Human retinal pigment epithelial cells in culture possess A₂-adenosine receptors

Adenosine agonists cause a marked stimulation in cyclic AMP accumulation in whole human retinal pigment epithelial (RPE) cells in the presence of adenosine deaminase and papaverine, a phosphodiesterase inhibitor. N-Ethylcarboxamidoadenosine (NECA) stimulate cyclic AMP accumulation 16.1-fold above basal with an EC₅₀ of 2.5 × 10^-7 M. It is also an effective (1.9-fold) stimulator of adenylate cyclase activity in RPE membrane preparations and a modest (1.22-fold) stimulator in the presence of forskolin in RPE cell membranes prepared from freshly isolated porcine RPE. N₆-Cyclopentyladenosine (CPA) and N₆-phenylisopropyladenosine (PIA) also increase cyclic AMP levels with EC₅₀s of 4.9 × 10⁶ M (8.9-fold above basal) and 3.5 × 10^-6 M (8.0-fold above basal) respectively. This potency order (NECA > PIA > CPA) is typical of A₂-adenosine receptors. The relatively A₁-selective agonist CPA and PIA, failed to decrease isoproterenol-stimulated cyclic AMP accumulation at concentrations ranging between 10^-10 and 10^-7 M indicating that RPE cells do not have A₁-receptors which inhibit adenylate cyclase. Three adenosine receptor antagonists, BW-A1433U, 8-cyclopentyltheophylline and 8-sulfophenyltheophylline, blocked the NECA-induced stimulation of cyclic AMP accumulation with IC₅₀s of 0.36 μM, 1.5 μM, and 75 μM respectively. Since alteration of cAMP levels has been demonstrated to affect several RPE functions, including cell migration, resorption of subretinal fluid, and phagocytosis, adenosine may play a significant regulatory role in RPE.