CARDIAC GLYCOSDES

CARDIAC GLYCOSDES

The plant steroids possessing the property of stimulating the unsufficient heart muscles are known as cardiac glycosides or  C23 glycosides. They possess bile acid-like steroidal carbon mework (tetracyclic 1.0, 13-dimethyl-cyclopentanoperhydroEnanthrene) with twenty three carbon atoms. These steroids as glycosides with sugars attached at the C-3 atom of the did nucleus. The steroid aglycones or genins are of two types

Cardenoides : These glycosides are found more abundantly in nature. They are C23 steroids having a 17β-side chain as an α,ß-unsaturated 5-membered lactone ring. Digitoxigenin, gitoxigenin, digoxigenin and ouabagenin are the cardemolides. Digitalis purpurea, D. lanata and Strophanthus are the herbal drugs which contain cardenolides.

Bufadienolides : The bufadienolides are C homologs of cardenolides and possess double bond containing 6membered lactone ring at the C-17 position. The compounds Ere less widely distributed in nature than are the ardenolides. They are found in some Liliaceae and Ranunculaceae and in the toad venoms. Therapeutically they are little used as cardiac drugs due to their low herapeutic index and producing side-effects.

Both cardenolides and bufadienolides possess the C/D ction in cis-configuration. For maximum cardiac activity, scone should possess an O.B-unsaturated lactone ring attached B at the 17-position of the steroidal skeleton. If coside is cleaved, the aglycone retains cardiac activity:

however, the sugar portion of the glycoside helps to dissolve the molecule which is important in its absorption and distribution in the body. Oxygen substitution on the steroid nucleus also influences the distribution of glycosides. If there are excess number of hydroxyl groups on the molecule, the onset of action and the subsequent dissipation from the body will be more rapid. In all the known biologically active cardiac glycosides the sugar residues are present in the pyranoside formand are joined in a B-glucoside linkage to the axial C-B-OH of the aglycone.

Cardiac glycosides have the ability to increase the force of systolic contraction. Due to increasing contractility, ventricle becomes more empty and a shortening in the length of systole takes place. Thus, the heart has more time to rest between contractions. Cardiac output and circulation are increased and the heart rate is decreased. When rapid digitalisation is required. digoxin or ouabain may be given by intravenous injection.