SLOW FEEDBACK INHIBITION OF CALCIUM RELEASE-ACTIVATED CALCIUM CURRENTBY CALCIUM-ENTRY

In many nonexcitable cells, depletion of the inositol 1,4,5-trisphosph ate-sensitive store activates Ca2+ influx, a process termed store-oper ated Ca2+ entry. In rat basophilic leukemia cells, emptying of the sto res activates a highly selective Ca2+ release-activated Ca2+ current ( CRAC), I-CRAC. We have recently found that I-CRAC activates in an esse ntially all-or-none manner when the current is evoked by receptor stim ulation, dialysis with inositol 1,4,5-trisphosphate via the patch pipe tte, or through the Ca2+ ATPase inhibitor thapsigargin (Parekh, A. B., Fleig, A., and Penner, R. (1997) Cell 89, 973-980), Regulatory mechan isms must therefore operate to control the overall amount of Ca2+ that enters through CRAC channels. Such mechanisms include membrane potent ial and protein kinase C. In the present study, we have investigated a dditional inhibitory pathways that serve to determine just how much Ca 2+ can enter through I-CRAC. We have directly measured the current usi ng the whole cell patch clamp technique. We report the presence of a s low Ca2+-dependent inactivation mechanism that curtails Ca2+ entry thr ough CRAC channels. This inactivation mechanism is switched on by Ca2 entering through CRAC channels, and therefore constitutes a slow nega tive feedback process. Although it requires a rise in intracellular Ca 2+ for activation, it maintains CRAC channels inactive even under cond itions that lower intracellular Ca2+ levels. The inactivation mechanis m does not involve store refilling, protein phosphorylation, G protein s, nor Ca2+-dependent enzymes. It accounts for up to 70% of the total inactivation of I-CRAC, and therefore appears to be a dominant inhibit ory mechanism. It is likely to be an important factor that shapes the profile of the Ca2+ signal in these non-excitable cells.