Monovalent cation permeability and Ca2+ block of the store-operated Ca2+ current ICRAC in rat basophilic leukemia cells

Like voltage-operated Ca2+ channels, store-operated CRAC channels become permeable to monovalent cations in the absence of external divalent cations. Using the whole-cell patch-clamp technique, we have characterized the permeation and selectivity properties of store-operated channels in the rat basophilic leukemia (RBL-1) cell line. Store depletion by dialysis with InsP(3) and 10 mM EGTA resulted in the rapid development of large inward currents in Na+- and Li+-based divalent-free solutions. Cs+ permeated the channels poorly (P-Cs/P-Na=0.01). Trimethylamine (TMA(+)), tetramethylammonium (TeMA(+)), tetraethylammonium (TEA(+)), N-methyl-D-glucamine (NMDG(+)) and TRIS+ were not measurably permeant. NH4+ was conducted well. We estimated the minimum pore diameter under divalent-free conditions to be between 0.32 nm and 0.55 nm. When cells were dialysed with buffered Ca2+ solution and I-CRAC activated by application of thapsigargin, P-Cs/P-Na was still low (0.08). Outward currents through CRAC channels were carried by intracellular Na+. K+ and, to a much lesser extent, by Cs+. Currents were unaffected by dialysis with Mg2+-free solution. The Na+ current was inhibited by external Ca2+ (half-maximal blocking concentration of 10 muM). This Ca2+-dependent block could be alleviated by hyperpolarization. The monovalent Na+ current was voltage dependent, increasing as the holding potential depolarized above 0 mV. Our results suggest that CRAC channels in RBL-1 cells have a smaller pore diameter than voltage-operated Ca2+ channels. discriminate between Group I cations, and differ markedly in their selectivity from CRAC channels reported in lymphocytes.