Adenine Molecule (Adenosine)

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Adenosine is a nucleoside formed when adenine is attached to a ribose ring (also known as a ribofuranose) via a ²-N₉-glycosidic bond.

Adenine is one of the two purine bases used in forming nucleotides of the nucleic acids DNA and RNA. In DNA, adenine (A) binds to thymine (T) to assist in stabilizing the nucleic acid structures. In RNA, adenine binds to uracil (U).

Adenine forms adenosine, a nucleoside, when attached to ribose, and deoxyadenosine when attached to deoxyribose, and it forms adenosine triphosphate (ATP), a nucleotide, when three phosphate groups are added to adenosine. Adenosine triphosphate is used in cellular metabolism as one of the basic methods of transferring chemical energy between reactions.

Adenosine plays an important role in biochemical processes, such as energy transfer - as adenosine triphosphate (ATP) and adenosine diphosphate (ADP) - as well as in signal transduction as cyclic adenosine monophosphate, cAMP. If adenine is attached to a deoxyribose ring, it is known as a deoxyadenosine.

Pharmacological effects

Action on the heart

When administered intravenously, adenosine causes transient heart block in the AV node of the heart. In individuals suspected of suffering from a supraventricular tachycardia (SVT), adenosine is used to help identify the rhythm. Certain SVTs can be successfully terminated with adenosine. This includes any re-entrant arrhythmias that require the AV node for the re-entry (ie: AV reentrant tachycardia (AVRT), AV nodal reentrant tachycardia (AVNRT)). In addition, atrial tachycardia can sometimes be terminated with adenosine.

Fast rhythms of the heart that are confined to the atria (i.e. atrial fibrillation, atrial flutter) or ventricles (ie: monomorphic ventricular tachycardia) and do not involve the AV node as part of the re-entrant circuit are not typically effected by adenosine. Because of the effects of adenosine on AV node-dependent SVTs, adenosine is considered a class V antiarrhythmic agent.The pharmacological effects of adenosine are blunted in individuals who are taking methylxanthines (ie: caffeine (even coffee) and theophylline)

Dosage

When given for the evaluation or treatment of an SVT, the initial dose is 6mg, given as a fast IV push. If this has no effect (ie: no evidence of transient AV block), a 12 mg dose can be given 1-2 minutes after the first dose.

According to guidelines, if the 12mg dose has no effect, a second 12mg dose can be administered 1-2 minutes after the previous dose. Some clinicians may administer a higher dose (typically 18mg), rather than repeat a dose that apparently had no effect.

Side effects

Many individuals experience facial flushing, lightheadedness, diaphoresis, or nausea after administration of adenosine. These symptoms are transitory, usually lasting less than one minute.

Metabolism

When adenosine enters the circulation, it is broken down by adenosine deaminase, which is present in red cells and the vessel wall. Dipyridamole, an inhibitor of adenosine deaminase, allows adenosine to accumulate in the blood stream. This causes an increase in coronary vasodilatation.