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

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 ).