Variations in the ensemble of potassium currents underlying resonance in turtle hair cells

1. Potassium currents were characterized in turtle cochlear hair cells by whole-cell voltage clamp during superfusion with the potassium channel antagonists, tetraethylammonium (TEA) and 4-aminopyridine (4-AP). The estimated resonant frequency f(o), was inferred from tau, the time constant of deactivation of outward current upon repolarization to -50 mV, according to the empirical relation, f(o) = k(1) tau(-1/2) + k(2). 2. Dose-response relations for TEA and 4-AP were obtained by exposing single cells to ten concentrations exponentially distributed over four orders of magnitude. Potassium current in cells tuned to low frequencies was carried by a single class of channels with an apparent affinity constant, K-i, for TEA of 35.9 mM. Half-blocking concentrations of 4-AP were correlated with the time constant of deactivation and varied between 26.2 and 102 mu M. In cells tuned to higher frequencies, K+ current was carried by a single class of channels with high affinity for TEA (K-i = 0.215 mM) and low affinity for 4-AP (K-i = 12.3 mM). This pharmacological profile suggests that K+ current in low frequency cells is purely voltage gated and in high frequency cells, it is gated by both Ca2+ and voltage. 3. For each current type, the voltage dependence of activation was determined from tail current amplitude at -50 mV. The purely voltage-gated current, I-K(V), was found to increase e-fold in 4.0 +/- 0.3 mV (n = 3) in low frequency cells exposed to TEA (25 mM). The Ca2+- and voltage-gated current, I-K(Ca), was more steeply voltage dependent, increasing e-fold in 1.9 mV (n = 2) in high frequency cells exposed to 4-AP (0.8 mM). 4. I-K(V) was found to inactivate slowly during prolonged voltage steps (similar to 10 s). Steady-state inactivation increased with depolarization from -70 mV and was incomplete such that on average I-K(V) did not fall below similar to 0.39 of its maximum value. 5. Superfusion of 4-AP (0.8 mM) reversibly depolarized a low frequency cell and eliminated steady voltage oscillations, while TEA (6 mM) had no effect. In a high frequency cell, voltage oscillations were abolished by TEA, but not by 4-AP. 6. The differential pharmacology of I-K(V) and I-K(Ca) was used to measure their contribution to K+ current in cells tuned to different frequencies. Both currents exhibited a frequency-dependent increase in maximum conductance. I-K(V) accounted for nearly all K+ current in cells tuned to less than 60 Hz, while I-K(Ca) was the dominant current in higher frequency cells. 7. Mapping resonant frequency onto epithelial position suggests an exponential relation between K+ current size and position. I-K(V) appeared to be limited to the apical or low frequency portion of the basilar papilla and coincided with maximal expression of a K+- selective inward rectifier, I-K(IR). This finding is consistent with the notion that low frequency resonance is produced by interaction of I-K(V) and I-K(IR) with the voltage-gated Ca2+ current, I-Ca, and the cell's capacitance. The ionic events underlying higher frequency resonance are dominated by the action of I-K(Ca) and I-Ca and include a contribution from I-K(IR).