Block of voltage-dependent K+ channels in insect muscle by the diacylhydrazine insecticide RH-5849, 4-aminopyridine,and quinidine

In calcium-free saline, voltage-clamped ventral longitudinal muscles of housefly larvae have maintained (I_K) and transient (I_A) voltage-dependent K⁺ currents. With 500 ms conditioning pulses, inactivation of I_A had a midpoint at ?53 mV and changed e-fold in 3.46 mV. I_A inactivated completely at ?40 mV, with a time constant of 71 ms, allowing the effects of various K⁺ channel blockers to be studied on I_K in isolation. RH-5849 (1,2-dibenzoyl-1-tert-butylhydrazine), a novel insect growth regulator, induces a lethal premature molt in insect larvae by mimicking the action of the molting hormone at ecdysone receptors. RH-5849 also causes acute neurotoxicity in some insects by selectively blocking of I_K in nerve and muscle. While most channel blockers have a Hill coefficient near 1, consistent with a simple one molecule per channel block mechanism, RH-5849 and the analog RH-1266 were found in the present study to block I_K channels in insect muscle with a Hill coefficient of 1.5. The lC₅₀ (concentration that caused 50% block) for block of I_K was 59 μM for RH-5849 and 40 μM for RH-1266. While tetraethylammonium blocked I_K by only 20% at 100 mM, 4-aminopyridine blocked the current with an lC₅₀ of 1.2 mM and a Hill coefficient of 0.97. Quinidine was the most potent blocker of I_K in this study, with an lC₅₀ of 20 μM. Block of I_K by either RH-5849 or 4-aminopyridine was independent of test pulse potential, but block by quinidine increased with depolarization. Block of I_K by RH-5849 and quinidine was time dependent, suggesting an open channel block mechanism, but the time course was too fast relative to channel activation for kinetic analysis. The lC₅₀ for block of I_K by RH-5849 decreased with temperature, with a Q₁₀ of 0.52. I_A was also blocked by RH-5849, but was less sensitive than I_K. The lC₅₀ for block of I_A by RH-5849 was 775 μM, 13-fold higher than the lC₅₀ for block of I_K. © 1992 Wiley-Liss, Inc.