Ca 2+ dynamics in the lumen of the endoplasmic reticulum in sensory neurons: direct visualization of Ca 2+ ‐induced Ca 2+ release triggered by physiological Ca 2+ entry

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