Atropine binds to and inhibit muscarinic acetylcholine receptors, producing a wide range of anticholinergic effects.
Atropine, a naturally occurring belladonna alkaloid, is a racemic mixture of equal parts of d- and l-hyoscyamine, whose activity is due almost entirely to the levo isomer of the drug. Atropine is commonly classified as an anticholinergic or antiparasympathetic (parasympatholytic) drug. More precisely, however, it is termed an antimuscarinic agent since it antagonizes the muscarine-like actions of acetylcholine and other choline esters. Adequate doses of atropine abolish various types of reflex vagal cardiac slowing or asystole. The drug also prevents or abolishes bradycardia or asystole produced by injection of choline esters, anticholinesterase agents or other parasympathomimetic drugs, and cardiac arrest produced by stimulation of the vagus. Atropine may also lessen the degree of partial heart block when vagal activity is an etiologic factor. Atropine in clinical doses counteracts the peripheral dilatation and abrupt decrease in blood pressure produced by choline esters. However, when given by itself, atropine does not exert a striking or uniform effect on blood vessels or blood pressure.
Much of the drug is destroyed by enzymatic hydrolysis, particularly in the liver. From 13 to 50% is excreted unchanged in the urine.
Oral, mouse: LD50 = 75 mg/kg. Symptoms of overdose includes widespread paralysis of parasympathetically innervated organs. Dry mucous membranes, widely dilated and nonresponsive pupils, tachycardia, fever and cutaneous flush are especially prominent, as are mental and neurological symptoms. In instances of severe intoxication, respiratory depression, coma, circulatory collapse and death may occur.