Inositol 1,4,5-trisphosphate induces bursts of calcium release inside Limulus ventral photoreceptors.

Injection of inositol 1,4,5-trisphosphate (InsP3) into dark-adapted Limulus ventral photoreceptors produces a series of discrete bursts of membrane depolarization. Prior injection of aequorin, a luminescent calcium indicator, reveals that the bursts of depolarization are accompanied by individual bursts of intracellular calcium elevation with a similar time course. Reduction of extracellular calcium increased rather than decreased the InsP3-induced rise in calcium. These results suggest that small numbers of InsP3 molecules can trigger discrete and rapid releases of large amounts of calcium from intracellular stores. In some cells, InsP3 injection induces a delayed and prolonged elevation of intracellular calcium in addition to the brief bursts.