Purinergic Receptors in Ocular Ciliary Epithelial Cells

Adenosine and ATP have been shown to activate separate cell surface purinergic receptors which have been designated P₁ for adenosine and P₂ for ATP. The pharmacological characterization of P₁ and P₂ purinergic receptor-mediated signal transduction has been performed in cultured cell lines of the ciliary epithelium. In ODM Clone-2, a cell line derived from human nonpigmented ciliary epithelium (NPE) and in a clone derived from bovine pigmented ciliary epithelium (PE), we observed that adenosine inhibits adenylate cyclase activity at high potency (nM) and stimulates adenylate cyclase activity at low potency (μM) suggesting the presence of P₁ subtypes on these cell membranes. The selective agonist cyclopentyladenosine (CPA) was effective at inhibiting forskolin-stimulated adenylate cyclase in these cells. The IC₅₀ for CPA in both NPE and PE was approximately 1 nM in the absence, and 11 nM in the presence of 3-isobutyl-1-methylxanthine (IBMX). In NPE, the selective agonist 2-[p-(2-carboxyethyl)phenethylamino]-5’-N-ethylcarboxamido adenosine (CGS 21680) stimulated adenylyl cyclase with an EC₅₀ of 11 ± 4 nM in the presence of 4-(3-butoxy-4-methyoxybenzyl)-2-imidazolidinone (RO-20-1724), a phosphodiesterase inhibitor devoid of adenosine receptor antagonism, and 61 ± 8 μM in the presence of IBMX. In PE cells, EC50 value of RO-20-1724 was 19 ± 5 nM (n = 3). The characterization of P2 receptors based upon the ability of ATP and its related analogues to stimulate inositol phosphate production reveal the presence of a putative P_2U, receptor in both cell types.