adenosiinia

Adenosine is a nucleoside formed by the condensation of adenine with a ribose sugar. It is a component of the nucleotides ATP, ADP, and AMP, and a building block of RNA, underpinning both energy transfer and genetic information storage in cells.

In cellular metabolism, adenosine participates in energy transfer and purine metabolism. It can be released into the extracellular space under stress or tissue damage, where it acts as a signaling molecule rather than just a metabolite.

Extracellular adenosine functions through four G protein–coupled receptors, known as A1, A2A, A2B, and A3. Activation of these receptors influences cardiac rhythm and coronary blood flow, modulates neural activity and sleep-wake regulation, and exerts anti-inflammatory and immunomodulatory effects. In a typical physiological setting, adenosine promotes vasodilation, can decrease oxygen demand, and contributes to protective responses during hypoxia.

Pharmacologically, caffeine and theophylline block adenosine receptors, contributing to alertness and cardiovascular effects. Clinically, intravenous adenosine is used to terminate certain types of supraventricular tachycardia by transiently slowing AV nodal conduction, and radiopharmaceutical adenosine or its analogs can aid imaging in myocardial perfusion studies.

Metabolism of adenosine involves phosphorylation to AMP by adenosine kinase and deamination to inosine by adenosine deaminase. Extracellular adenosine is generated by the action of ectonucleotidases on ATP and ADP and is subsequently cleared by cellular uptake and metabolism.

Adenosine is ubiquitous in tissues and can accumulate during hypoxia or inflammation, where it helps coordinate energy use, blood flow, and inflammatory responses.