Potassium (K⁺) in physiology
Potassium, along with sodium (Na⁺), is one of nature’s most important monovalent metal cations. Ion channels and transporters for K⁺ play a diverse range of functions from modulating neuronal activity to regulating solute balance. Potassium channels are by far the largest and most structurally diverse class of ion channels and much remains unknown about their functions in normal physiology and disease. An increasingly long list of K⁺ channels and K⁺ transporters are being associated with a wide variety of disorders.
Potassium channel and K⁺ transporter-targeted drugs already provide effective treatments for a wide breadth of indications including epilepsy, autism, type-2 diabetes, and diuresis. Unfortunately, K⁺ channels, particularly the Kv11.1 channel (a.k.a. hERG), may be the unintended target of other therapeutics resulting in sometimes deadly side-effects. As a result, pre-clinical testing of drug candidates to determine their effects on hERG channel activity is required.
Because expanding our knowledge of K⁺ channels and K⁺ transporters is of such intense interest to both basic and drug discovery researchers, a facile means to measure their activity is critical. Unfortunately, measuring intracellular and extracellular K⁺ levels and dynamics using fluorescent indicators is challenging. In most cells at rest, cytoplasmic K⁺ concentrations are above 100 mM while Na⁺ concentrations are in the 3 – 5 mM range. Conversely, extracellular K⁺ concentrations are typically in the 3-5 mM range while Na⁺ concentrations are above 100 mM. Therefore, an effective K⁺sensor needs to be able to discriminate between K⁺, Na⁺ and other common physiological cations while possessing a K⁺ affinity appropriate to detect physiologically relevant changes in K⁺ concentrations.
Fluorescent Potassium Indicators
Extensive research has afforded the ION Potassium Green (IPG) family of excellent K⁺ sensors, formerly known as Asante Potassium Green (APG). These molecules are small, synthetic fluorochromes which incorporate a K⁺-binding moiety. Under conditions where K⁺ is not bound, the fluorescence of the sensor is significantly quenched. When K⁺ is bound, the quenching is relieved, and the fluorescence of the sensor dramatically increases.
The IPG family of indicators are the best-in-class replacements for the legacy K⁺ indicator, PBFI. Unlike PBFI, the spectral properties of the IPG family of K⁺ indicators make them convenient to use with common filter sets (e.g. YFP and FITC) as well as multiphoton approaches, and the range of K⁺ affinities make them useful for applications as diverse as extracellular K⁺ sensing to monitoring intracellular K⁺ dynamics. The IPG family of K⁺ indicators is compatible with a wide variety of detectors including fluorescent microscopes, plate readers, flow cytometers, and fluorescent indicator-doped solid-state sensors.
(EX 525 nm, EM 545 nm, Kd 50 mM) is the lowest affinity of the ION Potassium Green indicators. It is the indicator that is best positioned to respond to modest changes in intracellular K⁺ concentrations resulting from activation of plasma membrane K⁺ channels.
(EX 525 nm, EM 545 nm, Kd 7 mM) has the highest K⁺ affinity of our single-wavelength indicators making it appropriate for conditions where there may be large changes in intracellular K⁺ (e.g. highly active synapses) or as an extracellular K⁺ sensor.
(EX 525 nm, EM 545 nm, Kd 18 mM) is the ION Potassium Green indicator with intermediate affinity. Currently, IPG-2 is our most popular intracellular K⁺ indicator.
(EX 340/380 nm, EM 505 nm, Kd 4 mM ) PBFI AM is a potassium indicator that is ratiometric and UV light-excitable. This acetoxymethyl (AM) ester form is useful for noninvasive measurements of intracellular potassium levels.
Thallium flux assays and Tl⁺-sensitive indicators
While the IPG family of dyes are excellent fluorescent K⁺ indicators, the challenges of discriminating between K⁺ and Na⁺ are not trivial, and together with the often modest changes in intracellular or extracellular K⁺ 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 K⁺-sensitive fluorescent indicators. The Tl⁺ flux assay takes advantage of a Tl⁺-selective fluorescent indicator, Thallos, and the fact that K⁺ channels and K⁺ transporters readily accept Tl⁺ as a K⁺ surrogate. When Tl⁺ is added to the outside of cells loaded with Thallos, Tl⁺ entering cells through K⁺ channels and K⁺ transporters results a dramatic increase in fluorescence. Today, the Tl⁺ assay is the most commonly used approach for large-scale high-throughput screening (HTS) of K⁺ channels and K⁺ transporters.
(EX 490 nm, EM 515 nm) Thallos is the best-in-class Tl⁺-sensitive fluorescent indicator. It offers outstanding cell loading and excellent compatibility with commonly available FITC filters.
(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.
Multi-instrument compatible assay solution for well plate-based, high-throughput measurements of thallium (Tl⁺) flux, through potassium (K⁺), sodium (Na⁺), 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 instrments. 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.
(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 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
The first assay solution for single-cell, high-throughput measurements of potassium (K⁺) channel activity. Powered by our patent-pending Snapshot assay technology, ion channel modulation generates a long-lasting signal so you can easily detect and quantify changes in ion channel or transporter activity using Flow Cytometry. This kit may also work for sodium (Na⁺) channels, non-selective cation channels, and some Na⁺ or K⁺ transporters. 200 assays
Potassium channel assays using thallium flux
Indicator forms and formats
We provide our IPG family of K⁺-sensitive fluorescent indicators 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 K⁺-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 and retention
Although the AM forms of ION’s fluorescent K⁺ 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 K⁺ and Tl⁺ indicators into cells, it is highly recommended.
Another factor that can affect cell loading and intracellular localization is the presence of transport proteins capable of transporting many ion-sensitive, fluorescent indicators. 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 K⁺-selective fluorescent indicators nearly impossible. Fortunately, inhibitors of these transport proteins can be an effective means of improving cytoplasmic localization and indicator retention. ION offers a convenient, 100X ION-Pro inhibitor solution that may be used in conjunction with any of our K⁺-sensitive fluorescent indicators.
Be Brilliant! Explore our line of Potassium Channel Assays and Indicators
Product Image | Product | Description | Ex/Em (nm): | Kd (mM): | Price | Quantity | Add to cart |
|---|---|---|---|---|---|---|---|
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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 |
$3,597.00
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 |
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 |
$211.00-$473.00
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 |
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 |
$211.00-$473.00
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 |
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 |
$211.00-$473.00
|
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 |
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 |
$211.00-$473.00
|
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 |
ION Biosciences’ Flow Cytometric Potassium Channel Assay is the first assay solution for single-cell, high-throughput measurements of potassium channel activity. Powered by our patent-pending Snapshot assay technology, ion channel modulation generates a long-lasting signal so you can easily detect and quantify changes in ion channel or transporter activity using Flow Cytometry. This kit may also work for sodium channels, non-selective cation channels, and some Na+ or K+ transporters. 200 assays. |
490/515 |
$473.00
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 |
IPG-1 AM is a yellow-green fluorescent, potassium indicator. Membrane-permeable. Lower potassium affinity than IPG-2 and IPG-4. Tailored to measure changes in potassium in environments with high potassium concentrations. |
525/545 |
50 |
$173.00-$391.00
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Yellow-green fluorescent, potassium indicator, membrane-impermeable. Lower potassium affinity (Kd = 50 mM) than IPG-2 and IPG-4. |
525/545 |
50 |
$334.00
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 |
IPG-2 is a yellow-green fluorescent, potassium indicator, membrane permeable. Lower potassium affinity than IPG-4, and higher affinity than IPG-1. |
525/545 |
18 |
$173.00-$391.00
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 |
IPG-2 is a yellow-green fluorescent, potassium indicator. Membrane-impermeable. Lower potassium affinity than IPG-4, and higher affinity than IPG-1. |
525/545 |
18 |
$334.00
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 |
Yellow-green fluorescent, potassium indicator, membrane permeable. Higher potassium affinity than IPG-1 and IPG-2, with improved selectivity for potassium over sodium. |
525/545 |
7 |
$173.00-$391.00
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 |
Yellow-green fluorescent, potassium indicator. Membrane-impermeable. Higher potassium affinity than IPG-1 and IPG-2, with improved selectivity for potassium over sodium. |
525/545 |
7 |
$334.00
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 |
PBFI AM is a ratiometric potassium indicator, membrane-permeable.
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340,380/505 |
5 |
$168.00-$420.00
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Ratiometric potassium indicator, membrane-impermeable.
|
340,380/505 |
5 |
$380.00
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 |
Best in class, green fluorescent, thallium (Tl⁺) indicator, membrane permeable. Works with potassium (K⁺), sodium (Na⁺), and monovalent cation channels, and transporters. |
490/515 |
$242.00-$545.00
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 |
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 |
$242.00-$545.00
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