Sodium and potassium indicators and ionophores
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Find the best indicator and ionophore for imaging Na+ and K+
Sodium and potassium channels are ion-selective pores that span the cell membrane and serve to regulate and establish membrane potential. They are typically classified into two types:
- Voltage-gated channels that open or close in response to changes in membrane potential
- Ligand-gated or ion-activated channels are triggered by the binding of ligand or ion
Sodium ions (Na+) and potassium ions (K+) are particularly important in excitable cells such as neurons and myocytes as they function to create the action potential and to rest the cell's resting membrane potential.
Na+ indicators at a glance
| Indicator | Excitation (nm) | Emission (nm) | Kd (mM) |
| Asante NaTRIUM Green | 488-517 | 540 | 20 |
| SBFI | 340/380 | 505 | 4 |
Asante NaTRIUM Green is a visible wavelength fluorescent indicator with a useful dynamic range for measuring cytosolic Na+ concentrations. Unlike SBFI, it loads readily into cells and is excited by visible light. Although Asante NaTRIUM Green excites maximally at 517 nm, its exceptional brightness enables excitation at the standard 488 nm settings used for the Fluo calcium indicators. Moreover, it works well for 2-photon excitation with near-infrared light. Asante NaTRIUM Green is also remarkable in its resistance to photobleaching and leakage.
SBFI is an ultraviolet (340 and 380 nm) excitation ratiometric Na+ indicator used to estimate Na+ gradients in isolated mitochondria, to measure intracellular Na+ levels and Na+ efflux in cells, and, in combination with other fluorescent indicators used to correlate changes in intracellular Na+ with Ca2+ and Mg2+ concentrations. Although the selectivity of SBFI for Na+ is less than that of calcium indicators such as Fura-2, it is sufficient for the detection of physiological concentrations of Na+ in the presence of other monovalent cations. The spectral response of SBFI upon ion binding permit excitation ratio measurements, and this indicator can be used with the same optical filters and equipment used for Fura-2.
K+ indicators at a glance
| Indicator | Excitation (nm) | Emission (nm) | Kd (mM) |
| Asante Potassium Green | 488 | 540 | 18 |
| PBFI | 340/390 | 500 | 5 |
Asante Potassium Green is a fluorescent indicator with a useful Kd for measuring cytosolic K+ concentration. It loads readily and is excited by visible light. Although non-ratiometric, its large fluorescence dynamic range allows sensing of even small changes in K+ concentration. Optimal excitation occurs at 517 nm, but the indicator can also be excited at the conventional wavelength of 488 nm. Asante Potassium Green works well with 2-photon excitation at near-infrared wavelengths and although not fully resistant to photobleaching it is more robust than other dyes. Like its Na+ counterpart, it is useful for confocal microscopy, flow cytometry, and screening.
PBFI is the K+ analog of the Na+ indicator SBFI. The Kd of PBFI for K+ is strongly dependent on whether Na+ is present, with a value of 5.1 mM in the absence of Na+ and 44 mM in solutions with a combined Na+ and K+ concentration of 135 mM (which approximates physiological ionic strength). Although the selectivity of PBFI for K+ is less than that of Ca2+ indicators such as Fura-2, it is sufficient for the detection of physiological concentrations of K+ in the presence of other monovalent cations. The spectral response of PBFI upon ion binding permit excitation ratio measurements, and this indicator can be used with the same optical filters and equipment used for Fura-2.
Na+ ionophores
Monensin sodium salt - Na+-selective carboxylic ionophore
SQI-Et (Na) - Effective at concentrations of ~ 40 µM
SQI-Pr (Na) - Effective at concentrations of 800-900 nM
SQI-Et and SQI-Pr have an intrinsic fluorescence that is excitable by UV light. Therefore, they are not well-suited for use with SBFI.
K+ ionophores
Valinomycin is a natural, lipid-soluble molecule that binds potassium ions (K+) and facilitates their transfer across lipid bilayers. It exhibits a 10,000-fold selectivity for K+ over Na+. Valinomycin is a potent antibiotic. It is commonly used as an agent to induce apoptosis and can also be used to calibrate potentiometric responses in membrane experiments involving potential-sensitive dyes.
Nigericin is selective for both K+ and hydrogen ions (H+). Therefore it is commonly used in the fluorescence calibration of cytosolic pH indicators.
Monovalent cation ionophore that displays selectivity for K+ and NH4+ (K+ = NH4+ > Na+ > Mg2+ > Li+>> Ca2+). Induces cation transport across artificial membranes.