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.