The
cardiac glycosides are basically steroids with an inherent ability to afford a very specific and
powerful action mainly on the cardiac muscle when administered through injection into man or animal. 

Small amount would exhibit a
much needed stimulation on a diseased heart, whereas an
excessive dose may cause even
death.

Lactones ring is very important for therapeutic activity. 

Removal of lactone ring or even a slight disturbance to the lactone ring means that the activity
of the activity
is lost. 

Cardiac glycosides contain a special sugar called digitoxose; through they
do carry
other sugar like glucose and
rhamnose. 

Very
often diuretic action is also associated with steroidal glycosides as they also
promote improved circulation
of blood through kidneys.

Classification of cardiac glycosides

Cardenolides:

They are C23 steroids that have a 17–β side chain and
α, unsaturated β–membered lactone ring.

Example:

Digitalis,
Quabain, Strophanthin,
etc

Bufadienolide:

The
bufadienolides are C24 homologues of the cardenolides and carry a doubly unsaturated 6– membered lactone ring 
at the 17–position. 

The bufadienolides derived 
their name from the genetic name for the
toad, Bufo (the prototype compound bufalin was isolated from the skin of toads).

Example:

Squill, etc

Chemical tests
for cardiac glycosides:

1. Raymond’s test:

To
the drug, add a few ml of 50% ethanol
and 0.1 ml of 1 % solution of m– dinitrobenzene in
ethanol.

To  this  solution,
 add  2–3  drops
 of
 20%  sodium  hydroxide  solution.
 Violet  colors
appears,
this is due to presence of active methylene group.