Adenosiini

Adenosiini, known in English as adenosine, is a nucleoside consisting of adenine attached to a ribose sugar via a β-N9-glycosidic bond. It is a component of RNA and a central metabolite in cellular energy transfer as part of ATP, ADP, and AMP. Extracellular adenosine is produced mainly by dephosphorylation of AMP through ecto-5'-nucleotidase (CD73) and is rapidly cleared by cellular uptake and by adenosine deaminase to inosine.

Adenosine signals through four G protein–coupled receptors: A1, A2A, A2B, and A3. A1 and A3 receptors inhibit adenylate cyclase via Gi/o, while A2A and A2B stimulate it via Gs, producing and regulating intracellular cAMP. Receptor distribution is tissue-dependent; A1 is abundant in brain and heart; A2A is prominent in the basal ganglia and immune cells; A2B and A3 have broader, lower-affinity expression.

Physiological roles include neuromodulation, cardiovascular regulation, and sleep-wake control. Endogenous adenosine promotes sleep by acting on A1 receptors in the brain and modulates coronary blood flow by vasodilation. It also modulates neurotransmitter release and has anti-inflammatory effects.

Clinically, intravenous adenosine is used to terminate certain paroxysmal supraventricular tachycardias due to transient AV nodal blockade, owing to its very short half-life. Pharmacologic stress testing uses adenosine or its analogue regadenoson to induce coronary hyperemia. Caffeine and other antagonists block adenosine receptors, influencing its effects.