CHARYBDOTOXIN BLOCK OF CA2-ACTIVATED K+ CHANNELS IN COLONIC MUSCLE DEPENDS ON MEMBRANE-POTENTIAL DYNAMICS()

Charybdotoxin (ChTX) is a specific blocker of Ca2+-activated K+ channe ls. The voltage-and time-dependent dynamics of ChTX black were investi gated using canine colonic myocytes and the whole cell patch-clamp tec hnique with step and ramp depolarization protocols. During prolonged s tep depolarizations, K+ current slowly increased in the continued pres ence of ChTX (100 nM). The rate of increase depended on membrane poten tial with an e-fold change for every 60 mV. During ramp depolarization s, the effectiveness of ChTX block depended significantly on the rate of the ramp (50% at 0.01 V/s to 80% at 0.5 V/s). Results are consisten t with a mechanism in which ChTX slowly ''unbinds'' in a voltage-depen dent manner. A simple kinetic model was developed in which ChTX binds to bath open and closed states. Slow unbinding is consistent with ChTX having little effect on electrical slow waves recorded from circular muscle while causing depolarization and contraction of longitudinal mu scle, which displays more rapid ''spikes.'' Resting membrane potential and membrane potential dynamics are important determinants of ChTX ac tion.