Current-dependent attenuation. (A) Di-8-ANEPPS transients recorded in response to a −120-mV (100-ms) pulse before (black noisy trace) and after blocking I_Cl with 9-ACA (red noisy trace). The solid lines are double exponential fits to the corresponding experimental records. The fiber was equilibrated with 70 mM [Cl]_i. (B) The black and red lines are the double exponential fits to the di-8-ANEPPS transients normalized to the maximal steady-state value and displayed at an expanded time scale. The current-dependent attenuation of the optical signal (green trace) was calculated by subtraction of these two traces and expressed as percentage change (see text). (C) Superimposed time course of the attenuation trace (green trace) presented inverted to allow for kinetic comparison with the 9-ACA–sensitive I_Cl (blue trace).