Characterization of muscarinic receptor binding by the novel radioligand, [3H]imidafenacin, in the bladder and other tissues of rats

Abstract The present study aimed to directly characterize specific binding sites of tritium ([ 3 H])-labeled imidafenacin, a new radioligand for labeling muscarinic receptors, in the bladder and other peripheral or central nervous tissues of rats. Muscarinic receptors in rat tissues were measured by radioligand binding assay using [ 3 H]imidafenacin. Specific [ 3 H]imidafenacin binding in rat tissues was saturable, reversible, and of high affinity. Estimated dissociation constants ( K d values) were significantly lower in submaxillary gland and prostate and higher in heart than in bladder, indicating lower K d values in M 1 and M 3 subtype- than M 2 subtype-dominating tissues. Unlabeled imidafenacin and clinically used antimuscarinic agents competed with [ 3 H]imidafenacin for binding sites in bladder and other tissues in a concentration-dependent manner, which indicated pharmacological specificity of [ 3 H]imidafenacin binding sites. Pretreatment with N -(2-chloroethyl)-4-piperidinyl diphenylacetate (4-DAMP mustard), an irreversible inactivating agent of M 3 subtype, significantly decreased the number of [ 3 H]imidafenacin binding sites in bladder, submaxillary gland, and colon, but not in heart. [ 3 H]imidafenacin labeled muscarinic receptors in M 1 and M 3 subtype-dominating tissues with higher affinity than [ N -methyl- 3 H]scopolamine methyl chloride (NMS). [ 3 H]imidafenacin is a useful radioligand to label muscarinic receptors in M 1 - and M 3 -dominating tissues with high affinity.