Variation of membrane properties in hair cells isolated from the turtle cochlea.

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Abstract
Hair cells were enzymatically isolated from identified regions of the turtle basilar papilla and studied with the patch-electrode technique. The experimental aim was to relate the resonance properties seen during current injection to the membrane currents measured in the same cell under whole-cell voltage clamp. Solitary hair cells had resting potentials of about -50 mV, and produced a damped oscillation in membrane potential at the onset and termination of a small current step; the resonant frequency varied from 9 to 350 Hz between cells, and was correlated with the region of papilla from which a cell had been isolated. The inferred frequency map was consistent with the tonotopic arrangement described previously in the intact papilla. Depolarizations from the testing potential under voltage clamp activated a large net outward current with a steep voltage dependence, and the steady-state current-voltage relationship was strongly rectified about the resting potential. Input resistances tended to be smaller in cells with higher resonant frequencies, although there was not concurrent variation in membrane area as inferred from the cell capacitance. The outward current abolished by extracellular application of 25 mM-tetraethylammonium chloride (TEA), or on exchange of Cs+ for K+ in the intracellular medium filling the recording electrode, each experiment supporting the contention that K+ is the major current carrier. Such treatments also removed the oscillations in membrane potential evoked by imposed current steps. Addition of TEA or intracellular perfusion with Cs+ also revealed a fast inward current with an ionic sensitivity consistent with its being carried by Ca2+. Like the K+ current, the Ca2+ current was activated by small depolarizations from the resting potential, and over this voltage range it was about five to ten times smaller than the K+ current. Its activation was more rapid than the fastest outward currents in high-frequency cells. The inward current could also be carried by Ba2+, which when substituted for external Ca2+ blocked the K+ current. Measurements on cells with resonant frequencies of 13-240 Hz indicated that the peak Ba2+ current increased systematically with resonant frequency. Manipulations such as external addition of Cd2+ which would be expected to reduce or abolish the Ca2+ current also blocked the K+ current, consistent with a previous suggestion (Lewis and Hudspeth, 1983b) that the hair-cell K+ conductance is gated by changes in intracellular Ca2+. The results support the following conclusions: (i) the resonance behaviour and tuning of turtle cochlear hair cells are governed by the interplay of membrane Ca2+ and K+ conductances; (ii) the resonant frequency is determined by the characteristics of the K+ conductance, an increase in frequency being achieved largely by faster kinetics, but also to some extent by an increase in the size of this conductance; (iii) the magnitude of the Ca2+ conductance increases with resonant frequency and this may be needed to enhance the sharpness of tuning; (iv) the membrane properties are graded monotonically with distance along the basilar papilla.