Fast calcium-dependent inactivation of calcium release-activated calcium current (CRAC) in RBL-1 cells

Fast inactivation of the Ca2+ release-activated Ca2+ current (ICRAC) was studied using whole cell patch-clamp recordings in rat basophilic leukemia (RBL-1) cells. Application of hyperpolarizing voltage steps from the holding potential of 0 mV revealed that ICRAC declined in amplitude over tens of milliseconds during steps more negative than -40 mV. This fast inactivation was predominantly Ca2+-dependent because first, it could be more effectively suppressed when BAPTA was included in the recording pipette instead of EGTA and second, replacing external Ca2+ with Sr2+ resulted in less inactivation. Recovery from inactivation was faster in the presence of BAPTA than EGTA. The extent of fast inactivation was independent of the whole cell ICRAC amplitude, compatible with the notion that the inactivation arose from a local feedback inhibition by permeating Ca2+ ions only on the channel it permeated. Ca2+ release from stores did not affect fast inactivation, nor did FCepsilonRI receptor stimulation. Current clamp recordings showed that the majority of RBL cells had a membrane potential close to -90 mV following stimulation of FCepsilonRI receptors. Hence fast inactivation is likely to impact on the extent of Ca2+ influx through CRAC channels under physiological conditions and appears to be an important negative feedback process that limits Ca2+ increases.