Relationship between intracellular calcium and its muffling measured by calcium iontophoresis in snail neurones

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Double-barreled and Concentric Microelectrodes for Measurement of Extracellular Ion Signals in Brain Tissue

Electrical activity in the brain is accompanied by significant ion fluxes across membranes, resulting in complex changes in the extracellular concentration of all major ions. As these ion shifts bear significant functional consequences, their quantitative determination is often required to understand the function and dysfunction of neural networks under physiological and pathophysiological conditions. In the present study, we demonstrate the fabrication and calibration of double-barreled ion-selective microelectrodes, which have proven to be excellent tools for such measurements in brain tissue. Moreover, so-called “concentric” ion-selective microelectrodes are also described, which, based on their different design, offer a far better temporal resolution of fast ion changes. We then show how these electrodes can be employed in acute brain slice preparations of the mouse hippocampus. Using double-barreled, potassium-selective microelectrodes, changes in the extracellular potassium concentration ([K⁺]_o) in response to exogenous application of glutamate receptor agonists or during epileptiform activity are demonstrated. Furthermore, we illustrate the response characteristics of sodium-sensitive, double-barreled and concentric electrodes and compare their detection of changes in the extracellular sodium concentration ([Na⁺]_o) evoked by bath or pressure application of drugs. These measurements show that while response amplitudes are similar, the concentric sodium microelectrodes display a superior signal-to-noise ratio and response time as compared to the double-barreled design. Generally, the demonstrated procedures will be easily transferable to measurement of other ions species, including pH or calcium, and will also be applicable to other preparations.

Speed, adaptation, and stability of the response to light in cone photoreceptors: the functional role of Ca-dependent modulation of ligand sensitivity in cGMP-gated ion channels

The response of cone photoreceptors to light is stable and reproducible because of the exceptional regulation of the cascade of enzymatic reactions that link visual pigment (VP) excitation to the gating of cyclic GMP (cGMP)-gated ion channels (cyclic nucleotide–gated [CNG]) in the outer segment plasma membrane. Regulation is achieved in part through negative feedback control of some of these reactions by cytoplasmic free Ca²⁺. As part of the control process, Ca²⁺ regulates the phosphorylation of excited VP, the activity of guanylate cyclase, and the ligand sensitivity of the CNG ion channels. We measured photocurrents elicited by stimuli in the form of flashes, steps, and flashes superimposed on steps in voltage-clamped single bass cones isolated from striped bass retina. We also developed a computational model that comprises all the known molecular events of cone phototransduction, including all Ca-dependent controls. Constrained by available experimental data in bass cones and cone transduction biochemistry, we achieved an excellent match between experimental photocurrents and those simulated by the model. We used the model to explore the physiological role of CNG ion channel modulation. Control of CNG channel activity by both cGMP and Ca²⁺ causes the time course of the light-dependent currents to be faster than if only cGMP controlled their activity. Channel modulation also plays a critical role in the regulation of the light sensitivity and light adaptation of the cone photoresponse. In the absence of ion channel modulation, cone photocurrents would be unstable, oscillating during and at the offset of light stimuli.

Mitochondria and chromaffin cell function

Chromaffin cells are an excellent model for stimulus-secretion coupling. Ca(2+) entry through plasma membrane voltage-operated Ca(2+) channels (VOCC) is the trigger for secretion, but the intracellular organelles contribute subtle nuances to the Ca(2+) signal. The endoplasmic reticulum amplifies the cytosolic Ca(2+) ([Ca(2+)](C)) signal by Ca(2+)-induced Ca(2+) release (CICR) and helps generation of microdomains with high [Ca(2+)](C) (HCMD) at the subplasmalemmal region. These HCMD induce exocytosis of the docked secretory vesicles. Mitochondria close to VOCC take up large amounts of Ca(2+) from HCMD and stop progression of the Ca(2+) wave towards the cell core. On the other hand, the increase of [Ca(2+)] at the mitochondrial matrix stimulates respiration and tunes energy production to the increased needs of the exocytic activity. At the end of stimulation, [Ca(2+)](C) decreases rapidly and mitochondria release the Ca(2+) accumulated in the matrix through the Na(+)/Ca(2+) exchanger. VOCC, CICR sites and nearby mitochondria form functional triads that co-localize at the subplasmalemmal area, where secretory vesicles wait ready for exocytosis. These triads optimize stimulus-secretion coupling while avoiding propagation of the Ca(2+) signal to the cell core. Perturbation of their functioning in neurons may contribute to the genesis of excitotoxicity, ageing mental retardation and/or neurodegenerative disorders.

The Ca(2+): H(+) coupling ratio of the plasma membrane calcium ATPase in neurones is little sensitive to changes in external or internal pH

To explore the effects of both external and internal pH (pH_o and pH_i) on the coupling between Ca²⁺ extrusion and H⁺ uptake by the PMCA activity in snail neurones H⁺ uptake was assessed by measuring surface pH changes (ΔpH_s) with pH-sensitive microelectrodes while Ba²⁺ or Ca²⁺ loads were extruded. Ru360 or ruthenium red injection showed that injected Ca²⁺ was partly taken up by mitochondria, but Ca²⁺ entering through channels was not. External pH was changed using a mixture of three buffers to minimise changes in buffering power. With depolarisation-induced Ca²⁺ or Ba²⁺ loads the ΔpH_s were not changed significantly over the pH range 6.5–8.5. With Ca²⁺ injections into cells with mitochondrial uptake blocked the ΔpH_s were significantly smaller at pH 8.5 than at 7.5, but this could be explained in part by the slower rate of activity of the PMCA. Low intracellular pH also changed the ΔpH_s responses to Ca²⁺ injection, but not significantly. Again this may have been due to reduced pump activity at low pH_i. I conclude that in snail neurones the PMCA coupling ratio is either insensitive or much less sensitive to pH than in red blood cells or barnacle muscle.

Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis

Our knowledge of the molecular mechanisms of intracellular homeostatic control of zinc ions is now firmly grounded on experimental findings gleaned from the study of zinc proteomes and metallomes, zinc transporters, and insights from the use of computational approaches. A cell's repertoire of zinc homeostatic molecules includes cytosolic zinc-binding proteins, transporters localized to cytoplasmic and organellar membranes, and sensors of cytoplasmic free zinc ions. Under steady state conditions, a primary function of cytosolic zinc-binding proteins is to buffer the relatively large zinc content found in most cells to a cytosolic zinc(ii) ion concentration in the picomolar range. Under non-steady state conditions, zinc-binding proteins and transporters act in concert to modulate transient changes in cytosolic zinc ion concentration in a process that is called zinc muffling. For example, if a cell is challenged by an influx of zinc ions, muffling reactions will dampen the resulting rise in cytosolic zinc ion concentration and eventually restore the cytosolic zinc ion concentration to its original value by shuttling zinc ions into subcellular stores or by removing zinc ions from the cell. In addition, muffling reactions provide a potential means to control changes in cytosolic zinc ion concentrations for purposes of cell signalling in what would otherwise be considered a buffered environment not conducive for signalling. Such intracellular zinc ion signals are known to derive from redox modifications of zinc-thiolate coordination environments, release from subcellular zinc stores, and zinc ion influx via channels. Recently, it has been discovered that metallothionein binds its seven zinc ions with different affinities. This property makes metallothionein particularly well positioned to participate in zinc buffering and muffling reactions. In addition, it is well established that metallothionein is a source of zinc ions under conditions of redox signalling. We suggest that the biological functions of transient changes in cytosolic zinc ion concentrations (presumptive zinc signals) complement those of calcium ions in both spatial and temporal dimensions.

Full system bifurcation analysis of endocrine bursting models

Plateau bursting is typical of many electrically excitable cells, such as endocrine cells that secrete hormones and some types of neurons that secrete neurotransmitters. Although in many of these cell types the bursting patterns are regulated by the interplay between voltage-gated calcium channels and calcium-sensitive potassium channels, they can be very different. We investigate so-called square-wave and pseudo-plateau bursting patterns found in endocrine cell models that are characterized by a super- or subcritical Hopf bifurcation in the fast subsystem, respectively. By using the polynomial model of Hindmarsh and Rose (Proceedings of the Royal Society of London B 221 (1222) 87-102), which preserves the main properties of the biophysical class of models that we consider, we perform a detailed bifurcation analysis of the full fast-slow system for both bursting patterns. We find that both cases lead to the same possibility of two routes to bursting, that is, the criticality of the Hopf bifurcation is not relevant for characterizing the route to bursting. The actual route depends on the relative location of the full-system's fixed point with respect to a homoclinic bifurcation of the fast subsystem. Our full-system bifurcation analysis reveals properties of endocrine bursting that are not captured by the standard fast-slow analysis.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

Differences of Ca2+ handling properties in identified central olfactory neurons of the antennal lobe

Information processing in neurons depends on highly localized Ca2+ signals. The spatial and temporal dynamics of these signals are determined by a variety of cellular parameters including the calcium influx, calcium buffering and calcium extrusion. Our long-term goal is to better understand how intracellular Ca2+ dynamics are controlled and contribute to information processing in defined interneurons of the insect olfactory system. The latter has served as an excellent model to study general mechanisms of olfaction. Using patch-clamp recordings and fast optical imaging in combination with the 'added buffer approach', we analyzed the Ca2+ handling properties of different identified neuron types in Periplaneta americana's olfactory system. Our focus was on two types of local interneurons (LNs) with significant differences in intrinsic electrophysiological properties: (1) spiking LNs that generate 'normal' Na+ driven action potentials and (2) non-spiking LNs that do not express voltage-activated Na+ channels. We found that the distinct electrophysiological properties from different types of central olfactory interneurons are strongly correlated with their cell specific calcium handling properties: non-spiking LNs, in which Ca2+ is the only cation that enters the cell to contribute to membrane depolarization, had the highest endogenous Ca2+ binding ratio and Ca2+ extrusion rate.

Calcium diffusion enhanced after cleavage of negatively charged components of brain extracellular matrix by chondroitinase ABC

The concentration of extracellular calcium plays a critical role in synaptic transmission and neuronal excitability as well as other physiological processes. The time course and extent of local fluctuations in the concentration of this ion largely depend on its effective diffusion coefficient (D*) and it has been speculated that fixed negative charges on chondroitin sulphate proteoglycans (CSPGs) and other components of the extracellular matrix may influence calcium diffusion because it is a divalent cation. In this study we used ion-selective microelectrodes combined with pressure ejection or iontophoresis of ions from a micropipette to quantify diffusion characteristics of neocortex and hippocampus in rat brain slices. We show that D* for calcium is less than the value predicted from the behaviour of the monovalent cation tetramethylammonium (TMA), a commonly used diffusion probe, but D* for calcium increases in both brain regions after the slices are treated with chondroitinase ABC, an enzyme that predominantly cleaves chondroitin sulphate glycans. These results suggest that CSPGs do play a role in determining the local diffusion properties of calcium in brain tissue, most likely through electrostatic interactions mediating rapid equilibrium binding. In contrast, chondroitinase ABC does not affect either the TMA diffusion or the extracellular volume fraction, indicating that the enzyme does not alter the structure of the extracellular space and that the diffusion of small monovalent cations is not affected by CSPGs in the normal brain ionic milieu. Both calcium and CSPGs are known to have many distinct roles in brain physiology, including brain repair, and our study suggests they may be functionally coupled through calcium diffusion properties.

The plasma membrane calcium ATPase (PMCA) of neurones is electroneutral and exchanges 2 H+ for each Ca2+ or Ba2+ ion extruded

The coupling between Ca2+ extrusion and H+ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca2+- and pH-sensitive microelectrodes in large voltage-clamped snail neurones, which have no Na+-Ca2+ exchangers. The recovery of [Ca2+]i and surface pH after a brief depolarization or Ca2+ injection was not slowed by hyperpolarization to -90 mV from a holding potential of -50 mV, consistent with a 1 Ca2+ : 2 H+ coupling ratio. Since Ca2+ injections proved difficult to quantify, and Ca2+ currents through channels were obscured by K+ currents, Ba2+ was used as a substitute. When the cell was bathed in Ca2+-free Ba2+ Ringer solution, the K+ currents were blocked and large inward currents were revealed on depolarization. The Ca2+-sensitive microelectrodes were sensitive to intracellular Ba2+ as well as Ca2+. With equal depolarizations Ba2+ entry appeared larger than Ca2+ entry and generated similar but slower pH changes. Ba2+ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca2+ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba2+ influx to that caused by unit charge of H+ injection was 0.85 +/- 0.08 (s.e.m., n = 8), corresponding to a Ba2+ : H+ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca2+ : H+ ratio is 1 : 2, ensuring that after a Ca2+ influx [Ca2+]i recovery is not influenced by the membrane potential and maximizes the conversion of Ca2+ influxes into possible pH signals.

Quantification of Mg2+ extrusion and cytosolic Mg2+-buffering in Xenopus oocytes

Intracellular Mg(2+) buffering and Mg(2+) extrusion were investigated in Xenopus laevis oocytes. Mg(2+) or EDTA were pressure injected and the resulting changes in the intracellular Mg(2+) concentration were measured simultaneously with Mg(2+)-selective microelectrodes. In the presence of extracellular Na(+), injected Mg(2+) was extruded from the oocytes with an estimated v(max) and K(M) of 74 pmol cm(-2)s(-1) and 1.28 mM, respectively. To investigate genuine cytosolic Mg(2+) buffering, measurements were carried out in the nominal absence of extracellular Na(+) to block Mg(2+) extrusion, and during the application of CCCP (inhibiting mitochondrial uptake). Under these conditions, Mg(2+) buffering calculated after both MgCl(2) and EDTA injections could be described by a buffer equivalent with a concentration of 9.8mM and an apparent dissociation constant, K(d-app), of 0.6mM together with an [ATP](i) of 0.9 mM with a K(d-app) 0.12 mM. Xenopus oocytes thus possess highly efficient mechanisms to maintain their intracellular Mg(2+) concentration.

Dynamic and static calcium gradients inside large snail (Helix aspersa) neurones detected with calcium-sensitive microelectrodes

We have used quartz Ca²⁺-sensitive microelectrodes (CASMs) in large voltage-clamped snail neurones to investigate the inward spread of Ca²⁺ after a brief depolarisation. Both steady state and [Ca²⁺]_i transients changed with depth of penetration. When the CASM tip was within 20 μm of the far side of the cell the [Ca²⁺]_i transient time to peak was 4.4 ± 0.5 s, rising to 14.7 ± 0.7 s at a distance of 80 μm. We estimate that the Ca²⁺ transients travelled centripetally at an average speed of 6 μm² s⁻¹ and decreased in size by half over a distance of about 45 μm. Cyclopiazonic acid had little effect on the size and time to peak of Ca²⁺ transients but slowed their recovery significantly. This suggests that the endoplasmic reticulum curtails rather than reinforces the transients. Injecting the calcium buffer BAPTA made the Ca²⁺ transients more uniform in size and increased their times to peak and rates of recovery near the membrane. We have developed a computational model for the transients, which includes diffusion, uptake and Ca²⁺ extrusion. Good fits were obtained with a rather large apparent diffusion coefficient of about 90 ± 20 μm² s⁻¹.This may assist fast recovery by extrusion.

The concept of "buffering" in systems and control theory: from metaphor to math

The paradigm of "buffering" is used increasingly for the description of diverse "systemic" phenomena encountered in evolutionary genetics, ecology, integrative physiology, and other areas. However, in this new context, the paradigm has not yet matured into a truly quantitative concept inasmuch as it lacks a corresponding quantitative measure of "systems-level buffering strength". Here, I develop such measures on the basis of a formal and general approach to the quantitation of buffering action. "Systems-level buffering" is shown to be synonymous with "disturbance rejection" in feedback-control systems, and can be quantitated by means of dimensionless proportions between partial flows in two-partitioned systems. The units allow either the time-independent, "static" buffering properties or the time-dependent, "dynamic" ones to be measured. Analogous to this "resistance to change", one can define and measure the "conductance to change"; this quantity corresponds to "set-point tracking" in feedback-control systems. Together, these units provide a systematic framework for the quantitation of buffering action in systems biology, and reveal the common principle behind systems-level buffering, classical acid-base buffering, and multiple other manifestations of buffering.

The effects of HCl and CaCl(2) injections on intracellular calcium and pH in voltage-clamped snail (Helix aspersa) neurons

To investigate the mechanisms by which low intracellular pH influences calcium signaling, I have injected HCl, and in some experiments CaCl(2), into snail neurons while recording intracellular pH (pH(i)) and calcium concentration ([Ca(2+)](i)) with ion-sensitive microelectrodes. Unlike fluorescent indicators, these do not increase buffering. Slow injections of HCl (changing pH(i) by 0.1-0.2 pH units min(-1)) first decreased [Ca(2+)](i) while pH(i) was still close to normal, but then increased [Ca(2+)](i) when pH(i) fell below 6.8-7. As pH(i) recovered after such an injection, [Ca(2+)](i) started to fall but then increased transiently before returning to its preinjection level. Both the acid-induced decrease and the recovery-induced increase in [Ca(2+)](i) were abolished by cyclopiazonic acid, which empties calcium stores. Caffeine with or without ryanodine lowered [Ca(2+)](i) and converted the acid-induced fall in [Ca(2+)](i) to an increase. Injection of ortho-vanadate increased steady-state [Ca(2+)](i) and its response to acidification, which was again blocked by CPA. The normal initial response to 10 mM caffeine, a transient increase in [Ca(2+)](i), did not occur with pH(i) below 7.1. When HCl was injected during a series of short CaCl(2) injections, the [Ca(2+)](i) transients (recorded as changes in the potential (V(Ca)) of the Ca(2+)-sensitive microelectrode), were reduced by only 20% for a 1 pH unit acidification, as was the rate of recovery after each injection. Calcium transients induced by brief depolarizations, however, were reduced by 60% by a similar acidification. These results suggest that low pH(i) has little effect on the plasma membrane calcium pump (PMCA) but important effects on the calcium stores, including blocking their response to caffeine. Acidosis inhibits spontaneous calcium release via the RYR, and leads to increased store content which is unloaded when pH(i) returns to normal. Spontaneous release is enhanced by the rise in [Ca(2+)](i) caused by inhibiting the PMCA.

The effect of calcium pump inhibitors on the response of intracellular calcium to caffeine in snail neurones

We have measured intracellular free calcium ([Ca(2+)]i) using Fura-2 or Ca(2+)-sensitive microelectrodes in voltage-clamped neurones of the snail, Helix aspersa. Caffeine-induced transient increases in [Ca(2+)]i were normally followed by a brief fall of [Ca(2+)]i below its pre-caffeine level. We investigated the cause of this undershoot by raising [Ca(2+)]i; and by inhibiting the plasma membrane or endoplasmic reticulum Ca ATPases (PMCA or SERCA respectively). When the cell membrane potential was decreased from -60 to -25mV, steady-state [Ca(2+)]i increased. The caffeine-induced transients were smaller while the undershoots were larger than in control conditions. When the PMCA was inhibited by high pH the steady-state [Ca(2+)]i increased by 100-400nM. The caffeine-induced [Ca(2+)]i increase and the subsequent undershoot both became larger. Injection of orthovanadate, which inhibits the PMCA and increases [Ca(2+)]i, did not block either effect of caffeine. But when the SERCA was inhibited by cyclopiazonic acid the undershoot disappeared. The phosphodiesterase inhibitor IBMX did not influence the undershoot. These results suggest that the undershoot is generated by the Ca(2+)] ATPase of the stores rather than that of the plasma membrane. Since the undershoot increased as [Ca(2+)]i increased, we conclude that at higher levels of [Ca(2+)]i the stores refill more rapidly.

Apparent intracellular Mg2+ buffering in neurons of the leech Hirudo medicinalis

The apparent intracellular Mg2+ buffering, or muffling (sum of processes that damp changes in the free intracellular Mg2+ concentration, [Mg2+](i), e.g., buffering, extrusion, and sequestration), was investigated in Retzius neurons of the leech Hirudo medicinalis by iontophoretic injection of H+, OH-, or Mg2+. Simultaneously, changes in intracellular pH and the intracellular Mg2+, Na+, or K+ concentration were recorded with triple-barreled ion-selective microelectrodes. Cell volume changes were monitored measuring the tetramethylammonium (TMA) concentration in TMA-loaded neurons. Control measurements were carried out in electrolyte droplets (diameter 100-200 microm) placed on a silver wire under paraffin oil. Droplets with or without ATP, the presumed major intracellular Mg2+ buffer, were used to quantify the pH dependence of Mg2+ buffering and to determine the transport index of Mg2+ during iontophoretic injection. The observed pH dependence of [Mg2+](i) corresponded to what would be expected from Mg2+ buffering through ATP. The quantity of Mg2+ muffling, however, was considerably larger than what would be expected if ATP were the sole Mg2+ buffer. From the decrease in Mg2+ muffling in the nominal absence of extracellular Na+ it was estimated that almost 50% of the ATP-independent muffling is due to the action of Na+/Mg2+ antiport.

Supralinear Ca2+ signaling by cooperative and mobile Ca2+ buffering in Purkinje neurons

Endogenous high-affinity Ca2+ buffering and its roles were investigated in mouse cerebellar Purkinje cells with the use of a low-affinity Ca2+ indicator and a high-affinity caged Ca2+ compound. Increases in the cytosolic Ca2+ concentration ([Ca2+]i) were markedly facilitated during repetitive depolarization, resulting in the generation of steep micromolar Ca2+ gradients along dendrites. Such supralinear Ca2+ responses were attributed to the saturation of a large concentration (0.36 mM) of a mobile, high-affinity (dissociation constant, 0.37 microM) Ca2+ buffer with cooperative Ca2+ binding sites, resembling calbindin-D28K, and to an immobile, low-affinity Ca2+ buffer. These data suggest that the high-affinity Ca2+ buffer operates as the neuronal computational element that enables efficient coincidence detection of the Ca2+ signal and that facilitates spatiotemporal integration of the Ca2+ signal at submicromolar [Ca2+]i.

Calcium binding capacity of the cytosol and endoplasmic reticulum of mouse pancreatic acinar cells

1. The droplet technique was used in this study to measure total calcium loss from pancreatic acinar cells due to calcium extrusion. The calcium binding capacity of the cytosol (kc) was measured as the ratio of the decrease in the total calcium concentration of the cytosol of the cell (Delta[Ca]c) and the synchronously occurring decrease in the free calcium ion concentration in the cytosol (Delta[Ca2+]c). The calcium dependency of the calcium binding capacity was determined by plotting values of kc against the corresponding [Ca2+]c. The rise in the cytosolic Ca2+ concentration of pancreatic acinar cells was triggered by stimulation with a supramaximal dose of cholecystokinin (CCK). The recovery of [Ca2+]c during continued exposure to the agonist was due to calcium extrusion from the cell. 2. The calcium binding capacity was about 1500-2000 for the [Ca2+]c range 150-500 nM. The mechanism of buffering was not investigated in this study. The calcium binding capacity of the cytosol did not vary significantly with [Ca2+]c in this range. The CCK-evoked decrease in the total calcium concentration in the lumen of the endoplasmic reticulum (ER) can be estimated from our data, taking into account previously published values for the volume of the ER in pancreatic acinar cells. Comparing the decrease in the total ER calcium concentration with our recently reported values for agonist-induced reductions in the free Ca2+ concentration inside the ER, we estimate that the calcium binding capacity of the ER is approximately 20. In pancreatic acinar cells we have therefore found a difference of two orders of magnitude in the efficiency of calcium buffering in the cytosol and the ER lumen.

Calcium buffering capacity of neuronal cell cytosol measured by flash photolysis of calcium buffer NP-EGTA

N1E-115 mouse neuroblastoma cells were injected with a calcium buffer/indicator solution to allow both ratiometric measurement of free calcium concentration and the release of calcium ions upon UV flash. The solution contained sulforhodamine, a marker dye used to estimate the volume injected; fluo-3, a calcium indicator, and NP-EGTA, a high affinity calcium-selective buffer that is converted by UV flash to products with negligible calcium affinity. The calcium increase recorded upon UV irradiation (Delta[Ca2+]i) was small for small injection volumes, increased with larger injection volumes, but approached a plateau at the largest injection volumes. From this relation we estimate the buffering capacity of the cytosol as 1700 ions bound per ion free.

Endogenous calcium buffering in motoneurones of the nucleus hypoglossus from mouse

1. Simultaneous patch clamp and rapid microfluorometric calcium measurements were performed on sixty-five motoneurones in slices of the nucleus hypoglossus in the brainstem of 2- to 6-day-old mice. 2. Hypoglossal motoneurones were particularly vulnerable to mechanical or metabolic stress during isolation of in vitro slice preparations. Therefore, experimental conditions were optimized for functional integrity, as judged by spontaneous rhythmic activity of hypoglossal nerves (XII). 3. Calcium concentrations in the cell soma were monitored with a temporal resolution in the millisecond time domain during depolarizing voltage steps. Ratiometric fluorescence measurements were made using a rapid monochromator (switching tau < 10 ms), a photomultiplier tube and the calcium sensitive dyes fura-2 and mag-fura-5. 4. Dynamics of somatic calcium transients were investigated as a function of the concentration of calcium indicator dye in the cell. Decays of calcium transients were approximated to a single exponential component and decay time constants showed a linear dependence on dye concentration. The extrapolated decay time in the absence of indicator dye was 0.7 +/- 0.2 s, suggesting rapid somatic calcium dynamics under physiological conditions. 5. By a process of back-extrapolation, the 'added buffer' method, a calcium binding ratio of 41 +/- 12 (9 cells) was obtained indicating that 98% of the calcium ions entering a hypoglossal motoneurone were bound by endogenous buffers. 6. Endogenous calcium binding ratios in hypoglossal motoneurones were small compared with those of other neurones with comparable size or geometry. Accordingly, our measurements suggest that the selective vulnerability of hypoglossal motoneurones to calcium-related excitotoxicity might partially result from low concentrations of calcium buffers in these cells.