[Ca 2+ ] Microdomains control agonist‐induced Ca 2+ release in intact HeLa cells

We have monitored specifically the [Ca 2+ ] in the lumen of the endoplasmic reticulum (ER) of intact HeLa cells using an ER‐targeted low‐Ca 2+ ‐affinity aequorin. The steady‐state [Ca 2+ ] in the ER was around 600 μM. Histamine induced a concentration‐dependent decrease in lumenal [Ca 2+ ], whose rate increased near one order of magnitude and became “quantal” when cytosolic [Ca 2+ ] ([Ca 2+ ] c ) was clamped with the Ca 2+ chelator BAPTA. This effect was not due to decreased Ca 2+ pumping because simultaneous addition of a SERCA inhibitor produced only additive effects. Given that inhibition by [Ca 2+ ] c of the inositol 1,4,5‐trisphosphate‐gated channels requires a [Ca 2+ ] c much higher than that observed in the bulk cytosol after histamine addition, we conclude that local [Ca 2+ ] c microdomains at the site of release strongly inhibit agonist‐induced Ca 2+ mobilization in intact cells. This effect should play a key role in the mechanism controlling cytosolic [Ca 2+ ] oscillations and waves, and therefore in the generation of spatio‐temporal Ca 2+ patterns.—Montero, M., Barrero, M. J., Alvarez, J. [Ca 2+ ] Microdomains control agonist‐induced Ca 2+ release in intact HeLa cells. FASEB J. 11, 881–885 (1997)