Increased frequency of calcium waves in Xenopus laevis oocytes that express a calcium-ATPase

Functional properties of intracellular calcium-release channels

Two major classes of intracellular calcium-release channels have been identified, the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor. These channels are the largest ion channels identified to date. Recent studies have established that approximately 90% of each of these proteins protrudes into the cytoplasm, presumably exposing many regulatory sites on the channel and allowing functional interactions with other cytoplasmic proteins. Current work is aimed at understanding the molecular mechanisms and cellular roles of these regulatory processes.

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.

Inositol trisphosphate receptor mediated spatiotemporal calcium signalling

Spatiotemporal Ca2+ signalling in the cytoplasm is currently understood as an excitation phenomenon by analogy with electrical excitation in the plasma membrane. In many cell types, Ca2+ waves and Ca2+ oscillations are mediated by inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channels in the endoplasmic reticulum membrane, with positive feedback between cytosolic Ca2+ and IP3-induced Ca2+ release creating a regenerative process. Remarkable advances have been made in the past year in the analysis of subcellular Ca2+ microdomains using confocal microscopy and of Ca2+ influx pathways that are functionally coupled to IP3-induced Ca2+ release. Ca2+ signals can be conveyed into the nucleus and mitochondria. Ca2+ entry from outside the cell allows repetitive Ca2+ release by providing Ca2+ to refill the endoplasmic reticulum stores, thus giving rise to frequency-encoded Ca2+ signals.

The lifetime of inositol 1,4,5-trisphosphate in single cells

In many eukaryotic cell types, receptor activation leads to the formation of inositol 1,4,5-trisphosphate (IP3) which causes calcium ions (Ca) to be released from internal stores. Ca release was observed in response to the muscarinic agonist carbachol by fura-2 imaging of N1E-115 neuroblastoma cells. Ca release followed receptor activation after a latency of 0.4 to 20 s. Latency was not caused by Ca feedback on IP3 receptors, but rather by IP3 accumulation to a threshold for release. The dependence of latency on carbachol dose was fitted to a model in which IP3 synthesis and degradation compete, resulting in gradual accumulation to a threshold level at which Ca release becomes regenerative. This analysis gave degradation rate constants of IP3 in single cells ranging from 0 to 0.284 s-1 (0.058 +/- 0.067 s-1 SD, 53 cells) and a mean IP3 lifetime of 9.2 +/- 2.2 s. IP3 degradation was also measured directly with biochemical methods. This gave a half life of 9 +/- 2 s. The rate of IP3 degradation sets the time frame over which IP3 accumulations are integrated as input signals. IP3 levels are also filtered over time, and on average, large-amplitude oscillations in IP3 in these cells cannot occur with period < 10 s.

Properties of intracellular Ca2+ waves generated by a model based on Ca(2+)-induced Ca2+ release

Cytosolic Ca2+ waves occur in a number of cell types either spontaneously or after stimulation by hormones, neurotransmitters, or treatments promoting Ca2+ influx into the cells. These waves can be broadly classified into two types. Waves of type 1, observed in cardiac myocytes or Xenopus oocytes, correspond to the propagation of sharp bands of Ca2+ throughout the cell at a rate that is high enough to permit the simultaneous propagation of several fronts in a given cells. Waves of type 2, observed in hepatocytes, endothelial cells, or various kinds of eggs, correspond to the progressive elevation of cytosolic Ca2+ throughout the cell, followed by its quasi-homogeneous return down to basal levels. Here we analyze the propagation of these different types of intracellular Ca2+ waves in a model based on Ca(2+)-induced Ca2+ release (CICR). The model accounts for transient or sustained waves of type 1 or 2, depending on the size of the cell and on the values of the kinetic parameters that measure Ca2+ exchange between the cytosol, the extracellular medium, and intracellular stores. Two versions of the model based on CICR are considered. The first version involves two distinct Ca2+ pools sensitive to inositol 1,4,5-trisphosphate (IP3) and Ca2+, respectively, whereas the second version involves a single pool sensitive both to Ca2+ and IP3 behaving as co-agonists for Ca2+ release. Intracellular Ca2+ waves occur in the two versions of the model based on CICR, but fail to propagate in the one-pool model at subthreshold levels of IP3. For waves of type 1, we investigate the effect of the spatial distribution of Ca(2+)-sensitive Ca2+ stores within the cytosol, and show that the wave fails to propagate when the distance between the stores exceeds a critical value on the order of a few microns. We also determine how the period and velocity of the waves are affected by changes in parameters measuring stimulation, Ca2+ influx into the cell, or Ca2+ pumping into the stores. For waves of type 2, the numerical analysis indicates that the best qualitative agreement with experimental observations is obtained for phase waves. Finally, conditions are obtained for the occurrence of "echo" waves that are sometimes observed in the experiments.

Diffusion of inositol 1,4,5-trisphosphate but not Ca2+ is necessary for a class of inositol 1,4,5-trisphosphate-induced Ca2+ waves

Combining a realistic model of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ oscillations with the diffusion of IP3 and buffered diffusion of Ca2+, we have found that diffusion of Ca2+ plays only a minor role in a class of agonist-induced Ca2+ wave trains. These waves are primarily kinematic in nature, with variable wavelengths and speeds that depend primarily on the phase differences between oscillators at different spatial points. The period is set by the steady-state value of IP3, while the wave speed approximately equals the wavelength/period. Ca2+ diffusion, which is much slower than that of IP3 because of endogenous buffers, is shown to have only a small effect on the wave trains and not to be necessary for the apparent wave propagation. Diffusion of IP3 sets the phase gradient responsible for these wave trains, which consist primarily of localized cycles of Ca2+ uptake and release. Our results imply a possible previously undisclosed role for IP3 in cell signaling.

Theoretical analysis of calcium wave propagation based on inositol (1,4,5)-trisphosphate (InsP3) receptor functional properties

In the presence of inositol (1,4,5)-trisphosphate (InsP3) repetitive waves of elevated cytosolic free Ca2+ (Ca waves) that travel through cellular cytoplasm are observed. Investigation of this phenomenon stimulated the view of cellular cytoplasm as 'an excitable medium composed of Ca release processes (InsP3R), coupled by a common stimulatory signal (Ca) through diffusion' [Lechleiter JD. Clapham DE. (1992) Molecular mechanisms of intracellular calcium excitability in Xenopus laevis oocytes. Cell, 69, 283-294]. Using a kinetic model of InsP3R gating, an analytical expression for the amplitude of Ca wave propagating through this excitable medium has been obtained. The amplitude of the Ca wave is determined by the combination of cell-specific parameters and the functional properties of a single InsP3R. An analytical expression for Ca wave propagation velocity has been also obtained using the Luther equation for diffusion-driven autocatalytic reaction. Both equations provided reasonable estimations for Ca wave amplitude (1.3 microM free Ca) and the velocity of the wave propagation (21 microns/s) for Ca waves in Xenopus oocytes when numerical values of parameters were used. The duration of refractory period has been shown to be determined mainly by the activity of CaATPase. Obtained results provide an insight into the mechanisms underlying the process of Ca wave propagation and define the interrelationship between different factors involved in this process. Some experimentally testable predictions can be done based on the analytical expressions obtained for Ca wave amplitude, the velocity of Ca wave propagation and the duration of refractory period.

Caffeine inhibits cytosolic calcium oscillations induced by noradrenaline and vasopressin in rat hepatocytes

The effects of caffeine on agonist-induced changes in intracellular Ca2+ concentration ([Ca2+]i) were studied in single fura 2-loaded cells and suspensions of rat hepatocytes. In single cells, caffeine (5-10 mM) inhibited [Ca2+]i oscillations induced both by noradrenaline (0.1 microM) and by vasopressin (0.1 nM). Caffeine shifted the dose-response curves of the [Ca2+]i rise induced by vasopressin (0.5 to 2 nM) and noradrenaline (from 80 to 580 nM) in suspensions of liver cells loaded with quin2. This inhibitory effect of caffeine was not due to inhibition of phosphodiesterase enzymes and elevation of cyclic AMP levels, because application of 3-isobutyl-1-methylxanthine, forskolin or 8-bromo cyclic AMP had no inhibitory effect on the intracellular Ca2+ rise induced by inositol 1,4,5-trisphosphate (InsP3)-dependent agonists. We demonstrate that the inhibitory effect of caffeine may result from at least three actions of caffeine: (1) inhibition of receptor-stimulated InsP3 formation; (2) inhibition of agonist-stimulated Ca2+ influx; and (3) direct inhibition of the InsP3-sensitive Ca(2+)-release channel.

Role of cytosolic Ca2+ in inhibition of InsP3-evoked Ca2+ release in Xenopus oocytes

1. Calcium liberation induced in Xenopus oocytes by flash photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor was monitored by confocal microfluorimetry. The object was to determine whether inhibition of Ca2+ release seen with paired flashes arose as a direct consequence of elevated cytosolic free [Ca2+]. 2. Responses evoked by just-suprathreshold test flashes were not inhibited by subthreshold conditioning flashes, but were strongly suppressed when conditioning flashes were raised above threshold. 3. Inhibition at first increased progressively as the inter-flash interval was lengthened to about 2 s and thereafter declined, with a half-recovery at about 4 s. 4. Intracellular injections of Ca2+ caused relatively slight inhibition of InsP3-evoked signals, even when cytosolic free [Ca2+] was elevated to levels similar to those at which strong inhibition was seen in paired-flash experiments. 5. Recovery from inhibition was not appreciably slowed when Ca2+ was injected to raise the free Ca2+ level between paired flashes. 6. We conclude that inhibition of InsP3-evoked Ca2+ liberation is not directly proportional to cytosolic free Ca2+ level and that recovery from inhibition in paired-pulse experiments involves factors other than the decline of cytosolic [Ca2+] following a conditioning response.

To boldly glow ... applications of laser scanning confocal microscopy in developmental biology

The laser scanning confocal microscope (LSCM) is now established as an invaluable tool in developmental biology for improved light microscope imaging of fluorescently labelled eggs, embryos and developing tissues. The universal application of the LSCM in biomedical research has stimulated improvements to the microscopes themselves and the synthesis of novel probes for imaging biological structures and physiological processes. Moreover the ability of the LSCM to produce an optical series in perfect register has made computer 3-D reconstruction and analysis of light microscope images a practical option.

Cellular and subcellular calcium signaling in gastrointestinal epithelium

Ca2+ is a critical second messenger in virtually all cell types, including the various epithelial cell types within the digestive system. When measured in cell populations, Ca2+ signals usually appear as a single transient or prolonged elevation. In individual epithelial cells, signaling patterns often vary from cell to cell and may contain more complex features such as Ca2+ oscillations. Subcellular Ca2+ signals show a further level of complexity, such as Ca2+ waves, and may relate to the polarized structure and function of epithelial cells. The approaches to detect cytosolic Ca2+ signals, the patterns and mechanisms of Ca2+ signaling, and the role of such signals in regulating the function of polarized epithelium within the gastrointestinal tract, pancreas, and liver are reviewed in this report.

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Action potentials, calcium transients and the control of differentiation of excitable cells

Calcium influx via action potentials in differentiating nerve and muscle is regulated principally by the expression of potassium currents. Transient elevations of intracellular calcium in spontaneously active cells are necessary for normal neuronal development. The mechanisms that connect calcium elevations to long term developmental change are likely to be utilized in the mature nervous system.

Transitions of latency time and oscillation phase on parameter surfaces from models of intracellular calcium ion dynamics

The dynamics of two classical elementary compartmental models stimulating intracellular calcium ion oscillatory behavior are examined in terms of parameter surfaces. It has been found that, along certain lines of instability on surfaces defined by model parameters, the highly non-linear nature of these models produces sharp transitions in the latency time which determines the phase of oscillations once they commence. This sensitivity to initial conditions in deterministic models, along with the stochastic variance inevitably present in actual biological systems, illustrates how two seemingly identical cells activated by identical synchronous stimulation can exhibit oscillatory responses which are out of phase with respect to each other.

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

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

Classes and mechanisms of calcium waves

The best known calcium waves move at about 5-30 microns/s (at 20 degrees C) and will be called fast waves to distinguish them from slow (contractile) ones which move at 0.1-1 microns/s as well as electrically propagated, ultrafast ones. Fast waves move deep within cells and seem to underlie most calcium signals. Their velocity and hence mechanism has been remarkably conserved among all or almost all eukaryotic cells. In fully active (but not overstimulated) cells of all sorts, their mean speeds lie between about 15-30 microns/s at 20 degrees C. Their amplitudes usually lie between 3-30 microM and their frequencies from one per 10-300 s. They are propagated by a reaction diffusion mechanism governed by the Luther equation in which Ca2+ ions are the only diffusing propagators, and calcium induced calcium release, or CICR, the only reaction; although this reaction traverses various channels which are generally modulated by IP3 or cADPR. However, they may be generally initiated by a second, lumenal mode of CICR which occurs within the ER. Moreover, they are propagated between cells by a variety of mechanisms. Slow intracellular waves, on the other hand, may be mechanically propagated via stretch sensitive calcium channels.

Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle

Spontaneous local increases in the concentration of intracellular calcium, called "calcium sparks," were detected in quiescent rat heart cells with a laser scanning confocal microscope and the fluorescent calcium indicator fluo-3. Estimates of calcium flux associated with the sparks suggest that calcium sparks result from spontaneous openings of single sarcoplasmic reticulum (SR) calcium-release channels, a finding supported by ryanodine-dependent changes of spark kinetics. At resting intracellular calcium concentrations, these SR calcium-release channels had a low rate of opening (approximately 0.0001 per second). An increase in the calcium content of the SR, however, was associated with a fourfold increase in opening rate and resulted in some sparks triggering propagating waves of increased intracellular calcium concentration. The calcium spark is the consequence of elementary events underlying excitation-contraction coupling and provides an explanation for both spontaneous and triggered changes in the intracellular calcium concentration in the mammalian heart.

A single-pool model for intracellular calcium oscillations and waves in the Xenopus laevis oocyte

We construct a minimal model of cytosolic free Ca2+ oscillations based on Ca2+ release via the inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channel (IP3R) of a single intracellular Ca2+ pool. The model relies on experimental evidence that the cytosolic free calcium concentration ([Ca2+]c) modulates the IP3R in a biphasic manner, with Ca2+ release inhibited by low and high [Ca2+]c and facilitated by intermediate [Ca2+]c, and that channel inactivation occurs on a slower time scale than activation. The model produces [Ca2+]c oscillations at constant [IP3] and reproduces a number of crucial experiments. The two-dimensional spatial model with IP3 dynamics, cytosolic diffusion of IP3 (Dp = 300 microns 2 s-1), and cytosolic diffusion of Ca2+ (Dc = 20 microns 2 s-1) produces circular, planar, and spiral waves of Ca2+ with speeds of 7-15 microns.s-1, which annihilate upon collision. Increasing extracellular [Ca2+] influx increases wave speed and baseline [Ca2+]c. A [Ca2+]c-dependent Ca2+ diffusion coefficient does not alter the qualitative behavior of the model. An important model prediction is that channel inactivation must occur on a slower time scale than activation in order for waves to propagate. The model serves to capture the essential macroscopic mechanisms that are involved in the production of intracellular Ca2+ oscillations and traveling waves in the Xenopus laevis oocyte.