The mammalian urinary bladder detrusor smooth muscle exhibits spontaneous activity in vivo and in vitro. Generation of spontaneous action potentials and subsequent contractions is dependent on Ca²⁺ influx through voltage-gated calcium channels. This is determined by the smooth muscle cell resting membrane potential (Em). Recently, two-pore domain K⁺ channels have been implicated in the setting of Em in vascular and gastrointestinal smooth muscle cells (Gardener et al., 2004; Cho et al., 2005). However, their role in detrusor smooth muscle remains unknown. Our aim was to investigate the expression and function of two-pore domain K⁺ channels in the rat detrusor. RT-PCR analysis of rat detrusor homogenates detected mRNAs for TWIK-1, TWIK-2, TREK-1, TASK-1 and THIK-1. TREK–2, TRAAK, TASK-2, and TASK-3 mRNAs were not detected. Subsequent experiments focused on acid-sensitive TASK-1 channels. Using an anti-TASK-1 antibody (Alomone Labs, Israel), Western blot analysis confirmed the expression of TASK-1 protein in rat detrusor homogenates. Immunohistochemistry showed that TASK-1 protein was expressed in both the detrusor smooth muscle and urothelium of the rat bladder. HEK 293 cells expressing TASK-1 and TASK-2 channels were used as controls for both techniques. The role of TASK-1 channels in the modulation of spontaneous activity was investigated. Male Wistar rats were killed by overdose of CO₂ followed by cervical dislocation. For membrane potential recordings, strips of bladder were superfused with HEPES-buffered Tyrode’s solution at 37°C and myocytes were impaled with a sharp microelectrode (50-80MΩ) filled with 3M KCl. Isometric tension recordings were performed on bladder strips that were incubated in HEPES-buffered Tyrode’s solution bubbled with 100% O₂ at 37°C. Strips were set at a tension of 10mN and allowed to equilibrate for 1 hour at which point the strips displayed spontaneous contractions. Changing bath solution pH from 7.4 to 8.4 hyperpolarised Em of the myocytes (-53.8 ± 1.1mV to –59.5 ± 0.9mV, P<0.01) with a change to 6.4 producing a depolarisation (–59.5 ± 0.9mV to –41.1 ± 0.9mV, P<0.001). pH 6.4 also produced an increase in the integrated force (AUC) and baseline tone of the spontaneous contractions (P<0.05). Similarly, the TASK-1 inhibitor anandamide (10μM; Maingret et al., 2001) produced a depolarisation of Em (-53.8 ± 0.4mV to –44.6 ± 1.0mV, P < 0.01) and an increase in the AUC and baseline tone of spontaneous contractions (P<0.05). These results confirm the expression of two-pore domain K⁺ channel subunits in the rat detrusor and implicate TASK-1 channels in the setting of Em and subsequent regulation of spontaneous electrical and contractile activity in the rat detrusor.