Role of cyclic ADP-Ribose in ATP-activated potassium currents in alveolar macrophages

There is growing evidence that extracellular ATP causes a dramatic change in the membrane conductance of a variety of inflammatory cells. In the present study, using the nystatin perforated patch recording configuration, we found that ATP (0.3-30 μM) induced a transient outward current in a concentration-dependent manner and that the reversal potential of the ATP- induced outward current was close to the K⁺ equilibrium potential, indicating that the membrane behaves like a K⁺ electrode in the presence of ATP. The first application of ATP to alveolar macrophages perfused with Ca²⁺-free external solution could induce the outward current, but the response to ATP was diminished with successive applications. Intracellular perfusion with a Ca²⁺ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid, also diminished the response. When cyclic ADP-ribose (cADPR) was applied to the macrophage cytoplasm, a transient outward current was elicited. Thereafter, the successive outward current was inhibited, suggesting the involvement of cADPR in the response. Intracellular perfusion with inositol 1,4,5-trisphosphate also induced a transient outward current, but the successive current was not inhibited. The ATP-induced outward current was abolished when 8-amino-cADPR (as a blocker of cADPR, 10^-6-10^-5 M) was introduced into the cytoplasm. Homogenates of alveolar macrophages showed both ADP-ribosyl cyclase and cADPR hydrolase activities, and CD38 (ADP- ribosyl cyclase/cADPR hydrolase) expression was confirmed by reverse transcriptase-polymerase chain reaction and Western blot analyses. These results indicate that ATP activates K⁺ currents by releasing Ca²⁺ from cADPR-sensitive internal Ca²⁺ stores.