Regulation of intracellular calcium in N1E-115 neuroblastoma cells: the role of Na(+)/Ca(2+) exchange

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.

Pharmacological Dissection of the Crosstalk between NaV and CaV Channels in GH3b6 Cells

Thanks to the crosstalk between Na⁺ and Ca²⁺ channels, Na⁺ and Ca²⁺ homeostasis interplay in so-called excitable cells enables the generation of action potential in response to electrical stimulation. Here, we investigated the impact of persistent activation of voltage-gated Na⁺ (Na_V) channels by neurotoxins, such as veratridine (VTD), on intracellular Ca²⁺ concentration ([Ca²⁺]_i) in a model of excitable cells, the rat pituitary GH3b6 cells, in order to identify the molecular actors involved in Na⁺-Ca²⁺ homeostasis crosstalk. By combining RT-qPCR, immunoblotting, immunocytochemistry, and patch-clamp techniques, we showed that GH3b6 cells predominantly express the Na_V1.3 channel subtype, which likely endorses their voltage-activated Na⁺ currents. Notably, these Na⁺ currents were blocked by ICA-121431 and activated by the β-scorpion toxin Tf2, two selective Na_V1.3 channel ligands. Using Fura-2, we showed that VTD induced a [Ca²⁺]_i increase. This effect was suppressed by the selective Na_V channel blocker tetrodotoxin, as well by the selective L-type Ca_V channel (LTCC) blocker nifedipine. We also evidenced that crobenetine, a Na_V channel blocker, abolished VTD-induced [Ca²⁺]_i elevation, while it had no effects on LTCC. Altogether, our findings highlight a crosstalk between Na_V and LTCC in GH3b6 cells, providing a new insight into the mode of action of neurotoxins.

Methodological aspects of measuring absolute values of membrane potential in human cells by flow cytometry

The bis-barbituric acid oxonol, DiBAC(4)(3) is used as a standard potentiometric probe in human cells. However, its fluorescence depends not only on membrane potential but also varies with nonpotential related changes in the amount of intracellular free and bound dye. This study demonstrates the influence of different experimental conditions on this nonspecific fluorescence proportion. IGR1 melanoma cells as a model were specifically altered in cell volume and protein content by depolarizing treatments or cell cycle synchronization. Flow cytometry was performed over a wide range of extracellular DiBAC(4)(3) concentrations. Fixation and increase in protein content led to a nonspecifically enhanced fluorescence, while changes in the amount of free intracellular dye as a result of altered cell volume proved to be negligible. To establish a calibration curve using totally depolarized cells, the pore-forming action of gramicidin should be preferred to fixation. Below 100 nM DiBAC(4)(3), the logarithmic relation between cell fluorescence and dye concentration turned into a virtually linear function intersecting with zero. Consequently, calibration can then be confined to determination of the fluorescence of depolarized cells stained with the same concentration as used for the actual measurement of membrane potential. Unexpectedly, quenching of fluorescence occurred in totally depolarized cells at concentrations higher than 6,250 nM. Linearity and quenching could be confirmed by additional experiments on Chinese hamster ovary CHO-K1 and B lymphoblastoid LCL-HO cells.

The Sodium Channel of Human Excitable Cells is a Target for Gambierol

BACKGROUND

Gambierol is a polycyclic ether toxin with the same biogenetic origin as ciguatoxins. Gambierol has been associated with neurological symptoms in humans even though its mechanism of action has not been fully characterized.

METHODS

We studied the effect of gambierol in human neuroblastoma cells by using bis-oxonol to measure membrane potential and FURA-2 to monitor intracellular calcium.

RESULTS

We found that this toxin: i) produced a membrane depolarization, ii) potentiated the effect of veratridine on membrane potential iii) decreased ciguatoxin-induced depolarization and iv) increased cytosolic calcium in neuroblastoma cells.

CONCLUSION

These results indicate that gambierol modulate ion fluxes by acting as a partial agonist of sodium channels.

Uptake of sodium in quince, sugar beet, and wheat protoplasts determined by the fluorescent sodium-binding dye benzofuran isophthalate

The uptake of sodium into protoplasts of quince (Cydonia oblonga Mill, clone BA29), sugar beet (Beta vulgaris L. cv. Monohill), and wheat (Triticum aestivum L. cv. Kadett) was determined by use of the acetoxy methyl ester of the fluorescent sodium-binding benzofuran isopthalate (SBFI-AM). In the presence of 1 mM CaCl2, little sodium was taken up in the cytosol of quince mesophyll cells compared to cytosols of sugar beet and wheat. Upon addition of 40 mM NaCl, approximately the same amount of sodium was taken up in leaf and root protoplasts of wheat, but no sodium was taken up in quince. However, in calcium-free medium, obtained by addition of ethylene glycol tetra acetic acid (EGTA), quince protoplasts transiently took up sodium in the cytosol when 200-400 mM NaCl was added to the protoplast medium. Moreover, after cultivation of quince in the presence of 200 mM sodium for 4 weeks, the cytosol of isolated protoplasts did not take up any sodium at all from a calcium-free medium. The results show that protoplasts from salt tolerant quince only temporarily take up sodium in the cytosol and that they have a mechanism for fast extrusion of sodium from that compartment. These mechanisms are probably important for the high salt tolerance of quince. Calcium blocks the sodium uptake into the cytosol of both quince and wheat protoplasts.

Chemosensitivity assay in mice prostate tumor: Preliminary report of flow cytometry, DNA fragmentation, ion ratiometric methods of anti-neoplastic drug monitoring

Flow cytometry, DNA fragmentation, ion ratiomateric analysis and NMR peaks characterized drug chemosensitivity of antineoplastic drugs. Hypotheses were: 1. The chemosensitive effect of different cancer cell lines is characteristic; 2. DNA fragmentation, ion ratiometric analysis suggest apoptosis status of tumor cells.