Caffeine-induced Ca$^2+$ sparks in mouse ventricular myocytes.
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Am. J. Physiol. Heart Circ. Physiol. 278 (2): H666-9 (February 2000)

Ca$^2+$ sparks are spatially localized intracellular Ca$^2+$ release events that were first described in 1993. Sparks have been ascribed to sarcoplasmic reticulum Ca$^2+$ release channel (ryanodine receptor, RyR) opening induced by Ca$^2+$ influx via L-type Ca$^2+$ channels or by spontaneous RyR openings and have been thought to reflect Ca$^2+$ release from a cluster of RyR. Here we describe a pharmacological approach to study sparks by exposing ventricular myocytes to caffeine with a rapid solution-switcher device. Sparks under these conditions have properties similar to naturally occurring sparks in terms of size and intracellular Ca$^2+$ concentration (Ca$^2+$(i)) amplitude. However, after the diffusion of caffeine, sparks first appear close to the cell surface membrane before coalescing to produce a whole cell transient. Our results support the idea that a whole cell Ca$^2+$(i) transient consists of the summation of sparks and that Ca$^2+$ sparks consist of the opening of a cluster of RyR and confirm that characteristics of the cluster rather than the L-type Ca$^2+$ channel-RyR relation determine spark properties.
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