Calcium-induced calcium release regulates action potential generation in guinea-pig sympathetic neurones.

Experiments were done using guinea-pig sympathetic neurones dissociated from the stellate ganglia to establish whether calcium-induced calcium release (CICR) modulated action potential (AP) generation in mammalian neurones. Using measurements of intracellular calcium ([Ca(2+)](i)) with the Ca(2+)-sensitive dye fluo-3, we demonstrated that 10 mM caffeine activated ryanodine receptors and caused a rise in [Ca(2+)](i) in both Ca(2+)-containing and Ca(2+)-deficient solutions. We also demonstrated that combined treatment with caffeine and 1 microm thapsigargin or caffeine and 20 microm ryanodine blocked subsequent caffeine-induced elevations of [Ca(2+)](i). Treatment with thapsigargin, ryanodine or 200 microM Cd(2+) to disrupt CICR decreased the latency to AP generation during 400 ms depolarizing current ramps using the perforated patch whole cell patch clamp in current clamp mode. Treatment with 500 microM tetraethylammonium also decreased the latency to AP generation during depolarizing current ramps in control cells, but not in cells pretreated with thapsigargin to deplete internal Ca(2+) stores. In summary, we propose that an outward current, carried at least in part through BK channels, is activated by CICR at membrane voltages approaching the threshold for AP initiation and that this current opposed depolarizing current ramps applied to guinea-pig sympathetic stellate neurones.