Nicotinic Receptors

Although the antimuscarinic drugs are normally selective for muscarinic cholinergic receptors, high concentrations of agents with a quaternary ammonium group (e.g., propantheline) can block nicotinic receptors on autonomic ganglia and skeletal muscles. However, these effects are generally not clinically important at usual therapeutic doses.

ABSORPTION, METABOLISM, AND EXCRETION

Both atropine and scopolamine are tertiary amines that cross biological membranes readily. They are well absorbed from the gastrointestinal tract and conjunctiva and can cross the blood-brain barrier. After the intravenous injection of atropine (dl-hyoscyamine), the biologically inactive isomer, d-hyoscyamine, is excreted unchanged in the urine. The active isomer, however, can undergo dealkylation, oxidation, and hydrolysis.

The quaternary ammonium derivatives of the belladonna alkaloids, as well as the synthetic quaternary ammonium compounds, are incompletely absorbed from the gastrointestinal tract. Consequently, greater amounts of these compounds are eliminated in the feces following oral administration. The blood-brain barrier prevents quaternary ammonium muscarinic blockers from gaining significant access to the CNS.

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