Voltage-dependent Ba2+ permeation through store-operated CRAC channels:: Implications for channel selectivity

Store-operated Ca2+ entry through CRAC channels is a major route for Ca2+, influx in non-excitable cells. Studies on store-operated channel selectivity using fluorescent dyes have found that the channels are impermeable to Ba2+,. Furthermore, in such studies, agonists have been reported to increase Ba2+, influx, leading to the conclusion that additional Ca2+ entry pathways (permeable to Ba2+) co-exist with the Ba2+ impermeable store-operated channels. However, patch clamp experiments demonstrate that CRAC channels are permeable to Ba2+. We have addressed this paradox using fluorescence measurements and whole cell patch clamp recordings Of ICRAC- In store-depleted cells loaded with fura 2, Ba2+ application results in a slower and smaller rise in fluorescence than is the case with Ca2+. Ba2+ unlike Ca2+, depolarises the membrane potential by similar to 40mV, due to rapid block of an inwardly rectifying K+ current. Although Ba2+, permeates CRAC channels at very negative potentials in patch clamp recordings, Ba2+, permeation is steeply voltage-dependent. This combination of Ba2+ -dependent depolarisation and voltage-dependent Ba2+ permeation accounts for the apparent lack of Ba2+ permeation through store-operated channels seen in fluorescence experiments. Our findings identify major limitations with the use of Ba2+ as a surrogate for Ca2+ in probing Ca2+ entry pathways and raise the possibility that some of the previous reports proposing multiple Ca2+ entry pathways based on Ba2+ entry into fura 2-loaded cells could be explained by voltage-dependent Ba2+ permeation through CRAC channels. (c) 2007 Elsevier Ltd. All rights reserved.