Effects of verapamil on the binding characteristics of muscarinic receptor subtypes

The calcium channel antagonist verapamil inhibited [3H]N-methyl scopolamine ([3H]NMS ) binding to muscarinic receptors from both rat brain cortex, an area rich in M1 receptors, and from pons-medulla, an area rich in M2 receptors. Verapamil reduced the dissociation kinetics of [3H]NMS binding in cortical receptors but had no detectable effects on the dissociation kinetics of receptors from the pons-medulla. These data suggest that receptors from the cortex have an allosteric site which regulates the binding kinetics, whereas receptors from the pons-medulla lack this site.