Characterization of outward currents in interstitial cells from the guinea pig bladder.

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.