Shaping of IP3-mediated Ca2+ signals by Ca2+ buffers and ...



Shaping of IP3-mediated Ca2+ signals by Ca2+ buffers and Ca2+ binding proteins.

Sheila L Dargan1, Angelo Demuro1, Nick Callamaras1, Jonathan Marchant1, Beat Schwaller2 and Ian Parker1

1UCI, Dept. Neurobiology & Behavior, Irvine, California 92697, 2Univ. Fribourg, Dept. Medicine, Fribourg, CH-1705 Switzerland

Local IP3-mediated Ca2+ release events (puffs) are recruited into global Ca2+ waves by successive cycles of diffusion and Ca2+ -induced Ca2+ release. The spatiotemporal properties of the Ca2+ signals are thus expected to be strongly influenced by cytosolic Ca2+ buffers. We studied this interaction using confocal linescan microscopy and photorelease of IP3 in Xenopus oocytes. EGTA (a buffer with slow ‘on-rate’) speeded Ca2+ signals and ‘Balkanized’ Ca2+ waves by dissociating them into discrete puffs. In contrast, BAPTA (a fast buffer with similar affinity) slowed Ca2+ responses and promoted ‘globalization’ of spatially uniform Ca2+ signals. These effects likely arise through changes in Ca2+ feedback at IP3 receptors, because Ca2+ signals evoked by influx through voltage gated channels were little affected. We then studied endogenous Ca2+ binding proteins. Parvalbumin (a slow buffer) closely mimicked the actions of EGTA, whereas calretinin (a fast buffer) produced biphasic Ca2+ signals with a fast initial decay (like EGTA) and a slow second component (like BAPTA). We propose that cell-specific expression of Ca2+ binding proteins with distinct kinetics (as observed in neurons) may shape the spatiotemporal patterns of IP3-mediated Ca2+ signals to serve specific physiological roles.

Supported by NIH GM 48071.

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