Fig. 7. Block of mechanotransduction currents by extracellular FM1-43. A, Mechano-electrical transducer currents in a P5 outer hair cell elicited by sinusoidal fluid-jet stimulation at 45 Hz. Driver voltage (DV) to the jet (25 V amplitude) is shown above the currents. The membrane potential was stepped between −104 and +96 mV in 20 mV increments from a holding potential of −84 mV. For clarity, only responses to every other voltage step are shown. All records are single traces and are offset so that the zero-transducer current levels (responses to inhibitory stimuli) are equally spaced.B, Superfusion with 6 μm FM1-43 rapidly reduced the transducer currents. Note that more current is shut off in response to the inhibitory phase of the sinusoid at −104 mV, pointing to an increase in the resting transducer current caused by the dye at this potential. C, Current–voltage curves for the peak-to-peak transducer currents recorded for the cell inA and B. The fits through the data are according to a simple single-energy-barrier model:I(V) =k [exp ((1 −γ)(V −Vr)/Vs) − exp (−γ(V -Vr)/Vs)], where k is a proportionality constant,Vr is the reversal potential,Vs is a measure for the steepness of the rectification, and γ is the fractional distance within the membrane's electrical field of an energy barrier, as measured from the outside. (●) k = 112 pA,Vr = 8.6 mV,Vs = 27 mV, and γ = 0.52; (○) k = 4 pA,Vr = −7.0 mV,Vs = 13 mV, and γ = 0.54. Cm, 5.8 pF;Rs, 4.2 MΩ; 24°C.D, Dose–response curves for block of transducer currents by FM1-43 at three different membrane potentials (top panel). The data were fitted with a logistic curve:I/IC = 1/(1 + ([D]/Kd)nH), where I is the current in the presence of the dye,IC is the control current,Kd = 2.4 ± 0.3 μm (○); 1.2 ± 0.1 μm(●); 3.0 ± 0.3 μm (▴).nH (Hill coefficient) = 1.2 ± 0.2 μm (○); 2.2 ± 0.3 μm(●); 1.2 ± 0.2 μm (▴). Bottom panel, Kd andnH both vary significantly as a function of voltage (p < 0.0001, ANOVA). E, Voltage dependence of the block of FM1-43 in the presence of 1.3 mm extracellular calcium. The ordinate represents fractional block, from 0 (no block) to 1 (complete block). ForD, E: 0.3, 1, and 2 μm, n = 3; 3 μm, n = 8; 6 μm, n = 4; 10 μm,n = 2; 20 μm, n = 1. F, Calcium sensitivity of block of the transducer currents by 3 μm FM1-43. For 0.1 mmCa2+, n = 3; 1.3 mm, n = 8; 5 mm,n = 5; 10 mm,n = 2.