Biphasic increase in intracellular calcium induced by platelet-activating factor in macrophages

In single mouse macrophages stimulated by platelet-activating factor (PAF), the intracellular calcium concentration (Cai) monitored with fura-2 at room temperature presents a biphasic increase, including a transient and a more sustained component. After pulse administration of PAF, the first phase lasts for a few seconds and reaches a peak value of 0.5-1 microM Ca2+ at high PAF concentration. The amplitude of this peak is independent of extracellular Ca2+ concentration, suggesting that the initial Ca2+ transient is due to the release of Ca2+ from intracellular stores. The second phase of the response lasts for several minutes; its maximum amplitude is reached 1-2 min after the brief initial PAF stimulation. This phase, suppressed in zero external Ca2+ and increased in 10 mM Ca2+, is probably due to influx of Ca2+ through the plasma membrane. This secondary Ca2+ increase is blocked by 10-50 microM lanthanum. At low PAF concentration, the initial Ca2+ transient is not followed by a second phase, showing that the initial rises of Ca2+ and of its activator (presumably inositol trisphosphate) are not sufficient to trigger the second phase of Ca2+ increase.