Model for receptor-controlled cytosolic calcium oscillations and for external influences on the signal pathway

Model analysis of nonlinear modification of neutrophil calcium homeostasis under the influence of modulated electromagnetic radiation of extremely high frequencies

The problem of resonance effects of electromagnetic radiation (EMR) on biological objects remained unsolved till now. Previously we demonstrated that low-intensity amplitude-modulated EMR of extremely high frequencies (EHF) modified the activity of mouse neutrophils in the synergistic reaction of calcium ionophore A23187 and phorbol ester PMA. The EHF EMR influence on the neutrophils was significant at the carrier frequencies of radiation within a narrow range of 41.8-42.05 GHz and at the modulation frequency of 1 Hz. The purpose of the work was the analysis of frequency-dependent modification of intracellular free calcium concentration ([Ca(2+)](i)) by modulated EHF EMR on the basis of a special model for [Ca(2+)](i) oscillations in the neutrophils. The calcium channels of plasma membrane were chosen as the action target of external modulation in the model. The computer simulation demonstrated the rise in [Ca(2+)](i) at the influence of the external field with a threshold dependence on the modulation amplitude. The effect depended heavily on a sequence of delivery of the chemical and electromagnetic stimuli. The narrow-band rise in [Ca(2+)](i) had a phase-frequency dependence. With the modulation amplitudes exceeding the threshold value, the rise in [Ca(2+)](i) of more than 50% of the initial level was observed at the frequency of about 1 Hz and in the phase range of 0.3-2.5 radians. The results of the model analysis are in good correspondence with the experimental data obtained before, namely, with the resonance modification of the neutrophil activity at the modulation frequency of 1 Hz and with the presence of the effect only at high concentrations of calcium ionophore.

Bioelectromagnetic medicine: the role of resonance signaling

Only recently has the critical importance of electromagnetic (EM) field interactions in biology and medicine been recognized. We review the phenomenon of resonance signaling, discussing how specific frequencies modulate cellular function to restore or maintain health. The application of EM-tuned signals represents more than merely a new tool in information medicine. It can also be viewed in the larger context of EM medicine, the all-encompassing view that elevates the EM over the biochemical. The discovery by Zhadin that ultrasmall magnetic intensities are biologically significant suggests that EM signaling is endogenous to cell regulation, and consequently that the remarkable effectiveness of EM resonance treatments reflects a fundamental aspect of biological systems. The concept that organisms contain mechanisms for generating biologically useful electric signals is not new, dating back to the nineteenth century discovery of currents of injury by Matteucci. The corresponding modern-day version is that ion cyclotron resonance magnetic field combinations help regulate biological information. The next advance in medicine will be to discern and apply those EM signaling parameters acting to promote wellness, with decreasing reliance on marginal biochemical remediation and pharmaceuticals.

G protein co-signaling and challenges for translational research

The Gq-linked G protein coupled receptors (GPCRs) and their signaling pathways are important clinical targets for the dementia of Alzheimer’s disease and cognitive decline with aging. Gq stimulates phospholipase C-β1 (PLC-β1) activity, increasing levels of inositol-1, 4, 5-trisphosphate (IP3) and diacylglycerol, to initiate mobilization of intracellular Ca2+ and activation of protein kinase C, respectively. While high concentrations of ligand typically evoke large sustained increases in cytosolic Ca2+ levels, it has long been appreciated that the dynamics of the Ca2+ increase are more complex and consistent with multiple levels of regulation. Physiologically relevant concentrations of Gq-ligands evoke rhythmic fluctuations or an oscillation in the level of cytosolic Ca2+. Downstream targets are tuned to respond to the frequency of the Ca2+ oscillations which in turn, reflect the oscillations in IP3 levels. Oscillatory behavior depends on the assembly of self-organizing interactions. The components that contribute to and regulate the Ca2+ oscillator have been unclear, precluding transfer of this fundamental knowledge from bench to bedside. Many GPCRs that signal with Gq also co-signal with G12. G protein co-signaling could therefore regulate the Ca2+ oscillator. This letter explores the potential relationship between Ca2+ oscillations, G protein co-signaling and cellular response in the context of our recent observations. We found that Gq efficacy is synergistic with phosphatidic acid, (PA), a signaling mediator generated downstream of activated G12 and RhoA. Regulation by PA depends on interaction with the unique PLC-β1 PA binding region. G protein co-signaling is therefore a mechanism for GPCRs to collectively assemble self-organizing interactions that regulate the Ca2+ oscillator.

Disulphide bridges of phospholipase C of Chlamydomonas reinhardtii modulates lipid interaction and dimer stability

BACKGROUND

Phospholipase C (PLC) is an enzyme that plays pivotal role in a number of signaling cascades. These are active in the plasma membrane and triggers cellular responses by catalyzing the hydrolysis of membrane phospholipids and thereby generating the secondary messengers. Phosphatidylinositol-PLC (PI-PLC) specifically interacts with phosphoinositide and/or phosphoinositol and catalyzes specific cleavage of sn-3- phosphodiester bond. Several isoforms of PLC are known to form and function as dimer but very little is known about the molecular basis of the dimerization and its importance in the lipid interaction.

PRINCIPAL FINDINGS

We herein report that, the disruption of disulphide bond of a novel PI-specific PLC of C. reinhardtii (CrPLC) can modulate its interaction affinity with a set of phospholipids and also the stability of its dimer. CrPLC was found to form a mixture of higher oligomeric states with monomer and dimer as major species. Dimer adduct of CrPLC disappeared in the presence of DTT, which suggested the involvement of disulphide bond(s) in CrPLC oligomerization. Dimer-monomer equilibrium studies with the isolated fractions of CrPLC monomer and dimer supported the involvement of covalent forces in the dimerization of CrPLC. A disulphide bridge was found to be responsible for the dimerization and Cys7 seems to be involved in the formation of the disulphide bond. This crucial disulphide bond also modulated the lipid affinity of CrPLC. Oligomers of CrPLC were also captured in in vivo condition. CrPLC was mainly found to be localized in the plasma membrane of the cell. The cell surface localization of CrPLC may have significant implication in the downstream regulatory function of CrPLC.

SIGNIFICANCE

This study helps in establishing the role of CrPLC (or similar proteins) in the quaternary structure of the molecule its affinities during lipid interactions.

Features of anti-inflammatory effects of modulated extremely high-frequency electromagnetic radiation

Using a model of acute zymosan-induced paw edema in NMRI mice, we test the hypothesis that anti-inflammatory effects of extremely high-frequency electromagnetic radiation (EHF EMR) can be essentially modified by application of pulse modulation with certain frequencies. It has been revealed that a single exposure of animals to continuous EHF EMR for 20 min reduced the exudative edema of inflamed paw on average by 19% at intensities of 0.1-0.7 mW/cm(2) and frequencies from the range of 42.2-42.6 GHz. At fixed effective carrier frequency of 42.2 GHz, the anti-inflammatory effect of EHF EMR did not depend on modulation frequencies, that is, application of different modulation frequencies from the range of 0.03-100 Hz did not lead to considerable changes in the effect level. On the contrary, at "ineffective" carrier frequencies of 43.0 and 61.22 GHz, the use of modulation frequencies of 0.07-0.1 and 20-30 Hz has allowed us to restore the effect up to a maximal level. The results obtained show the critical dependence of anti-inflammatory action of low-intensity EHF EMR on carrier and modulation frequencies. Within the framework of this study, the possibility of changing the level of expected biological effect of modulated EMR by a special selection of combination of carrier and modulation frequencies is confirmed.

Effects of magnetic fields on intracellular calcium oscillations

Based on the model of the two calcium stores developed by Goldbeter, and the viewpoint that the cell membrane is a major site of interaction for extremely-low-frequency (ELF) electromagnetic fields, the permeability of ions can be changed by electromagnetic fields, a theoretical model of cytosolic calcium oscillations influenced by magnetic fields is presented. It is shown that, the field frequency condition will influence the pattern of calcium oscillation while the field strength was given, and the magnetic field strength will change the pattern of calcium oscillation when the frequency was 50Hz and 100Hz. Therefore, intracellular calcium oscillations can be influenced by ELF magnetic fields and then a series cellular response could be transformed.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Effects of extremely low frequency electromagnetic fields on the adenylate kinase activity of rod outer segment of bovine retina

Extremely low frequency electromagnetic fields (ELF-EMFs) of 75 Hz with amplitudes above a threshold of about 125 microT have a dramatic effect on the adenylate kinase (AK) activity of the rod outer segment (ROS) membranes. In fact, the ATP production by ROS membranes or by purified disk membranes placed in the field decreased by approximately 54%. The decrease in enzymatic activity was independent of the time of exposure to the field and was completely reversible. When disk membranes were solubilized with Triton or a soluble isoform of AK was used, negligible effects of the field were obtained on the enzymatic activity, suggesting that the membrane has an important role in determining the conditions for the enzyme inactivation.

The DC electrical-field-induced Ca2+ response and growth stimulation of multicellular tumor spheroids are mediated by ATP release and purinergic receptor stimulation

It has been demonstrated that adenosine 5′-triphosphate (ATP) is actively secreted by cells, thereby eliciting Ca2+-dependent signal transduction cascades in an autocrine and paracrine manner. In the present study the effects of direct current (DC) electrical fields on ATP release, the intracellular Ca2+ concentration [Ca2+]i and growth of multicellular prostate tumor spheroids were investigated. Treatment of multicellular tumor spheroids by a single DC electrical field pulse with a field strength of 750 Vm-1 for 60 seconds resulted in a transient Ca2+ response, activation of c-Fos and growth stimulation. The initial [Ca2+]i signal was elicited at the anode-facing side of the spheroid and spread with a velocity of approximately 12 μm per second across the spheroid surface. The electrical-field-evoked Ca2+ response as well as c-Fos activation and growth stimulation of tumor spheroids were inhibited by pretreatment with the anion channel blockers NPPB, niflumic acid and tamoxifen. Furthermore, the Ca2+ response elicited by electrical field treatment was abolished following purinergic receptor desensitivation by repetitive treatment of tumor spheroids with ATP and pretreatment with the purinergic receptor antagonist suramin as well as with apyrase. Electrical field treatment of tumor spheroids resulted in release of ATP into the supernatant as evaluated by luciferin/luciferase bioluminescence. ATP release was efficiently inhibited in the presence of anion channel blockers. Our data suggest that electrical field treatment of multicellular tumor spheroids results in ATP release, which concomitantly activates purinergic receptors, elicits a Ca2+ wave spreading through the tumor spheroid tissue and stimulates tumor growth.

Serum plays a critical role in modulating [Ca2+]c of primary culture bone cells exposed to weak ion-resonance magnetic fields

Primary-culture bone cells were exposed to ion-resonance (IR) magnetic fields tuned to Ca2+. Cytosolic calcium concentration, [Ca2+]c, was measured by using fura-2 during field exposure. The fields investigated were 20 muT static + muT p-p at either 15.3 or 76.6 Hz, and 0.13 mT static + either 0.5 or 1.0 mT p-p at 100 Hz. Other parameters included field orientation, culture age (2 or 5 days after plating), and the presence of serum (0 or 2%) during exposure. Total experiment time was 29.5 min: The field was applied after 2 min, and bradykinin was added as an agonist control after 22 min. The data were quantified on a single-cell basis during the 2-22 min exposure period in terms of the magnitude of the largest occurring [Ca2+]c spike normalized to local baseline. Field-exposed and control groups were characterized in terms of the percent of cells exhibiting spike magnitudes above thresholds of 100 or 66% over baseline and were compared by using Fisher's exact test. Without serum, there was little evidence that IR magnetic fields altered [Ca2+]c. However, in the presence of 2% serum, 3 of the 16 experiments exhibited significant effects at the 100% threshold. Reducing this threshold to 66% resulted in five experiments exhibiting significant effects. Most strikingly, in all of these cases, the field acted to enhance [Ca2+]c activity as opposed to suppressing [Ca2+]c activity. These findings suggest a role for serum or for constituents within serum in mediating the effects of IR magnetic fields on cells and may provide a resolution pathway to the dilemma imposed by theoretical arguments regarding the possibility of such phenomena. Possible roles of serum and future studies are discussed.

Extremely low frequency pulsed DC electric fields promote neutrophil extension, metabolic resonance and DNA damage when phase-matched with metabolic oscillators

Application of extremely low frequency pulsed DC electric fields that are frequency- and phase-matched with endogenous metabolic oscillations leads to greatly exaggerated neutrophil extension and metabolic resonance wherein oscillatory NAD(P)H amplitudes are increased. In the presence of a resonant field, migrating cell length grows from 10 to approximately 40 microm, as does the overall length of microfilament assemblies. In contrast, cells stop locomotion and become spherical when exposed to phase-mismatched fields. Although cellular effects were not found to be dependent on electrode type and buffer, they were sensitive to temporal constraints (phase and pulse length) and cell surface charge. We suggest an electromechanical coupling hypothesis wherein applied electric fields and cytoskeletal polymerization forces act together to overcome the surface/cortical tension of neutrophils, thus promoting net cytoskeletal assembly and heightened metabolic amplitudes. Metabolic resonance enhances reactive oxygen metabolic production by neutrophils. Furthermore, cellular DNA damage was observed after prolonged metabolic resonance using both single cell gel electrophoresis ('comet' assay) and 3'-OH DNA labeling using terminal deoxynucleotidyl transferase. These results provide insights into transmembrane signal processing and cell interactions with weak electric fields.

Intra- and intercellular Ca2+ signaling in paraneurons and other secretory cells

Paraneurons are endocrine and sensory cells which share structural, functional, and metabolic features with neurons. They produce identical with or related to neurotransmitters or neurohormones, which are synthesized and secreted by regulated secretion. They are receptoconductile-secretory in function, which is shared by specific proteins distributed at proper regions of cell membrane. A substantial advance has been made in the molecular machinery underlying protein sorting and transport within the endoplasmic reticulum and Golgi apparatus, and the mechanism of targeted membrane fusion by constitutive secretion. Various patterns of [Ca2+]c dynamics play cardinal signaling roles in stimulus-secretion coupling in individual secretory cells. Long-lived recurrent Ca2+ spikes or oscillation may maintain prolonged secretory responses, ATP synthesis in mitochondria, cell growth, differentiation, and division. In the neurons and the paraneurons of neuroectodermal origin, action potentials propagate along a conductile region to the secretory region of each cell and hardly be transmitted to the adjacent cells. In the paraneurons of gut endodermal origin, intracellular signalling including Ca2+ spikes can be propagated to the adjacent cells, and in turn may maintain coordination of individual cells forming a cell society.

A realistic model of biphasic calcium transients in electrically nonexcitable cells

In many electrically nonexcitable cells, the release of calcium from internal stores is followed by a much slower phase in which the intracellular calcium concentration decreases gradually to a sustained value higher than the concentration before stimulation. This elevated calcium plateau has been shown to be the result of calcium influx. The model presented in this work describes a system consisting of a cytoplasmic calcium store and a plasma membrane calcium channel, both excitable by a membrane receptor; a fast cytoplasmic calcium buffer; and calcium pumps in both the calcium store and cellular membranes. Inherent difficulties in the numerical evaluation of the model, caused by very large calcium fluxes across the store membrane, were overcome by analytically separating the fast processes of calcium release from the slower processes of calcium cycling across the plasma membrane. This enabled the simulation of realistic biphasic calcium transients similar to those observed experimentally. The model predicted 1) a strong correlation between the rate of calcium cycling across the plasma membrane and the rate of calcium decay; and 2) a dependence on the level of cell excitation of the maximum rise in cytoplasmic calcium concentration, the level of the elevated calcium plateau, and the rate of calcium decay. Using the model, we simulated the washout of agonist from the bathing solution and the depletion of the calcium store by a pharmacological agent (such as thapsigargin) under several experimental conditions.

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.

Calcium and cell cycle progression: possible effects of external perturbations on cell proliferation

Exit from the phase of cellular division appears to be driven by a calcium signal that triggers a cascade of events leading to the completion of mitosis. Here we propose a model that relates the dynamics of cytosolic calcium to progression through mitosis, G1 and G2 phases of the cell cycle. To this end, the assumption has been made that the transient rise ir cytosolic calcium concentration during mitosis is induced by inositol(1,4,5)triphosphate (IP3), which in turn is released at high levels of mitosis-promoting factor (MPF). On this basis, a system of ordinary differential equations is proposed to simulate the evolution of ten cell-cycle-specific molecular species, including cyclins A and B, MPF, IP3, Ca2+, the CaMKII holoenzyme, and the ubiquitination complex. The influence on the cell proliferation capacity exerted by external perturbations, like calcium microinjections, depletion of intracellular calcium stores, electromagnetic fields, or stimulation/inhibition of different calcium currents through the plasma membrane, can be studied by appropriate modulation of the parameters involved in the signal transduction pathway.

Extracellular calcium concentration controls the frequency of intracellular calcium spiking independently of inositol 1,4,5-trisphosphate production in HeLa cells

Stimulation of single HeLa cells with histamine evoked repetitive increases of the intracellular calcium ion concentration (Ca2+ spikes). The frequency of Ca2+ spiking increased as the extracellular hormone concentration was elevated. In addition, the frequency of Ca2+ spiking could be accelerated by increasing the extracellular Ca2+ concentration ([Ca2+]0) in the presence of a constant hormone concentration. The range of [Ca2+]0 over which the spiking frequency could be titrated was nominally-zero to 10mM, being half-maximally effective at approx. 1 and 2.5mM for 37 and 22 degrees C respectively. The effect of [Ca2+]0 on inositol phosphates production was also examined. Changes of [Ca2+]0 over a range which had been found to affect the frequency of Ca2+ spiking did not have any effect on the rate of myo-inositol 1,4,5-trisphosphate (InsP3) production, although an increase in inositol phosphates production was observed as [Ca2+]0 was increased from zero to values giving less than half-maximal Ca2+ spike frequency. These data suggest that at low Ca2+ spike frequency, Ca2+-stimulated activation of phospholipase C may contribute to Ca2+ spiking in HeLa cells, but under some conditions the availability of Ca2+ to the intracellular stores, rather than changes in the rate of InsP3 production, determines the Ca2+ spike frequency.

The influence of 50-Hz magnetic fields on cytoplasmic Ca2+ oscillations in human leukemia T-cells

We have studied the effects of 50-Hz 100-microT rms magnetic fields on intracellular Ca2+ concentration in the Jurkat T lymphocyte variant E6.1 using fluorescent probes Indo-1 and Fura-2. We found, however, that the pattern of intracellular Ca2+ fluctuations also depended on the agent used for cell attachment, in our case the polypeptide poly-L-lysine. In order to isolate possible effects of magnetic field exposure from those of poly-L-lysine, the action of polypeptide on cytosolic Ca2+ was studied as well. It was found that a 10(-7)% concentration of polypeptide triggered prolonged Ca2+ spiking. Higher (10(-4)%) concentrations induced rapid increases in intracellular Ca2+ followed by high, unstable Ca2+ levels. The response of these cells to the monoclonal antibody anti-CD3 was also inhomogeneous, similar to one caused by poly-L-lysine. The effect of magnetic field exposure was studied on cells initially exhibiting (1) non-oscillating, low Ca2+ concentration and (2) prolonged Ca2+ concentration oscillations. In case (1) the result was negative. In case (2), statistically significant changes were found: the oscillation amplitude was reduced on average by 30%, and the frequency composition was shifted towards higher frequencies.

Activation-dependent and biphasic electromagnetic field effects: model based on cooperative enzyme kinetics in cellular signaling

Experiments on filed exposure effects of extremely-low-frequency electric and magnetic fields (EMFs) on biological systems have shown that, in many cases, the biological-functional status is of fundamental importance for an effective interaction. For example, studies of calcium uptake regulation in cells of the immune system, particularly in T lymphocytes, have revealed that, depending on the degree of cellular activation, either stimulatory, inhibitory, or no field exposure effects are observed for identical field parameters. A brief summary of the experimental findings is given, and a theoretical approach is presented that accounts in a qualitative manner for EMF exposure effects 1) that depend on the degree of cellular activation and 2) that exhibit a biphasic response behavior (stimulation/ inhibition). In the model, biochemical stimulation of the cell results in activation of specific signaling pathways that regulate calcium dynamics in the cell (calcium release from intracellular calcium stores and capacitative calcium entry). We assume that, controlled by these pathways, a specific EMF-sensitive enzyme system becomes activated. The activated enzyme, in turn, exhibits a feedback control on the signal processes, thus leading to a modulation of calcium entry. This modulation may affect other cellular processes that are calcium dependent (e.g., DNA synthesis). Magnetic field exposure is assumed to alter the kinetics of a specific step within the enzyme-reaction cycle in accord with the radical-pair mechanism, although the formulism is not restricted to this specific example. Results show that inclusion of cooperative steps within the enzyme-reaction cycle provides a theoretical basis that enables a simple description of a biphasic response behavior to EMF exposure.

Membrane potential changes in activated cells: connection with cytosolic calcium oscillator

The activation of many cells by hormones, growth factors or neurotransmitters can lead to a periodic variation of the cell membrane potential. A theoretical model that describes this behaviour on the basis of the cytosolic calcium oscillator is developed. The main assumption of this model stressed that the plasma membrane potential is mainly a K+ diffusion potential. It is shown that the oscillations of the intracellular calcium concentration can induce solely the membrane potential oscillations, without any further interaction mechanisms between fluxes through the ionic channels from the plasma membrane. A one-pool model was used to account for Ca2+ release from the intracellular stores. The model and the numerical calculations are based on the theory of nonlinear dynamical systems.

Stimulus-Induced Oscillations in Guard Cell Cytosolic Free Calcium

Ca2+ is implicated as a second messenger in the response of stomata to a range of stimuli. However, the mechanism by which stimulus-induced increases in guard cell cytosolic free Ca2+ ([Ca2+]i) are transduced into different physiological responses remains to be explained. Oscillations in [Ca2+]i may provide one way in which this can occur. We used photometric and imaging techniques to examine this hypothesis in guard cells of Commelina communis. External Ca2+ ([Ca2+]e), which causes an increase in [Ca2+]i, was used as a closing stimulus. The total increase in [Ca2+]i was directly related to the concentration of [Ca2+]e, both of which correlated closely with the degree of stomatal closure. Increases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of [Ca2+]e. At 0.1 mM, [Ca2+]e induced symmetrical oscillations. In contrast, 1.0 mM [Ca2+]e induced asymmetric oscillations. Oscillations were stimulus dependent and modulated by changing [Ca2+]e. Experiments using Ca2+ channel blockers and Mn2+-quenching studies suggested a role for Ca2+ influx during the oscillatory behavior without excluding the possible involvement of Ca2+ release from intracellular stores. These data suggest a mechanism for encoding the information required to distinguish between a number of different Ca2+-mobilizing stimuli in guard cells, using stimulus-specific patterns of oscillations in [Ca2+]i.

Model for external influences on cellular signal transduction pathways including cytosolic calcium oscillations

Experiments on the effects of extremely-low-frequency (ELF) electric and magnetic fields on cells of the immune system, T-lymphocytes in particular, suggest that the external field interacts with the cell at the level of intracellular signal transduction pathways. These are directly connected with changes in the calcium-signaling processes of the cell. Based on these findings, a theoretical model for receptor-controlled cytosolic calcium oscillations and for external influences on the signal transduction pathway is presented. We discuss the possibility that the external field acts on the kinetics of the signal transduction between the activated receptors at the cell membrane and the G-proteins. It is shown that, depending on the specific combination of cell internal biochemical and external physical parameters, entirely different responses of the cell can occur. We compare the effects of a coherent (periodic) modulation and of incoherent perturbations (noise). The model and the calculations are based on the theory of self-sustained, nonlinear oscillators. It is argued that these systems form an ideal basis for information-encoding processes in biological systems.