Cardioacceleratory effect of muscarinic blocking agents in the dog

To test whether the tachycardia in response to atropine after adrenergic blockade is partly due to a central excitatory action, the effects of atropine, methylatropine and methylscopolamine were compared in dogs in neurolept-anesthesia. The latter two agents proved to have effect, similar to atropine. A central action is therefore improbable. It was possible to partly abolish the tachycardia by hexamethonium. The cardioacceleration by atropine, methylatropine and methylscopolamine, so far as it is not caused by muscarinic receptor blockade, can be explained by the unmasking of an underlying acceleratory tone.