Charybdotoxin block of Ca2+-activated K+ channels in colonic muscle depends on membrane potential dynamics

Charybdotoxin(ChTX) is a specific blocker ofCa²⁺-activatedK⁺ channels. The voltage- andtime-dependent dynamics of ChTX block were investigated using caninecolonic myocytes and the whole cell patch-clamp technique with step andramp depolarization protocols. During prolonged step depolarizations,K⁺ current slowly increased in thecontinued presence of ChTX (100 nM). The rate of increase depended onmembrane potential with an e-foldchange for every 60 mV. During ramp depolarizations, the effectivenessof ChTX block depended significantly on the rate of the ramp (50% at0.01 V/s to 80% at 0.5 V/s). Results are consistent with a mechanismin which ChTX slowly "unbinds" in a voltage-dependent manner. Asimple kinetic model was developed in which ChTX binds to both open andclosed states. Slow unbinding is consistent with ChTX having littleeffect on electrical slow waves recorded from circular muscle whilecausing depolarization and contraction of longitudinal muscle, whichdisplays more rapid "spikes." Resting membrane potential andmembrane potential dynamics are important determinants of ChTX action.