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

Fast inactivation of the Ca2+ release-activated Ca2+ current (I-CRAC) 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 I-CRAC 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 I-CRAC 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 FC epsilon RI receptor stimulation. Current clamp recordings showed that the majority of RBL cells had a membrane potential close to -90 mV following stimulation of FC epsilon RI 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.