Spatiotemporal patterning of IP3‐mediated Ca2+ signals in Xenopus oocytes by Ca2+‐binding proteins

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Abstract
Ca2+‐binding proteins (CaBPs) are expressed in a highly specific manner across many different cell types, yet the physiological basis underlying their selective distribution patterns remains unclear. We used confocal line‐scan microscopy together with photo‐release of IP3 in Xenopus oocytes to investigate the actions of mobile cytosolic CaBPs on the spatiotemporal properties of IP3‐evoked Ca2+ signals. Parvalbumin (PV), a CaBP with slow Ca2+‐binding kinetics, shortened the duration of IP3‐evoked Ca2+ signals and ‘balkanized’ global responses into discrete localized events (puffs). In contrast, calretinin (CR), a presumed fast buffer, prolonged Ca2+ responses and promoted ‘globalization’ of spatially uniform Ca2+ signals at high [IP3]. Oocytes loaded with CR or PV showed Ca2+ puffs following photolysis flashes that were subthreshold in controls, and the spatiotemporal properties of these localized events were differentially modulated by PV and CR. In comparison to results we previously obtained with exogenous Ca2+ buffers, PV closely mimicked the actions of the slow buffer EGTA, whereas CR showed important differences from the fast buffer BAPTA. Most notably, puffs were never observed after loading BAPTA, and this exogenous buffer did not show the marked sensitization of IP3 action evident with CR. The ability of Ca2+ buffers and CaBPs with differing kinetics to fine‐tune both global and local intracellular Ca2+ signals is likely to have significant physiological implications.