Sodium Indicators

Sodium indicators are molecules that exhibit an increase in fluorescence upon binding Na⁺.

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.


Na⁺-sensitive fluorescent 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).

Thallium (Tl⁺)-sensitive fluorescent 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 Tl⁺ 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 Na⁺ permeable channels and Na⁺ 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 Na⁺ permeable channels and Na⁺ 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.

Indicator forms and formats 

We provide ING-2 in membrane permeable and membrane impermeable forms. 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 Indicators

Our collection of membrane permeable and impermeable ion indicators targeting Na⁺.

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High-throughput, no wash thallium (Tl⁺) assay. An optimal solution for measuring flux through potassium (K⁺), sodium (Na⁺), and non-selective cation channels and their effectors including transporters and GPCRs. 100 plates


High-throughput, no wash thallium (Tl⁺) assay. An optimal solution for measuring flux through potassium (K⁺), sodium (Na⁺), and non-selective cation channels and their effectors including transporters and GPCRs. 10 plates


Yellow-green fluorescent, sodium (Na⁺) indicator, membrane permeable. Higher Na⁺ affinity than ING-1



Yellow-green fluorescent, sodium (Na⁺) indicator, membrane impermeable. Higher Na⁺ affinity than ING-1



Best in class, green fluorescent, thallium (Tl⁺) indicator, membrane permeable. Works with potassium (K⁺), sodium (Na⁺), and monovalent cation channels, and transporters.

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