MRP transporters as membrane machinery in the bradykinin-inducible export of ATP

Adenosine triphosphate (ATP) plays the role of an autocrine/paracrine signal molecule in a variety of cells. So far, however, the membrane machinery in the export of intracellular ATP remains poorly understood. Activation of B2-receptor with bradykinin-induced massive release of ATP from cultured taenia coli smooth muscle cells. The evoked release of ATP was unaffected by gap junction hemichannel blockers, such as 18α-glycyrrhetinic acid and Gap 26. Furthermore, the cystic fibrosis transmembrane regulator (CFTR) coupled Cl − channel blockers, CFTR(inh)172, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, Gd3 + and glibenclamide, failed to suppress the export of ATP by bradykinin. On the other, the evoked release of ATP was greatly reduced by multidrug resistance protein (MRP) transporter inhibitors, MK-571, indomethacin, and benzbromarone. From western blotting analysis, blots of MRP 1 protein only, but not MRP 2 and MRP 3 protein, appeared at 190 kD. However, the MRP 1 protein expression was not enhanced after loading with 1 μM bradykinin for 5 min. Likewise, niflumic acid and fulfenamic acid, Ca2 + -activated Cl − channel blockers, largely abated the evoked release of ATP. The possibility that the MRP transporter system couples with Ca2 + -activated Cl − channel activities is discussed here. These findings suggest that MRP transporters, probably MRP 1, unlike CFTR-Cl − channels and gap junction hemichannels, may contribute as membrane machinery to the export of ATP induced by G-protein-coupled receptor stimulation.