Slow feedback inhibition of calcium release-activated calcium current by calcium entry.

In many nonexcitable cells, depletion of the inositol 1,4, 5-trisphosphate-sensitive store activates Ca2+ influx, a process termed store-operated Ca2+ entry. In rat basophilic leukemia cells, emptying of the stores activates a highly selective Ca2+ release-activated Ca2+ current (CRAC), ICRAC. We have recently found that ICRAC activates in an essentially all-or-none manner when the current is evoked by receptor stimulation, dialysis with inositol 1, 4,5-trisphosphate via the patch pipette, or through the Ca2+ATPase inhibitor thapsigargin (Parekh, A. B., Fleig, A., and Penner, R. (1997) Cell 89, 973-980). Regulatory mechanisms must therefore operate to control the overall amount of Ca2+ that enters through CRAC channels. Such mechanisms include membrane potential and protein kinase C. In the present study, we have investigated additional inhibitory pathways that serve to determine just how much Ca2+ can enter through ICRAC. We have directly measured the current using the whole cell patch clamp technique. We report the presence of a slow Ca2+-dependent inactivation mechanism that curtails Ca2+ entry through CRAC channels. This inactivation mechanism is switched on by Ca2+ entering through CRAC channels, and therefore constitutes a slow negative feedback process. Although it requires a rise in intracellular Ca2+ for activation, it maintains CRAC channels inactive even under conditions that lower intracellular Ca2+ levels. The inactivation mechanism does not involve store refilling, protein phosphorylation, G proteins, nor Ca2+-dependent enzymes. It accounts for up to 70% of the total inactivation of ICRAC, and therefore appears to be a dominant inhibitory mechanism. It is likely to be an important factor that shapes the profile of the Ca2+ signal in these nonexcitable cells.