Outward currents were characterized from cells resembling interstitial cells of Cajal (ICCs) isolated from the detrusor of the guinea pig bladder. ICC-like cells were studied using the whole cell patch clamp technique and K+ filled pipettes. Outward currents were evoked by stepping positively from a holding potential of -80 mV. ICC-like cells were distinguished from smooth muscle cells by the presence of lateral branches and an inability to contract spontaneously or when depolarized. Depolarization elicited large outward currents. Penitrem A, a blocker of large conductance, Ca activated K+ channels, significantly decreased the outward current. Its Ca dependence was demonstrated by significant inhibition with nifedipine and Ca-free solution. When large conductance, Ca activated K+ and Ca currents were blocked with penitrem A and nifedipine, a voltage dependent current was unmasked, which activated positive to -50 mV and displayed voltage dependent inactivation with half-maximal inactivation occurring at -71 mV. It was blocked in concentration dependent fashion by tetraethylammonium but unaffected by 4-aminopyridine, charybdotoxin or apamin, suggesting that small and intermediate conductance, calcium activated potassium channels, and Kv1.2 and Kv1.3 channels are unlikely to be involved. At maximal concentrations of tetraethylammonium a portion of the voltage dependent K+ current remained that was not affected by any of the blockers tested. ICC-like cells from the detrusor possess calcium activated and voltage dependent K+ currents.
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