Cocaine-induced closures of single batrachotoxin-activated Na+ channels in planar lipid bilayers.
Batrachotoxin (BTX)-activated Na+ channels from rabbit skeletal muscle were incorporated into planar lipid bilayers. These channels appear to open most of the time at voltages greater than -60 mV. Local anesthetics, including QX-314, bupivacaine, and cocaine when applied internally, induce different durations of channel closures and can be characterized as "fast" (mean closed duration less than 10 ms at +50 mV), "intermediate" (approximately 80 ms), and "slow" (approximately 400 ms) blockers, respectively. The action of these local anesthetics on the Na+ channel is voltage dependent; larger depolarizations give rise to stronger binding interactions. Both the dose-response curve and the kinetics of the cocaine-induced closures indicate that there is a single class of cocaine-binding site. QX-314, though a quaternary-amine local anesthetic, apparently competes with the same binding site. External cocaine or bupivacaine application is almost as effective as internal application, whereas external QX-314 is ineffective. Interestingly, external Na+ ions reduce the cocaine binding affinity drastically, whereas internal Na+ ions have little effect. Both the cocaine association and dissociation rate constants are altered when external Na+ ion concentrations are raised. We conclude that (a) one cocaine molecule closes one BTX-activated Na+ channel in an all-or-none manner, (b) the binding affinity of cocaine is voltage sensitive, (c) this cocaine binding site can be reached by a hydrophilic pathway through internal surface and by a hydrophobic pathway through bilayer membrane, and (d) that this binding site interacts indirectly with the Na+ ions. A direct interaction between the receptor and Na+ ions seems minimal.
Wang
G K
GK
Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts 02115.
D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D001496:Batrachotoxins / Q000494:pharmacology*; D001665:Binding Sites; D002045:Bupivacaine / Q000494:pharmacology; D003042:Cocaine / Q000493:pharmacokinetics / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D008012:Lidocaine / Q000031:analogs & derivatives / Q000494:pharmacology; D008051:Lipid Bilayers; D009132:Muscles / Q000502:physiology*; D011817:Rabbits; D015222:Sodium Channels / Q000187:drug effects*
D000779:Anesthetics, Local; D001496:Batrachotoxins; D008051:Lipid Bilayers; D015222:Sodium Channels; C012647:QX-314; D008012:Lidocaine; D003042:Cocaine; D002045:Bupivacaine