Continuous Fluorescent Monitoring of Cellular Calcium Fluxe: A Novel Perfusion System for the Invest

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 –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 ).