Fig. 2. Na+ current inhibition by ephedrine (n = 5 cells/group; data are presented as mean ± SEM). The respective pulse protocol is inserted above the representative tracings. ( A ) Voltage-dependent block of Na+ channels by ephedrine. Conditioning prepulses ranging in amplitude from −160 to −15 mV were applied. Na+ currents were evoked by the delivery of the test pulse to +30 mV. Normalized Na+ current in the absence (control) or presence of 1,000 μM ephedrine was plotted against conditioning prepulse potential. Data were fitted well with a Boltzmann function. The average V0.5 value (50% availabilities) and K E (a slope factor) values for the fitted Boltzmann functions were −81.5 ± 0.2 and −73.8 ± 0.3 mV for control and ephedrine, respectively, and 8.3 ± 0.2 and 7.6 ± 0.2 mV, respectively. ( B ) Dose–response curves for hyperpolarized and depolarized sodium channels. The hyperpolarized state affinity for ephedrine on Na+ channels was measured with a prepulse of −150 mV for 10 s, and the depolarized state affinity was measured with prepulse of −60 mV for 10s. Pulses were delivered at 30-s intervals. Lines connecting data points represent fits to the data with the Hill equation. The similarities of blocking properties between HEK cells expressing Nav1.4 (skeletal muscle Na+ channel isoform) and GH3 cells expressing Nav1.1, 1.2, and 1.3 (neuronal Na+ channel isoforms) are illustrated by the dose–response curve of ephedrine at −150 mV ( dashed line ) as well as in the representative tracing. ( C ) Use-dependent block of Na+ channels by ephedrine. A test pulse of −30 mV was evoked for 24 ms at 5 Hz. Lines drawn through the data points are the best fit of a single exponential function.