Inositol 1,4,5-triphosphate-triggered Ca²⁺ release from bovine adrenal medullary secretory vesicles

The effect of inositol 1,4,5-triphosphate (Ins(1,4,5)-P₃) and calcium ionophore A23187 on Ca²⁺ release from bovine adrenal medullary secretory vesicles and microsomes was examined. Ins(1,4,5)P₃ released 3.5 nmol of Ca²⁺/mg protein from secretory vesicles and 1.5 nmol of Ca²⁺/mg protein from microsomes as measured by a Ca²⁺-selective electrode. However, A23187 promoted Ca²⁺ uptake into vesicles while releasing Ca²⁺ from microsomes. Ins(1,4,5)P₃-induced Ca²⁺ release from secretory vesicles was rapid, but the released Ca²⁺ was absorbed within 3 min during which the Ins(1,4,5)P₃-releasable pools were refilled. The in situ calcium content of secretory vesicle measured by atomic absorption spectrometry was 112 ± 6.3 nmol/mg protein indicating the potential importance of secretory vesicles as an intracellular Ca²⁺ store. The high Ca²⁺-buffering capacity of secretory vesicles is presumed to be due to the high Ca²⁺-binding capacity of chromograninin A, the major intravesicular protein, which has calsequestrin-like properties.