Properties of single-channel and whole cell Cl - currents in guinea pig detrusor smooth muscle cells.

Multiple types of Cl ^- channels regulate smooth muscle excitability and contractility in vascular, gastrointestinal, and airway smooth muscle cells. However, little is known about Cl ^- channels in detrusor smooth muscle (DSM) cells. Here, we used inside-out single channel and whole cell patch-clamp recordings for detailed biophysical and pharmacological characterizations of Cl ^- channels in freshly isolated guinea pig DSM cells. The recorded single Cl ^- channels displayed unique gating with multiple subconductive states, a fully opened single-channel conductance of 164 pS, and a reversal potential of -41.5 mV, which is close to the E _Cl of -65 mV, confirming preferential permeability to Cl ^- . The Cl ^- channel demonstrated strong voltage dependence of activation (half-maximum of mean open probability, V _0.5 , ~-20 mV) and robust prolonged openings at depolarizing voltages. The channel displayed similar gating when exposed intracellularly to solutions containing Ca ²⁺ -free or 1 mM Ca ²⁺ . In whole cell patch-clamp recordings, macroscopic current demonstrated outward rectification, inhibitions by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid, and insensitivity to chlorotoxin. The outward current was reversibly reduced by 94% replacement of extracellular Cl ^- with I ^- , Br ^- , or methanesulfonate (MsO ^- ), resulting in anionic permeability sequence: Cl ^- >Br ^- >I ^- >MsO ^- . While intracellular Ca ²⁺ levels (0, 300 nM, and 1 mM) did not affect the amplitude of Cl ^- current and outward rectification, high Ca ²⁺ slowed voltage-step current activation at depolarizing voltages. In conclusion, our data reveal for the first time the presence of a Ca ²⁺ -independent DIDS and niflumic acid-sensitive, voltage-dependent Cl ^- channel in the plasma membrane of DSM cells. This channel may be a key regulator of DSM excitability.