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

Abstract

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 (InsP3)- producing agonists released only 60Ð80%. Both InsP3 and caffeine emptied completely the ER in digitoninpermeabilized 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 [C 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 InsP3 receptors or CICR.