Continuous Fluorescent Monitoring of Cellular Calcium Fluxe: A Novel Perfusion System for the Investigation of Inositol(1,4,5)trisphosphate-Dependent Quantal Calcium Release Using Immobilized, Electropermeabilized Cells

Inositol(1,4,5)trisphosphate [Ins(1,4,5)P3] is a well-characterized second messenger that interacts specifically with a family of Ins(1,4,5)P3 receptor-operated Ca2+ channels to mobilize nonmitochondrial intracellular Ca2+ stores (1). Ins(1,4,5)P3-induced Ca2+ mobilization (IICM) exhibits fascinating biphasic kinetics, whereby suboptimal concentrations of Ins(1,4,5)P3 (2) induce a rapid release of only a fraction of the total Ins(1,4,5)P3-sensitive Ca2+ pool, followed by a prolonged phase of slow Ca2+ efflux, with no significant further Ca2+ release until additional Ins(1,4,5)P3 is added. This widespread phenomenon (reviewed in refs. 3, 4, 5) has been labeled “quantal Ca2+ release” (QCR) (2,6) or “incremental detection” (7) and directly contrasts with the behavior of other agents that mobilize intracellular Ca2+ stores. For example, suboptimal concentrations of ionophores such as ionomycin exhibit slower Ca2+ efflux, which nevertheless ultimately produces the same net release as maximally effective concentrations (8).