Sodium Channel Assays

Sodium and thallium indicators, which exhibit an increase in fluorescence upon binding Na⁺ or Tl⁺, can be used to measure the activity of sodium channels and transporters.

Sodium (Na⁺) in physiology

Sodium, along with potassium (K⁺), is one of the most important monovalent metal cations in living organisms. Sodium channels, Na⁺-permeable non-selective monovalent cation channels, and Na⁺ transporters play critical roles including modulating neuronal activity, powering transport of nutrients and signaling molecules, and regulating solute balance. Na⁺-permeable channel and Na⁺ transporter-targeted drugs provide effective treatments for a diversity of indications: epilepsy, pain, bipolar disorder, depression, diuresis, and many others. An increasingly long list of Na⁺-permeable channels, and Na⁺ transporters are being associated with a wide variety of disorders. As a result, interest in Na⁺-permeable channels and Na⁺ transporters as drug targets remains high.

Because expanding our understanding of Na⁺ permeable channels and Na⁺ transporters are of such intense interest to both basic and drug discovery researchers, a facile means to measure their activity is required. Unfortunately, measuring intracellular and extracellular Na⁺ levels and dynamics using fluorescent indicators is challenging. In most cells at rest, cytoplasmic Na⁺ concentrations are ~5 mM while K⁺ concentrations are above 100 mM. Conversely, extracellular Na⁺ concentrations are typically >100 mM while K⁺ concentrations are in the 3-5 mM range. Therefore, an effective Na⁺ sensor needs to be able to discriminate between K⁺, Na⁺ and other common physiological cations while possessing an appropriate Na⁺ affinity to detect physiologically relevant changes in Na⁺ concentrations.

 

Representative sodium channel (Nav1.3) data acquired using Brilliant sodium. Veratridine, an inhibitor of Nav channel inactivation, yields a dose dependent increase in intracellular fluorescence.

Sodium channel assays and sodium indicators

ION Natrium Green-2 (ING-2), formerly known as Asante Natrium Green (ANG-2), is a small, synthetic fluorochrome fused with a Na⁺-binding moiety. Under conditions where Na⁺ is not bound, the fluorescence of the sensor is significantly quenched. When Na⁺ is bound, the quenching is relieved, and the fluorescence of the sensor dramatically increases.  

ING-2 (EX 525 nm, EM 545 nm, Kd 20 mM) is the best-in-class fluorescent Na⁺ sensor replacing previous Na⁺ indicators such as SBFI and CoroNa Green. Its affinity for Na⁺ is well suited to respond to changes in intracellular Na⁺ concentrations resulting from activation of plasma membrane Na⁺ permeable channels and Na⁺ transporters. It is compatible with a wide variety of detectors including fluorescent microscopes, plate readers, and flow cytometers using common filter sets like sets (e.g. YFP and FITC) as well as multiphoton approaches. ING-2 has also proven useful for high-throughput screening (HTS).

ING-1 (EX 525 nm, EM 545 nm, Kd 92 mM) is a yellow-green fluorescent, sodium indicator with a lower affinity for sodium than ING-2.

Brilliant Sodium is the first kit designed to measure intracellular sodium in an HTS-compatible format, so you can discover and characterize the effects of many tens of thousands of compounds and environmental factors on effectors of sodium channels and transporters.

Thallium flux assays and thallium indicators

While ING-2 is an excellent Na⁺-sensitive, fluorescent indicator, the challenges of discriminating between K⁺ and Na⁺ are not trivial, and together with the often modest changes in intracellular Na⁺ results in relatively small signals compared to those observed with the Ca2⁺-sensitive fluorescent indicators, such as Fluo-4

In many instances, the thallium flux assay offers an outstanding alternative to ING-2 and other Na⁺-sensitive fluorescent indicators. The Tl⁺ flux assay takes advantage of a Tl⁺-selective fluorescent indicator, Thallos, and the fact that sodium channels and transporters readily accept Tl⁺ as a Na⁺ surrogate. When Tl⁺ is added to the outside of cells loaded with Thallos, Tl⁺ entering cells through Na⁺ permeable channels and Na+ transporters results in a dramatic increase in fluorescence. Although the Tl⁺ assay is most commonly associated with high-throughput screening (HTS) of K⁺ channels, it is equally suitable for HTS of sodium channels and transporters.

Thallos, (EX 490 nm, EM 515 nm) is the best-in-class Tl⁺-sensitive fluorescent indicator. It boasts outstanding cell loading and excellent compatibility with a wide variety of detectors including fluorescent microscopes, plate readers, and flow cytometers using commonly available FITC filters.

(EX 490 nm, EM 515 nm) High-throughput, no wash thallium flux assay. An optimal solution for measuring the activity of potassium channels and transporters, as well as sodium channels, non-selective cation channels, and GPCRs. 10 plates

(EX 530 nm, EM 550 nm) Brilliant Thallium Gold includes Thallos Gold – ION’s gold fluorescent thallium (Tl+) indicator. Red-shifted excitation and emission enable multiplexing with GFP-expressing cells or other green fluorescent indicators. Useful for fluorescence-based potassium channel assays and for studying a wide variety of monovalent cation channels (sodium channels), transporters, and GPCRs.

(EX 490 nm, EM 515 nm) A thallium flux assay adaptation designed to work with standard plate readers, microscopes, flow cytometers, and more, for measuring flux through potassium (K⁺), sodium (Na⁺), and non-selective cation channels and their effectors including transporters and GCPRs. 10 plates 

Indicator forms and formats 

We provide ING-2 in membrane permeable and membrane impermeable forms, as well as our Brilliant Sodium Assays. We offer Thallos as part of our convenient Brilliant Thallium Assay kits and, unlike other vendors, as a stand-alone product for investigators who prefer to use their own solutions and reagents.

Cell permeable, acetoxymethyl group (AM) esters

The native forms of the Na⁺-sensitive and Tl⁺-sensitive fluorescent indicators are negatively charged and membrane impermeable. However, masking the negative charge using non-polar, ester-linked moieties (AM esters) allow the molecules to enter cells through passive diffusion. Once inside the cell, ubiquitous intracellular esterase enzymes promote rapid hydrolysis of the AM esters leaving the active, highly polar form of the indicator trapped inside the cell.

Reagents for improving indicator loading, retention and background suppression

Although the AM forms of ION’s fluorescent, Na⁺ and Tl⁺ indicators readily cross the plasma membrane, there are other factors that can affect how well the indicators load into cells. Because AM forms of the indicators are quite hydrophobic, it is common to use the non-ionic surfactant, Pluronic F-127, to improve their solubility in aqueous solution thus improving their loading into cells. Pluronic F-127 at the concentrations employed for loading indicators into cells is generally well tolerated. While Pluronic F-127 isn’t absolutely essential for loading most fluorescent Na⁺ and Tl⁺ indicators into cells, it is highly recommended. 

Another factor that can affect cell loading and intracellular localization is the presence of anion transport proteins in both plasma and intracellular membranes. In many cell types, the activity of these transporters can result in compartmentalization of the indicators into intracellular compartments as well as extrusion of the indicators outside of the cell. Both of these processes can dramatically reduce the signal-to-background levels and in some cell types (e.g. CHO cells), make adequate loading of Na⁺ and Tl⁺ sensitive fluorescent indicators nearly impossible. Fortunately, probenecid, an organic anion transport inhibitor can be an effective means of inhibiting the transporters, improving cytoplasmic localization and indicator retention. ION offers a convenient, 100X probenecid solution that may be used in conjunction with our Na⁺ and Tl⁺-sensitive fluorescent indicators. While probenecid is generally well-tolerated over the time course of typical experiments using Na⁺ and Tl⁺-sensitive fluorescent indicators, effects on the activity of TRP channels has been reported.

Be Brilliant! Explore our line of Sodium Channel Assays and Indicators

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Brilliant thallium flux assay express packaging, dark background
A potassium channel assay, and so much more. ION’s high-throughput, no wash thallium (Tl⁺) flux assay is an optimal solution for measuring the activity of potassium (K⁺), sodium (Na⁺), and non-selective cation channels and their effectors including transporters and GPCRs. Our Express kit can also be customized for your screening needs - reducing waste, increasing reproducibility, and maximizing convenience. 100 plates
490/515
FITC, GFP
$3,270.00
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Min: 1
Step: 1
Brilliant thallium gold thallium flux assay product packaging
A potassium channel assay, and so much more. Brilliant Thallium Gold includes Thallos Gold - ION’s gold fluorescent (Ex/Em = 528/552 nm) thallium (Tl+) indicator. Red-shifted excitation and emission enable multiplexing with GFP-expressing cells or other green fluorescent indicators. Useful for fluorescence-based potassium (K+) channel assays and for studying a wide variety of monovalent cation channels (sodium channels), transporters, and GPCRs.
530/550
YFP
$192.00-$430.00
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Min: 1
Step: 1
Brilliant Thallium Gold Snapshot is a multi-instrument compatible assay solution for well plate-based, high-throughput measurements of thallium flux through potassium, sodium, and non-selective cation channels, as well as some Na+ or K+ transporters. Our patent-pending Snapshot technology generates a long-lasting signal so you can easily detect and quantify changes in ion channel or transporter activity on most fluorescence-capable instruments. The inclusion of Thallos Gold AM, our red-shifted thallium indicator, makes this kit the ideal option when using GFP-expressing cells to run potassium channel assays. 
530/550
YFP
$192.00-$430.00
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Brilliant thallium snapshot packaging with an image demonstration microscope compatibility
An endpoint-compatible potassium channel assay. ION's Brilliant Thallium Snapshot kit is a thallium flux assay adaptation designed to work with standard plate readers, microscopes, flow cytometers, and more, for measuring flux through potassium (K⁺), sodium (Na⁺), and non-selective cation channels, transporters, and their effectors, including GPCRs.
490/515
FITC, GFP
$192.00-$430.00
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Min: 1
Step: 1
Thallium flux assay packaging
A potassium channel assay, and so much more. ION's high-throughput, no wash thallium (Tl⁺) flux assay is an optimal solution for measuring the activity of potassium (K⁺), sodium (Na⁺), and non-selective cation channels, transporters, and their effectors such as GPCRs. 10 plates
490/515
FITC, GFP
$192.00-$430.00
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Min: 1
Step: 1
Thallos AM product packaging with thallium (Tl+) ion sticker, dark background
Best in class, green fluorescent, thallium (Tl⁺) indicator, membrane permeable. Works with potassium (K⁺), sodium (Na⁺), and monovalent cation channels, and transporters.
490/515
DMSO
FITC, GFP
$220.00-$495.00
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Min: 1
Step: 1
Thallos gold, thallium indicator for thallium flux assay product packaging
Thallos Gold is a gold fluorescent (Ex/Em = 528/552 nm) intracellular thallium (Tl+) indicator. Red-shifted excitation and emission enable multiplexing with GFP-expressing cells or other green fluorescent indicators. Useful for fluorescence-based potassium channel assays and for studying a wide variety of monovalent cation channels (sodium channels), transporters, and GPCRs.
530/550
DMSO
YFP
$220.00-$495.00
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Min: 1
Step: 1
Brilliant sodium assay kit packaging for sodium channel assays
Brilliant Sodium is the first high-throughput, no wash sodium channel assay designed to measure changes in intracellular sodium caused by effectors of sodium channels, sodium transporters, and non-selective cation channels.
525/545
GFP, FITC, YFP
$190.00-$3,150.00
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ING-1 AM is a yellow-green fluorescent, sodium indicator. Membrane-permeable. Lower sodium affinity than ING-2.  
525/545
92
DMSO
GFP, FITC, YFP
$150.00-$340.00
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Packaging for ING-2, a fluorescent sodium indicator
ING-2 AM is a yellow-green fluorescent, sodium indicator. Membrane-permeable. Higher sodium affinity than ING-1. Suitable for high throughput screening of sodium channels and transporters.  
525/545
20
DMSO
GFP, FITC, YFP
$150.00-$340.00
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Yellow-green fluorescent sodium indicator, membrane-impermeable. Higher sodium affinity than ING-1. Suitable for high throughput screening of sodium channels and transporters.  
525/545
20
H2O
GFP, FITC, YFP
$290.00
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Min: 1
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SBFI AM, ratiometric sodium indicator, packaging with blue background and sodium (Na+) ion sticker
SBFI AM is a ratiometric sodium indicator. UV-excitable. Membrane-permeable.  
340,380/505
4
DMSO
Fura-2
$144.00-$360.00
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