Muscarinic antagonists have no intrinsic activity, and they can produce effects only by blocking the activation of muscarinic receptors by muscarinic agonists or by neuronally released ACh. Therefore, the magnitude of the response produced by muscarinic antagonists depends on the existing level of cholinergic activity or on the presence of muscarinic agonists. Also, the nature of the response of an organ to the administration of a muscarinic antagonist will depend on the organ's pattern of innervation; for example, some organs receive dual innervation from adrenergic and cholinergic pathways. At these locations, block of the activation of mus-carinic receptors can increase the tone provided by the adrenergic input.
The effects of muscarinic blocking drugs on various human organ systems are summarized in Table 13.1.The tissues or systems affected will depend on the dose administered, the drug's pharmacokinetic properties (e.g., increased entry into the CNS at higher concentrations), and the differential sensitivity of muscarinic receptors in various organs to individual blocking agents. Although muscarinic agonists typically do not exhibit selectivity among muscarinic receptors (see Chapter 12), some muscarinic antagonists are selective in their ability to block subtypes of muscarinic receptors.
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