Membrane Potential and Channels

Membrane potential is the difference in voltage between the interior and exterior of a cell. The membrane potential allows a cell to function as a battery, providing power to operate a variety of molecular devices embedded in the membrane. In electrically excitable cells such as neurons, membrane potential is used for transmitting signals between different parts of a cell. Opening or closing of ion channels at one point in the membrane produces a local change in the membrane potential, which causes an electric current to flow rapidly to other points in the membrane. Ion channels have been identified as important drug discovery targets.

The plasma membrane of a cell typically has a transmembrane potential of approximately ?70 mV (negative inside) as a consequence of K⁺, Na⁺ and Cl^- concentration gradients that are maintained by active transport processes. Potentiometric probes offer an indirect convenient method of detecting the translocation of these ions although the fluorescent ion indicators can be used to directly measure changes in specific ion concentrations. Potentiometric optical probes enable researchers to perform membrane potential measurements in organelles and in cells that are too small for microelectrodes. Moreover, in conjunction with imaging techniques, these probes can be employed to map variations in membrane potential across excitable cells, in perfused organs and ultimately in the brain in vivo with spatial resolution and sampling frequency that cannot be obtained using microelectrodes. Increases and decreases in membrane potential play a central role in many physiological processes, including nerve-impulse propagation, muscle contraction, cell signaling and ion-channel gating.

Potentiometric probes are important tools for studying these processes, as well as for visualizing mitochondria (which exhibit transmembrane potentials of approximately ?150 mV, negative inside matrix), for assessing cell viability and for high-throughput screening of new drug candidates. Potentiometric probes include the cationic or zwitterionic styryl dyes, the cationic carbocyanines and rhodamines, and the anionic oxonols. The class of dye determines factors such as accumulation in cells, response mechanism and toxicity. Selecting the best potentiometric probe for a particular application can be complicated by the substantial variations in their optical responses, phototoxicity and interactions with other molecules. There are two classes of membrane potential probes based on their response mechanisms: fast response and slow response membrane potential dyes.

Fast Response Membrane Potential Probes

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Fast-response probes have their fluorescence in response to a change in the surrounding electric field. Their optical response is sufficiently fast to detect transient (millisecond) potential changes in excitable cells, including single neurons, cardiac cells and intact brains. However, the magnitude of their potential-dependent fluorescence change.

Slow Response Membrane Potential Probes

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Slow-response probes exhibit potential-dependent changes in their transmembrane distribution that are accompanied by a fluorescence change. The magnitude of their optical responses is much larger than that of fast-response probes, typically a 1% fluorescence change per mV. Slow-response probes, which include cationic carbocyanines and rhodamines and anionic oxonols, are suitable for detecting changes in average membrane potentials of nonexcitable cells caused by respiratory activity, ion-channel permeability, drug binding and other factors.