To probe the sensitivity of GIRK2 channel activation by brain PIP2, we examined the effect of neomycin, which is a competitive inhibitor of PIP2 40, on the K+ flux with GIRK2 channels. Increasing concentrations of neomycin applied acutely to GIRK2-containing liposomes with brain PIP2 attenuated the rate of quenching (Fig. 2c). The IC50 for neomycin inhibition is 11.1 ± 2.4 μM (n = 4), with a Hill coefficient of 1.3 ± 0.3 (Fig. 2d). Taken together, these experiments demonstrate that both brain PIP2 and soluble diC8-PIP2 activate GIRK2 channels in a dose-dependent manner, in the absence of G proteins or other ligands, indicating that GIRK2 channels are directly gated by PIP2 in the presence of Na+.
