Eosinophils are white blood cells that attack pathogens by releasing toxic proteins, including major basic protein (MBP) and eosinophil peroxidase. Quite how stimuli such as platelet-activating factor (PAF) induce the release of these from intracellular granules has been unclear. Scott O'Grady and co-workers now reveal the sequence of events that follows PAF stimulation (see p. 5749). They show that PAF signalling causes a rise in the level of intracellular Ca2+ ([Ca2+]i). Fluorescent pH indicators reveal that this leads to alkalinization of the cytoplasm, which can also be induced by compounds such as ionomycin and thapsigargin that increase [Ca2+]i by other mechanisms. Significantly, the authors find that the alkalinization of the cytoplasm is accompanied by acidification of the granules. Speculating that PAF-induced Ca2+ signalling might stimulate a proton pump in the granule membrane, they go on to demonstrate that an H...

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