Interstitial cells (IC) isolated from the guinea-pig bladder possess several potassium currents including BK and voltage-dependent delayed rectifier currents(1). The aim of the present investigation was to investigate whether KCNQ channels could be contributing to the total outward potassium current in guinea-pig bladder interstitial cells. Such channels have been reported in other smooth muscle cells including portal vein(2). Bladders were removed from guinea-pigs, killed by cervical dislocation. Bladders were opened longitudinally, mucosa removed and underlying detrusor was chopped into mm3 pieces. Cells were isolated enzymatically as previously described 1 and studied using the whole-cell or amphotericin B perforated patch clamp technique. IC were depolarized from -60mV to +40mV to elicit outward currents. This current was reduced from 814±79pA to 709±71pA (n=23, p<0.001) by the KCNQ channel blocker, XE991 (10µM). In the presence of penitrem A (100nM) to block BK channels, XE991 significantly reduced the current at +40mV from 375±69pA to 259±56pA, n=4, p=0.014. To address the concern that penitrem A could be blocking KCNQ channels, experiments were carried out with EGTA (5mM) and ATP (3mM) in the pipette solution to remove BK and K-ATP currents; an approach used by Evans et al(3). Under these conditions, XE991 reduced the outward current from 542±45pA to 472±40pA (n=12 cells, p=0.001). Increasing the concentration of XE991 to 30µM further decreased the current to 419±39pA, n=12, p=0.007). Similar effects were found using another KCNQ blocker, linopirdine, although this drug was less effective than XE991 (690±149pA reduced to 549±144pA, n=4, p=0.065). The KCNQ channel opener, flupirtine, increased outward currents across the voltage range from -60mV to 0mV (n=7, p<0.05). This effect was not significant at more positive potentials. Cells were held at -20 mV and stepped to hyperpolarizing potentials from -90mV to -30mV to evoke deactivation tail currents. In 60mM external potassium solution to enhance current amplitude and 5mM 4AP to block delayed rectifier current, XE991 (10µM) significantly reduced currents when stepping to potentials ranging from -90mV to -50mV. At -90mV, the current of -143±22pA was reduced to -89±19pA, n=7, p=0.0015. These data suggest that KCNQ channels are responsible for a proportion of the total outward current in guinea-pig bladder interstitial cells. Functional experiments on strips of guinea-pig bladder in our laboratory (see presentation by Carson and McCloskey) show that these channels are involved in the regulation of spontaneous contractile activity in this tissue.