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 Ca(2+) influx, a process termed store-operated Ca(2+) entry. In rat basophilic leukemia cells, emptying of the stores activates a highly selective Ca(2+) release-activated Ca(2+) current (CRAC), I(CRAC). We have recently found that I(CRAC) 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 Ca(2+) 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 Ca(2+) 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 Ca(2+) can enter through I(CRAC). We have directly measured the current using the whole cell patch clamp technique. We report the presence of a slow Ca(2+)-dependent inactivation mechanism that curtails Ca(2+) entry through CRAC channels. This inactivation mechanism is switched on by Ca(2+) entering through CRAC channels, and therefore constitutes a slow negative feedback process. Although it requires a rise in intracellular Ca(2+) for activation, it maintains CRAC channels inactive even under conditions that lower intracellular Ca(2+) levels. The inactivation mechanism does not involve store refilling, protein phosphorylation, G proteins, nor Ca(2+)-dependent enzymes. It accounts for up to 70% of the total inactivation of I(CRAC), and therefore appears to be a dominant inhibitory mechanism. It is likely to be an important factor that shapes the profile of the Ca(2+) signal in these non-excitable cells.