Concepts of single-channel analysis: inferring function from fluctuations

Abstract

Abstract The defining act of an ion channel protein is to allow an electrical current of ions to flow passively across a cell membrane. Thus, among the variety of approaches discussed in this text, perhaps the most basic measurement of channel function is its unitary current. As described in Chapters 1 and 2, single channel currents can be recorded with techniques that involve patch pipettes or artificial bilayers. Depending on the noise characteristics of the system, a patch-clamp amplifier allows one to measure unitary currents as small as ∼0.1 pA. Regardless of how the single-channel recording is obtained, the primary objective is to interpret or make sense of it in terms of channel function. For this formidable task, the record supplies only two basic types of information: current amplitudes and dwell times. These pieces of information are illustrated for a recording of a Ca2+ -activated K+ channel in Figure I. The first trace in Figure I shows that this particular channel fluctuates between a well-defined closed (down) and open (up) state. The unitary current, i, of the channel is measured as the difference between the closed and open levels. A dwell time in the open state is defined as the duration of an event in the open state between two consecutive closing events. Likewise, a dwell time in the closed state is that between two consecutive openings.