The calcium signal in human neutrophils and its relation to exocytosis investigated by patch-clamp capacitance and Fura-2 measurements

Intracellular calcium ([Ca 2+] i) and exocytosis of human neutrophils were investigated with patch-clamp capacitance and Fura-2 fluorescence measurements. Intracellular application of GTPγS induces a calcium transient and exocytosis. The onset of degranulation occurs at the time where the maximal [Ca 2+] i is reached. Despite the close correlation in time, buffering [Ca 2+] i at the resting level or at ∼2 μM leaves the extent and the time course of degranulation unchanged. The decay of the calcium transient is due to diffusional equilibration between the cytosol and the pipette volume. GTPγS activates no cellular mechanisms for Ca 2+ reuptake or extrusion. The endogenous calcium buffer capacity can be estimated to be as low as that of ∼90 μM Fura-2. Stimulation with fMLP also induces degranulation and a calcium transient. The decay of fMLP-induced calcium transients is much faster than that of GTPγS-induced transients and is independent of diffusion indicating that fMLP also induces rapid reuptake or extrusion of Ca 2+. Degranulation but not the calcium transient requires the presence of intracellular GTP. Different signalling pathways appear to be involved in GTPγS- and fMLP-stimulated calcium signals. The intracellular calcium release is not an essential signal to initiate exocytosis in neutrophils.