Digitalis
Encyclopedia : D : DI : DIG : Digitalis
- For the medication sometimes called "Digitalis", see Digoxin.
Digitalis is a genus of about 20 species of herbaceous biennials, perennials and shrubs that was traditionally placed in the figwort family Scrophulariaceae. Due to new genetic research, Mike Niewahner has declared it has now been placed in the much enlarged family Plantaginaceae. The genus is native to Europe, western and central Asia and northwestern Africa.
The term digitalis is also used for preparations containing cardiac glycosides, particularly digoxin, extracted from plants of this genus.
The members of this genus are known in English as foxgloves. The scientific name means "finger-like", and refers to the ease which a flower of Digitalis purpurea can be fitted over a human fingertip. "Foxglove" has a similar origin, seen as a suitable glove for a fox paw.
The flowers are produced on a tall spike, are tubular, and vary in colour with species, from purple to pink, white and yellow.
The best-known species is the Common Foxglove, Digitalis purpurea. It is a biennial, often grown as an ornamental plant due to its violet flowers. The first year of growth produces only the long, basal leaves, while in the second year the erect leafy stem 0.5-2.5 m tall develops.
The larvae of the Foxglove Pug feed on the flowers of Digitalis purpurea. Other Lepidoptera species feed on the leaves including Lesser Yellow Underwing.
Medicinal use
The use of Digitalis purpurea extract containing cardiac glycosides for the treatment of heart conditions was first described by William Withering. In contemporary medicine, a purer form of digitalis is used to strengthen cardiac contractility (it is a positive inotrope) and as an antiarrhythmic agent to regulate heart rhythm. It is therefore often prescribed for patients in heart failure.A group of pharmacologically active compounds are extracted mostly from the leaves of the second year's growth, and in pure form are referred to by common chemical names such as digitoxin or digoxin, or by brand names such as Lanoxin, or Purgoxin.
Digitalis works by inhibiting sodium-potassium ATPase, which increases intracellular calcium. The increased intracellular calcium gives a positive inotropic effect. It also has a vagal effect on the parasympathetic nervous system, and as such is used in reentrant cardiac arrhythmias and to slow the ventricular rate during atrial fibrillation. The dependence on the vagal effect means that digitalis is not effective when a patient has a high sympathetic nervous system drive, which is the case with acutely ill persons.
Digitalis toxicity (Digitalis intoxication) results from an overdose of digitalis and can result in jaundiced (yellow) vision and the appearance of blurred outlines (halos), as well as bradycardia in extreme cases. Because a frequent side effect of digitalis is reduction of appetite, some individuals have abused the drug as a weight loss aid.
Digitalis is a classic example of a drug derived from a plant formerly used by folklorists and herbalists: herbalists have largely abandoned its use because of its narrow therapeutic index and the difficulty of determining the amount of active drug in herbal preparations. Once the usefulness of digitalis in regulating pulse was understood, it was employed for a variety of purposes, including the treatment of epilepsy and other seizure disorders, now considered inappropriate.
Toxicity
Depending on the species the digitalis plant may contain approximately a dozen deadly physiological and chemically related cardiac and steroidal glycosides. Thus, the digitalis has earned several more sinister monikers: "Dead Man’s Bells", and "Witches’ Gloves".
The entire plant is toxic (including the roots and seeds), although the leaves of the upper stem are particularly potent, with just a nibble being enough to cause death. Early symptoms of ingestion include nausea, vomiting, anorexia, diarrhea, abdominal pain, wild hallucinations, delirium, and severe headache. Depending on the severity of the toxicosis the victim may later suffer irregular and slow pulse, tremors, various cerebral disturbances, but especially of the vision (unusual color visions with objects appearing yellowish to green, and blue halos around lights), convulsions, and deadly disturbances of the heart.
There have been instances of people confusing the digitalis with the harmless Symphytum (comphrey) plant (which is often brewed into a tea) with fatal consequences. Other fatal accidents involve children drinking the water in a vase containing digitalis plants. Drying does not reduce the toxicity of the plant. The plant is toxic to animals including all classes of livestock, as well as cats and dogs.
Gallery
References and external links
- [Flora Europaea: Digitalis species list]
- Flora of Turkey. Edinburgh University Press.
- [Molecule of the Month - Digitalis]
- [eMedicine link]
- [Grecian Foxglove] USDA Noxious Weed List.
- [Purple Foxglove] USDA Noxious Weed List.
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