A Ca2+ Current Activated by Release of Intracellular Ca2+ Stores in Rat Basophilic Leukemia Cells (RBL-1)

We have characterized a Ca²⁺ current activated by depletion of intracellular Ca²⁺ stores (capacitative Ca²⁺ entry current) as a first step to investigate the mechanisms underlying communication between the intracellular Ca²⁺ stores and the plasma membrane Ca²⁺ permeability. Whole cell currents in response to voltage ramps from −125 to +60 mV from a holding potential of −40 mV were recorded in rat basophilic leukemia cells (RBL-1 cells) in solutions designed to optimize detection of a Ca²⁺ current. An inwardly rectifying current could be activated upon dialysis of the cell interior with pipette solutions devoid of Ca²⁺ and containing 20 mm BAPTA, a procedure expected to passively deplete intracellular Ca²⁺ stores. The current was maximally activated within 2 min, was sensitive to extracellular Ca²⁺ concentration and was abolished by removal of extracellular Ca²⁺. The current was markedly reduced in the presence of Ni²⁺ or La³⁺. The pathway activated by this protocol was permeant to Ba²⁺, displaying complex permeability characteristics at negative potentials. A small inward Mn²⁺ current consistent with a finite permeability of the pathway to Mn²⁺ was detected. In contrast Ni²⁺ displayed no detectable current carrying ability. Extracellular Na⁺ permeated the pathway in the absence of extracellular Ca²⁺. Under conditions designed to reduce passive depletion of intracellular Ca²⁺ stores, a Ca²⁺ current indistinguishable from that described above was activated by addition of ionomycin. This observation is consistent with the activation of the Ca²⁺ influx pathway occurring as a result of events associated with depletion of intracellular Ca²⁺ stores. Importantly, application of extracellular Ni²⁺ in the presence of ionomycin irreversibly inhibited the current. The presence of an inwardly rectifying K⁺ current in RBL cells could confound studies of the capacitative Ca²⁺ entry current when recorded using pipette solutions devoid of K⁺ since this current would be inward over the voltage range used to investigate the capacitative Ca²⁺ entry current. This study compares an inward rectifying K⁺ current and the capacitative Ca²⁺ entry current in RBL cells and highlights some similarities and differences between the two currents. The results demonstrate that caution should be exercised in interpreting recordings made using extracellular solutions containing even modest amounts of K⁺ when studying the capacitative Ca²⁺ entry current in RBL cells. Received: 12 September 1995/Revised: 18 June 1996