Roles of outward potassium currents in the action potentials of guinea pig ureteral myocytes

Outward currents of freshly dissociated ureteral myo-cytes consist mainly of Ca²⁺-activated K⁺ current (I_KCa) and a transient outward current (I_TO)- No delayed rectifier current was apparent. I_KCa is small and nondecaying and fluctuates actively and irregularly. Blocking I_KCa decreased resting membrane conductance and prolonged action potential plateaus, showing its roles in maintaining the resting potential and in repolarizing action potentials. It is also responsible for the membrane potential fluctuations on action potential plateaus. Neither 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride nor caffeine reduced the fluctuations in the outward current or in the action potentials, indicating that internal Ca²⁺ storage contributes little to the fluctuations. I_TO has fast activation and inactivation kinetics with inactivation time constants of ∼15 and 150 ms, respectively. Its highly negative voltage-availability relationship (V_0.5 = -70.5 mV) suggests a low availability (<5%) at normal resting potentials. It has only trivial effects on action potentials.