Extracellular ATP and cAMP as Paracrine and Interorgan Regulators of Renal Function Renal Microvascular Effects of P2 Receptor Stimulation

The physiological role of the adenosine A3 receptor is being investigated using newly synthesized, selective ligands. Recently, in addition to agonists, selective antagonists have been developed that belong to three distinct, non-purine chemical classes: flavonoids, 1,4-dihydropyridine derivatives (e.g. MRS1191, which is 1300-fold selective for human adenosine A3 vs A1/A2A receptors, with a Ki value of 31 nM) and the triazoloquinazolines (e.g. MRS1220, which has a Ki value of 0.65 nM). The A3 receptor has proven enigmatic in terms of antagonist ligand specificity, coupling to second messengers, and biological effects in the CNS, inflammatory system and cardiovascular system. A3 receptors are also potentially involved in apoptosis. It appears that intense, acute activation of A3 receptors acts as a lethal input to cells, while low concentrations of A3 receptor agonists protect against apoptosis. Here, Kenneth Jacobson describes how A3 receptor agonists might be useful in treating inflammatory conditions, possibly through their inhibition of tumour necrosis factor alpha (TNF-alpha) release, which has been shown in macrophages. A3 receptor antagonists might be useful in treating asthma or acute brain ischaemia. Recently, the versatility of A3 receptor agonists, administered either before or during ischaemia, in eliciting potent cardioprotection has been shown.

The effects of lipopolysaccharide (LPS), a potent inflammatory mediator, on the shear stress stimulated release of adenosine triphosphate (ATP) were investigated on endothelial cells from human umbilical vein in primary culture. Human umbilical vein endothelial cells (HUVEC) in primary cultures were subjected to shear stress using a cone and plate apparatus. ATP released by the cells was measured by luminometry, using a luciferin-luciferase assay. Under conditions of shear stress alone (25dyn/cm2), ATP accumulates into the culture medium and reaches a maximum after 3 to 5 min of stimulation (121.7+/-13.2 pmol/ml). The shear stress-stimulated release of ATP was significantly increased after a 4 h pre-incubation of endothelial cells with 50 microg/ml (314.4+/-26.7 pmol/ml) and 10microg/ml lipopolysaccharide (207.7+/-22.2 pmol/ml). Dexamethasone, an anti-inflammatory glucocorticoid, inhibited the effects of lipopolysaccharide. These results show that non-damaged endothelial cells release ATP under experimental inflammatory conditions and support an early role of extracellular ATP in the inflammatory process.