Investigation of noncalcium interactions of fura-2 by classical and synchronous fluorescence spectroscopy

Synchronous force and Ca2+ measurements for repeated characterization of excitation-contraction coupling in human myocardium

Dysfunctional Ca2+ signaling affects the myocardial systole and diastole, may trigger arrhythmia and cause transcriptomic and proteomic modifications in heart failure. Thus, synchronous real-time measurement of Ca2+ and force is essential to investigate the relationship between contractility and Ca2+ signaling and the alteration of excitation-contraction coupling (ECC) in human failing myocardium. Here, we present a method for synchronized acquisition of intracellular Ca2+ and contraction force in long-term cultivated slices of human failing myocardium. Synchronous time series of contraction force and intracellular Ca2+ were used to calculate force-calcium loops and to analyze the dynamic alterations of ECC in response to various pacing frequencies, post-pause potentiation, high mechanical preload and pharmacological interventions in human failing myocardium. We provide an approach to simultaneously and repeatedly investigate alterations of contractility and Ca2+ signals in long-term cultured myocardium, which will allow detecting the effects of electrophysiological or pharmacological interventions on human myocardial ECC.

Cellular effects of BAPTA: Are they only about Ca2+ chelation?

Intracellular Ca2+ signals play a vital role in a broad range of cell biological and physiological processes in all eukaryotic cell types. Dysregulation of Ca2+ signaling has been implicated in numerous human diseases. Over the past four decades, the understanding of how cells use Ca2+ as a messenger has flourished, largely because of the development of reporters that enable visualization of Ca2+ signals in different cellular compartments, and tools that can modulate cellular Ca2+ signaling. One such tool that is frequently used is BAPTA; a fast, high-affinity Ca2+-chelating molecule. By making use of a cell-permeable acetoxymethyl ester (AM) variant, BAPTA can be readily loaded into the cytosol of cells (referred to as BAPTAi), where it is trapped and able to buffer changes in cytosolic Ca2+. Due to the ease of loading of the AM version of BAPTA, this reagent has been used in hundreds of studies to probe the role of Ca2+ signaling in specific processes. As such, for decades, researchers have almost universally attributed changes in biological processes caused by BAPTAi to the involvement of Ca2+ signaling. However, BAPTAi has often been used without any form of control, and in many cases has neither been shown to be retained in cells for the duration of experiments nor to buffer any Ca2+ signals. Moreover, increasing evidence points to off-target cellular effects of BAPTA that are clearly not related to Ca2+ chelation. Here, we briefly introduce Ca2+ signaling and the history of Ca2+ chelators and fluorescent Ca2+ indicators. We highlight Ca2+-independent effects of BAPTAi on a broad range of molecular targets and describe some of BAPTAi's impacts on cell functions that occur independently of its Ca2+-chelating properties. Finally, we propose strategies for determining whether Ca2+ chelation, the binding of other metal ions, or off-target interactions with cell components are responsible for BAPTAi's effect on a particular process and suggest some future research directions.

Time-Resolved Synchronous Fluorescence for Biomedical Diagnosis

This article presents our most recent advances in synchronous fluorescence (SF) methodology for biomedical diagnostics. The SF method is characterized by simultaneously scanning both the excitation and emission wavelengths while keeping a constant wavelength interval between them. Compared to conventional fluorescence spectroscopy, the SF method simplifies the emission spectrum while enabling greater selectivity, and has been successfully used to detect subtle differences in the fluorescence emission signatures of biochemical species in cells and tissues. The SF method can be used in imaging to analyze dysplastic cells in vitro and tissue in vivo. Based on the SF method, here we demonstrate the feasibility of a time-resolved synchronous fluorescence (TRSF) method, which incorporates the intrinsic fluorescent decay characteristics of the fluorophores. Our prototype TRSF system has clearly shown its advantage in spectro-temporal separation of the fluorophores that were otherwise difficult to spectrally separate in SF spectroscopy. We envision that our previously-tested SF imaging and the newly-developed TRSF methods will combine their proven diagnostic potentials in cancer diagnosis to further improve the efficacy of SF-based biomedical diagnostics.

Nanomaterials-Tools, Technology and Methodology of Nanotechnology Based Biomedical Systems for Diagnostics and Therapy

Nanomedicine helps to fight diseases at the cellular and molecular level by utilizing unique properties of quasi-atomic particles at a size scale ranging from 1 to 100 nm. Nanoparticles are used in therapeutic and diagnostic approaches, referred to as theranostics. The aim of this review is to illustrate the application of general principles of nanotechnology to select examples of life sciences, molecular medicine and bio-assays. Critical aspects relating to those examples are discussed.

Asante Calcium Green and Asante Calcium Red--novel calcium indicators for two-photon fluorescence lifetime imaging

For a comprehensive understanding of cellular processes and potential dysfunctions therein, an analysis of the ubiquitous intracellular second messenger calcium is of particular interest. This study examined the suitability of the novel Ca²⁺-sensitive fluorescent dyes Asante Calcium Red (ACR) and Asante Calcium Green (ACG) for two-photon (2P)-excited time-resolved fluorescence measurements. Both dyes displayed sufficient 2P fluorescence excitation in a range of 720–900 nm. In vitro, ACR and ACG exhibited a biexponential fluorescence decay behavior and the two decay time components in the ns-range could be attributed to the Ca²⁺-free and Ca²⁺-bound dye species. The amplitude-weighted average fluorescence decay time changed in a Ca²⁺-dependent way, unraveling in vitro dissociation constants K_D of 114 nM and 15 nM for ACR and ACG, respectively. In the presence of bovine serum albumin, the absorption and steady-state fluorescence behavior of ACR was altered and its biexponential fluorescence decay showed about 5-times longer decay time components indicating dye-protein interactions. Since no ester derivative of ACG was commercially available, only ACR was evaluated for 2P-excited fluorescence lifetime imaging microscopy (2P-FLIM) in living cells of American cockroach salivary glands. In living cells, ACR also exhibited a biexponential fluorescence decay with clearly resolvable short (0.56 ns) and long (2.44 ns) decay time components attributable to the Ca²⁺-free and Ca²⁺-bound ACR species. From the amplitude-weighted average fluorescence decay times, an in situ K_D of 180 nM was determined. Thus, quantitative [Ca²⁺]_i recordings were realized, unraveling a reversible dopamine-induced [Ca²⁺]_i elevation from 21 nM to 590 nM in salivary duct cells. It was concluded that ACR is a promising new Ca²⁺ indicator dye for 2P-FLIM recordings applicable in diverse biological systems.

Synthesis and properties of Asante Calcium Red--a novel family of long excitation wavelength calcium indicators

Although many synthetic calcium indicators are available, a search for compounds with improved characteristics continues. Here, we describe the synthesis and properties of Asante Calcium Red-1 (ACR-1) and its low affinity derivative (ACR-1-LA) created by linking BAPTA to seminaphthofluorescein. The indicators combine a visible light (450-540 nm) excitation with deep-red fluorescence (640 nm). Upon Ca2+ binding, the indicators raise their fluorescence with longer excitation wavelengths producing higher responses. Although the changes occur without any spectral shifts, it is possible to ratio Ca(2+)-dependent (640 nm) and quasi-independent (530 nm) emission when using visible (< 490 nm) or multiphoton (∼780 nm) excitation. Therefore, both probes can be used as single wavelength or, less dynamic, ratiometric indicators. Long indicator emission might allow easy [Ca2+]i measurement in GFP expressing cells. The indicators bind Ca2+ with either high (Kd = 0.49 ± 0.07 μM; ACR-1) or low affinity (Kd = 6.65 ± 0.13 μM; ACR-1-LA). Chelating Zn2+ (Kd = 0.38 ± 0.02 nM) or Mg2+ (Kd∼5mM) slightly raises and binding Co2+ quenches dye fluorescence. New indicators are somewhat pH-sensitive (pKa = 6.31 ± 0.07), but fairly resistant to bleaching. The probes are rather dim, which combined with low AM ester loading efficiency, might complicate in situ imaging. Despite potential drawbacks, ACR-1 and ACR-1-LA are promising new calcium indicators.

TAT-mediated photochemical internalization results in cell killing by causing the release of calcium into the cytosol of cells

BACKGROUND

Lysis of endocytic organelles is a necessary step in many cellular delivery methodologies. This is achieved efficiently in the photochemical internalization approach but the cell death that accompanies this process remains a problem.

METHODS

We investigate the mechanisms of cell death that accompanies photochemical internalization of the fluorescent peptide TMR-TAT.

RESULTS

TMR-TAT kills cells after endocytosis and light irradiation. The lysis of endocytic organelles by TMR-TAT causes a rapid increase in the concentration of calcium in the cytosol. TMR-TAT co-localizes with endocytic organelles containing calcium prior to irradiation and photochemical internalization leads to the release of the lumenal content of these organelles. Ruthenium red and cyclosporin A, inhibitors of calcium import in mitochondria and of the mitochondria permeability transition pore, inhibit cell death.

CONCLUSIONS

TMR-TAT mediated photochemical internalization leads to a disruption of calcium homeostasis. The subsequent import of calcium in mitochondria is a causative factor of the cell death that accompanies photochemical internalization. General significance Understanding how the lysis of endocytic organelles affects cellular physiology and causes cell death is crucial to the development of optimal delivery methodologies.

Imaging ensemble activity in arthropod olfactory receptor neurons in situ

We show that lobster olfactory receptor neurons (ORNs), much like their vertebrate counterparts, generate a transient elevation of intracellular calcium (Ca(i)) in response to odorant activation that can be used to monitor ensemble ORN activity. This is done in antennal slice preparation in situ maintaining the polarity of the cells and the normal micro-environment of the olfactory cilia. The Ca(i) signal is ligand-specific and increases in a dose-dependent manner in response to odorant stimulation. Saturating stimulation elicits a robust increase of up to 1 μM free Ca(i) within 1-2s of stimulation. The odor-induced Ca(i) response closely follows the discharge pattern of extracellular spikes elicited by odorant application, with the maximal rise in Ca(i) matching the peak of the spike generation. The Ca(i) signal can be used to track neuronal activity in a functional subpopulation of rhythmically active ORNs and discriminate it from that of neighboring tonically active ORNs. Being able to record from many ORNs simultaneously over an extended period of time not only allows more accurate estimates of neuronal population activity but also dramatically improves the ability to identify potential new functional subpopulations of ORNs, especially those with more subtle differences in responsiveness, ligand specificity, and/or transduction mechanisms.

Ionic mechanism of ouabain-induced swelling of leech Retzius neurons

By using electrophysiological and microfluorimetric methods, we found that leech Retzius neurons swell after inhibition of the Na(+)-K(+) pump by the cardiac glycoside ouabain. To explore the mechanism of this swelling, we measured the effect of ouabain on [Na(+)](i), [K(+)](i), and [Cl(-)](i), as well as on the membrane potential, by applying triple-barrelled ion-sensitive microelectrodes. As shown previously, ouabain induced a marked [Na(+)](i) increase, a [K(+)](i) decrease, and a membrane depolarization, and it also evoked an increase in [Cl(-)](i). The analysis of the data revealed a net uptake of NaCl, which quantitatively explained the ouabain-induced cell swelling. In the absence of extracellular Na(+) or Cl(-), NaCl uptake was excluded, and the cell volume remained unaffected. Likewise, NaCl uptake and, hence, cell swelling did not occur when the Na(+)-K(+) pump was inhibited by omitting bath K(+). Also, in K(+)-free solution, [Na(+)](i) increased and [K(+)](i) dropped, but [Cl(-)](i) slightly decreased, and after an initial, small membrane depolarization, the cells hyperpolarized for a prolonged period. It is concluded that the ouabain-induced NaCl uptake is caused by the depolarization of the plasma membrane, which augments the inwardly directed electrochemical Cl(-) gradient.

Mag-indo1 affinity for Ca(2+), compartmentalization and binding to proteins: the challenge of measuring Mg(2+) concentrations in living cells

A physicochemical study of the Mag-indo1 binding to Ca(2+) in solution showed that: (i) the characteristic fluorescence spectra of Ca(2+)-bound and Mg(2+)-bound Mag-indo1 are identical; (ii) two successive equilibria occur for increasing Ca(2+) concentrations; and (iii) the value of the dissociation constant of the first one, as determined by using a probe dilution protocol, amounts to 780 nM. In order to investigate the fluorescence level of Mag-indo1 trapped in cell organelles, fluorescence spectra of Mag-indo1-loaded fibroblasts were recorded before and after a digitonin permeabilization. Their resolution into cation-bound, protein-bound, and free Mag-indo1 characteristic spectra allowed measurement of the fluorescence intensities of these species. The intensities emitted from whole cells were compared to those emitted from organelles (assumed to be endoplasmic reticulum according to a DiOC(6) loading). The cation-bound Mag-indo1 fluorescence resulted partially (20 to 50%) from the cytosol for 30% of the cells, and totally from compartments for 70% of the cells. We found a concentration value of 500 nM for compartmentalized Ca(2+) and concluded that the Mag-indo1 binding to Ca(2+) is likely to affect drastically the Mg(2+) concentration measurements in cells. Moreover, we showed that the amount variation of protein-bound Mag-indo1 also affects Mg(2+) measurements when using the two-wavelength ratio method.

Fluorescence lifetime microscopy of the sodium indicator sodium-binding benzofuran isophthalate in HeLa cells

The behavior of the sodium indicator sodium-binding benzofuran isophthalate (SBFI) is investigated in HeLa cells by time-resolved fluorescence microscopy. The fluorescence relaxation of SBFI in HeLa cells can be described by a triexponential for intracellular sodium concentration ([Na(+)](i)) between 0 and 90 mM. Changes in [Na(+)](i) affect neither the fluorescence relaxation times (0.21, 0. 60, and 2.7 ns) nor the average decay time (2.2 ns). The preexponential factor of the shortest decay time is negative. However, the ratio of the fluorescence excitation signal at 340 nm to that at 380 nm increases with [Na(+)](i). To elucidate the behavior of SBFI in cells, experiments are performed on SBFI in buffer at various concentrations of sodium, potassium, and bovine serum albumin (BSA) and at various viscosities. The fluorescence decay is triexponential only in the presence of BSA. The relaxation times are independent of [Na(+)] and [BSA]. The preexponential factor of the shortest decay time is negative from a certain [BSA] on, which depends on [Na(+)]. The data indicate that interactions with intracellular components rather than microviscosity influence the SBFI behavior in cells. A model is suggested in which the fluorescence intensities are mainly determined by the signals from the Na(+) subsetSBFI and SBFI subsetprotein complexes.

Ionic selectivity of low-affinity ratiometric calcium indicators: mag-Fura-2, Fura-2FF and BTC

Accurate measurement of elevated intracellular calcium levels requires indicators with low calcium affinity and high selectivity. We examined fluorescence spectral properties and ionic specificity of three low-affinity, ratiometric indicators structurally related to Fura-2: mag-Fura-2 (furaptra), Fura-2FF, and BTC. The indicators differed in respect to their excitation wavelengths, affinity for Ca2+ (Kd approximately 20 microM, 6 microM and 12 microM respectively) and selectivity over Mg2+ (Kd approximately 2 mM for mag-Fura-2, > 10 mM for Fura-2FF and BTC). Among the tested indicators, BTC was limited by a modest dynamic range upon Ca2+ binding, susceptibility to photodamage, and sensitivity to alterations in pH. All three indicators bound other metal ions including Zn2+, Cd2+ and Gd3+. Interestingly, only in the case of BTC were spectral differences apparent between Ca2+ and other metal ions. For example, the presence of Zn2+ increased BTC fluorescence 6-fold at the Ca2+ isosbestic point, suggesting that this dye may be used as a fluorescent Zn2+ indicator. Fura-2FF has high specificity, wide dynamic range, and low pH sensitivity, and is an optimal low-affinity Ca2+ indicator for most imaging applications. BTC may be useful if experimental conditions require visible wavelength excitation or sensitivity to other metal ions including Zn2+.

Two dye two wavelength excitation calcium imaging: results from bovine adrenal chromaffin cells

We tested a mixture of Calcium-Green-1 (CG-1) and Brilliantsulfaflavine (BS) for dual excitation ratiometric measurements of the intracellular free calcium concentration ([Ca2+]i) in bovine adrenal chromaffin cells. Dyes were coloaded (without being molecularly linked to each other) in the whole-cell configuration of the patch clamp technique. We compared the loading time-courses of CG-1 and BS, investigated their intracellular distribution patterns and studied the time course of photobleaching. We determined the apparent dissociation constant of CG-1, both optically and by potentiometric titration. Our findings indicate that: (i) with excitation at 420/488 nm, calibrated fluorescence signals could be derived using a Grynkiewicz-type equation; (ii) BS is an ideal reference dye that displayed no interaction with CG-1 or cellular constituents; and (iii) that calibration requires diffusional equilibration between pipette and the accessible volume of the cell. Spatially resolved recordings of fluorescence excitation spectra revealed elevated fluorescence of CG-1 in the nucleus such that reported [Ca2+]i levels seemed 25% higher compared to cytosolic values. Comparing fluorescence emission from in vitro dye solutions with in vivo values, we could estimate the accessible volume fraction and amount of Ca(2+)-insensitive dye.

Steady-state and time-resolved fluorescence measurements for studying molecular interactions: interaction of a calcium-binding probe with proteins

The binding of 2-[(2-bis-[carboxymethyl]amino-5-methylphenoxy)-methyl] 6-methoxy-8-bis[carboxymethyl] aminoquinoline, the fluorescent calcium probe Quin2, to serum albumin and several other proteins has been investigated. Changes in fluorescence emission spectra and fluorescence anisotropy revealed interactions between Quin2 and several proteins including human serum albumin, bovine serum albumin, aldolase, phosphoglucose isomerase, glyceraldehyde-3-phosphate dehydrogenase, and alkaline phosphatase. Protein-probe interactions were inhibited by the presence of calcium. Binding was also measured by resonance energy transfer and gel permeation chromatography. Equilibrium binding constants for Quin2 were quantitated by the application of the recently-developed "SPECTRABIND' program to spectroscopic data (D. Toptygin and L. Brand, Anal. Biochem., 224 (1995) 330-338). Binding of Quin2 to human serum albumin is discussed in terms of the published X-ray crystal structure of human serum albumin (X.M. He and D.C. Carter, Nature, 358 (1992) 209-215).

Non-hyperbolic calcium calibration curve of Fura-2: implications for the reliability of quantitative Ca2+ measurements

The fluorescence probe Fura-2 is widely applied for the quantitative determination of cellular free Ca2+ concentration. Generally, a hyperbolic calibration curve has been taken as a basis. The in vitro calibration of Fura-2 performed with free Ca2+ concentrations ranging from 10 nM to about 9 mM demonstrates a non-hyperbolic curve. Assuming two Ca2+ binding sites of Fura-2 dissociation constants of 190 nM and 176 microM were estimated. The analysis of the calibration data by the equation introduced by Grynkiewicz et al. [Grynkiewicz G., Poenie M., Tsien R.Y. A new generation of Ca2+ indicators with greatly improved fluorescence properties. J Biol Chem 1985; 260: 3440-3450] shows that the inclusion of Ca2+ concentrations in the range of the low-affinity binding site leads to an overestimation of the dissociation constant normally used within this method. The Rmax value conventionally estimated at a nominal saturating free Ca2+ concentration increases if concentrations in the range of the low-affinity binding site are selected for this purpose. Provided that millimolar free Ca2+ concentrations are applied for the determination of Rmax, the experimentally-estimated free Ca2+ concentration should be considerably lower than the real cellular concentration. This bias is especially to be expected if free Ca2+ concentrations exceeding the high-affinity Ca2+ binding site of Fura-2 are measured. With the use of approximately 10 microM free Ca2+ for the determination of Rmax the contribution of the low-affinity binding site is negligible. Data for cellular free Ca2+ calculated on the basis of Rmax values estimated with Ca2+ concentrations in the millimolar range should be considered with caution.

Inhibition of calmodulin expression prevents low-pH-induced gap junction uncoupling in Xenopus oocytes

The relationship among intracellular pH (pHi), -log10 intracellular Ca2+ concentration (pCai) and gap junctional conductance, the participation of Ca2+ stores, and the role of calmodulin in channel regulation have been studied in Xenopus oocytes, expressing the native connexin (Cx38), exposed to external solutions bubbled with 100% CO2. The time courses of pHi [measured with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorscein (BCECF)], pCai (measured with the membrane-associated fura-C18) and junctional conductance (measured with a double voltage-clamp protocol) were compared. The data obtained confirm previous evidence for a closer relationship of junctional conductance with pCai than with pHi. Evidence for an inhibitory effect of intracellularly injected ruthenium red or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) on CO2-induced uncoupling, coupled to negative results with Ca2+-free external solutions, point to a low-pHi -induced Ca2+ release from internal stores, likely to be primarily mitochondria. The hypothesis proposing a participation of calmodulin in channel gating was tested by studying the effects of calmodulin expression inhibition by intracellular injection of oligonucleotides antisense to the two calmodulin mRNAs expressed in the oocytes. Calmodulin mRNA was permanently eliminated in 5h. The oocytes injected with the antisense nucleotides progressively lost the capacity to uncouple with CO2 within 72 h. The effect of CO2 on junctional conductance was reduced by approximately 60% in 24 h, by approximately 76% in 48 h and by approximately 93% in 72 h. Oocytes that had lost gating sensitivity to CO2 partially recovered gating competency following calmodulin injection. The data suggest that lowered pHi uncouples gap junctions by a Ca2+- calmodulin-mediated mechanism.

Maintenance of Fura-2 fluorescence in glial cells and neurons of the leech central nervous system

Identified glial cells and neurones of the leech central nervous system (CNS) were injected iontophoretically with the calcium indicator dye Fura-2 to measure intracellular Ca2+, while simultaneously recording the membrane potential using a double-barrelled theta-type microelectrode. Both glial cells and neurones responded with Ni(2+)-sensitive Ca2+ transients upon membrane depolarization, indicating Ca2+ influx through voltage-gated Ca2+ channels. In contrast to neurones, the glial cells showed a rapid loss of fluorescence with a half-time of 6.3 +/- 1.8 min (n = 6) after dye injection. Both kinetics and amplitudes of the stimulus-induced Ca2+ transients were affected by this rapid dye loss. The anion exchange inhibitor probenicid (2 mM) significantly reduced, but did not prevent, the loss of Fura-2 fluorescence, suggesting that some dye left the glial cell via an anion exchanger. In order to compensate this fluorescence loss, we injected Fura-2 throughout the experiment. Under this condition, similar Ca2+ transients could be elicited repeatedly for more than 1 h. In Retzius neurones single injections of Fura-2 yielded enough intracellularly trapped dye to allow measurement of intracellular Ca2+ for up to 30 min after the end of injection without large decrease in absolute fluorescence.

Are intracellular ionic concentrations accessible using fluorescent probes? The example of Mag-indo-1

The study of the physicochemical properties of Mag-indo-1, a fluorescent probe used for intracellular magnesium measurements, has shown that in a biological environment the deprotonated form of this probe is in simultaneous equilibrium with a protonated form, a protein and a magnesium-bound form. The complex emission fluorescence spectrum emitted by a single living cell was analyzed using a computerized method, allowing the evaluation of the contribution of each species of the Mag-indo-1 to the cellular fluorescence. This approach used to evaluate intracellular Mg2+ concentration has also shown the variability of the important participation of protein-bound Mag-indo-1 to the cellular fluorescence. Thus the widely used ratioing method, unable to take into account this variability, cannot afford a reliable evaluation of [Mg2+]. Whatever the technique used for investigation (microfluorimetry, flow cytometry, etc.) the evaluation of [Mg2+]i using the fluorescent probe Mag-indo-1 requires a method able to quantify, in complex fluorescence, the fluorescence intensity of the forms involved in the equilibrium with Mg2+.

Free calcium concentrations in bullfrog rods determined in the presence of multiple forms of Fura-2

We employed the fluorescent calcium indicator Fura-2, loaded into intact retinas of the bullfrog Rana catesbeiana, to measure free calcium concentrations in the rod outer-segment cytosol. We determined that traditional methods of calculation yielded erroneous values of calcium. This error results from the presence of at least two distinct pools of Fura-2 in rod outer segments. Application of manganese quenches each pool, but quenching occurs at different rates. Using this fact, we show that the pools can be isolated by brief exposure to manganese and examined separately. One of these pools has the same fluorescent properties as the free salt of Fura-2 we use in our in vitro calibrations. The other source of fluorescence has more unusual properties. Although insensitive to calcium concentrations in the physiological range, it contributes significant anomalous fluorescence when cytosolic free calcium concentrations are elevated by application of IBMX. Nevertheless, the experimentally isolated, classic pool of Fura-2 is well behaved and allows us to calculate calcium concentrations relative to the Kd of Fura-2 by the usual ratio method. We show that when rods are exposed to saturating light, the free calcium concentration in their outer segments falls to a level not significantly different from zero within 20-30 s.

Cytosolic free magnesium concentration in human platelets

Although the magnesium ion (Mg2+) is considered to play an important role in cell activation, information is limited by the lack of suitable methods for measuring cytosolic free Mg2+ concentration ([Mg2+]i). We measured [Mg2+]i in resting and activated human platelets using a new fluorescent Mg(2+)-indicator, mag-fura-2. [Mg2+]i was 0.54 +/- 0.14 mM in resting platelets from 15 healthy volunteers. [Mg2+]i was elevated to 1.33 +/- 0.44 mM and 0.92 +/- 0.37 mM in platelets stimulated with thrombin and collagen, respectively. Increased Mg2+ was considered to be derived chiefly from intracellular Mg2+ mobilization. These results suggest that platelet activation is associated with the increase in [Mg2+]i. The estimation of [Mg2+]i using this method is useful for the investigation of mechanism for various cell activation.