Foxglove (Digitalis purpurea)
SCROPHULARIACEAE, Figwort Family
Unlike many medicinal plants, which have a long history of uses, foxglove (Digitalis purpurea) was not an important medicine until the late 18th century. In 1776, William Withering was a physician with a large country practice in England. A lady was dying from a disease called dropsy, or edema, in which liquids accumulate in the body and cause swelling of tissues and body cavities. He left her, expecting her to die shortly, but he later learned that she had recovered after taking an old cure of a garden plant called foxglove. For ten years, Withering conducted experiments to demonstrate the uses of foxglove and discovered that dropsy is actually a symptom of heart disease in which the heart does not pump hard enough to get rid of urine. He showed that foxglove stimulated urination by pumping more liquids to the kidneys. Withering died before his results appeared in print, so he never got to see how foxglove, or digitalis, became a lifesaver for heart disease patients.
Digitalis is very toxic and fatal with an overdose, so its potency was measured very carefully against an unusual standard, some crystals of a substance from another plant, Strophanthus gratus. The active chemical in foxglove, which is isolated from the leaves, is a steroid glycoside called digitoxin, but there are at least thirty other cardiac glycosides, of which five are important. Digitoxin is a stimulant that improves heart tone and rhythm, which then improves circulation. Technically, it appears that digitoxin increases myocardial contractility, as do its derivatives, such as digoxin, which is now the principal medicine. Digoxin presumably binds to the membranes of the muscle cells and aids in the pumping of sodium and potassium ions. This then slows heart rate and also reduces heart size, which lessens myocardial oxygen demand.
Crude extracts were hard to use, because dosage of the active ingredients could not be precisely determined. Digoxin is used instead of crude extracts of foxglove leaves because it has a short half-life in the body and is, therefore, easier to treat for toxicity. Digitalis is not now universally accepted for treatment of heart disease, as it once was, and now dopamine and other alkaloids are also being used. Digoxin is also used as a treatment to slow muscular dystrophy and to reduce pressure in the eye of glaucoma victims.
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