[65] The use of optical probes to monitor membrane potential

The importance of the electrical activity of cell membranes has been recognized by physiologists for many years. Classically, the electrical potential difference between the aqueous phases separated by a natural membrane has been measured directly by suitably placed electrodes. Microelectrodes have been developed, which can be used in small cells and some subcellular organelles but the general use of electrodes is still restricted to relatively few systems. Recently, extensive use has been made of membrane probes that respond optically to transmembrane potential to provide a nondestructive technology to follow the electrical activity of many different membrane preparations. Intrinsic probes are natural membrane chromophores that respond directly to the membrane potential. The most widely studied intrinsic probes of membrane potential are the carotenoid pigments found in many photosynthetic membranes. The chapter identifies the probes that have proved most useful in studies of membrane potential. It discusses (1) the principles involved in selecting a useful probe of membrane potential, (2) the sources of the probes, (3) the techniques and instrumentation necessary to follow the probe signals, (4) the calibration of the observed signals with the magnitude of the membrane potential, and (5) the validity of the probe method for determining the membrane potential.