Regulation of ryanodine receptor opening by lumenal Ca2+ underlies quantalCa2+ release in PC12 cells

Graded or "quantal" Ca2+ release from intracellular stores has been observe d in various cell types following activation of either ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (InsP(3)R). The mechanism c ausing the release of Ca2+ stores in direct proportion to the strength of s timulation is unresolved. We investigated the properties of quantal Ca2+ re lease evoked by activation of RyR in PC12 cells, and in particular whether the sensitivity of RyR to the agonist caffeine was altered by lumenal Ca2+. Quantal Ca2+ release was observed in cells stimulated with 1 to 40 mM caff eine, a range of caffeine concentrations giving a >10-fold change in lumena l Ca2+ content. The Ca2+ load of the caffeine-sensitive stores was modulate d by allowing them to refill for varying times after complete discharge wit h maximal caffeine, or by depolarizing the cells with K+ to enhance their n ormal steady-state loading. The threshold for RyR activation was sensitized similar to 10-fold as the Ca2+ load increased from a minimal to a maximal loading. In addition, the fraction of Ca2+ released by low caffeine concent rations increased. Our data suggest that RyR are sensitive to lumenal Ca2over the full range of Ca2+ loads that can be achieved in an intact PC12 ce ll, and that changes in RyR sensitivity may be responsible for the terminat ion of Ca2+ release underlying the quantal effect.