Characterization of outward K(+) currents in isolated smooth muscle cells from sheep urethra

The perforated-patch techniquewas used to measure membrane currents in smooth muscle cells from sheepurethra. Depolarizing pulses evoked large transient outward currentsand several components of sustained current. The transient current anda component of sustained current were blocked by iberiotoxin, penitremA, and nifedipine but were unaffected by apamin or 4-aminopyridine,suggesting that they were mediated by large-conductanceCa²⁺-activated K⁺ (BK) channels. When the BKcurrent was blocked by exposure to penitrem A (100 nM) andCa²⁺-free bath solution, there remained a voltage-sensitiveK⁺ current that was moderately sensitive to blockade withtetraethylammonium (TEA; half-maximal effective dose = 3.0 ± 0.8 mM) but not 4-aminopyridine. Penitrem A (100 nM) increasedthe spike amplitude and plateau potential in slow waves evoked insingle cells, whereas addition of TEA (10 mM) further increased theplateau potential and duration. In conclusion, bothCa²⁺-activated and voltage-dependent K⁺currents were found in urethral myocytes. Both of these currents arecapable of contributing to the slow wave in these cells, suggesting that they are likely to influence urethral tone under certain conditions.