Ca2+ modulation of volume-regulated anion channels: evidence for colocalization with store-operated channels

Ca2+ regulation of Cl- current induced by cell swelling (I(CI,swell)) in response to hypotonicity was studied in human prostate cancer epithelial cells (LNCaP) by using the patch-clamp technique. Increase of global intracellular Ca2+ ([Ca2+]in) to 1 mM as well as variations of the extracellular Ca2+ ([Ca2+]out) in the 0 to 10 mM range did not affect time course of the development, maximal amplitude, rectification properties, and kinetics of I(CI,swell). However, the presence of 0.1 mM thapsigargin (TG), an inhibitor of endoplasmic reticulum (ER) Ca2+ pump, resulted in a more than 50% inhibition of ICI,swell. The blockade of plasma membrane store-operated channels (SOCs), activated in the presence of TG, by 2 mM Ni2+ prevented TG-conferred I(CI,swell) inhibition by extracellular Ca2+. In the presence of TG and Ca2+, the cells failed to exhibit regulatory volume decrease. We conclude that interaction between volume-regulated anion channels (VRACs) carrying I(CI,swell) and Ca2+ occurs in the microdomains from the inner surface of the membrane that are not accessible to the changes in [Ca2+]in, but can be readily reached by Ca2+ entering the cell via plasma membrane, especially through SOCs. Preferred access of SOC-transported Ca2+ to VRAC suggests colocalization of these channels in the cell membrane.