Silica increases cytosolic free calcium ion concentration of alveolar macrophages in vitro

Rat alveolar macrophages were exposed to silica dust (quartz) suspended in culture medium (SiO2, dry particle size less than 5 μm in diameter) and fluctuation in their cytosolic free calcium content ([Ca2+]i) was detected in cell monolayers with a fluorescent calcium probe (Indo-1AM). Cytosolic free calcium content was correlated with lactate dehydrogenase (LDH) release, an index of cell damage. SiO2 induced a concentration- and time-dependent increase of cytosolic free Ca2+ ion concentration and LDH release. [Ca2+]i was increased about fivefold when cells were exposed to 200 μg of SiO2 per milliliter (3 ml per dish) for 2 hr. [Ca2+]i changed within 15 min of SiO2 treatment, whereas LDH release was measurably increased only after 30 min. Chelation of extracellular Ca2+ by 2 mm ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetate did not prevent SiO2-induced fluctuation of macrophage [Ca2+]i, but did partially prevent the SiO2-induced increase in LDH release (p < 0.01). We conclude that a very early event in SiO2-induced damage of alveolar macrophages involves mobilization of intracellular calcium pools to increase [Ca2+]i. These results suggest that SiO2-induced macrophage damage, a key event in the development of silicosis, may involve perturbation of intracellular calcium homeostasis.