Smoothly Graded Ca2+ Release from Inositol 1,4,5-trisphosphate-sensitive Ca2+ Stores

Stimulation of cells with Ca(2+)-mobilizing hormones often leads to the generation of temporally and spatially complex changes in the intracellular Ca2+ ion concentration ([Ca2+]i). To understand the mechanisms regulating Ca2+ release from intracellular stores more clearly, we investigated the ability of histamine to release Ca2+ stores under different experimental conditions, using video imaging of single Fura-2-loaded HeLa cells. In Ca(2+)-free medium, stepwise increases in histamine concentration released an increasing proportion of the intracellular Ca2+ pool. This pattern of Ca2+ release is analogous to the "quantal" release of Ca2+ previously observed using permeabilized cells. Quantal Ca2+ release was observed at both 20 and 37 degrees C and was not due to inactivation or desensitization of the Ca2+ release mechanism, since application of histamine in a pulsatile manner, which avoided desensitization of the Ca(2+)-release mechanism, still produced a quantal response. In Ca(2+)-containing medium at both 20 and 37 degrees C, stepwise increases in histamine concentration evoked [Ca2+]i responses where the amplitude was smoothly graded in direct proportion to the histamine concentration. Similar smoothly graded responses were observed from HeLa cells in Ca(2+)-free medium. These data indicate that hormone-evoked Ca2+ release from intracellular stores is limited by the hormone concentration, and that the mechanisms underlying complex [Ca2+]i signals do not lead to an all-or-none release of Ca2+ from the entire intracellular Ca2+ pool. We suggest that the hormone-sensitive intracellular Ca2+ pool is composed of functionally discrete units that are recruited by agonists in a concentration-dependent manner.