A(3) adenosine receptors regulate Cl- channels of nonpigmented ciliary epithelial cells

Adenosine stimulates Cl- channels of the nonpigmented (NPE) cells of the ci liary epithelium. We sought to identify the specific adenosine receptors me diating this action. Cl- channel activity in immortalized human (HCE) NPE c ells was determined by monitoring cell volume in isotonic suspensions with the cationic ionophore gramicidin present. The A(3)-selective agonist N-6-( 3-iodobenzyl)-adenosine-5'-N-methyluronamide(IB-MECA) triggered shrinkage ( apparent K-d = 55 +/- 10 nM). A(3)-selective antagonists blocked IB-MECA-tr iggered shrinkage, and A(3)-antagonists (MRS-1097, MRS-1191, and MRS-1523) also abolished shrinkage produced by 10 mu M adenosine when all four known receptor subtypes are occupied. The Al-selective agonist N-6-cyclopentylade nosine exerted a small effect at 100 nM but not at higher or lower concentr ations. The AZA agonist CGS-21680 triggered shrinkage only at high concentr ation (3 mu M), an effect blocked by MRS-1191. IB-MECA increased intracellu lar Ca2+ in HCE cells and also stimulated short-circuit current across rabb it ciliary epithelium. A(3) message was detected in both HCE cells and rabb it ciliary processes using RT-PCR. We conclude that human HCE cells and rab bit ciliary processes possess As receptors and that adenosine can activate Cl- channels in NPE cells by stimulating these A(3) receptors.