Fluorescence primer v4.doc Page 1/11 19/11/2008 Fluorescence The term fluorescence was coined by Stokes circa 1850 to name a phenomenon resulting in the emission of light at longer wavelength than the absorbed light, and his name is still used to describe the wavelength shift (Figure 1A). Figure 1 The following general rules of fluorescence (see Jameson et al., 20031) are of practical importance: 1) In a pure substance existing in solution in a unique form, the fluorescence spectrum is invariant, remaining the same independent of the excitation wavelength. 2) The fluorescence spectrum lies at longer wavelengths than the absorbtion. 3) The fluorescence spectrum is, to a good approximation, a mirror image of the absorption band of least frequency These rules follow from quantum optical considerations depicted in the Perrin-Jablonski diagram (Figure 1B). Note that although the fluorophore may be excited into different singlet state energy levels (S1, S2, etc.) rapid thermal relaxation invariably brings the fluorophore to the lowest vibrational level of the first excited electronic state (S1) and emission occurs from that level. This fact explains the independence of the emission spectrum from the excitation wavelength. The fact that ground state fluorophores (at room temperature) are predominantly in the lowest vibrational level of the ground electronic state accounts for the Stokes shift. Finally, the fact that the spacings of the vibrational energy levels of S0 and S1 are usually similar accounts for the fact that the emission and the absorption spectra are approximately mirror images. The presence of appreciable Stokes shift is principally important for practical applications of fluorescence because it allows to separate (strong) excitation light from (weak) emitted fluorescence using appropriate optics. A standard optical configuration includes excitation light source, excitationFluorescence primer v4.doc Page 2/11 19/11/2008 filter, dichroic mirror, emission filter and the optical sensor (see Figure 2 depicting optical setup for imaging voltage-sensitive dye Di-4-ANBDQBS in isolated myocytes). The right angle between the excitation light and the direction of observation of the emitted light allows for a more complete elimination of exciting light from the observed signal. In order for a fluorophore to be used as a reporter of a biological signal in real time, its fluorescence should respond to changes in the measured parameter in a sufficiently fast, reversible and calibratable manner. Changes in the measured biological parameter can alter quantum yield (the ratio between incident and emitted photons), spectral properties, or localization of the fluorophore with respect to certain cellular compartments. Figure 2 In the EP imaging lab we will use a voltage-sensitive and a calcium-sensitive fluorescent probes for real-time monitoring the action potential and intracellular calcium concentration in cardiac myocytes, and the propagated action potential in whole hearts. The purpose of this document is to provide a brief introduction into relevant fluorescent methods. Fast potentiometric probes Sensing mechanism The most popular potentiometric probes used in cardiac electrophysiology (e.g., Di-4-ANEPPS) respond to changes in the transmembrane potential (Vm) due to electrochromic mechanism (Figure 3, see Ref.2). These dyes belong to a structural class of naphthylstyryl. They are amphiphilic membrane staining dyes which usually have a pair of hydrocarbon chains acting as membrane anchors and a hydrophilic group which aligns the chromophore perpendicular to the membrane/aqueous interface. The naphthydtyryl pyridinium chromophore has most of the positive charge localized near the piridinium nitrogen in the ground state, and near the arylamino nitrogen in the excited state. Thus, a dipole moment of the molecule changes upon excitation by a photon. A change in the voltage across the membrane causes a spectral shift resulting from a direct interaction between the field and the ground and excited state dipole moments. Specifically, a decrease in Vm (corresponding to the event of the action potential) shifts the absorption spectrum to longer wavelength and the emission spectrum to shorter wavelength (the emission spectrum shift is illustrated in Figure 4). Note that the peak fluorescence does not change. In order to track changes in Vm, fluorescence can be collected either on the short-wavelength wing of the spectrum, or the long-wavelength wing of the spectrum, or both (see Fig. 4). In the latter case, the measurement is ratiometric (see below). The sign of the change in fluorescence corresponding to the action potential will depend on the chosen emission wavelength (see Fig. 4, bottom). Observe that an overly broad emission filter capturing the entire emission spectrum may fail to detect voltage-dependent changes in fluorescence, because theFluorescence primer v4.doc Page 3/11 19/11/2008 increase in the short-wavelength part of the spectrum and the decrease in the long-wavelength part of the spectrum will cancel each other. It should be noted also that fast potentiometric probes emit fluorescence even in the absence of any change in electrical field and that this “passive” background fluorescence (F) is large as compared to the “active” signal (∆F). The ratio ∆F/F is an important parameter of fast potentiometric probes and a recently introduced probe Di-4-ANBDQBS which we will test in this lab has an improved ∆F/F in addition to other valuable features (see Table 1). groundexcited Ratiometry The fact that the fluorescence spectrum shifts in response to change in Vm allows for ratiometric measurement of voltage-sensitive fluorescence (see Table 1 and Fig. 4). For example, di-4-ANEPPS signal can be recorded simultaneously at 540 nm and above 610 nm with excitation at 510 nm.3 The main motivation for ratiometry in cardiac studies is removing motion artifact which should have the same direction in the two recording channels, whereas the changes related to the AP have opposite directions.4 In addition, ratiometry helps to compensate for dye bleaching, internalization and washout. Note however that motion artifact is by far the greatest problem for voltage-sensitive imaging in beating hearts and cardiac tissues. Figure 3 Calibration For all practical purposes, change in fluorescence (∆F) of potentiometric dyes such as