Stereoselectivity of channel inhibition by secondary alkanol enantiomers at nicotinic acetylcholine receptors.

BACKGROUND

At the nicotinic acetylcholine receptor, long chain alkanols reduce, whereas short chain alkanols augment endplate currents. Using 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 channel, stabilizing the open state and enhancing the apparent affinity of the agonist for channel opening. Long chain alkanols would bind the same site and simply inhibit without affecting the agonist's apparent affinity.

METHODS

Agonist-stimulated 86Rb+ efflux from acetylcholine 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 86Rb+ efflux elicited 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 enantiomers of 2-butanol differed twofold, but the other enantiomers exhibited no stereoselectivity. The enantiomers of 2-octanol caused a concentration-dependent depression of carbamylcholine-stimulated 86Rb+ efflux without significantly altering the agonist's apparent dissociation constant. In contrast, the enantiomers of 2-butanol caused: (1) a nonstereoselective decrease in carbachol's apparent dissociation constant and (2) the expected stereoselective decrease in maximal carbamylcholine-stimulated 86Rb+ efflux.

CONCLUSIONS

The alkanol site that modulates the apparent agonist affinity for channel opening is distinct from the site that results in inhibition of cation flux through the channel.