A CA2-CELLS (RBL-1)( CURRENT ACTIVATED BY RELEASE OF INTRACELLULAR CA2+ STORES IN RAT BASOPHILIC LEUKEMIA)

We have characterized a Ca2+ current activated by depletion of intrace llular Ca2+ stores (capacitative Ca2+ entry current) as a first step t o investigate the mechanisms underlying communication between the intr acellular 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 C a2+ current. An inwardly rectifying current could be activated upon di alysis of the cell interior with pipette solutions devoid of Ca2+ and containing 20 mM BAPTA, a procedure expected to passively deplete intr acellular Ca2+ stores. The current was maximally activated within 2 mi n, 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 N i2+ displayed no detectable current carrying ability. Extracellular Na + permeated the pathway in the absence of extracellular Ca2+. Under co nditions designed to reduce passive depletion of intracellular Ca2+ st ores, a Ca2+ current indistinguishable from that described above was a ctivated by addition of ionomycin. This observation is consistent with the activation of the Ca2+ influx pathway occurring as a result of ev ents associated with depletion of intracellular Ca2+ stores, Important ly, application of extracellular Ni2+ in the presence of ionomycin irr eversibly inhibited the current. The presence of an inwardly rectifyin g K+ current in RBL cells could confound studies of the capacitative C a2+ entry current when recorded using pipette solutions devoid of K+ s ince this current would be inward over the voltage range used to inves tigate the capacitative Ca2+ entry current. This study compares an inw ard rectifying K+ current and the capacitative Ca2+ entry current in R BL 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 contain ing even modest amounts of K+ when studying the capacitative Ca2+ entr y current in RBL cells.