Mechanism of action of benzilylcholine mustard at the muscarinic receptor

The irreversible antagonism exerted by benzilylcholine mustard (BCM) on acetylcholine induced contractions of the isolated guinea pig ileum can best be characterized by multiple sites of alkylation. BCM (50 μM, for 15 min) completely abolishes the response to acetylcholine, but the maximum responsiveness of the tissue recovers with time according to a first order loss of tritium from tissues blocked with [³H]BCM (t 1/2 of 38.9 min). The rightward shift of the dose response curves observed after recovery of maximum response (acetylcholine dose ratio = 390) is stable over 8 hr, although the dose ratio would be expected to return toward control values if the receptor reserve model was applicable in this tissue. The data can best be interpreted on the basis that BCM alkylates two sites at the muscarinic receptor: an allosteric site which stabilizes an inactive form of the receptor, resulting in an apparent decrease in affinity for the agonist, and a second site which is postulated to be the agonist recognition site proper. Alkylation of the second site results in decreased maximum response.