CALCIUM AND BARIUM PERMEATION THROUGH CALCIUM RELEASE-ACTIVATED CALCIUM (CRAC) CHANNELS

A Ca2+ current activated by store depletion has been described recentl y in several cell types and has been termed I-CRAC (for Ca2+ release-a ctivated Ca2+ current). In this paper, the Ca2+ and Ba2+ permeability of CRAC channels is investigated in mast cells, rat basophilic leukaem ia cells (RBL) and human T-lymphocytes (Jurkat). The selectivity of CR AC channels for Ca2+ over monovalent cations is identical in all three cell types and is at least as high as that of voltage-operated Ca2+ ( VOC) channels in the various tissues tested. The amplitude of Ba2+ cur rents relative to Ca2+ currents (I-Ba/l(Ca)) through CRAC channels was found to be strongly dependent on the membrane potential and was much smaller in Jurkat cells compared to mast and RBL cells. An anomalous mole-fraction behavior was observed at very negative membrane potentia ls in all three cell types when using different mixtures of external C a2+ and Ba2+. In contrast to VOC channels, the anomalous mole-fraction effect was not observed at potentials positive to -20 mV.