Coupling of voltage-dependent gating and Ba++ block in the high-conductance, Ca++-activated K+ channel.
Voltage-dependent Ca++-activated K+ channels from rat skeletal muscle were reconstituted into planar lipid bilayers, and the kinetics of block of single channels by Ba++ were studied. The Ba++ association rate varies linearly with the probability of the channel being open, while the dissociation rate follows a rectangular hyperbolic relationship with open-state probability. Ba ions can be occluded within the channel by closing the channel with a strongly hyperpolarizing voltage applied during a Ba++-blocked interval. Occluded Ba ions cannot dissociate from the blocking site until after the channel opens. The ability of the closed channel to occlude Ba++ is used as an assay to study the channel's gating equilibrium in the blocked state. The blocked channel opens and closes in a voltage-dependent process similar to that of the unblocked channel. The presence of a Ba ion destabilizes the closed state of the blocked channel, however, by 1.5 kcal/mol. The results confirm that Ba ions block this channel by binding in the K+-conduction pathway. They further show that the blocking site is inaccessible to Ba++ from both the cytoplasmic and external solutions when the channel is closed.
D000818:Animals; D001464:Barium / Q000378:metabolism / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology*; D004594:Electrophysiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D051381:Rats
D007473:Ion Channels; D001464:Barium; D011188:Potassium; D002118:Calcium