Imaging the activity and localization of single voltage-gated Ca2+ channels by total internal reflection fluorescence microscopy

The patch-clamp technique has enabled functional studies of single ion channels, but suffers limitations including lack of spatial information and inability to independently monitor currents from more than one channel. Here, we describe the use of total internal reflection fluorescence microscopy as an alternative, noninvasive approach to optically monitor the activity and localization of multiple Ca2+-permeable channels in the plasma membrane. Images of near-membrane Ca2+ signals were obtained from >100 N-type channels expressed within restricted areas (80x80 mum) of Xenopus oocytes, thereby permitting simultaneous resolution of their gating kinetics, voltage dependence, and localization. Moreover, this technique provided information inaccessible by electrophysiological means, demonstrating that N-type channels are immobile in the membrane, show a patchy distribution, and display diverse gating kinetics even among closely adjacent channels. Total internal reflection fluorescence microscopy holds great promise for single-channel recording of diverse voltage- and ligand-gated Ca2+-permeable channels in the membrane of neurons and other isolated or cultured cells, and has potential for high-throughput functional analysis of single channels.