Cooking with calcium: the recipes for composing global signals from elementary events

Recent studies have suggested that global intracellular Ca2+ signals arise from the summation and coordination of subcellular elementary release events (e.g., "Ca2+ puffs"), although the modes of recruitment of such signals are unknown. In order to understand how cells utilize elementary Ca2+ release events, we imaged Ca2+ transients evoked through the phosphoinositide pathway in HeLa cells using confocal microscopy. During the pacemaker phase leading to the global Ca2+ signal, elementary Ca2+ release events were recruited in (1) frequency, (2) amplitude, and (3) spatial domains. Since each digital elementary event contributes to a small change of the analog cytosolic Ca2+ concentration, the net effect of the advancement in the three domains is to drive the ambient Ca2+ concentration toward a threshold where the signal becomes regenerative, resulting in a global Ca2+ wave.