Ca(2+)-induced Ca(2+) Release in Chromaffin Cells Seen from inside the ER with Targeted Aequorin

The presence and physiological role of Ca(2+)-induced Ca(2+) release (CICR) in nonmuscle excitable cells has been investigated only indirectly through measurements of cytosolic [Ca(2+)] ([Ca(2+)](c)). Using targeted aequorin, we have directly monitored [Ca(2+)] changes inside the ER ([Ca(2+)](ER)) in bovine adrenal chromaffin cells. Ca(2+) entry induced by cell depolarization triggered a transient Ca(2+) release from the ER that was highly dependent on [Ca(2+)](ER) and sensitized by low concentrations of caffeine. Caffeine-induced Ca(2+) release was quantal in nature due to modulation by [Ca(2+)](ER). Whereas caffeine released essentially all the Ca(2+) from the ER, inositol 1,4,5-trisphosphate (InsP(3))- producing agonists released only 60–80%. Both InsP(3) and caffeine emptied completely the ER in digitonin-permeabilized cells whereas cyclic ADP-ribose had no effect. Ryanodine induced permanent emptying of the Ca(2+) stores in a use-dependent manner after activation by caffeine. Fast confocal [Ca(2+)](c) measurements showed that the wave of [Ca(2+)](c) induced by 100-ms depolarizing pulses in voltage-clamped cells was delayed and reduced in intensity in ryanodine-treated cells. Our results indicate that the ER of chromaffin cells behaves mostly as a single homogeneous thapsigargin-sensitive Ca(2+) pool that can release Ca(2+) both via InsP(3) receptors or CICR.