Inhibition of slow-wave repolarization and Ca(2+)-activated K+ channels by quaternary ammonium ions

• Published in: The American journal of physiology Vol. 264; no. 3 Pt 1; pp. C625 - C631
• Main Authors: Carl, A, Frey, B W, Ward, S M, Sanders, K M, Kenyon, J L
• Format: Journal Article
• Language: English
• Published: United States 01-03-1993
• Subjects: Ammonia - pharmacology; Animals; Calcium - pharmacology; Cell Membrane - physiology; Cell Membrane - ultrastructure; Dogs; Dose-Response Relationship, Drug; In Vitro Techniques; Membrane Potentials - physiology; Muscle, Smooth - physiology; Muscle, Smooth - ultrastructure; Potassium Channels - drug effects; Potassium Channels - physiology; Quaternary Ammonium Compounds - pharmacology; Tetraethylammonium; Tetraethylammonium Compounds - pharmacology
• ISSN: 0002-9513
• DOI: 10.1152/ajpcell.1993.264.3.c625

We studied the effects of the K+ channel blocker tetrapentylammonium (TPeA) on the electrical activity of intact circular smooth muscle from canine colon. TPeA (10 and 20 microM) increased slow-wave duration and "locked" the membrane potential around -30 mV plateau potential after several minutes of application, suggesting that K+ channels are essential for termination of colonic slow waves. Repolarization and normal slow-wave activity resumed after 20-30 min of washout. The patch-clamp technique was used to study the block of large-conductance Ca(2+)-activated K+ channels (BK channels) by TPeA and tetraethylammonium (TEA) in excised and cell-attached patches from isolated colonic smooth muscle cells. Channel block was characterized by a voltage-dependent dissociation constant [Kd(V)] for the binding of TEA and TPeA to a blocking site located a fraction of the distance across the membrane field (delta). The extracellular TEA binding site had a Kd(0) of 0.33 mM and a delta of 0.23. The extracellular TPeA binding site had a Kd(0) of 2.2 mM but showed significantly less voltage dependence (delta = 0.02). The intracellular binding site for TEA was of low affinity [Kd(0) = 76 mM]. Intracellular TPeA was the most potent blocker of BK channel current [Kd(0) = 11.7 microM]. The voltage dependence of block by intracellular TPeA (delta = -0.21) was not significantly different from that of intracellular TEA (delta = -0.3). Internal TPeA (10 microM) also blocked a 70-pS K+ channel and a 23-pS K+ channel.