2-Aminoethoxydiphenyl borate (2-APB) inhibits capacitative calcium entry independently of the function of inositol 1,4.5-trisphosphate receptors

Capacitative calcium entry (CCE), the mechanism that replenishes intracellular calcium stores after depletion, is essential to intracellular calcium signaling. CCE is mediated by the channels in the plasma membrane generally referred to as "store operated channels (SOCs)". However, the molecular identity of the SOCs has never been determined, and the mechanism of the activation of SOCs remains to be elucidated. Recent studies have demonstrated that 2-aminoethoxydiphenyl borate (2-APB), which has been found to be an antagonist of inositol 1,4,5-trisphosphate receptors (IP₃Rs), inhibits CCE, suggesting that IP₃Rs channel activity is essential to the generation of CCE. However, CCE has also been reported to occur normally in IP₃R-deficient cells. In order to resolve this discrepancy, we investigated the effect of 2-APB on CCE in IP₃Rs-deficient cells. In response to store depletion with thapsigargin or N,N,N',N'-tetrakis (2-pyridylmethyl) ethylene diamine (TPEN), CCE was generated in IP₃Rs-deficient cells the same as in wild-type cells, however, 2-APB abolished CCE in IP₃Rs-deficient cells, despite the fact that this cell line does not possess functional IP₃Rs. We also examined the effect of 2-APB on several types of TRP Ca²⁺ channels, which exhibit properties similar to those of SOCs. 2-APB had a different inhibitory effect on spontaneous and thapsigargin-induced Ba²⁺ influx in cells that transiently expressed individual TRP subtypes. These results suggest that the channel activity of IP₃Rs is not essential to the generation of CCE in this cell line and that 2-APB inhibits CCE independently of the function of IP₃Rs.