Dual effect of carbachol on the muscarinic receptor.

The carbachol-mediated inhibition of cAMP synthesis in membranes of rat cerebral cortex revealed maximum in 0.3 mM carbachol solution, while further increases of the drug concentration blocked the inhibitory effect and restored the adenylate cyclase activity. The bell-shaped dose-response curve was analyzed assuming that carbachol binds with two distinct sites on muscarinic receptor, which are responsible for agonistic and antagonistic effects, related to this receptor. The pKagon and pKant values for carbachol were 5.0 and 2.1, respectively.