Ca2+ dynamics in the lumen of the endoplasmic reticulum in sensory neurons:: direct visualization of Ca2+-induced Ca2+ release triggered by physiological Ca2+ entry

In cultured rat dorsal root ganglia neurons, we measured membrane currents, using the patch-clamp whole-cell technique, and the concentrations of free Ca2+ in the cytosol ([Ca2+](i)) and in the lumen of the endoplasmic reticulum (ER) ([Ca2+](L)), using high- (Fluo-3) and low- (Mag-Fura-2) affinity Ca2+-sensitive fluorescent probes and video imaging. Resting [Ca2+](L) concentration varied between 60 and 270 muM. Activation of ryanodine receptors by caffeine triggered a rapid fall in [Ca2+](L) levels, which amounted to only 40-50% of the resting [Ca2+](L) value. Using electrophysiological depolarization, we directly demonstrate the process of Ca2+-induced Ca2+ release triggered by Ca2+ entry through voltage-gated Ca2+ channels. The amplitude of Ca2+ release from the ER lumen was linearly dependent on I-Ca.