Silica Induces Changes in Cytosolic Free Calcium, Cytosolic pH, and Plasma Membrane Potential in Bovine Alveolar Macrophages

The mineral-dust induced activation of pulmonary phagocytes is thought to be involved in the induction of severe lung diseases. The activation of bovine alveolar macrophages (BAM) by silica was investigated by flow cytometry. Short-term incubation (< 10 min) of BAM with silica gel and quartz dust particles induced increases in the cytosolic free calcium concentration ([Ca2+]i), decreases in intracellular pH (pHi), and increases in plasma membrane potential (PMP). The extent of these changes was concentration dependent, related to the type of dust and was due to Ca2+ influx from the extracellular medium. An increase in [Ca2+]i was inhibited, when extracellular Ca2+ was removed. Furthermore the calcium signal was quenched by Mn2+ and diminished by the calcium channel blocker verapamil. The protein kinase C specific inhibitor bisindolylmaleimide II (GF 109,203 X) did not inhibit the silica-induced [Ca2+]i rise. In contrast, silica-induced cytosolic acidification and depolarization were inhibited by GF 109,203 X but not by removal of extracellular calcium. Addition of TiO2 particles or heavy metal-containing dusts had no effect on any of the three parameters. Our data suggest the existence of silica-activated transmembrane ion exchange mechanisms in BAM, which might be involved in the specific cytotoxicity of silica by Ca(2+)-dependent and independent pathways.