Fig. 4.

A, comparison of the development of use-dependent block of Na⁺ channels by 250 μM internal QX-314. Currents are elicited by voltage steps from −100 to −10 mV and normalized to the first pulse in the train for the wild-type (●, ○), Q399C (■), and F1236C (▲). B, representative whole-cell Na⁺ currents in the presence of QX-314. Currents elicited by the first (a), 20th (b), and 60th (c) pulses for the wild-type, Q399C, and F1236C mutants are shown. The current amplitudes are reduced, but the time constant of current decay is not changed by QX-314. There is no significant difference in the time constant of decay between the wild type and any of the mutant channels except for L396C (see Table 2). C, bar plot of the extent of use-dependent block that develops at steady state with 250 μM QX-314 in the pipette at a stimulation frequency of 1 Hz. The height of the bars represents the fraction of current remaining at 1 min. The magnitude of use-dependent block increases with higher concentrations of QX-314. There is significantly less use-dependent block of the Q399C, L396C, and F1236C mutants (p < 0.05).