ACTIVATION OF STORE-OPERATED CALCIUM INFLUX AT RESTING INSP(3) LEVELSBY SENSITIZATION OF THE INSP(3) RECEPTOR IN RAT BASOPHILIC LEUKEMIA-CELLS

1. Patch clamp and fura-2 AM measurements were performed to study the effects of sensitizing the inositol 1,4,5-trisphosphate (InsP(3)) rece ptor to InsP(3) on the activation of Ca2+ release-activated Ca2+ curre nt (I-CRAC) in rat basophilic leukaemia (RBL) cells. 2. The sensitizin g agent thimerosal (1 mu M) triggered Ca2+ release, and this was follo wed by Ca2+ influx. With no added InsP(3) in the patch pipette, thimer osal activated I-CRAC; this was prevented by heparin. I-CRAC activated by thimerosal was very similar to that evoked by InsP(3) or ionomycin . 3. Dialysing cells either for short (30 s) or long (600 s) periods o f time prior to application of thimerosal did not affect the subsequen t activation of I-CRAC, even though no InsP(3) was included in the pat ch pipette. 4. These results suggest that sensitizing the InsP(3) rece ptor can result in large Ca2+ influx in the presence of resting InsP(3 ), and that stores closer to the membrane may contribute more to activ ation of I-CRAC than stores further away.