Nucleotide stimulation of Cl^- secretion in the pigmented rabbit conjunctiva

We evaluated the role of extracellular UTP and other nucleotides in the regulation of active ion transport across the pigmented rabbit conjunctiva. When added to the mucosal side of the conjunctiva, UTP (0.01-1000 μM), increased the short-circuit current by up to 14.6 ± 2.1 μA/cm². The half- maximal concentration was 11.4 ± 2.3 μM. The serosal absence of Cl^-, serosal presence of 10 μM bumetanide, and mucosal presence of 0.3 mM N- phenylanthranilic acid significantly reduced the change in the short-circuit current (Δlsc) induced by 10 μM UTP by 78, 77, and 42%, respectively. Mucosal 10 μM UTP significantly increased ³⁶Cl flux in the serosal-to- mucosal direction by 0.17 μEq/cm²/h, while not affecting mucosal-to-serosal ³⁶Cl flux. By contrast, ²²Na transport in either direction was unaffected. The rank order of Δlsc elicited by adenosine and nucleotides was consistent with the predominant involvement of P2Y purinergic receptors in the UTP effect on conjunctival ion transport. Moreover, the Δlsc elicited by UTP was inhibited by 0.05 and 1 mM suramin (a P2-purinergic receptor antagonist), resulting in a rightward shift of the half-maximal concentration to 106.7 ± 1.3 μM. In conclusion, the primary effect of UTP on ion transport in the pigmented rabbit conjunctiva is stimulation of Cl^- secretion, possibly at the P2Y₂ and/or the P2Y₄ receptor on the mucosal side of the tissue. Because of the coupling of fluid flow with Cl^- secretion, UTP or its analogs may be considered for stimulating transconjunctival fluid flow in the dry-eye state.