[View Larger Version of this Image]

Figure S1. Example of a control experiment to compare the time course and amplitude of membrane current and Na⁺ efflux, under zero-trans conditions, during an ∼40-min-long washout (starting at the solution change artifact, a), and reapplication (at b), of 200 nM TTX. The current and ²²Na⁺ efflux are expressed in the same units (µA cm^-2) so that shifts in their magnitude can be compared directly. These changes agree well throughout the entire 40-min period shown. The stability and resolution of the method is good, given that peak Na⁺ channel gating current (Armstrong, C.M., and F. Bezanilla. 1973. Nature. 242:459-461) is much larger, on the order of 50 µA cm^-2. Experimental conditions: axon (550 µm diameter) internally dialyzed with 170 mM Na⁺ and 250 µM veratridine, superfused with Na⁺- and K⁺-free solution, and held at -20 mV using the end-pool voltage clamp technique described previously (Rakowski, R.F. 1989. Biophys. J. 55:663-671). To facilitate direct comparison of the current and flux records a D.C. offset was added to the current trace so that its value at the end of the current record (50 min) was equivalent to the measured flux.