Adenosine is a nucleoside that occurs naturally in all cells of the body. Chemically it is 6-amino-9-beta-D-ribofuranosyl-9-H-purine.
Structure and blood levels
Adenosine is made up of adenine attached to a ribose sugar molecule (ribofuranose) moiety. The bond that attaches the adenine and the ribose sugar is called a β-N9-glycosidic bond.
The usual plasma adenosine level is between 0.04 and 0.2 micromoles.
Physiological and pharmacological role of Adenosine
In the body, adenosine helps in cellular energy transfer by forming molecules like adenosine triphosphate (ATP) and adenosine diphosphate (ADP).
Adenosine also plays a role in signalling various pathways and functions in the body by forming signally molecules like cyclic adenosine monophosphate (cAMP).
Adenosine in the brain
In the brain adenosine is an inhibitory neurotransmitter. This means, adenosine can act as a central nervous system depressant. In normal conditions, it promotes sleep and suppresses arousal. When awake the levels of adenosine in the brain rise each hour.
Adenosine in the heart
In the heart adenosine causes dilation of the coronary blood vessels that improves blood circulation to the heart. Adenosine also increases the diameter of blood vessels in the peripheral organs.
In the heart adenosine decreases heart rate and in blood it has an anti-platelet action. Antiplatelet action prevents platelet aggregation and coagulation.
Adenosine in the blood
In blood adenosine is broken down by adenosine deaminase. This enzyme is present in red cells and the vessel wall. The drug Dipyridamole is an inhibitor of the enzyme adenosine deaminase and thus raises the levels of adenosine in blood. This leads to blood vessel dilatation and improved blood flow through the coronary blood vessels that supply the heart muscles.
Adenosine in the kidneys, lungs and liver
In the kidneys adenosine decreases renal blood flow and decrease the production of rennin from the kidneys.
In the lungs it causes constriction of airways and in the liver it leads to constriction of blood vessels and increases breakdown of glycogen to form glucose.