Differential effects of extracellular ATP on chloride transport in cortical collecting duct cells.

Extracellular ATP in the cortical collecting duct can inhibit epithelial sodium channels (ENaC) but also stimulate calcium-activated chloride channels (CACC). The relationship between ATP-mediated regulation of ENaC and CACC activity in cortical collecting duct cells has not been clearly defined. We used the mpkCCD(c14) cortical collecting duct cell line to determine effects of ATP on sodium (Na(+)) and chloride (Cl(-)) transport with an Ussing chamber system. ATP, at a concentration of 10(-6) M or less, did not inhibit ENaC-mediated short-circuit current (I(sc)) but instead stimulated a transient increase in I(sc). The macroscopic current-voltage relationship for ATP-inducible current demonstrated that the direction of this ATP response changes from positive to negative when transepithelial voltage (V(te)) is clamped to less than -10 mV. We hypothesized that this negative V(te) might be found under conditions of aldosterone stimulation. We next stimulated mpkCCD(c14) cells with aldosterone (10(-6) M) and then clamped the V(te) to -50 mV, the V(te) of aldosterone-stimulated cells under open-circuit conditions. ATP (10(-6) M) induced a transient increase in negative clamp current, which could be inhibited by flufenamic acid (CACC inhibitor) and BAPTA-AM (calcium chelator), suggesting that ATP stimulates Cl(-) absorption through CACC. Together, our findings suggest that the status of ENaC activity, by controlling V(te), may dictate the direction of ATP-stimulated Cl(-) transport. This interplay between aldosterone and purinergic signaling pathways may be relevant for regulating NaCl transport in cortical collecting duct cells under different states of extracellular fluid volume.