The interaction of the enantiomers of aceclidine with subtypes of the muscarinic receptor.

The pharmacological activity of the enantiomers of aceclidine was investigated in Chinese hamster ovary cells transfected with the M1 through M5 subtypes of the muscarinic receptor and also in the rat heart and parotid gland that express primarily M2 and M3 receptors, respectively. When measured by stimulation of phosphoinositide hydrolysis in Chinese hamster ovary cells transfected with the M1, M3 and M5 muscarinic subtypes, the potency of S-(+)-aceclidine was approximately 2- to 4-fold greater than that of R-(-)-aceclidine, whereas the maximal response of the R-(-)-isomer was only 44 to 64% that of the S-(+)-isomer. When measured by inhibition of forskolin-stimulated cyclic AMP accumulation in Chinese hamster ovary cells transfected with the M2 and M4 muscarinic subtypes, the potency of S-(+)-aceclidine was approximately 3.5-fold greater than that of R-(-)-aceclidine. In cells transfected with the M2 muscarinic receptor, the maximal responses of the enantiomers were the same, whereas the maximal response of R-(-)-aceclidine was 86% that of S-(+)-aceclidine in cells transfected with the M4 muscarinic subtype. The activities of the enantiomers of aceclidine at native M2 and M3 muscarinic receptors coupled to inhibition of adenylyl cyclase activity in the heart and stimulation of phosphoinositide hydrolysis in the parotid gland, respectively, were similar to those observed in Chinese hamster ovary cells transfected with the corresponding receptor subtypes. We devised a simple quantitative method for using our data in Chinese hamster ovary cells to predict the relative potencies of agonists in a more sensitive assay in which the agonists produce a full maximum response. By using this method, we were able to predict the relative potencies of the enantiomers for eliciting contractions in the guinea pig ileum, an M3 muscarinic response, from their activity in Chinese hamster ovary cells transfected with the M3 muscarinic subtype. Our method of analysis should have application in a variety of studies in which transfected cells are used to determine the pharmacological activity of agonists.