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 technique. Dialysis of the cells with 10 μM free Ca2+ and 300 μM guanosine 5'-O-(3-thiotriphosphate) (GTP[γ-S]) resulted in prominent capacitance increases. However, dialysis with either Ca2+ (225 nM to 10 μM) or GTP[γ-S] alone failed to induce a capacitance change. Under 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 (InsP3) failed to induce a capacitance increase even in the presence of GTP[γ-S]. However, when Ca2+-ATPases were inhibited by thapsigargin, InsP3 and GTP[γ-S] led to a pronounced elevation in membrane capacitance. This increase was dependent on a rise in intracellular Ca2+ because it was abolished when cells were dialysed with a high level of EGTA (10 mM) in the recording pipette. The increase was also dependent on Ca2+ influx because it was effectively suppressed when external Ca2+ was removed. Our results demonstrate that I(CRAC) represents an important source of Ca2+ for triggering a secretory response.