Endogenous purine nucleoside that depresses the sinoatrial node, blocks A-V conduction, inhibits Purkinje pacemaker activity, and decreases the effects of catecholamines.

Produced and released from ischemic myocardial cells.

Competitive antagonist is aminophylline.

A nucleoside that occurs naturally in all cells of the body.

Chemically it is 6-amino-9-beta-D-ribofuranosyl-9-H-purine.

It is made up of adenine attached to a ribose sugar molecule.

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.

It helps in cellular energy transfer by forming molecules like adenosine triphosphate (ATP) and adenosine diphosphate (ADP).

Plays a role in signalling various pathways and functions by forming signally molecules like cyclic adenosine monophosphate (cAMP).

In the brain adenosine it is an inhibitory neurotransmitter, acting as a central nervous system depressant.

In normal conditions, it promotes sleep and suppresses arousal.

Levels increase each hour while awake.

In the heart adenosine causes dilation of the coronary blood vessels that improves blood circulation to the heart, and decreases heart rate and has an anti-platelet action

It increases the diameter of blood vessels in the peripheral organs.

 Adenosine is responsible for constriction of afferent arterioles in the kidney and reduction in GFR. 

In the heart adenosine decreases heart rate and in blood it has an anti-platelet action. Antiplatelet action prevents platelet aggregation and coagulation.

It is broken down in the blood by adenosine deaminase.

Adenosine deaminase enzyme is present in red cells and the vessel wall.

Dipyridamole is an inhibitor of the enzyme adenosine deaminase and raises the levels of adenosine in blood.

Dipyridamole leads to blood vessel dilatation and improved blood flow through the coronary blood vessels that supply the heart muscles.

In the kidneys adenosine decreases renal blood flow and decrease the production of rennin from the kidneys.

Adenosine in the lungs causes constriction of airways.

In the liver it leads to constriction of blood vessels and increases breakdown of glycogen to form glucose.

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