Ca(2+)-induced Ca(2+) Release Phenomena in Mammalian Sympathetic Neurons Are Critically Dependent on the Rate of Rise of Trigger Ca(2+)

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