to this role, we suggest that these chaperones dynamically modulate the conformation of mature glycoproteins, thereby affecting their function.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Ca2+ wave dispersion and spiral wave entrainment in Xenopus laevis oocytes overexpressing Ca2+ ATPases

Complex, spatiotemporal patterns of intracellular Ca2+ release in Xenopus oocytes can be accounted for by the operation of two fundamental processes: Ca2+ release from the endoplasmic reticulum (ER) via the inositol 1,4,5-trisphosphate receptor (IP3R) with its inherent dependency on cytosolic Ca2+, and Ca2+ uptake via Ca2+ ATPases. Overexpression of sarco-endoplasmic reticulum Ca2+ ATPases (SERCAs) in Xenopus oocytes, increases IP3-induced Ca2+ wave frequency and amplitude [1-3]. This effect can be attributed to an increased removal of cytoplasmic Ca2+ and more efficient refilling of Ca2+ stores. By overexpressing SERCA isoforms, we report here that Ca2+ waves exhibit dispersion [4-6]. At wavelengths greater than 60 microns, wave velocity is constant. However, wave velocity and amplitude progressively decreases at smaller wavelengths. Below gamma approximately 20 microns, Ca2+ waves disperse and fail to propagate. In oocytes exhibiting both spiral and target patterns of Ca2+ release, spiral waves had higher frequencies and showed entrainment of the surrounding regions. These properties are characteristic of a classical excitable medium [4-6].

Endothelial Ca2+ waves preferentially originate at specific loci in caveolin-rich cell edges

Stimulation of endothelial cells (ECs) with ATP evoked an increase in intracellular Ca2+ concentration ([Ca2+]i). In a single bovine aortic EC, the [Ca2+]i rise started at a specific peripheral locus and propagated throughout the entire cell as a Ca2+ wave. The initiation locus was constant upon repeated stimulation with ATP or other agonists (bradykinin and thrombin). The Ca2+ wave was unaffected by the removal of extracellular Ca2+, demonstrating its dependence on intracellular Ca2+ release. Microinjection of heparin into the cell inhibited the ATP-induced Ca2+ responses, indicating that the Ca2+ wave is at least partly mediated by the inositol 1,4, 5-trisphosphate receptor. Immunofluorescence staining revealed that caveolin, a marker protein for caveolae, is distributed heterogeneously in the cell and that Ca2+ waves preferentially originate at caveolin-rich cell edges. In contrast to caveolin, internalized transferrin and subunits of the clathrin-associated adaptor complexes such as adaptor protein-1 and -2 were diffusely distributed. Disruption of microtubules by Colcemid led to redistribution of caveolin away from the edges into the perinuclear center of the cell, and the ATP-induced [Ca2+]i increase was initiated on the rim of the centralized caveolin. Thus, caveolae may be involved in the initiation of ATP-induced Ca2+ waves in ECs.

CNS effects of heavy particle irradiation in space: behavioral implications

Research from several sources indicates that young (3 mo) rats exposed to heavy particle irradiation (56Fe irradiation) produces changes in motor behavior as well as alterations in neuronal transmission similar to those seen in aged (22-24 mo) rats. These changes are specific to neuronal systems that are affected by aging. Since 56Fe particles make up approximately 1-2% of cosmic rays, these findings suggest that the neuronal effects of heavy particle irradiation on long-term space flights may be significant, and may even supercede subsequent mutagenic effects in their mission capabilities. It is suggested that among other methods, it may be possible to utilize nutritional modification procedures to offset the putative deleterious effects of these particles in space.

Impact of cytoplasmic calcium buffering on the spatial and temporal characteristics of intercellular calcium signals in astrocytes

The impact of calcium buffering on the initiation and propagation of mechanically elicited intercellular Ca2+ waves was studied using astrocytes loaded with different exogenous, cell membrane-permeant Ca2+ chelators and a laser scanning confocal or video fluorescence microscope. Using an ELISA with a novel antibody to BAPTA, we showed that different cell-permeant chelators, when applied at the same concentrations, accumulate to the same degree inside the cells. Loading cultures with BAPTA, a high Ca2+ affinity chelator, almost completely blocked calcium wave occurrence. Chelators having lower Ca2+ affinities had lesser affects, as shown in their attenuation of both the radius of spread and propagation velocity of the Ca2+ wave. The chelators blocked the process of wave propagation, not initiation, because large [Ca2+]i increases elicited in the mechanically stimulated cell were insufficient to trigger the wave in the presence of high Ca2+ affinity buffers. Wave attenuation was a function of cytoplasmic Ca2+ buffering capacity; i.e., loading increasing concentrations of low Ca2+ affinity buffers mimicked the effects of lesser quantities of high-affinity chelators. In chelator-treated astrocytes, changes in calcium wave properties were independent of the Ca2+-binding rate constants of the chelators, of chelation of other ions such as Zn2+, and of effects on gap junction function. Slowing of the wave could be completely accounted for by the slowing of Ca2+ ion diffusion within the cytoplasm of individual astrocytes. The data obtained suggest that alterations in Ca2+ buffering may provide a potent mechanism by which the localized spread of astrocytic Ca2+ signals is controlled.

Elevation of intracellular calcium in smooth muscle causes endothelial cell generation of NO in arterioles

It is well known that vascular smooth muscle tone can be modulated by signals arising in the endothelium (e.g., endothelium-derived relaxing factor, endothelium-derived hyperpolarizing factor, and prostaglandins). Here we show that during vasoconstriction a signal can originate in smooth muscle cells and act on the endothelium to cause synthesis of endothelium-derived relaxing factor. We studied responses to two vasoconstrictors (phenylephrine and KCl) that act by initiating a rise in smooth muscle cell intracellular Ca2+ concentration ([Ca2+]i) and exert little or no direct effect on the endothelium. Fluo-3 was used as a Ca2+ indicator in either smooth muscle or endothelial cells of arterioles from the hamster cheek pouch. Phenylephrine and KCl caused the expected rise in smooth muscle cell [Ca2+]i that was accompanied by an elevation in endothelial cell [Ca2+]i. The rise in endothelial cell [Ca2+]i was followed by increased synthesis of NO, as evidenced by an enhancement of the vasoconstriction induced by both agents after blockade of NO synthesis. The molecule involved in signal transmission from smooth muscle to endothelium is as yet unknown. However, given that myoendothelial cell junctions are frequent in these vessels, we hypothesize that the rise in smooth muscle cell Ca2+ generates a diffusion gradient that drives Ca2+ through myoendothelial cell junctions and into the endothelial cells, thereby initiating the synthesis of NO.

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.

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 for a non-capacitative Ca2+ entry during [Ca2+] oscillations

Current models for the agonist-induced activation of Ca2+ entry from the extracellular medium in non-excitable cells generally emphasize a capacitative mechanism whereby Ca2+ entry is activated simply as a result of the emptying of intracellular Ca2+ stores, without any direct involvement of inositol phosphates. To date, the activation and control of Ca2+ entry have generally been studied under conditions where the agonist-sensitive stores undergo a profound and sustained depletion. However, responses under more normal physiological conditions typically involve the cyclical release and refilling of the stores associated with oscillations in [Ca2+], and the nature and control of entry under these conditions has received relatively little attention. In this study, using isolated cells from the exocrine avian nasal gland as a model system, we show that: (a) the agonist-enhanced rate of Mn2+ quench is independent of the cyclical emptying and refilling of the agonist-sensitive Ca2+ pool during oscillations; (b) the Ca2+ entry pathway is maintained in an activated state for extended periods following inhibition of oscillations under conditions in which agonist-sensitive stores can be shown to be full; (c) no Ca2+ entry could be detected in oscillating cells in experiments that followed a definitive protocol for the demonstration of capacitative entry; and (d) on initial exposure to low agonist concentrations, activation of Ca2+ entry preceded any detectable release of Ca2+ from the stores. We conclude that the essential characteristics of the control of Ca2+ entry during oscillations are incompatible with current capacitative models.

Crosstalk between cellular morphology and calcium oscillation patterns. Insights from a stochastic computer model

Agonist-induced oscillations in the concentration of intracellular free calcium ([Ca2+]i) display a wide variety of temporal and spatial patterns. In non-excitable cells, typical oscillatory patterns are somewhat cell-type specific and range from frequency-encoded, repetitive Ca2+ spikes to oscillations that are more sinusoidal in shape. Although the response of a cell population, even to the same stimulus, is often extremely heterogeneous, the response of the same cell to successive exposures can be remarkably similar. We propose that such "Ca2+ fingerprints' can be a consequence of cell-specific morphological properties. The hypothesis is tested by means of a stochastic computer simulation of a two-dimensional model for oscillatory Ca2+ waves which encompasses the basic elements of the two-pool oscillator introduced by Goldbeter et al. (Goldbeter A., Dupont G., Berridge M.J. Minimal model for signal-induced Ca(2+)-oscillations and for their frequency encoding through protein phosphorylation. Proc Natl Acad Sci USA 1990; 87: 1461-1465). In the framework of our extended spatiotemporal model, single cells can display various oscillation patterns which depend on the agonist dose, Ca2+ diffusibility, and several morphological parameters. These are, for example, size and shape of the cell and the cell nucleus, the amount and distribution of Ca2+ stores, and the subcellular location of the inositol(1,4,5)-trisphosphate-generating apparatus.

Nonlinear propagation of spherical calcium waves in rat cardiac myocytes

Spontaneous calcium waves in enzymatically isolated rat cardiac myocytes were investigated by confocal laser scanning microscopy (CLSM) using the fluorescent Ca2+-indicator fluo-3 AM. As recently shown, a spreading wave of enhanced cytosolic calcium appears, most probably during Ca2+ overload, and is initiated by an elementary event called a "calcium spark." When measured by conventional fluorescence microscopy the propagation velocity of spontaneous calcium waves determined at several points along the cardiac myocyte was previously found to be constant. More precise measurements with a CLSM showed a nonlinear propagation. The wave velocity was low, close to the focus, and increased with increasing time and propagation length, approaching a maximum of 113 microns/s. This result was surprising, inasmuch as for geometrical reasons a decrease of the propagation velocity might be expected if the confocal plane is not identical with that plane where the focus of the wave was localized. It is suggested that the propagation velocity is essentially dependent on the curvature of the spreading wave. From the linear relationship of velocity versus curvature, a critical radius of 2.7 +/- 1.4 microns (mean +/- SD) was worked out, below which an outward propagation of the wave will not take place. Once released from a sufficiently extended cluster of sarcoplasmic reticulum release channels, calcium diffuses and will activate its neighbors. While traveling away, the volume into which calcium diffuses becomes effectively smaller than at low radii. This effect is the consequence of the summation of elementary events (Ca2+ sparks) and leads to a steeper increase of the cytosolic calcium concentration after a certain diffusion path length. Thus the time taken to reach a critical threshold of [Ca2+]i at the neighboring calcium release sites decreases with decreasing curvature and the wave will propagate faster.

Ca2+ entry modulates oscillation frequency by triggering Ca2+ release

As in many cells, the frequency of agonist-induced cytosolic Ca2+ concentration ([Ca2+]1) oscillations in exocrine avian nasal gland cells is dependent on the rate of Ca2+ entry. Experiments reveal that the initiation of each oscillatory spike is independent of the relative fullness of the stores and, furthermore, the oscillating pool is normally fully refilled by the end of each [Ca2+]1 spike. Therefore, contrary to current models, the interspike interval (which essentially sets the frequency) does not reflect the time taken to recharge the oscillating stores. Instead, the data show that it is the previously demonstrated role that Ca2+ entry plays in triggering the repetitive release of Ca2+ from the oscillating stores, rather than the recharging of those stores, that provides the basis for the observed effects of Ca2+ entry rate on oscillation frequency.

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Hepatocyte growth factor-induced calcium waves in hepatocytes as revealed with rapid scanning confocal microscopy

Cytosolic Ca2+ transients induced by hepatocyte growth factor (HGF) were imaged in primary cultured rat hepatocytes using newly developed rapid scanning confocal microscopes and indo-1. HGF (40 ng/ml) increased cytosolic free Ca2+ concentration ([Ca2+]i) in about 60% of hepatocytes, in 45% of which the increases were oscillatory. In each of the oscillatory hepatocytes, the repetitive increases in [Ca2+]i originated from a specific same region adjacent to the cell membrane and propagated across the cell like waves. Phenylephrine (10 microM) also induced Ca2+ waves. The locus where HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves were originated was the same, and there was a correlation in the peak height between HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves in each cell, although the mechanisms of inositol 1,4,5-trisphosphate (ins(1,4,5)P3) formation induced by HGF should be different from those by phenylephrine. On the other hand, there was no correlation between sensitivity of each cell to HGF and that to phenylephrine which were measured as latent periods prior to Ca2+ rises after an addition of the agonists. These results suggested the following: the spatial patterns of Ca2+ waves were decided by a common mechanism, probably not the propagation of ins(1,4,5)P3 but the distribution of ins(1,4,5)P3-sensitive Ca2+ pools; sensitivities of each cell to the agonists did not mainly depend on the common mechanism.

On the roles of Ca2+ diffusion, Ca2+ buffers, and the endoplasmic reticulum in IP3-induced Ca2+ waves

We have investigated the effects of Ca2+ diffusion, mobile and stationary Ca2+ buffers in the cytosol, and Ca2+ handling by the endoplasmic reticulum on inositol 1,4,5-trisphosphate-induced Ca2+ wave propagation. Rapid equilibration of free and bound Ca2+ is used to describe Ca2+ sequestration by buffers in both the cytosol and endoplasmic reticulum (ER) lumen. Cytosolic Ca2+ regulation is based on a kinetic model of the inositol 1,4,5-trisphosphate (IP3) receptor of De Young and Keizer that includes activation and inhibition of the IP3 receptor Ca2+ channel in the ER membrane and SERCA Ca2+ pumps in the ER. Diffusion of Ca2+ in the cytosol and the ER and the breakdown and diffusion of IP3 are also included in our calculations. Although Ca2+ diffusion is severely limited because of buffering, when conditions are chosen just below the threshold for Ca2+ oscillations, a pulse of IP3 or Ca2+ results in a solitary trigger wave that requires diffusion of Ca2+ for its propagation. In the oscillatory regime repetitive wave trains are observed, but for this type of wave neither the wave shape nor the speed is strongly dependent on the diffusion of Ca2+. Local phase differences lead to waves that are predominately kinematic in nature, so that the wave speed (c) is related to the wavelength (lambda) and the period of the oscillations (tau) approximately by the formula c = lambda/tau. The period is determined by features that control the oscillations, including [IP3] and pump activity, which are related to recent experiments. Both solitary waves and wave trains are accompanied by a Ca2+ depletion wave in the ER lumen, similar to that observed in cortical preparations from sea urchin eggs. We explore the effect of endogenous and exogenous Ca2+ buffers on wave speed and wave shape, which can be explained in terms of three distinct effects of buffering, and show that exogenous buffers or Ca2+ dyes can have considerable influence on the amplitude and width of the waves.

Decoding of cytosolic calcium oscillations in the mitochondria

Frequency-modulated oscillations of cytosolic Ca2+ ([Ca2+]c) are believed to be important in signal transduction, but it has been difficult to correlate [Ca2+]c oscillations directly with the activity of Ca(2+)-regulated targets. We have studied the control of Ca(2+)-sensitive mitochondrial dehydrogenases (CSMDHs) by monitoring mitochondrial Ca2+ ([Ca2+]m) and the redox state of flavoproteins and pyridine nucleotides simultaneously with [Ca2+]c in single hepatocytes. Oscillations of [Ca2+]c induced by IP3-dependent hormones were efficiently transmitted to the mitochondria as [Ca2+]m oscillations. Each [Ca2+]m spike was sufficient to cause a maximal transient activation of the CSMDHs and [Ca2+]m oscillations at frequencies above 0.5 per minute caused a sustained activation of mitochondrial metabolism. By contrast, sustained [Ca2+]c increases yielded only transient CSMDH activation, and slow or partial [Ca2+]c elevations were ineffective in increasing [Ca2+]m or stimulating CSMDHs. We conclude that the mitochondria are tuned to oscillating [Ca2+]c signals, the frequency of which can control the CSMDHs over the full range of potential activities.

A role for phospholipase C activity but not ryanodine receptors in the initiation and propagation of intercellular calcium waves

Mechanical stimulation of a single cell in an airway epithelial culture initiates an increase in intracellular Ca2+ concentration ([Ca2+]i) that propagates from cell to cell as an intercellular Ca2+ wave. These Ca2+ waves appear to require an increase in intracellular inositol 1,4,5-trisphosphate (IP3) concentration ([IP3]i) in the stimulated cell and are propagated between cells by the diffusion of IP3 through gap junctions. To test the hypothesis that the activation of phospholipase C (PLC) contributes to the elevation of [IP3]i and initiation of an intercellular Ca2+ wave, changes in [Ca2+]i induced by mechanical stimulation were measured by digital fluorescence microscopy in the presence of the PLC inhibitor, aminosteroid U73122. Following exposure to U73122 mechanical stimulation elevated [Ca2+]i of the stimulated cell, but did not initiate the propagation of an intercellular Ca2+ wave. By contrast, in the presence of U73343, a similar aminosteroid that does not inactivate PLC, mechanical stimulation increased the [Ca2+]i of the stimulated cell and initiated an intercellular Ca2+ wave. U73122 also blocked the elevation of [Ca2+]i of airway epithelial cells in response to ATP, a P2-receptor agonist that activates PLC to elevate [IP3]i and [Ca2+]i. In addition, the propagation of intercellular Ca2+ waves was not affected by the ryanodine-receptor agonists, caffeine or ryanodine. The hypotheses that: (1) an elevation of [IP3]i is required to initiate intercellular Ca2+ waves; (2) mechanical stimulation activates PLC; and (3) Ca2+ wave propagation in airway epithelial cells involves Ca2+ release from intracellular stores primarily via IP3 receptors are supported by these results.

Sperm-induced calcium oscillations of human oocytes show distinct features in oocyte center and periphery

Temporal and spatial characteristics of explosive periodic increases (spikes) of intracellular free Ca2+ concentration ([Ca2+]i) induced by sperm in human oocytes (Ca2+ oscillations) were analyzed by confocal laser scanning microscopy and compared to Ca2+ oscillations induced in oocytes by the thiol reagent thimerosal. During the steady-state period of sperm-induced Ca2+ oscillations, each individual [Ca2+]i spike invariably began from a focus in oocyte periphery and spread throughout the entire peripheral region before propagating to the central ooplasm. This peripheral Ca2+ wave was immediately followed by an explosive [Ca2+]i increase in the central ooplasm. However, this central [Ca2+]i rise only peaked when [Ca2+]i in the peripheral ooplasm was already on the decline. Moreover, the peak [Ca2+]i values were always considerably higher in the oocyte center than in the periphery. In contrast, thimerosal-induced Ca2+ oscillations did not show this particular form of propagation. These data show that sperm-induced Ca2+ oscillations have a unique pattern of spatial dynamics and suggest that the bulk of Ca2+ mobilized during each spike is released from stores that have a relatively high threshold for Ca(2+)-induced Ca2+ release (CICR). These stores are poorly developed, if not absent, in the oocyte cortex, and CICR from them is triggered by previous CICR from another type of store with a lower threshold that are preferentially located in the oocyte cortex and act as a detonator.

Efficacy of peak Ca2+ currents (ICa) as trigger of sarcoplasmic reticulum Ca2+ release in myocytes from the guinea-pig coronary artery

1. Increments in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured in single smooth muscle cells from guinea-pig coronary artery together with the density of peak Ca2+ currents (ICa) in response to clamp steps from -50 to 0 mV. The comparison of depolarization- with caffeine-induced delta[Ca2+]c was used to define the efficacy by which ICa can trigger Ca2+ release from the sarcoplasmic reticulum (SR). 2. At 2.5 mM extracellular calcium concentration ([Ca2+]o), depolarization induced a rapid rise of delta[Ca2+]c followed by a slow creep. Peak [Ca2+]c occurred within ca 30 s and could be followed by an undershoot and a second rise in [Ca2+]c. The creep was blocked by ryanodine but was insensitive to block of InsP3 receptors with heparin. The creep was not observed in Cs(+)-filled cells. After disappearance of the creep, a tonic delta[Ca2+]c became unmasked. 3. At 2.5 mM [Ca2+]o, peak ICa was -0.80 +/- 0.17 microA cm-2. delta[Ca2+] peaked at the end of the 6 s pulse at 202 +/- 98 nM while caffeine-induced delta[Ca2+]c peaked at 1330 +/- 410 nM. The ratio of depolarization- to caffeine-induced delta[Ca2+]c was 10 +/- 6%. 4. In media containing 10 mM [Ca2+]o plus 1 microM Bay K 8644, peak ICa was -2.6 +/- 1.1 microA cm-2 and delta[Ca2+]c peaked within 2.5 s at 451 +/- 194 nM. Paired measurements yielded the ratio of depolarization- to caffeine induced delta[Ca2+]c as 30 +/- 10%. Depolarization-induced delta[Ca2+]c was nearly blocked by caffeine and reduced by ryanodine to 30%, suggesting the contribution of Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ stores. 5. Trypsin (1 mg ml-1) in the electrode solution (10 mM [Ca2+]o plus 1 microM Bay K 8644) increased peak ICa up to 12.5 microA cm-2. ICa induced a delta[Ca2+]c of 990 +/- 210 nM and was accompanied by a 'hump' of IK,Ca. When applied briefly after peak delta[Ca2+]c, caffeine increased [Ca2+]c only moderately. The results suggest that a peak ICa can trigger a synchronized whole-cell Ca2+ release only if ICa is strongly augmented. 6. Amplitude and rate of rise of delta[Ca2+]c were graded by test step potentials along a bell-shaped voltage-dependent curve, similar to that of L-type ICa. Steps to +80 mV induced no delta[Ca2+]c when the electrode solution contained 10 mM Na+. However, with 150 mM intrapipette Na+, pulses to +80 mV induced delta[Ca2+]c.(ABSTRACT TRUNCATED AT 400 WORDS)