Barium block of the muscarinic potassium current in guinea-pig atrial cells

Abstract

Block of the muscarinic K⁺ current (i _K,ACh) by Ba²⁺ has been studied in guinea-pig atrial cells using the whole-cell patch-clamp technique. The dose-response curve for the block of i _K,ACh can be fitted assuming that a muscarinic K⁺ channel is blocked when a single Ba²⁺ ion binds to it (apparent dissociation constant, K _d=125 μM at 0 mV). Block was voltage and time dependent. The voltage dependence can be explained by Ba²⁺ binding to a site within the pore of the channel, 36% across the width of the membrane electric field (from the outside). Raising the bathing K⁺ concentration reduced Ba²⁺ block of i _K,ACh which suggests that Ba²⁺ and K⁺ compete for a common binding site. When Ba²⁺ was added during an exposure to ACh (muscarinic K⁺ channel open), block of i _K,ACh developed rapidly, but when Ba²⁺ was added prior to an exposure to ACh (muscarinic K⁺ channel closed), little block of i _K,ACh was evident when ACh was first applied. This suggests that when the muscarinic K⁺ channel is closed in the absence of ACh, Ba²⁺ does not have access to the binding site within the pore of the channel. In conclusion, Ba²⁺ block of i _K,ACh is concentration, voltage, time, K⁺ and state dependent.