Effect of 13 cholinergic antagonists on M-cholinergic receptors in rat hearts

By means of radioligand receptor binding assay of M-cholinergic receptors from rat heart membranes, the results showed that [3H]QNB bound in a saturable manner to M-cholinergic receptors with an equilibrium dissociation constant(KD)=0.36 nM and the maximum number of binding sites(Bmax)=29.8 fmol/mg protein. The affinities and modes of action of 13 cholinergic antagonists on M-cholinergic receptor in rat heart were measured from inhibition of [3H]QNB binding to M-cholinergic receptors. By the dose-reponse equation and Hill equation, the calculated kinetics parameters were as follows. 1. The order of affinity constants(Ki) was QNB>7923, scopolamine>atropine>B-7601>M-8225>benzhexol>S-18. M-8218>7810>S-27>anisodamine>anisodine. 2. Their slopes(b) were all close to 1. It suggested that the antagonists acted on the same M-cholinergic receptor sites. 3. The Hill numbers(nH)of most antagonists approximated to 1, which indicated receptor binding of antagonists likely to be bimolecular reaction without cooperative interactions.