Quantal Ca2+ mobilization stimulated by inositol 1,4,5-trisphosphate in permeabilized hepatocytes

In several cell types, including hepatocytes, submaximal concentrations of Ins(1,4,5)P3 stimulate an initial rapid mobilization of intracellular Ca2+ stores that is followed by either no further Ca2+ release or very much slower release. Further additions of Ins(1,4,5)P3 then evoke further Ca2+ mobilization. Such 'incremental' responses [Meyer & Stryer (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3841-3845] could result from all-or-nothing emptying of stores that differ in their sensitivities to Ins(1,4,5)P3 or from partial emptying of stores that are more uniformly sensitive, but unable to release all of their Ca2+ because the response to Ins(1,4,5)P3 rapidly attenuates. By measuring unidirectional 45Ca2+ efflux from intracellular stores stimulated with Ins(1,4,5)P3 under conditions where they continue to sequester 40Ca2+, we provide evidence suggesting that Ins(1,4,5)P3 stimulates all-or-nothing emptying of stores that differ in their sensitivities to Ins(1,4,5)P3, a quantal response pattern.