Ca 2+ ‐induced exocytosis in individual human neutrophils: high‐ and low‐affinity granule populations and submaximal responses

We have investigated Ca 2+ ‐induced exocytosis from human neutrophils using the whole cell patch–clamp capacitance technique. Microperfusion of Ca 2+ buffer solutions (<30 nM to 5 mM free Ca 2+ ) through the patch–clamp pipette revealed a biphasic activation of exocytosis by Ca 2+ . The first phase was characterized by high affinity (1.5–5 μM) and low apparent cooperativity (⩽2) for Ca 2+ , and the second phase by low affinity (∼100 μM) and high cooperativity (>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 Ca 2+ concentrations, only a fraction of a given granule population was released. This submaximal release cannot simply be explained by Ca 2+ 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 Ca 2+ 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 Ca 2+ sensitivity of secretion is granule‐, rather than cell‐specific.