The Natural Flavone Acacetin Blocks Small Conductance Ca(2+)-Activated K(+) Channels Stably Expressed in HEK 293 Cells

The natural flavone acacetin inhibits several voltage-gated potassium currents in atrial myocytes, and has anti-atrial fibrillation (AF) effect in experimental AF models. The present study investigates whether acacetin inhibits the Ca(2+)-activated potassium (K(Ca)) currents, including small conductance (SK(Ca)1, SK(Ca)2, and SK(Ca)3), intermediate conductance (IK(Ca)), and large-conductance (BK(Ca)) channels stably expressed in HEK 293 cells. The effects of acacetin on these K(Ca) channels were determined with a whole-cell patch voltage-clamp technique. The results showed that acacetin inhibited the three subtype SK(Ca) channel currents in concentration-dependent manner with IC(50) of 12.4 μM for SK(Ca)1, 10.8 μM for SK(Ca)2, and 11.6 μM for SK(Ca)3. Site-directed mutagenesis of SK(Ca)3 channels generated the mutants H490N, S512T, H521N, and A537V. Acacetin inhibited the mutants with IC(50) of 118.5 μM for H490N, 275.2 μM for S512T, 15.3 μM for H521N, and 10.6 μM for A537V, suggesting that acacetin interacts with the P-loop helix of SK(Ca)3 channel. However, acacetin at 3–10 μM did not decrease, but induced a slight increase of BK(Ca) (+70 mV) by 8% at 30 μM. These results demonstrate the novel information that acacetin remarkably inhibits SK(Ca) channels, but not IK(Ca) or BK(Ca) channels, which suggests that blockade of SK(Ca) by acacetin likely contributes to its anti-AF property previously observed in experimental AF.