- Cardiac glycoside
Cardiac glycosides are drugs used in the treatment of
congestive heart failureand cardiac arrhythmia. These glycosides are found as secondary metabolites in several plants, but also in some animals. Some of these compounds ( ouabainand some frog poisons) are used in Africa as arrow-poisons for hunting.
Cardiac glycosides work by inhibiting the Na+/K+ pump. This causes an increase in the level of
sodium ions in the myocytes, which then leads to a rise in the level of calciumions. This inhibition increases the amount of Ca2+ ions available for contraction of the heart muscle, improves cardiac output and reduces distention of the heart.
They do this by stabilizing the E2-P transition state of the Na+/K+ pump. The proposed mechanism is the following: inhibition of the Na+/K+ pump leads to increased Na+ levels, which in turn slows down the extrusion of Ca2+ via the Na+/Ca2+ exchange pump. Increased amounts of Ca2+ are then stored in the
sarcoplasmic reticulumand released by each action potential.
They have an
antiarrhythmiceffect by prolonging the refractory period of the AV node ( Atrioventricular node), reducing the number of impulses reaching the ventricles. Cardiac output is restored but atrial fibrillationor atrial flutter are not abolished.
Examples of plants producing cardiac glycosides:
Strophanthus" – ouabaing/k/e-strophanthin
Digitalislanata" and "Digitalis purpurea" – digoxin, digitoxin
* "Scilla maritima" – proscillaridine A
* "Adonis vernalis, Adonis aestivalis"
* "Acokanthera oblongifolia"
Convallaria" (lily of the valley)
Some frog-poisons contain
bufalin, marinobufageninand bufadienolides, cardiac glycosides.
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