ATP-sensitive K$^{+}$-channel run-down is Mg$^{2+}$ dependent

ATP-sensitive K$^{+}$-channel currents were recorded from isolated membrane patches and voltage-clamped CRI-G1 insulin-secreting cells. Internal Mg$^{2+}$ ions inhibited ATP-K$^{+}$ channels by a voltage-dependent block of the channel current and decrease of open-state probability. The run-down of ATP-K$^{+}$ channel activity was also shown to be [Mg$^{2+}$]$_{\text{i}}$ dependent, being almost abolished in Mg$^{2+}$-free conditions. Substitution of Mn$^{2+}$ for Mg$^{2+}$ did not prevent run-down, nor did the presence of phosphate-donating nucleotides, a protease or phosphatase inhibitor or replacement of Cl$^{-}$ by gluconate.