Propylbenzilylcholine mustard-sensitive and -resistant muscarinic receptors in cardiac muscle.

1. A left atrium of guinea pig driven electrically was used as a test organ containing M2-cholinoceptors. 2. Concentration-response curves of carbachol and butyltrimethylammonium, muscarinic full agonists, were progressively inhibited by 10 and 30 min treatments of the atrium with propylbenzilylcholine mustard (PrBCM; 10(-6) M). The 50 min treatment with PrBCM had no further significant inhibitory effect on their curves. 3. The 30 min treatment of the atrium with PrBCM completely inhibited the concentration-response curve of pilocarpine, a partial agonist. In the atrium after the 30 min treatment with PrBCM, pilocarpine shifted the concentration-response curves of the full agonists, suggesting a competitive antagonism. 4. These results suggest an existence of two subtypes of M2-receptors; PrBCM-sensitive and -resistant ones, and that the full agonists inhibit the twitch through an interaction of both the receptors, while the partial agonist produce inhibition through an activation of PrBCM-sensitive ones.