CA2-DEPENDENT CAPACITANCE INCREASES IN RAT BASOPHILIC LEUKEMIA-CELLS FOLLOWING ACTIVATION OF STORE-OPERATED CA2+ ENTRY AND DIALYSIS WITH HIGH-CA2+-CONTAINING INTRACELLULAR SOLUTION()

Ca2+-dependent vesicular fusion was studied in single whole-cell patch -clamped rat basophilic leukemia (RBL) cells using the capacitance tec hnique. Dialysis of the cells with 10 mu M free Ca2+ and 300 mu M guan osine 5'-O-(3-thiotriphosphate) (GTP[gamma-S]) resulted in prominent c apacitance increases. However, dialysis with either Ca2+ (225 nM to 10 mu M) or GTP[gamma-S] alone failed to induce a capacitance change. Un der conditions of weak Ca2+ buffering (0.1 mM EGTA), activation of Ca2 +-release-activated Ca2+ (CRAC) channels by dialysis with inositol 1,4 ,5-trisphosphate (InsP(3)) failed to induce a capacitance increase eve n in the presence of GTP[gamma-S]. However, when Ca2+ ATPases were inh ibited by thapsigargin, InsP(3) and GTP[gamma-S] led to a pronounced e levation in membrane capacitance. This increase was dependent on a ris e in intracellular Ca2+ because it was abolished when cells were dialy sed with a high level of EGTA (10 mM) in the recording pipette. The in crease was also dependent on Ca2+ influx because it was effectively su ppressed when external Ca2+ was removed. Our results demonstrate that I-CRAC represents an important source of Ca2+ for triggering a secreto ry response.