Cardiac glycosides are a class of drugs and the constituents of a very small handful of herbs that have very strong effects on the heart. In small, prescribed doses, this group of drugs helps the heart work more efficiently, but can prove to be very unsafe in larger, more toxic amounts. The cardiac glycosides are prescribed for people who have life-threatening cardiovascular diseases.
Structure
The cardiac glycoside group has two important features, according to a Hampshire College report -- there is a sugar-containing group and a steroid, or cholesterol-based portion. The sugar component is known as the glycone, while the cholesterol portion is known as the aglycone. The activity of the steroid portion is dependent on a specific "U" shaped configuration of the rings that form the cholesterol portion, according to a Virginia Commonwealth University report.
Effects
The effects of cardiac glycosides are caused by a manipulation of the electrolytes that stimulate muscle contraction and nerve cell conduction. The Hampshire College document notes that changes in calcium, potassium and sodium ions create effects that can be beneficial in treating the heart disease known as congestive heart failure (CHF) and in some cases, abnormal heart rhythms, known as arrhythmias.
The effects produced by cardiac glycosides are commonly known as a positive inotrophic and negative chronotropic. The 3rd edition of "Understanding Pharmacology for Health Professionals," by Susan M. Turley describes the inotropic effects as the ability to strengthen the strength of each heart contraction. By intensifying each heart contraction, a once compromised heart may more efficiently pump blood to the rest of the body. The negative chronotropic effect will slow the heart down, which makes it efficient for fast paced heart rhythm disorders and increases its ability to fill up more completely before the blood is ejected.
Common Drugs and Herbs
The most popular pharmaceutical cardiac glycoside is the drug digoxin. The trade name of this drug is Lanoxin. Digoxin was originally isolated from the herb known as foxglove, according to Turley. The Hampshire report notes that the cardiac glycoside group is found in 11 plant families in total, and its purpose in plants is to defend the herb from plant-eating animals. The foxglove plant, also known as Digitalis has been used since 1500 B.C., according to Virginia Commonwealth. In those times, the herb was used for medicinal and poisonous purposes.
Side Effects
The same poisonous aspects of cardiac glycosides that were once used to adorn arrows in hunting or to induce violent vomiting are the same unwanted effects that can accompany its prescriptive use without proper caution. Digitalis toxicity is the terminology used by Turley to denote the serious side effects that can accompany cardiac glycosides. Cardiac glycosides have two properties that make them potentially toxic. They exhibit a narrow margin between a safe and therapeutic dose and a toxic dose. The cardiac glycosides have a low therapeutic index. The glycosides also have a long half-life, according to Turley, which means that they can stay and circulate around the body for long periods of time. The elderly and people with kidney disease are especially susceptible to the effects of a long half-life.
Antidotes
The toxic potential of cardiac glycosides encouraged drug makers to find substances that would bind up and render the substance ineffective in the blood, if overdose or an accident occurred. The antidotes belong to the class of drugs known for their cholesterol-binding capacity. They are drugs often used to lower cholesterol in people with high cholesterol as well. Turley notes that the antidotes known as cholestyramine, colestipol and digoxin immune fab are able to inactivate the cardiac glycosides in either the bloodstream or digestive tract.
References
- Hampshire College: Steroid Stereochemistry and Cardiac Glycosides
- Virginia Commonwealth University: Cardiac Glycosides
- "Understanding Pharmacology for Health Professionals"; Susan M. Turley; 2003
