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Caffeine keeps you awake by blocking adenosine receptors. Each type of adenosine receptor has different functions, although with some overlap. [3] For instance, both A 1 receptors and A 2A play roles in the heart, regulating myocardial oxygen consumption and coronary blood flow, while the A 2A receptor also has broader anti-inflammatory effects throughout the body. [4]
The adenosine A2A receptor has also been shown to play a regulatory role in the adaptive immune system. In this role, it functions similarly to programmed cell death-1 (PD-1) and cytotoxic t-lymphocyte associated protein-4 ( CTLA-4 ) receptors, namely to suppress immunologic response and prevent associated tissue damage.
A 1 receptors are implicated in sleep promotion by inhibiting wake-promoting cholinergic neurons in the basal forebrain. [6] A 1 receptors are also present in smooth muscle throughout the vascular system. [7] The adenosine A 1 receptor has been found to be ubiquitous throughout the entire body. [citation needed]
In neutrophils, tissue adenosine can either activate or inhibit various neutrophil functions, depending on the inflammatory microenvironment, the expression of adenosine receptors on the neutrophil, and the affinity of these receptors for adenosine. Micromolar concentrations of adenosine activate A2A and A2B receptors.
A 2A receptor antagonists may prevent hepatic cirrhosis, and pentoxifylline may inhibit phosphodiesterase and provide renal protection. [6]The A 2A receptor antagonists may be used for treatment of attention deficit hyperactivity disorder (), because of the receptors ability to regulate neurotransmission in the basal ganglia and cortex, particularly dopaminergic and glutamatergic signaling.
Alpha 2 receptors are associated with sympatholytic properties. Alpha-adrenergic agonists have the opposite function of alpha blockers . Alpha adrenoreceptor ligands mimic the action of epinephrine and norepinephrine signaling in the heart, smooth muscle and central nervous system, with norepinephrine being the highest affinity.
Blood vessels with α 1-adrenergic receptors are present in the skin, the sphincters [4] of gastrointestinal system, kidney (renal artery) [5] and brain. [6] During the fight-or-flight response vasoconstriction results in decreased blood flow to these organs. This accounts for the pale appearance of the skin of an individual when frightened.
Two subtypes of adenosine A 2 receptors are known. Both are G protein-coupled adenosine receptors: Adenosine A 2A receptor; Adenosine A 2B receptor