Ca2+-induced Ca2+ Release Phenomena in Mammalian Sympathetic Neurons Are Critically Dependent on the Rate of Rise of Trigger Ca2+

The role of ryanodine-sensitive intracellular Ca²⁺ stores present in nonmuscular cells is not yet completely understood. Here we examine the physiological parameters determining the dynamics of caffeine-inducedCa²⁺ release in individual fura-2–loaded sympathetic neurons. Two ryanodine-sensitive release components weredistinguished: an early, transient release (TR) and a delayed, persistent release (PR). The TR component showsrefractoriness, depends on the filling status of the store, and requires caffeine concentrations ≥10 mM. Furthermore, it is selectively suppressed by tetracaine and intracellular BAPTA, which interfere with Ca²⁺-mediated feedback loops, suggesting that it constitutes a Ca²⁺-induced Ca²⁺-release phenomenon. The dynamics of release ismarkedly affected when Sr²⁺ substitutes for Ca²⁺, indicating that Sr²⁺ release may operate with lower feedbackgain than Ca²⁺ release. Our data indicate that when the initial release occurs at an adequately fast rate, Ca²⁺ triggers further release, producing a regenerative response, which is interrupted by depletion of releasable Ca²⁺ andCa²⁺-dependent inactivation. A compartmentalized linear diffusion model can reproduce caffeine responses:When the Ca²⁺ reservoir is full, the rapid initial Ca²⁺ rise determines a faster occupation of the ryanodine receptor Ca²⁺ activation site giving rise to a regenerative release. With the store only partially loaded, the slower initialCa²⁺ rise allows the inactivating site of the release channel to become occupied nearly as quickly as the activatingsite, thereby suppressing the initial fast release. The PR component is less dependent on the store''s Ca²⁺ content.This study suggests that transmembrane Ca²⁺ influx in rat sympathetic neurons does not evoke widespread amplification by CICR because of its inability to raise [Ca²⁺] near the Ca²⁺ release channels sufficiently fast to overcometheir Ca²⁺-dependent inactivation. Conversely, caffeine-induced Ca²⁺ release can undergo considerable amplification especially when Ca²⁺ stores are full. We propose that the primary function of ryanodine-sensitive stores inneurons and perhaps in other nonmuscular cells, is to emphasize subcellular Ca²⁺ gradients resulting from agonist-induced intracellular release. The amplification gain is dependent both on the agonist concentration and onthe filling status of intracellular Ca²⁺ stores.