Cardiac glycosides

(Just a little extract from my vet Physiology assignment. Still needs a lot of editing so there are probably some errors)

Cardiac Glycosides

To understand the act of cardiac glycosides on the heart you need to understand how the myocardial muscle contracts and relaxes. An action potential initiates contraction by opening voltage gated calcium channels on the outer membrane 14. Calcium rushes into the cell releasing the sarcoplasmic reticulum calcium stores. The calcium is key in binding to protein called troponin in the muscle fibre structure to initiate contraction 14.

It is then in the relaxation of the muscle that the cardiac glycoside disrupts the normal physiological process. During normal relaxation calcium would dissociate from triponin and be taken back up into the SR and the rest would be moved out with a Na+/Ca2+ exchange transporter protein in the cell membrane 14. To keep the sodium levels down in the cell to allow this pump to work it interacts with the Na+/K+ ATPase pump which takes the excess sodium out in exchange for potassium into the cell 14.

When the cardiac glycoside binds extracellularly to the Na+/K+ pump it inhibits the pump 3,4,7. Due to this pump’s key role in the Na+ balance the inhibition also effects the Na+/Ca2+ pump causing a build up of extra Ca2+ in the cell 3. This means the next time the heart contracts there will be more Ca2+ in the cells and this will cause call depolarisation and stronger heart contraction contraction. This is why cardiac glycoside poising initially slows the heart rate but increases the hearts contraction forces 3,7.

Lily of the Valley is a member of the Asparagaceae family (and not the Liliaceae family, as you might think), Its scientific name is Convalaria majalis, and it is found across the Northern Hemisphere. It is toxic, and it contains a range of cardiac glycosides. These are compounds which can cause congestive heart failure, and the plants produce them as a defense. 

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Foxglove may be one of the most easily recognizable flowers commonly found in the garden. Scientifically known as Digitalis purpurea, foxglove is in the family Plantaginaceae and is native to most of temperate Europe, but has been naturalized in many areas around the world. While the striking variegation of the flowers is what makes foxglove so popular, it is it’s health effects that give this plant notoriety. Foxglove contains a cocktail of cardiac glycosides, most notably the compounds digitoxin and digoxin. These molecules inhibit the sodium-potassium ATPase pump found in all animal cells. This pump is responsible for many physiological processes, including regulating the electrical conductivity of nerve cells. If any part of the foxglove plant is ingested, the cardiac glycosides act to inhibit the electrical impulses that regulate our heartbeat, leading to heart palpitations, and eventually cardiac arrest. So while foxglove is a beautiful addition to the landscape, extreme caution should be taken to ensure poisoning does not occur.

Why does Digoxin toxicity result in increased automaticity?

Hey everyone!

Digitalis and other cardiac glycosides are known to cause an AV nodal delay.

Then why does too much Digoxin result in some arrhythmias that are due to increased automaticity?

Brady arrhythmias are explainable. But why tachy arrhythmias?

You see, cardiac glycosides reversibly inhibit the sodium-potassium-ATPase, causing an increase in intracellular sodium and a decrease in intracellular potassium. The increase in intracellular sodium prevents the sodium-calcium antiporter from expelling calcium from the myocyte, which increases intracellular calcium. The net increase in intracellular calcium augments inotropy.

Excessive intracellular calcium may cause delayed after-depolarizations, which may in turn lead to premature contractions and trigger arrhythmias. Cardiac glycosides shorten repolarization of the atria and ventricles, decreasing the refractory period of the myocardium, thereby increasing automaticity and the risk for arrhythmias.

That’s all!

-IkaN

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Nerium oleander is in the milkweed family Apocynaceae. The Oleander plant has been cultivated for so long that botanists are uncertain of its origins, although Asia has been named as part of its native range. Oleander is also one of the most toxic common garden plants in cultivation today. All parts of the plant contain hazardous toxic compounds including Oleandrin, a cardiac glycoside. These compounds cause cardiac arrest and respiratory failure and make ingestion of any part of this plant potentially fatal.

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