Two types of K⁺ currents in marginal cells cultured from rat stria vascularis

Membrane currents in marginal cells cultured from rat stria vascularis were studied using the whole-cell patch-clamp technique. Two types of voltage-dependent whole-cell currents were observed in the voltage range from -150 mV to +50 mV: an outwardly rectifying current and an inwardly rectifying current. The outwardly rectifying current, which was activated by depolarizing pulses more positive than -30 mV, was sensitive to TEA (20 mM) and relatively not to Ba²⁺ (0.5 mM). Tail current analysis revealed that the outward currents were primarily K⁺-selective. The conductance of the current was half-maximal at 0.5 mV and a substantial portion of current was not inactivated by the depolarizing prepulses from -30 mV to +20 mV. The inwardly rectifying current with rapid exponential activation was observed with hyperpolarizing voltage pulses. The zero-current potential of this current was dependent on extracellular K⁺ concentration. In contrast to the outwardly rectifying current, this current was blocked by extracellular application of Ba²⁺, not by TEA. The conductance of this current increased with the increase in external K⁺ concentration. Our data suggest that marginal cells cultured from rat stria vascularis express at least two types of voltage-dependent K⁺ currents which may serve as K⁺ secretory pathways into endolymph.