Binding of the S(+)- and R(−)-enantiomers of bethanechol to muscarinic receptors in jejunal smooth muscle, nasal mucosa, atrial and ventricular myocardium of the rat

Abstract Both the enantiomers of bethanechol displaced R(−)-[ 3 H]quinuclidinyl benzilate from muscarinic binding sites in homogenates from rat jejunum, nasal mucosa, atrial and ventricular myocardium. Stereoselectivity of receptor binding for S(+)- and R(−)-bethanecol was found in all tissues investigated (K i S(+) i R(−)). Different proportions of high and low affinity binding sites for bethanechol enantiomers and S(+)-methacholine in jejunal smooth muscle, atrial and ventricular muscle and the presence of only low affinity binding sites in nasal mucosa point to differences in receptor-effector coupling in these tissues. The relative insensitivity of cardiac tissue for bethanechol compared to methacholine might be related to the much smaller proportion of high affinity binding sites found for S(+)-bethanechol in heart ventricular muscle.