Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs - fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca2+ permeable ion channel using Ca2+ indicators like fluo-3. These Single Channel Ca2+ Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca2+ sparks and Ca2+ puffs caused by Ca2+ release from intracellular stores (due to the opening of ryanodine receptors and IP3 receptors, respectively). In contrast to intracellular Ca2+ release events, SCCaFrs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca2+ handling in the vicinity of a channel with a known Ca2+ influx, to obtain the Ca2+ current passing through plasma membrane cation channels in near physiological solutions, to localize Ca2+ permeable ion channels on the plasma membrane, and to estimate the Ca2+ currents underlying those elementary events where the Ca2+ currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca2+ channels, and stretch-activated channels. For the L-type Ca2+ channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca2+ currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase. (C) 2004 Elsevier Ltd. All rights reserved.