1. The effects of externally applied ATP and neomycin on whole cell currents of isolated guinea pig cochlear outer hair cells (OHCs) were studied using the whole cell voltage-clamp technique. In OCHs held at -70 mV, ATP activated a large inward current. In the presence of neomycin, the ATP-induced whole cell current activated along a relatively unaltered time course, but the current then decreased to a reduced steady level. The neomycin inhibition of the ATP-induced current was dose dependent. The half-inhibitory concentration (IC50) of neomycin measured at steady state was estimated to be 90 microM. 2. Neomycin inhibition of the ATP response could not be reversed by increasing the concentration of ATP, indicating that the effect was noncompetitive. The inhibition was voltage dependent and was greatly reduced when OHCs were held at positive potentials. 3. Cells treated with 100 microM ATP gave maximal current responses. Addition of neomycin substantially increased membrane current noise of the 100 microM ATP responses. When neomycin concentration was varied from 10 to 500 microM, the current noise level peaked between 50 and 100 microM. The noise increase was observed at negative holding potentials but not at positive potentials. 4. The neomycin-induced whole cell current noise was used to estimate the size of the underlying elementary current. The ATP-induced single channel current of OHCs at -70 mV was estimated to be approximately 0.3 pA. The number of ATP-activated channels in a single OHC was estimated to be in the range of a few thousand. 5. The characteristics of the neomycin inhibition of ATP-induced currents were consistent with an open channel blocking mechanism. Analysis of the voltage dependence of the steady state neomycin inhibition suggested a neomycin binding site at an electrical distance of 0.3 from the extracellular side.
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