2-Aminoethoxydiphenyl borate directly inhibits store-operated calcium entry channels in human platelets.

In this study, we examined 2-aminoethoxydiphenyl borate (2APB) as an inhibitor of Ca(2+) influx in human platelets. 2APB was found to inhibit thrombin-mediated intracellular Ca(2+) mobilization rapidly in platelets incubated in the absence of extracellular Ca(2+). This result supports an intracellular action of 2APB on inositol 1,4,5-trisphosphate (IP(3))-receptor Ca(2+) channels. 2APB was without effect on the ability of thapsigargin to mobilize intracellular Ca(2+). This result suggests that the efflux of Ca(2+) from the endoplasmic reticulum mediated by thapsigargin is not via IP(3) Ca(2+) channels. However, 2APB was able to prevent the entry of Ca(2+) and Sr(2+) through thapsigargin-activated, store-operated Ca(2+) channels (SOCC). This result supports a direct inhibitory effect of 2APB on SOCC. 2APB was also able to block the entry of Sr(2+), Ba(2+), and Mn(2+) entry into unstimulated platelets, which suggests that 2APB was inhibiting the Ca(2+) influx channels directly. The capacity of 2APB to prevent Ca(2+) influx and Sr(2+) influx was rapid because it occurred immediately upon addition to the platelets. The inhibition of Ca(2+) and Sr(2+) influx by 2APB was similar to that seen with the cell-impermeable nonselective Ca(2+)-channel blocker La(3+) or the Ca(2+) chelator EGTA. Diphenylboronic anhydride and 2,2-diphenyltetrahydrofuran, two compounds that are structurally similar to 2APB, also inhibited Ca(2+) influx. It was concluded that 2APB was a rapid and effective direct inhibitor of SOCC in human platelets; as such, it cannot be used to support the involvement of IP(3) receptors in the activation of SOCC.