ROLES OF OUTWARD POTASSIUM CURRENTS IN THE ACTION-POTENTIALS OF GUINEA-PIG URETERAL MYOCYTES

Outward currents of freshly dissociated ureteral myocytes consist main ly af Ca2+-activated K+ current (I-KCa) and a transient outward curren t (ITO) NO delayed rectifier current was apparent. I-KCa is small and nondecaying and fluctuates actively and irregularly. Blocking I-KCa de creased resting membrane conductance and prolonged action potential pl ateaus, showing its roles in maintaining the resting potential and in repolarizing action potentials. It is also responsible for the membran e potential fluctuations on action potential plateaus. Neither 8-(diet hylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride nor caffeine red uced the fluctuations in the outward current or in the action potentia ls, indicating that internal Ca2+ storage contributes little to the fl uctuations. ITO has fast activation and inactivation kinetics with ina ctivation time constants of similar to 15 and 150 ms, respectively. It s highly negative voltage-availability relationship (V-0.5 = -70.5 mV) suggests a low availability (<5%) at normal resting potentials. It ha s only trivial effects on action potentials.