STEREOSELECTIVITY OF CHANNEL INHIBITION BY SECONDARY ALKANOL ENANTIOMERS AT NICOTINIC ACETYLCHOLINE-RECEPTORS

Background: At the nicotinic acetylcholine receptor, long chain alkano ls reduce, whereas short chain alkanols augment endplate currents. Usi ng the enantiomers of five members of a homologous series of secondary alkanols (2-butanol through 2-octanol), we tested the hypothesis that these actions occur at a single hydrophobic site in the lumen of the channel. Small alkanols would bind to this site without blocking the c hannel, stabilizing the open state and enhancing the apparent affinity of the agonist for channel opening. Long chain alkanols would bind th e same site and simply inhibit without affecting the agonist's apparen t affinity. Methods: Agonist-stimulated Rb-86+ efflux from acetylcholi ne receptor-rich vesicles from Torpedo nobiliana was studied by adding agonist and allowing efflux to proceed for 10 s before termination by filtration. Results: All of the 2-alkanols inhibited Rb-86+ efflux el icited by a maximally stimulating concentration of agonist. Inhibitory potency increased logarithmically with the number of carbon atoms in the hydrocarbon chain of the alkanol. The inhibitory potency of the en antiomers of 2-butanol differed twofold, but the other enantiomers exh ibited no stereoselectivity. The enantiomers of 2-octanol caused a con centration-dependent depression of carbamylcholine-stimulated Rb-86+ e fflux without significantly altering the agonist's apparent dissociati on constant. In contrast, the enantiomers of 2-butanol caused: (1) a n onstereoselective decrease in carbachol's apparent dissociation consta nt and (2) the expected stereoselective decrease in maximal carbamylch oline-stimulated Rb-86+ efflux. Conclusions: The alkanol site that mod ulates the apparent agonist affinity for channel opening is distinct f rom the site that results in inhibition of cation flux through the cha nnel.