The role of calcium in the initiation of superoxide release from alveolar macrophages

The role of calcium in the release of superoxide anion (O 2−) was examined in alveolar macrophages after stimulation with the Soluble stimuli: concanavalin A (Con A), N‐formyl methionyl phenylalanine (FMP), and the calcium ionophore, A23187. The release of O 2− by Con A was unaffected over a wide range of extracellular calcium concentrations (20 μM to 3 mM), whereas increasing the extracellular calcium above 2 mM inhibited FMP‐stimulated O 2− release. In contrast, A23187 did not stimulate O 2− release in calcium‐free medium (≤ 30 μM). The addition of EGTA (50 μM) to calcium‐free medium had no effect on Con A stimulation of O 2− release or FMP‐stimulated O 2− release. These results suggest that, for the three soluble stimuli, there are different roles for Ca⁺² in the activation and transmission of stimulatory signals across the cell membrane. Con A‐ or FMP‐stimulated calcium efflux from calcium‐loaded cells in either calcium‐free medium or 0.5 mM calcium‐containing medium. In calcium‐free medium, FMP transiently retarded ⁴⁵Ca⁺² uptake, while in 0.5 mM calcium‐containing medium, FMP transiently stimulated ⁴⁵Ca⁺² uptake. For either Con A or FMP, calcium efflux preceded O 2− release by 30–45 sec. Quinine, an agent that blocks membrane hyperpolarization in macrophages, completely blocked O 2− release by concanavalin A or FMP and inhibited ⁴⁵Ca⁺² efflux by 50% or more for both agents. These results support the hypothesis that redistribution of cellular Ca⁺² is one of the initial steps leading to the release of O 2−.