CA2-INDUCED EXOCYTOSIS IN INDIVIDUAL HUMAN NEUTROPHILS - HIGH-AFFINITY AND LOW-AFFINITY GRANULE POPULATIONS AND SUBMAXIMAL RESPONSES()

We have investigated Ca2+-induced exocytosis from human neutrophils us ing the whole cell patch-clamp capacitance technique, Microperfusion o f Ca2+ buffer solutions (<30 nM to 5 mM free Ca2+) through the patch-c lamp pipette revealed a biphasic activation of exocytosis by Ca2+, The first phase was characterized by high affinity (1.5-5 mu M) and low a pparent cooperativity (less than or equal to 2) for Ca2+, and the seco nd phase by low affinity (similar to 100 mu M) and high cooperativity (>6), Only the second phase was accompanied by loss of myeloperoxidase , suggesting that the low-affinity exocytosis reflected release of per oxidase-positive (primary) granules, while the high-affinity exocytosi s reflected release of peroxidase-negative (secondary and tertiary) gr anules, 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 sug gests that the secretory response of individual cells is adjusted to t he strength of the stimulus. The Ca2+ dependence of the high-and low-a ffinity phases of neutrophil exocytosis bears a resemblance to endocri ne and neuronal exocytosis, respectively, The occurrence of such high- and low-affinity exocytosis in the same cell is novel, and suggests th at the Ca2+ sensitivity of secretion is granule-, rather than cell-spe cific.