IXN (0–30 µM) or verapamil (200 µM) were incubated with recombinant human ABCB1 membranes (25 µg) in the presence of 5 mM MgATP at 37 ◦C for 40 min. Then, the remaining ATP was detected according to the manufacture's protocol, using spectraMax M5 (Molecular Devices, ...