Figure 5.

Pharmacological dissection of the roles of Sh and Shab K⁺ channel subunits. A, In the bath containing the Shab channel blocker quinidine (100 μm), WT displayed a sudden jump in ejp amplitudes (b) during high-frequency nerve stimulation (underline, 5 Hz), but the basal release revealed by individual test stimuli (dots, 0.2 Hz) remained small (a), as in Shab mutants (Fig. 2). WT larvae treated with the Sh channel blocker 4-AP (1 mm) displayed large ejps (a) after single stimulation (dots, 0.2 Hz). Furthermore, the ejps remained similar in amplitude (b) during high-frequency stimulation (underline, 10 Hz), reminiscent of Sh ejps. B, Synergistic effects of Sh and Shab channel blockade by mutations and drugs. Bath application of 4-AP (1 mm) on Shab, quinidine (20 μm) on Sh, and 4-AP (200 μm) + quinidine (20 μm) on WT, produced large ejps with a prolonged duration in response to single nerve stimuli (dots). [Ca²⁺] = 0.1 mm. The number of muscles was 12–21 for each condition except for three in WT + 4-AP.