Voltage-gated potassium currents in rat vas deferens smooth muscle cells

Voltage-gated components of the outward current in single smooth muscle cells isolated from the epididymal part of the rat vas deferens were studied using amphotericin B perforated patch-clamp techniques. The complex kinetics of the net outward current elicited by positive voltage steps from −80 mV to +40 mV suggested the presence of several components. Bath application of 200 nM charybdotoxin, a potent blocker of large-conductance, Ca²⁺-dependent K⁺ channels (BK_Ca), reduced the current amplitude significantly. When BK_Ca channels were suppressed, fast-inactivating (I _K,f) and delayed rectifying (I _K,dr) components of the outward current were identified. I _K,f was characterized by fast kinetics of current decay, negative steady-state activation and inactivation dependencies and sensitivity to 4-aminopyridine with an apparent K _d of 0.32 mM, properties similar to those of the A-type K⁺ current. In contrast, I _K,dr activated and inactivated at more positive potentials. The time constant of activation of I _K,dr was voltage dependent with an e-fold decrease per 21 mV depolarization. I _K,dr was inhibited by clofilium, a blocker of voltage-gated K⁺ channels, with an IC₅₀ of 12 µM and was not blocked by 5 mM 4-aminopyridine. The possible significance of the voltage-gated currents is discussed.