Ca2+-induced exocytosis in individual human neutrophils: high- and low-affinity granule populations and submaximal responses

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Abstract
We have investigated Ca2+‐induced exocytosis from human neutrophils using the whole cell patch–clamp capacitance technique. Microperfusion of Ca2+ buffer solutions (6). Only the second phase was accompanied by loss of myeloperoxidase, suggesting that the low‐affinity exocytosis reflected release of peroxidase‐positive (primary) granules, while the high‐affinity exocytosis reflected release of peroxidase‐negative (secondary and tertiary) granules. At submaximal Ca2+ concentrations, only a fraction of a given granule population was released. This submaximal release cannot simply be explained by Ca2+ modulation of the rate of exocytosis, and it suggests that the secretory response of individual cells is adjusted to the strength of the stimulus. The Ca2+ dependence of the high‐ and low‐affinity phases of neutrophil exocytosis bears a resemblance to endocrine and neuronal exocytosis, respectively. The occurrence of such high‐ and low‐affinity exocytosis in the same cell is novel, and suggests that the Ca2+ sensitivity of secretion is granule‐, rather than cell‐specific.