Techniques for Membrane Capacitance Measurements

Abstract

Understanding the process whereby cells transduce an external signal to a secretory response (“stimulus—secretion coupling”: Douglas, 1968) has been an important topic of research for many years. The understanding of early events in the cascade in excitable cells, whereby an external signal evokes an electrical response mediated by ion channels, has certainly been revolutionized by the development of the patch-clamp technique. Extensions of the technique, however, have also provided surprising flexibility in reporting events late in the cascade whereby intracellular Ca²⁺ and other second messengers lead to exocytosis. In 1982, Neher and Marty reported that the patch-clamp technique together with basic impedance analysis could be used to monitor membrane (electrical) capacitance as a single-cell assay of exocytosis and endocytosis. Since exocytosis involves the fusion of secretory granule membrane with the plasma membrane and a corresponding increase in surface area, an increase in membrane capacitance is observed. The excess membrane is reclaimed in the process of endocytosis, which leads to a corresponding decrease in capacitance. Present techniques can detect changes in capacitance on the order of a femtofarad, allowing the fusion of single secretory granules with diameters greater that about 200 nm to be resolved. The temporal resolution possible is on the order of milliseconds (e.g., Breckenridge and Almers, 1987); therefore, capacitance-recording techniques can almost achieve the resolution of synaptic preparations, where the electrical response of a postsynaptic cell serves as a reporter of secretion. Resolution limits of membrane capacitance estimation techniques are discussed in greater detail in Section 5.