Using the perforated patch voltage clamp, we investigated swelling -activated ionic channels (SACs) in rat urinary bladder smooth muscle cells. Hypo -osmotic (60%) bath solution increased a membrane current which was inhibited by the SAC inhibitor, gadolinium. The reversal potential of the hypotonicity -induced current shifted in the positive direction by increasing external K + concentration. The hypotonicity -induced current was inhibited by extracellular acidic pH, phorbol ester and forskolin. These pharmacological properties are identical to those of arachidonic acid -induced current present in these cells, suggesting the presence of TREK -1, a four -transmembrane two pore domain K + channel. Using RT -PCR we screened rat bladder smooth muscles and cerebellum for expression of TREK -1, TREK -2 and TRAAK mRNAs. Only TREK -1 mRNA was expressed in the bladder, while all three were expressed in the cerebellum. We conclude that a mechanosensitive K + channel is present in rat bladder myocytes, which is activated by arachidonic acid and most likely is TREK -1. This K + channel may have an important role in the regulation of bladder smooth muscle tone during urine storage.