Actions of pentobarbital enantiomers on nicotinic cholinergic receptors

The enantiomers of pentobarbital had four different actions on the nicotinic receptor-rich membranes from Torpedo electroplaques. (i) Both inhibited cholinergically stimulated cation flux through the receptor's channel, with IC50 values of approximately 25 microM and extremely weak stereoselectivity. (ii) (R)-(+)-[14C]Pentobarbital bound to a saturable site with an apparent dissociation constant of 100 microM, a Hill coefficient of 1.2, and a stoichiometry of 1:1 with the acetylcholine binding sites. (S)-(-)-Pentobarbital also displaced (+)-[14C]pentobarbital but its IC50 was 4-fold higher than that of the (+)-enantiomer under the same conditions. (iii) Both enantiomers caused a stereoselective allosteric inhibition of [3H]acetylcholine binding, which occurred over the same concentration range and with the same stereoselectivity as barbiturate binding. (iv) Above 1 mM, pentobarbital caused an unexpected and sudden increase in [3H]acetylcholine binding, which lacked significant stereoselectivity. These results are consistent with a model where low concentrations of pentobarbital act on the receptor by binding to allosteric sites that have higher affinity but lower stereoselectivity for the open channel conformation than for the resting conformation, whereas the highest concentrations of pentobarbital act by nonspecific mechanisms mediated by general membrane perturbations.