Calcium-dependent enhancement of depletion-activated calcium current in Jurkat T lymphocytes

We have obtained evidence that the Ca(2+)-selective current activated by Ca2+ store depletion (Ca2+ release-activated Ca2+ current; Icrac) in Jurkat T lymphocytes is augmented in a time-dependent manner by Ca2+ itself. Whole cell patch clamp experiments employed high cytosolic Ca(2+)-buffering conditions to passively deplete Ca2+ stores. Rapidly switching to nominally Ca(2+)-free extracellular buffer instantaneously reduced Icrac measured at -100 mV to leak current level. Unexpectedly, readmission of 2 mM Ca2+ instantaneously restored only 38 +/- 5% (mean +/- SEM, n = 9) of the full Icrac amplitude. The remainder reappeared in a monotonic time-dependent manner over 10 to 20 sec. Rapid vs. slow intracellular Ca2+ chelators did not alter this process, and inorganic Icrac blockers did not regenerate it, arguing against an intracellular site of action. The effect was specific to Ca2+: introduction of the permeant ions, Ba2+ or Sr2+, failed to invoke time-dependent Icrac reappearance. Moreover, equimolar substitution of Ba2+ for Ca2+ initially produced Ba2+ current of similar magnitude to the full Ca2+ current, but the Ba2+ current decayed monotonically to < 50% of its initial amplitude in < 20 sec. Conversely, return to Ca2+ produced a time-dependent increase in Icrac to its larger Ca2+ permeation level. Thus Ca2+ appears to selectively promote a reversible transition of Icrac that results in larger current flux, and at least partially explains the selectivity of this current for Ca2+ over other divalent ions.