Adenosine is a purine present normally in the body. This molecule however has several physiological and pharmacological roles to play in functioning of the normal body as well as in some diseased conditions.
There are several pharmacological aspects of this molecule.
Effects on the heart and blood vessels
In the heart and blood vessels, adenosine has profound effects. It helps to dilate or expand the blood vessels that supply the heart (coronary blood vessels) and thereby enhances blood supply to the heart muscles.
Blood vessels all over the body also dilate when adenosine is administered.
In the heart adenosine decreases heart rate and also decreases the speed with which impulses flow between the heart muscles to bring about a contraction.
Adenosine acts in opposition to adrenaline and also possesses anti-platelet action that prevents platelets from aggregating.
Effects on the kidneys
In the kidneys adenosine reduces blood flow, glomerular filtration rate and decreases secretion of rennin.
Effects on the lungs
In the lungs, adenosine constricts the airways but decreases the blood flow resistance in the lungs and thus may be used to reduce pulmonary artery pressure.
Effects on the liver
In the liver adenosine causes constriction of blood vessels and increases the breakdown of glycogen to form glucose. It also prevents fat breakdown or lipolysis and improves the uptake of glucose.
Effects on the central nervous system
In the brain, adenosine is a depressant neurotransmitter.
Effects on the adrenal glands
In the adrenal glands secretions and production of steroid hormones are raised when adenosine is given.
Effects on immunity
Adenosine suppresses immunity and immunological functions to a certain extent.
Use in heart disease
The United States Food and Drugs Administration approved adenosine for the treatment of supraventricular tachycardia (SVT).
In the heart adenosine acts on the sinus node that is responsible for firing new impulses that bring about contraction of the heart. From the sinus node the impulse passes via the atrioventricular (AV) node.
Adenosine prevents firing of new impulses and also prevents conduction of the impulse via the AV node. Thus it is active in SVT.
The normal level of adenosine in blood ranges between 0.04 and 0.2 micromoles.
There are two adenosine receptors. A1 receptors are found in cardiomyocytes of heart muscle cells. Binding to these receptors inhibits adenyl cyclase activity which lowers cyclic adenosine monophosphate (cAMP). Thus the firing of new impulses at the sinus node is prevented and there is slowing of AV node conduction.
The A2 receptors are found in endothelial cells and smooth muscle cells that line the blood vessels. These work in contrary to the A1 receptors by enhancement of adenylyl cylase activity and increased cyclic AMP. This rise in cAMP causes dilatation of blood vessels.
What are the uses of adenosine in therapy?
Adenosine can help patients with SVT as well as some with re-entrant pathways. Adenosine may also be used to diagnose wide complex SVT and compare SVT with ventricular tachycardia (VT).
Adenosine can also unmask and help detect atrial flutter and atrial fibrillation. Adenosine can also be used for blood pressure control particularly during anesthesia and may also act as an antiplatelet agent.
Side effects of adenosine use
Adenosine is a very short acting drug with duration of action less than a minute. Side effects thus are also very short lasting. Common side effects include reddening or flushing of face, chest discomfort, tightening of airways, headache, falling blood pressure etc.
Certain drugs can increase blood levels of adenosine and enhance its actions. These include drugs like Dypyridamole. Yet others oppose its effects like theophylline and caffeine.