Intracellular Mg2+ is important for mediating enzymatic reactions, DNA synthesis, hormonal secretion and muscular contraction. To facilitate the investigation of magnesium's role in these and other cellular functions, we offer several different fluorescent indicators for measuring intracellular Mg2+ concentration. They include furaptra,ref which we refer to as mag-fura-2 to denote the similarity of its structure and spectral response with the Ca2+ indicator fura-2; and mag-indo-1, with a structure and spectral response similar to that of indo-1. For applications such as confocal laser-scanning microscopy and flow cytometry, we offer the Magnesium Green and mag-fluo-4 indicators. The various methods for measuring intracellular Mg2+ have been reviewed.ref

Mg2+ indicators are generally designed to maximally respond to the Mg2+ concentrations commonly found in cells, typically ranging from about 0.1 mM to 6 mM. Intracellular free Mg2+ levels have been reported to be ~0.3 mM in synaptosomes,ref 0.37 mM in hepatocytes ref and 0.5–1.2 mM in cardiac cells,ref whereas the concentration of Mg2+ in normal serum is ~0.44–1.5 mM.ref Measurements using fluorescent Mg2+ indicators are somewhat more demanding than intracellular Ca2+ determinations because physiological changes in Mg2+ concentration are relatively small. Compartmentalization and binding to proteins can also be a problem in use of these indicators in cells.ref Mg2+ indicators also bind Ca2+; however, typical physiological Ca2+ concentrations (10 nM–1 µM) usually do not interfere with Mg2+ measurements because the affinity of these indicators for Ca2+ is low. For ultrasensitive Mg2+ measurement applications, intracellular Ca2+ background can be suppressed using BAPTA AM (B1205, B6769; B1205, B6769; Chelators, Calibration Buffers, Ionophores and Cell-Loading Reagents—Section 19.8).ref AlthoughCa2+ binding by Mg2+ indicators can be a complicating factor in Mg2+ measurements,ref this property can also be exploited for measuring high Ca2+ concentrations (1–100 µM);ref see Fluorescent Ca2+ Indicators Excited with UV Light—Section 19.2 and Fluorescent Ca2+ Indicators Excited with Visible Light—Section 19.3 for further examples.

For intracellular calibration of Mg2+ indicators, we offer the ionophores A-23187 and the nonfluorescent 4-bromo A-23187 (A1493, B1494; Chelators, Calibration Buffers, Ionophores and Cell-Loading Reagents—Section 19.8), which are preferred over ionomycin (I24222) because they transport Mg2+ more effectively.ref Solutions used to calibrate Mg2+ indicators should be initially free of heavy metals such as Mn2+ that can interact with the indicators. These metals can be removed by treating the solution with the divalent cation chelator TPEN (T1210, Chelators, Calibration Buffers, Ionophores and Cell-Loading Reagents—Section 19.8).

Magnesium Indicators Excited by UV Light

Mag-Fura-2 and Mag-Indo-1

The dissociation constant for Mg2+ of mag-indo-1 is 2.7 mM, slightly higher than that of mag-fura-2, which is 1.9 mM. The lower-affinity mag-indo-1 indicator is sensitive to somewhat higher spikes in intracellular Mg2+.ref The affinities of mag-fura-2 and mag-indo-1 for Mg2+ are reported to be essentially invariant at pH values between 5.5 and 7.4 and at temperatures between 22°C and 37°C.ref A detailed study of the photophysics of mag-fura-2 has been published.ref Comparisons of intracellular and solution dissociation constants for mag-fura-2 have been published by Günther ref and by Tashiro and Konishi.ref

As with their Ca2+ indicator analogs, mag-fura-2 undergoes an appreciable shift in excitation wavelength upon Mg2+ binding (Figure 19.6.1), and mag-indo-1 exhibits a shift in both its excitation and emission wavelengths (Figure 19.6.2). Equipment, optical filters and calibration methods are very similar to those required for the Ca2+ indicators. The excitation-ratioable mag-fura-2 indicator is most useful for fluorescence microscopy, whereas the emission-ratioable mag-indo-1 indicator is preferred for flow cytometry.ref Many applications of mag-fura-2 involve estimation of the affinity and selectivity of Mg2+ binding to proteins.ref Displacement of bound Mg2+ by Li+ provides a surrogate assay for Li+ transport, a process for which few direct detection methods exist.ref Researchers have used mag-fura-2 to measure intracellular Mg2+ in a wide variety of cells, organelles and tissues, including:

  • Cortical neurons ref
  • Isolated mitochondria ref
  • Platelets ref
  • Rat hepatocytes ref
  • Rat ventricular myocytes ref
  • Xenopus oocytesref

In addition to the cell-impermeant potassium salt of mag-fura-2 (M1290), we offer the cell-permeant AM esters of mag-fura-2 and mag-indo-1 as a set of 20 vials, each containing 50 µg (M1292, M1295). The special packaging is recommended when small quantities of the dyes are to be used over a long period of time. Mag-fura-2 AM is also available in a single vial containing 1 mg (M1291).

Figure 19.6.1 A) Fluorescence excitation and B) fluorescence emission spectra of mag-fura-2 (M1290) in solutions containing 0–35 mM Mg2+.

Figure 19.6.2 A) Fluorescence excitation and B) fluorescence emission spectra of mag-indo-1 in solutions containing 0–100 mM Mg2+.

Magnesium Indicators Excited by Visible Light

We also offer visible light–excitable Mg2+ indicators, including the Magnesium Green and mag-fluo-4 indicators. As with mag-fura-2 and mag-indo-1, these visible light–excitable Mg2+ indicators can also be used as low-affinity Ca2+ indicators (Fluorescent Ca2+ Indicators Excited with Visible Light—Section 19.3) and may be useful as indicators for Zn2+ and other metals (Fluorescent Indicators for Zn2+ and Other Metal Ions—Section 19.7).

Magnesium Green Indicator

The Magnesium Green indicator exhibits a higher affinity for Mg2+ (Kd ~1.0 mM) than does mag-fura-2 (Kd ~1.9 mM) or mag-indo-1 (Kd ~2.7 mM); this indicator also binds Ca2+ with moderate affinity (Kd for Ca2+ in the absence of Mg2+ is ~6 µM, measured at 22°C using our Calcium Calibration Buffer Kits). The spectral properties of the Magnesium Green indicator are similar to those of the Calcium Green indicators. Upon binding Mg2+, Magnesium Green exhibits an increase in fluorescence emission intensity without a shift in wavelength (Figure 19.6.3). The Magnesium Green indicator has been used to investigate the binding of free Mg2+ by the bacterial SecA protein ref and by protein tyrosine kinases.ref By exploiting the fact that ATP has greater Mg2+-binding affinity than ADP, researchers have used Magnesium Green to indirectly measure ATP in pancreatic acinar cells,ref in bullfrog hair cells,ref in cultured Xenopus spinal neurons ref and in isolated mitochondria.ref Magnesium Green is available as a cell-impermeant potassium salt (M3733) or as a cell-permeant AM ester (M3735).


Figure 19.6.3 Mg2+-dependent fluorescence emission spectra of Magnesium Green (M3733).


Mag-fluo-4 is an analog of fluo-4 with a Kd for Mg2+ of 4.7 mM and a Kd for Ca2+ of 22 µM (measured at 22°C using our Calcium Calibration Buffer Kits), making it useful as an intracellular Mg2+ indicator as well as a low-affinity Ca2+ indicator (Fluorescent Ca2+ Indicators Excited with Visible Light—Section 19.3). Mag-fluo-4 has a much more sensitive fluorescence response to Mg2+ binding than does our Magnesium Green indicator. Because physiological fluctuations of intracellular Mg2+ concentration are typically small, this increased sensitivity is a considerable advantage.ref Like fluo-4, mag-fluo-4 is essentially nonfluorescent in the absence of divalent cations and exhibits strong fluorescence enhancement with no spectral shift upon binding Mg2+ (Figure 19.6.4). Mag-fluo-4 is available as a cell-impermeant potassium salt (M14205) or as a cell-permeant AM ester (M14206).


Figure 19.6.4 Fluorescence emission spectra of mag-fluo-4 (M14205) in solutions containing 0–50 mM Mg2+.

Data Table

       Low Mg2+           High Mg2+     
Cat #MWStorageSolubleAbsECEmSolventAbsECEmSolventProductKdNotes
M1290586.68F,D,LpH >636922,000511H2O33024,000491H2O/Mg2+ 1.9 mM1, 2, 3, 4
M1291722.57F,D,LDMSO36631,000475EtOAc    M1290  
M1292722.57F,D,LDMSO36631,000475EtOAc    M1290  
mag-indo-1594.74F,D,LpH >634938,000480H2O33033,000417H2O/Mg2+ 2.7 mM1, 2, 3, 4
M1295730.63F,D,LDMSO35437,000472MeOH    mag-indo-1  
M3733915.90F,D,LpH >650677,000531H2O50675,000531H2O/Mg2+ 1.0 mM1, 2, 3, 4, 5
M37351025.71F,DDMSO30216,000noneMeOH    M3733  
M14205681.77F,D,LpH >649074,000see NotesH2O49375,000517H2O/Mg2+ 4.7 mM1, 2, 3, 4, 6
M14206817.66F,D,LDMSO45725,000see NotesMeOH    M14205 7
  1. Dissociation constants are known to vary considerably depending on the temperature, pH, ionic strength, viscosity, protein binding, presence of other ions (especially polyvalent ions), instrument setup and other factors. It is strongly recommended that these values be verified under user-specific experimental conditions.
  2. This indicator binds Ca2+ with higher affinity than Mg2+, producing a similar spectral response.
  3. Kd(Mg2+) values have been determined in 115 mM KCl, 20 mM NaCl, 10 mM Tris, pH 7.05, 0 to 35 mM Mg2+ at 22°C.
  4. Spectra measured in aqueous buffers containing zero or 35 mM Mg2+, indicated as H2O and H2O/Mg2+ respectively.
  5. This indicator exhibits fluorescence enhancement in response to ion binding, with essentially no change in absorption or emission wavelengths.
  6. Fluorescence of the free indicator is very weak and is enhanced >100-fold on binding Mg2+.
  7. Fluorescence of this AM ester derivative is very weak and is enhanced only after hydrolytic cleavage followed by binding of divalent cations to the anionic indicator.

For Research Use Only. Not for use in diagnostic procedures.