Tonic mechanosensitivity of outer hair cells after loss of tip links

Tip links - the extracellular connectors between the distal ends of adjacent stereocilia - are essential for the fast mechanical gating of hair-cell transducer channels. Transduction in the absence of tip links was investigated for outer hair cells of the adult guinea-pig cochlea by patch-clamp recordings of the whole-cell current during mechanical stimulation of the hair bundle. Loss of tip links induced by application of BAPTA led to permanently opened transducer channels, as evidenced by a constant inward current, loss of response to sinusoidal mechanical deflection of the hair bundle and block by the open-channel blocker dihydrostreptomycin (100 μM). Step deflection of the hair bundle (200-500 nm) in the inhibitory direction exponentially reduced this current to a constant value with time constant, τon, of the order of seconds. The current returned exponentially to the pre-stimulus level with time-constant, τoff, also of the order of seconds. τon was dependent on the inter-stimulus interval, Δt, such that reducing this interval below about 40 s resulted in an exponentially faster response. τoff was independent of Δt. Application of the calcium ionophore, ionomycin (10 μM), showed that τon became independent of Δt after saturating elevation of the intracellular Ca2+ concentration. Flash-photolytic release of intracellular caged calcium (25-μM NP-EGTA/AM) showed that τon is dependent on intracellular Ca2+ concentration. These experiments imply an intracellular, calcium-dependent gating mechanism for hair-cell transducer channels.