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

The perforated-patch technique was used to measure membrane currents in smooth muscle cells from sheep urethra. Depolarizing pulses evoked large transient outward currents and several components of sustained current. The transient current and a component of sustained current were blocked by iberiotoxin, penitrem A, and nifedipine but were unaffected by apamin or 4-aminopyridine, suggesting that they were mediated by large-conductance Ca2+-activated K+ (BK) channels. When the BK current was blocked by exposure to penitrem A (100 nM) and Ca2+-free bath solution, there remained a voltage-sensitive K+ current that was moderately sensitive to blockade with tetraethylammonium (TEA; half-maximal effective dose = 3.0 +/- 0.8 mM) but not 4-aminopyridine. Penitrem A (100 nM) increased the spike amplitude and plateau potential in slow waves evoked in single cells, whereas addition of TEA (10 mM) further increased the plateau potential and duration. In conclusion, both Ca2+ activated and voltage-dependent K+ currents were found in urethral myocytes. Both of these currents are capable of contributing to the slow wave in these cells, suggesting that they are likely to influence urethral tone under certain conditions.