Release of Ca²⁺ from a non-mitochondrial store site in peritoneal macrophages treated with saponin by inositol 1,4,5-trisphosphate

The effects of inositol 1,4,5-trisphosphate, prepared from human erythrocyte ghosts, on Ca²⁺ release from intracellular store sites were studied in saponin-treated guinea pig peritoneal macrophages. Micromolar concentrations of inositol 1,4,5-trisphosphate released Ca²⁺ within 1 min from store sites which had accumulated Ca²⁺ in the presence of 10 mM-NaN₃. In the presence of 10 mM-NaN₃, the Ca²⁺ accumulated in the presence of oxalate was seen in the endoplasmic reticulum of saponin-treated macrophages by electron microscopy, indicating that the site of Ca²⁺ released by inositol 1,4,5-trisphophate may be endoplasmic reticulum-like membranes. When the concentrations of free Ca²⁺ were over 3.5x10^-6M, the release of Ca²⁺ by this agent was inhibited. This inhibition may be due to either the higher concentration of extra-vesicular free Ca²⁺ or the larger accumulation of Ca²⁺ into the store site or perhaps both effects. MgCl₂ also had an inhibitory effect on the Ca²⁺ release. Inositol 1,4,5-trisphosphate also released Ca²⁺ from cardiac sarcoplasmic reticulum, but not from erythrocyte inside-out vesicles.