Inhibition of slow-wave repolarization and Ca(2+)-activated K+ channels by quaternary ammonium ions.
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.(ABSTRACT TRUNCATED AT 250 WORDS)
Carl
A
A
Department of Physiology, University of Nevada School of Medicine, Reno 89557-0046.
Frey
B W
BW
Ward
S M
SM
Sanders
K M
KM
Kenyon
J L
JL
D000641:Ammonia / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000502:physiology / Q000648:ultrastructure; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology / Q000648:ultrastructure; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D000641:Ammonia; C027435:tetrapentylammonium; D002118:Calcium