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Adenosine receptor



The adenosine receptors (or P1 receptors[1]) are a class of purinergic receptors, G-protein coupled receptors with adenosine as endogenous ligand.[2]

In humans, there are four adenosine receptors. Each is encoded by a separate gene and has different functions, although also overlapping. For instance, both A1 receptors and A2a play roles in the heart, regulating myocardial oxygen consumption and coronary blood flow.

Additional recommended knowledge

Contents

Comparison of subtypes

Adenosine recetptors
Receptor Gene Mechanism [3] Effects Agonists Antagonists
A1 ADORA1 Gi/o --> cAMP↑/↓
  • decrease heart rate
A2a ADORA2A Gs --> cAMP
  • CGS21680
  • ATL-146e
A2b ADORA2B Gs --> cAMP
  • theophylline
A3 ADORA3 Gi/o -->
  • cardioprotective in cardiac ischemia
  • inhibition of neutrophil degranulation
  • Cl-IB-MECA
  • MRS3558
  • theophylline
  • MRS1191
  • MRS1523
  • MRE3008F20

A1 adenosine receptor

Main article: Adenosine A1 receptor

The adenosine A1 receptor has been found to be ubiquitous throughout the entire body.

Mechanism

This receptor has an inhibitory function on most of the tissues in which it rests. In the brain, it slows metabolic activity by a combination of actions. Presynaptically, it reduces synaptic vesicle release while post synaptically it has been found to stabilize the magnesium on the NMDA receptor.

Antagonism and agonism

Caffeine, along with theophylline have been found to antagonize both A1 and A2a receptors in the brain. Specific antagonists include 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), and Cyclopentyltheophylline‎ (CPT) or 8-cyclopentyl-1,3-dipropylxanthine‎ (CPX), while specific agonists include 2-chloro-N(6)-cyclopentyladenosine (CCPA).

In heart

The A1, together with A2a receptors, of endogenous adenosine are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow. Stimulation of the A1 receptor has a myocardial depressant effect by decreasing the conduction of electrical impulses and suppressing pacemaker cell function, resulting in a decrease in heart rate. This makes adenosine a useful medication for treating and diagnosing tachyarrhythmias, or excessively fast heart rates. This effect on the A1 receptor also explains why there is a brief moment of cardiac standstill when adenosine is administered as a rapid IV push during cardiac resuscitation. The rapid infusion causes a momentary myocardial stunning effect.

In normal physiological states, this serves as protective mechanisms. However, in altered cardiac function, such as hypoperfusion caused by hypotension, heart attack or cardiac arrest caused by nonperfusing bradycardias, adenosine has a negative effect on physiological functioning by preventing necessary compensatory increases in heart rate and blood pressure that attempt to maintain cerebral perfusion.

In neonatal medicine

Adenosine antagonists are widely used in neonatal medicine;

Because a reduction in A1 expression appears to prevent hypoxia-induced ventriculomegaly and loss of white matter and therefore raise the possibility that pharmacological blockade of A1 may have clinical utility.

Theophylline and caffeine are nonselective adenosine antagonists that are used to stimulate respiration in premature infants.

However, we are unaware of clinical studies that have examined the incidence of periventricular leukomalacia (PVL) as related to neonatal caffeine use. Caffeine may reduce cerebral blood flow in premature infants, possibly by blocking vascular A2 ARs. Thus, it may prove more advantageous to use selective A1 antagonists to help reduce adenosine-induced brain injury.

A2A adenosine receptor

Main article: Adenosine A2a receptor

As with the A1, the A2a receptors are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow.

Mechanism

The activity of A2A adenosine receptor, a G-protein coupled receptor family member, is mediated by G proteins which activate adenylyl cyclase. It is abundant in basal ganglia, vasculature and platelets and it is a major target of caffeine.[4]

Function

The A2a receptor is responsible for regulating myocardial blood flow by vasodilating the coronary arteries, which increases blood flow to the myocardium, but may lead to hypotension. Just as in A1 receptors, this normally serves as a protective mechanism, but may be destructive in altered cardiac function.

Agonists and antagonists

Specific antagonists include KW6002 and SCH-58261, while specific agonists include CGS21680 and ATL-146e.[5]

A2B adenosine receptor

Main article: Adenosine A2b receptor

This integral membrane protein stimulates adenylate cyclase activity in the presence of adenosine. This protein also interacts with netrin-1, which is involved in axon elongation.

A3 adenosine receptor

Main article: Adenosine A3 receptor

It has been shown in studies to inhibit some specific signal pathways of adenosine. It allows for the inhibition of growth in human melanoma cells. Specific antagonists include MRS1191, MRS1523 and MRE3008F20, while specific agonists include Cl-IB-MECA and MRS3558.[5]

References

  1. ^ Fredholm BB, Abbracchio MP, Burnstock G, Dubyak GR, Harden TK, Jacobson KA, Schwabe U, Williams M (1997). "Towards a revised nomenclature for P1 and P2 receptors". Trends Pharmacol. Sci. 18 (3): 79–82. doi:10.1016/S0165-6147(96)01038-3. PMID 9133776.
  2. ^ Fredholm BB, IJzerman AP, Jacobson KA, Klotz KN, Linden J (2001). "International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors". Pharmacol. Rev. 53 (4): 527–52. PMID 11734617.
  3. ^ Unless else specified in boxes, then ref is:senselab
  4. ^ Entrez Gene: ADORA2A adenosine A2a receptor.
  5. ^ a b Jacobson KA, Gao ZG (2006). "Adenosine receptors as therapeutic targets". Nature reviews. Drug discovery 5 (3): 247–64. doi:10.1038/nrd1983. PMID 16518376.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Adenosine_receptor". A list of authors is available in Wikipedia.
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