Ca2+ is an interesting intracellular messenger because local rises in its concentration – `sparks' – can trigger global Ca2+ waves that spread across the cell. Frequently, this involves Ca2+-induced Ca2+ release (CICR), in which Ca2+ sparks stimulate release of further Ca2+ from intracellular stores through ryanodine receptors. Llewelyn Roderick and co-workers now show that in cardiomyocytes the subcellular geography of the Ca2+ signalling machinery usually prevents this so the cells do not contract inappropriately (see p. 6327). Combining confocal microscopy and electrophysiology, they show the peripheral sparks generated during normal twitching do not produce Ca2+ waves that spread into the interior of the cell – which would stimulate excessive contraction. Instead Ca2+ pumps on mitochondria and the sarcoplasmic reticulum mop up the Ca2+ signal before it can spread. When the authors inhibit these pumps, global Ca2+ waves...

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