Adenosine Receptor Classification:Quo Vadimus?

A2 adenosine receptors and vascular pathologies

Cardiovascular disease, a leading cause of death and morbidity, is regulated, among various factors, by inflammation. The level of the metabolite adenosine is augmented under stress, including inflammatory, hypoxic, or injurious events. Adenosine has been shown to affect various physiological and pathological processes, largely through 1 or more of its 4 types of receptors: the A1 and A3 adenylyl cyclase inhibitory receptors and the A2A and A2B adenylyl cyclase stimulatory receptors. This article focuses on reviewing common and distinct effects of the 2 A2-type adenosine receptors on vascular disease and the mechanisms involved. Understanding the pathogenesis of vascular disease mediated by these receptors is important to the development of therapeutics and to the prevention and management of disease.

Adenosine and blood platelets

Adenosine is an important regulatory metabolite and an inhibitor of platelet activation. Adenosine released from different cells or generated through the activity of cell-surface ectoenzymes exerts its effects through the binding of four different G-protein-coupled adenosine receptors. In platelets, binding of A(2) subtypes (A(2A) or A(2B)) leads to consequent elevation of intracellular cyclic adenosine monophosphate, an inhibitor of platelet activation. The significance of this ligand and its receptors for platelet activation is addressed in this review, including how adenosine metabolism and its A(2) subtype receptors impact the expression and activity of adenosine diphosphate receptors. The expression of A(2) adenosine receptors is induced by conditions such as oxidative stress, a hallmark of aging. The effect of adenosine receptors on platelet activation during aging is also discussed, as well as potential therapeutic applications.

High level of adenosine A1 receptor-like immunoreactivity in the CA2/CA3a region of the adult rat hippocampus

We describe the immunocytochemical distribution of adenosine A1 receptors in the rat hippocampus. Adenosine A1 receptor-like immunoreactivity was seen on the cell soma and dendrites of pyramidal cells and the cell soma and proximal part of dendrites of granule cells, but not on glial cells. Developmentally, adenosine A1 receptor-like immunoreactivity was diffuse on postnatal day 7 and increased in intensity in individual cells by day 21. In the CA2/CA3a region, the adult pattern of A1 receptor distribution was established by day 28. In the adult rat hippocampus, rostrocaudal inspection revealed that immunoreactivity in CA2/CA3a was greatest. Confocal microscopy revealed differences in the staining patterns for the adenosine A receptor and synaptophysin, a marker of presynaptic terminals. This result suggests that the adenosine A1 receptor might have postsynaptic physiological functions. Double-labeling of adenosine A1 receptors and anterogradely-labeled fibers from the supramammillary nucleus showed that the fibers from the supramammillary nucleus terminate directly on the cell soma of the A1 receptor-immunopositive neurons in CA2/CA3a and the dentate gyrus. These results indicate that the adenosine A 1 receptor in CA2/CA3a and the dentate gyrus are in a position to regulate hippocampal theta activity and that resultant strong synaptic depression in CA2/CA3a could play a role in regulating the intrinsic signal flow between CA3 and CA1.

Adenosine receptors: new opportunities for future drugs

This review summarises current knowledge on adenosine receptors, an important G protein-coupled receptor. The four known adenosine receptor subtypes A1, A2A, A2B, and A3 are discussed with special reference to the opportunities for drug development.

Synthesis and A1 and A2A adenosine binding activity of some pyrano[2,3-c]pyrazol-4-ones

A series of pyrano[2,3-c]pyrazol-4-ones was synthesized and evaluated for bovine brain adenosine A1 and A2A receptor binding affinity. Substituents at positions 5 and/or 6 were varied in order to define the structure-activity relationships in these new kinds of adenosine receptor ligands. The most selective and potent ligand among the reported compounds was the 1,4-dihydro-1-phenyl-3-methyl-6-(3-aminophenyl)-pyrano[2,3-c]pyraz ol-4-one 11 which showed a 27-fold selectivity for A1 receptor and a Ki value of 84 nM.

Acute effects of progesterone on glucose metabolism in rat adipocytes: are they modulated by endogenous adenosine?

Progesterone rapidly inhibits glucose oxidation of isolated rat adipocytes. Because this inhibition is triggered by endogenous adenosine, the present study was designed to examine the effect of the steroid on cyclic adenosine monophosphate (cAMP) accumulation, its relation to lipolysis, and the possible participation of adenosine. The results strongly indicate that physiological concentrations of progesterone increase the release of adenosine by isolated adipocytes, with maximal release at the end of a 20-minute incubation. Progesterone decreased both cAMP levels and lipolysis in quiescent adipocytes or in adipocytes stimulated by isoproterenol. The increase of endogenous adenosine may explain the decline of cAMP and glycerol levels observed with progesterone. The effects of the steroid on lipolysis disappeared when adenosine was hydrolyzed by adenosine deaminase (ADA). On the other hand, in the absence of endogenous adenosine, the effect of progesterone on the cAMP level was decreased only in isoproterenol-stimulated cells. The inhibitory effects of progesterone on cAMP and glycerol production seem not to be related directly to the adenosine A1 receptor, for selective A1 receptor antagonists (8-cyclopentyl-1,3-dipropylxanthine [DPCPX] and CP 68,247) did not counteract these effects. However, mechanisms mediated by guanyl nucleotide binding proteins cannot be excluded. The decrease of cAMP and of lipolysis may be related to a stimulation of phosphodiesterases (PDEs). When PDEs I [Ca(2+)-calmodulin-regulated PDE family) were blocked by a selective inhibitor (CP 41,757), the progesterone inhibitory effect persisted, suggesting that PDEs I are not regulated by the steroid. On the other hand, the progesterone effect on cAMP accumulation but not on lipolysis disappeared in the presence of a selective inhibitor of the PDE IV family (cAMP-dependent-specific family). Ro 20.1724. When the specific inhibitor of PDE I or PDE IV was combined with ADA, the progesterone effect on cAMP disappeared. Taken together, these results suggest that the progesterone inhibitory action on cAMP levels was not mediated through A1 receptors or through activation of PDE I, but may be related to PDE IV activities. The progesterone effect on lipolysis seemed not to be directly related to changes in cAMP levels; an effect on PDE III activities in relation with the increase of adenosine release cannot be excluded.