Summary
Adenosine receptors are divided into two main subtypes, designated A1 and A2. Although the two subtypes are presently defined by structure-activity relationships, the original definition based on adenylate cyclase is still useful: A1 receptors inhibit adenylate cyclase, whereas A2 receptors stimulate the enzyme. The two receptors also contribute in different ways to adenosine’s role in maintaining oxygen supply/demand balance: A1 responses generally bring about a reduction in oxygen demand (e. g., reductions in heart rate and contractility, inhibition of locomotor activity, hypothermia, inhibition of lipolysis), whereas A2 responses generally increase oxygen supply (e. g., vasodilation, inhibition of platelet aggregation).
A1 receptor binding assays are now well-characterized, and we routinely use 3H-N6γ-cyclohexyl-adenosine for this purpose. To determine the relative A1/A2 affinities of compounds, we developed an A2 receptor binding assay, based on the 3H-NECA binding assay of Yeung and Green. 3H-NECA binding was performed in rat striatal membranes, with 50 nM N6-cyclopenty-ladenosine (CPA) included in the assay to eliminate A1 binding of 3H-NECA. Under these conditions, 3H-NECA binding showed the characteristics expected of an A2 receptor. Affinities of six adenosine agonists in 3H-NECA binding were closely correlated with literature values for affinities in the dog coronary artery system (r = 0.986, P < 0.001), whereas no such correlation was seen for 3H-CHA binding (r = 0.530, n. s.). CPA was the most highly A1-selective agonist (780-fold), and 2-(phenylamino)adenosine (CV-1808) was the most highly A2-selective (5-fold). The antagonist PD 116,948 (8-cyclopentyl-1,3-dipropylxanthine) showed 740-fold A1-selectivity. PD 115,199, N-[2-(dimethylamino)ethyl]-N-methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)benzenesulfonamide, a moderately soluble xanthine derivative, had a high affinity in A2 binding but was equally potent in A1 binding. 3H-PD 116,948 and 3H-PD 115,199 have been used to label A1 and A2 receptors, respectively.
Several groups have observed that A2 receptors can differ markedly in their affinity for adenosine, and it has been proposed that A2 receptors can be divided into high- and low-affinity subclasses. The high-affinity A2 receptors, which we propose to call A2a, stimulate broken-cell adenylate cyclase and are localized to the striatum, whereas the low-affinity A2b receptors stimulate cyclic AMP accumulation in brain slices and are widely distributed in the brain. The 3H-NECA binding site has a high affinity for adenosine agonists and is highly localized to the striatum, indicating that it belongs to the A2a subclass. 2-(4-Methoxyphenyl) adenosine (CV-1674) appears to be quite selective for the A2a receptor: it has 600 nM affinity in 3H-NECA binding but is inactive at 1 mM in human fibroblasts (A2b response). The antagonist ligand 3H-PD 115,199 does not show any detectable binding to the A2b receptor, confirming that this receptor is not simply an antagonist-preferring coupling state of the A2a receptor.
Some new structure-activity criteria to distinguish A1 and A2 receptors are proposed.
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© 1987 Springer-Verlag Berlin Heidelberg
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Bruns, R.F., Lu, G.H., Pugsley, T.A. (1987). Adenosine Receptor Subtypes: Binding Studies. In: Gerlach, E., Becker, B.F. (eds) Topics and Perspectives in Adenosine Research. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-45619-0_6
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DOI: https://doi.org/10.1007/978-3-642-45619-0_6
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