Atropine

atropine, poisonous crystalline substance belonging to a class of compounds known as alkaloids and used in medicine. Atropine occurs naturally in belladonna (Atropa belladonna), from which the crystalline compound was first prepared in 1831. Since then, a number of synthetic and semisynthetic substitutes have been developed for atropine, owing to its general nonselectivity in action and adverse effects.

Despite atropine’s lack of therapeutic selectivity, the drug continues to find use in modern medicine. It is applied locally to the eye to dilate the pupil in the examination of the retina or to break up or prevent adhesions between the lens and the iris. It gives symptomatic relief from hay fever and head colds by drying up nasal and lachrymal secretions, and it can be given prior to surgery to reduce the production of saliva and airway secretions. Atropine also is used as an antidote for poisoning with organophosphate nerve toxins, including tabun and sarin. Because atropine relaxes intestinal spasms resulting from stimulation of the parasympathetic portion of the autonomic nervous system, it is prescribed in certain types of bowel distress and is included in a number of proprietary cathartics.

Atropine has been used in the treatment of childhood bed-wetting, and it has been occasionally employed to relieve ureteral and biliary spasms. However, the drug’s effectiveness in treating those conditions is disputed, and its adverse effects may outweigh its benefits. Atropine is no longer used as a respiratory stimulant. In the treatment of asthma to relax bronchial spasms, it has been largely replaced by epinephrine.

Specific effects of atropine include the arrest of secretion of sweat, mucus, and saliva; inhibition of the vagus nerve, which results in an increased heart rate; dilation of the pupil and paralysis of accommodation of the lens of the eye; and relaxation of bronchial, intestinal, and other smooth muscles. Central effects include excitement and delirium followed by depression and paralysis of the medulla oblongata, a region of the brain continuous with the spinal cord.

The ubiquitousness of the effects of atropine is a distinct disadvantage in its clinical use; as a result, a number of synthetic and semisynthetic substitutes with more-specific effects have been introduced. Homatropine, for example, has a more transient action in the eye and little or no effect on the central nervous system; dicyclomine exerts direct relaxant effects on the gastrointestinal tract and is used in the treatment of irritable bowel syndrome; and oxybutynin acts on the smooth muscles of the urinary bladder and is used in the treatment of overactive bladder.

Atropine occurs naturally as a racemic mixture of D- and L-hyoscyamine in plants such as belladonna, henbane (Hyoscyamus niger), jimsonweed (Datura stramonium), the mandrake Mandragora officinarum, and Scopolia, all of the family Solanaceae. Atropine forms a series of well-crystallized salts, of which the sulfate is principally used in medicine. Both atropine and hyoscyamine have been synthesized from tropine.