Effects of lipophilic ions on outer hair cell membrane capacitance and motility

The outer hair cell (OHC) from the mammalian organ of Corti possesses a bell-shaped voltage-dependent capacitance function. The nonlinear capacitance reflects the activity of membrane bound voltage sensors associated with membrane motors that control OHC length. We have studied the effects of the lipophilic ions, tetraphenylborate (TPB-) and tetraphenylphosphonium (TPP+), on nonlinear capacitance and motility of isolated guinea-pig OHCs. Effects on supporting cells were also investigated. TPB- produced an increase in the peak capacitance (Cmpk) and shifted the voltage at peak capacitance (VpkCm) to hyperpolarized levels. Washout reversed the effects. Perfusion of 0.4 micrometer TPB- caused an average increase in Cmpk of 16.3 pF and VpkCm shift of 13.6 mV. TPP+, on the other hand, only shifted VpkCm in the positive direction, with no change in Cmpk. The contributions from native OHC and TPB--induced capacitance were dissected by a double Boltzmann fitting paradigm, and by blocking native OHC capacitance. While mechanical response studies indicate little effect of TPB- on the motility of OHCs which were in normal condition or treated with salicylate or gadolinium, the voltage at maximum mechanical gain (VdeltaLmax) was shifted in correspondence with native VpkCm, and both changed in a concentration-dependent manner. Both TPB--induced changes in Cmpk and VpkCm were affected by voltage prepulses and intracellular turgor pressure. TPB- induced a voltage-dependent capacitance in supporting cells whose characteristics were similar to those of the OHC, but no indication of mechanical responses was noted. Our results indicate that OHC mechanical responses are not simply related to quantity of nonspecific nonlinear charge moved within the membrane, but to the effects of motility voltage-sensor charge movement functionally coupled to a mechanical effector.