Dual effects of ATP on K+ currents of mouse pancreatic beta-cells

K+ currents through ATP-dependent channels were recorded from inside-out patches of beta-cell membrane as previously described (Rorsman and Trube 1985). Channels were opened by removing ATP from the intracellular side of the membrane. The open probability and/or the number of active channels declined spontaneously ("run-down") when ATP was absent for periods longer than about 30 s. Channels subject to the run-down could be activated again after applying a blocking concentration (greater than 0.1 mM) of ATP in presence of 1 mM MgCl2 for at least 2 min. ATP in absence of Mg and the ATP-analogues AMP-PNP, AMP-PCP and ATP gamma S were ineffective in reactivating the channels. This suggests that phosphorylation of the channels or associated proteins or hydrolysis of ATP may be necessary for keeping the channels available. In contrast to the differential effects on the run-down, ATP in presence and absence of Mg and the ATP analogues were similarly effective in blocking the channels at concentrations above 0.1 mM. Using an experimental protocol avoiding the run-down the dose-inhibition curve for ATP was found to reach 50% at 18 microM.