Abstract

The large-conductance calcium-activated potassium channel (BK_Ca channel) plays critical roles in smooth muscle relaxation. In urinary bladder smooth muscle, BK_Ca channel activity underlies the maintenance of the resting membrane potential and repolarization of the spontaneous action potential triggering the phasic contraction. To identify novel BK_Ca channel activators, we screened a library of natural compounds using a cell-based fluorescence assay and a hyperactive mutant BK_Ca channel (Lee et al., 2013). From 794 natural compounds, kurarinone, a flavanone from Sophora flavescens, strongly potentiated BK_Ca channels. When treated from the extracellular side, this compound progressively shifted the conductance-voltage relationship of BK_Ca channels to more negative voltages and increased the maximum conductance in a dose-dependent manner. Whereas kurarinone strongly potentiated the homomeric BK_Ca channel composed of only the α subunit, its effects were much smaller on heteromeric channels coassembled with auxiliary β subunits. Although the activation kinetics was not altered significantly, the deactivation of BK_Ca channels was dramatically slowed by kurarinone treatment. At the single-channel level, kurarinone increased the open probability of the BK_Ca channel without affecting its single-channel conductance. Kurarinone potently relaxed acetylcholine-induced contraction of rat bladder smooth muscle and thus decreased the micturition frequency of rats with overactive bladder symptoms. These results indicate that kurarinone can directly potentiate BK_Ca channels and demonstrate the therapeutic potentials of kurarinone and its derivatives for developing antioveractive bladder medications and supplements.