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 FCɛRI receptor stimulation. Current clamp recordings showed that the majority of RBL cells had a membrane potential close to −90 mV following stimulation of FCɛRI 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. Received: 28 August 1998/Revised: 30 November 1998