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Intracellular Ions

The steady-state maintenance of disproportionate concentrations of inorganic cations and anions is a feature of living cells. Homeostatic regulation of these ionic gradients across different compartments is critical for most cellular functions. Measuring concentrations of these ions with both spatial and temporal resolution has become very critical in understanding the physiology of the cells in the research community. Ion probes provide a means of correlating ion channel activation with subsequent changes in intracellular ion concentration. Flux measured by these type of probes also reflects the cells' membrane potential. We offer many ion probes which effectively measures respected ion efficiently.



Calcium Ion Detection

Calcium acts as a universal second messenger in a variety of cells. Numerous functions of all types of cells are regulated by Ca2+, thus calcium measurement is critical for various biological investigations. In the 1980s, Tsien and colleagues produced a variety of fluorescent indicators. Among them Indo-1, Fura-2, Fluo-3 and Rhod-2 have been the most valuable dyes for measuring Ca2+. In recent years, AAT Bioquest has introduced the most robust calcium probes: Fluo-8®, Cal-520®and Calbryte™ 520 all of which enable high throughput screening of GPCR and calcium channel drug discovery targets by monitoring calcium flux. FLIPR® and FlexStation® instruments of Molecular Devices, FDSS®/?Cell of Hamamatsu and NOVOstar of BMG Technologies have further accelerated the high throughput measurement of calcium for GPCR and ion channel research.

Fluorescent Single-Wavelength Calcium Indicators

Among visible light-excitable calcium indicators, Fluo-8®, Fluo-4, Fluo-3, Rhod-2 and Rhod-4™ are most commonly used. Fluo-8® indicators are widely used in flow cytometry, confocal laser-scanning microscopy and for high throughput screening of GPCR targets. Fluo-8® is essentially nonfluorescent unless bound to Ca2+ and exhibits a quantum yield of ~0.15 in the presence of saturating Ca2+ and a Kd of 390 nM for Ca2+. Cal-520® is by far the best 488 nm-excitable green fluorescent calcium indicator with a significantly improved signal/background ratio and intracellular retention.

The long-wavelength Rhod-4™, Cal-590™ and Cal-630™ are valuable alternative Ca2+ indicators to the green fluorescent Fluo-8®, Fluo-4 and Fluo-3 for experiments in cells and tissues that have high levels of autofluorescence. Rhod-5N has a lower binding affinity for Ca2+ than any other BAPTA-based indicator (Kd = ~320 µM) and is suitable for Ca2+ measurements from 10 µM to 1 mM. Like the parent Rhod-2 indicator, Rhod-5N is essentially nonfluorescent in the absence of divalent cations and exhibits strong fluorescence enhancement with no spectral shift upon binding Ca2+. All Fluo, Cal and Rhod indicators are available as cell-impermeant potassium salts or as cell-permeant AM esters.

Superior Single-Wavelength Calcium Indicators

Cal-500®, AM Cal-520®, AM Calbryte™ 520, AM Cal-590™, AM Calbryte™ 590, AM Cal-630™, AM Calbryte™ 630, AM
Readout Large increase in fluorescence emission intensity upon binding to calcium (minimal fluroescence at resting calcium levels)
Ex/Em (nm) 386/481 492/514 492/514 573/588 580/592 607/625 607/624
Filter Set DAPI FITC TRITC Texas Red
Kd 303 nM 320 nM 1200 nM 561 nM 1400 nM 792 nM 1200 nM
Permeability Permeable
Dye Loading Load in cell media
Unit Size 10x50 µg 1 mg 1 mg 1 mg 1 mg 1 mg 1 mg
Cat No. 20412 21131 20653 20512 20702 20532 20722

Classic Single-Wavelength Calcium Indicators

Cal-Green™-1, AM Fluo 3, AM Fluo 4, AM Rhod 2, AM Rhod 5N, AM
Readout Increase in fluorescence emission intensity upon binding to calcium (minimal fluroescence at resting calcium levels)
Ex/Em (nm) 506/531 506/526 494/516 549/578 551/577
Kd 190 nM 390 nM 345 nM 570 nM 0.3 mM
Permeability Permeable
Dye Loading Load in cell media
Unit Size 1 mg 1 mg 1 mg 20x50 µg 1 mg
Cat No. 20502 21011 20550 21064 21070

Fluorescent Ratiometric Calcium Indicators

Ratiometric, or dual-wavelength, ion indicators are a subcategory of fluorescent dyes utilized for their ability to quantitatively measure intracellular ion concentrations. Compared to single-wavelength indicators, dual-wavelength indicators possess unique spectral properties which are triggered in response to binding its target ion. In the case of ratiometric calcium indicators, such dyes undergo a shift in either their optimum absorption or emission wavelength intensities when binding to free Ca2+. For example, dual-excitation Ca2+ indicators exhibit two peak excitation wavelengths when either bound or free of Ca2+. An increase in Ca2+ concentration initiates an increase and decrease in the fluorescence emission intensities of the dual-excitation indicator when excited at the Ca2+-bound and Ca2+-free peak excitation wavelengths, respectively. Using ratios derived from the photometric data gathered, researchers can accurately determine intracellular Ca2+ concentrations. Ratioing techniques are advantageous because it reduces effects indicative of uneven dye loading, poor dye retention and photobleaching. Since their introduction in 1985 by Tsien and collaborators, ratiometric indicators have been cited in countless scientific papers and have aided in advances in investigating the role of calcium in cellular regulation (Grynkiewicz et al. 1985).

Quin-2, AM BTC, AM Fura-2, AM Fura-FF, AM Fura-8™, AM Fura Red, AM Indo-1, AM Cal Red™ R525/650, AM
Readout Ratiometric excitation wavelength changes in response to calcium binding Ratiometric emission wavelength changes in response to calcium binding
Detection Method Excitation wavelength changes Excitation wavelength changes
Zero Calcium Ex/Em (nm) 353/495 464/533 363/512 363/512 386/532 490/656 346/475 492/650
High Calcium Ex/Em (nm) 333/495 401/529 335/505 339/507 354/524 458/597 330/401 492/525
Kd 60 nM 7 µM 145 nM 5.5 µM 260 nM 400 nM 230 nM 330 nM
Permeability Cell permeable
Dye Loading Load in cell media
Unit Size 1 mg 1 mg 1 mg 10x50 µg 1 mg 10x50 µg 1 mg 1 mg
Cat No. 21050 21054 21021 21027 21055 21048 21032 20590

FLIPR® Calcium Assays

Calcium flux assays are preferred methods in drug discovery for screening G protein coupled receptors (GPCRs). Cells expressing a GPCR of interest that signals through calcium are pre-loaded with our proprietary Fluo-8® AM, Calbryte™ 520, AM, Cal-590™, AM and Rhod-4™ AM which can cross cell membrane. Screen Quest™ Calcium Assay Kits provide an optimized assay method for monitoring GPCRs and calcium channels. These assays can be performed in a convenient 96-well or 384-well microtiter-plate format, are readily adaptable to automation and do not require a wash step.

Table 1. Ordering Info for Screen Quest Calcium Assays Products

Product Name
Unit Size
36200Screen Quest™ Calbryte-590 Probenecid-Free and Wash-Free Calcium Assay Kit1 Plate310
36201Screen Quest™ Calbryte-590 Probenecid-Free and Wash-Free Calcium Assay Kit10 Plates998
36202Screen Quest™ Calbryte-590 Probenecid-Free and Wash-Free Calcium Assay Kit100 Plates7298
36307Screen Quest™ Fluo-8 Medium Removal Calcium Assay Kit *Optimized for Difficult Cell Lines*1 Plate100
36308Screen Quest™ Fluo-8 Medium Removal Calcium Assay Kit *Optimized for Difficult Cell Lines*10 Plates363
36309Screen Quest™ Fluo-8 Medium Removal Calcium Assay Kit *Optimized for Difficult Cell Lines*100 Plates2573
36314Screen Quest™ Fluo-8 No Wash Calcium Assay Kit1 Plate100
36315Screen Quest™ Fluo-8 No Wash Calcium Assay Kit10 Plates363
36316Screen Quest™ Fluo-8 No Wash Calcium Assay Kit100 Plates3098
36317Screen Quest™ Calbryte-520 Probenecid-Free and Wash-Free Calcium Assay Kit1 Plate205

Luminescent Calcium Detection: Coelenterazine and Derivatives

The aequorin complex is comprised of a 22,000-dalton apoaequorin protein, molecular oxygen and the luminophore coelenterazine. When three Ca2+ ions bind to this complex, coelenterazine is oxidized to coelenteramide, with a concomitant release of carbon dioxide and blue light. The approximately third-power dependence of aequorin's bioluminescence on Ca2+ concentration allows the measurement of Ca2+ concentrations with a broad detection range from ?0.1 µM to >100 µM. Unlike fluorescent Ca2+ indicators, Ca2+-bound aequorin can be detected without illuminating the sample, thereby eliminating the interference from autofluorescence.

AAT Bioquest offers coelenterazine and several synthetic coelenterazine analogs for reconstituting aequorin in cells that have been transfected with apoaequorin cDNA. In addition to native coelenterazine, we also offer a few derivatives of coelenterazine that confer different Ca2+ affinities and spectral properties on the aequorin complex. Recombinant apoaequorin reconstituted with coelenterazine hcp is reported to have the best luminescence overall, with both a high quantum yield and a fast response time. However, intracellular reconstitution of aequorin from coelenterazine analogs can be relatively slow. Aequorins containing the cp, f or h form of coelenterazine exhibit 10-20 times stronger luminescence than that of apoaequorin reconstituted with native coelenterazine. Coelenterazine h has been predominantly used in HTS screening assay for GPCRs.

Coelenterazine Coelenterazine cp Coelenterazine f Coelenterazine h Coelenterazine hcp Coelenterazine n
Readout Coelenterazine, when bound to calcium, is oxidized to coelenteramide resulting in release of carbon dioxide and 'blue' light emission
Em (nm) 466 442 472 466 445 468
Relative Luminescence 1 28 20 16 500 0.5
Half Rise Time (ms) 6-30 2-5 6-30 6-30 2-5 6-30
Unit Size 1 mg 1 mg 1 mg 1 mg 250 µg 1 mg
Cat No. 21156 21157 21158 21159 21154 21161

Zinc Ion Detection

Zinc is the second most abundant transition metal in living organisms after iron. While most Zn2+ in the brain is tightly bound, such that free Zn2+ levels extracellularly and intracellularly are likely to be picomolar, a subset of glutamatergic neurons possess weakly bound zinc in presynaptic boutons which is released at micromolar levels in response to a variety of stimuli. The intracellular concentration of free Zn2+ is extremely low in most cells.

Mounting evidence indicates that zinc has multiple roles in cell biology, as a part of metalloenzyme catalytic sites, as a structural component of gene regulatory proteins, and as a free signal ion, particularly in the cortex of the brain. It is of particular importance in the regulation of gene expression, as Zn2+ binding proteins account for nearly 50% of the transcription regulatory proteins in the human genome. Zn2+ is also functionally active in pancreatic insulin secretion and is a contributory factor in neurological disorders including epilepsy and Alzheimer's disease. Free Zn2+ is released from metalloprotein complexes during oxidative stress.

The key to the further progress in understanding the multiple roles of zinc is the availability of fluorescent Zn2+ indicator systems that permits quantitative determination and imaging of zinc fluxes and levels over a broad concentration range both intracellularly and extracellularly using fluorescence microscopy and flow cytometry. AAT Bioquest offers the classic TSQ, zinquin, as well as our newly developed sensitive Metal Fluor™ Zn 520 to help elucidate the role of Zn2+ release and the localization of free or chelatable Zn2+ in cells.

Zinc Indicators Selection Guide

TSQ Zinquin, AM Metal Fluor™ Zn 520 AM Metal Fluor™ Zn 520 K+
Readout Indicators respond to saturating levels of zinc with a significant increase in fluorescence emission
Ex/Em (nm) 344/385 360/480 490/514 490/514
Filter Set DAPI FITC
Permeability Permeant Permeant Permeant Impermeant
Dye Loading Load in cell media Load by microinjection, patch pipette, or pinocytic loading agent
Unit Size 5 mg 1 mg 1 mg 1 mg
Cat No. 21254 21261 21263 21262