cAMP inhibits transepithelial chloride secretion across bovine ciliary body/epithelium

PURPOSE. To investigate the potential significance of cAMP in the regulation of Cl- transport across the bovine ciliary body/epithelium (CBE). METHODS. Fresh native bovine CBE preparation was mounted in a modified Ussing chamber. The effects of cAMP-stimulating agents on short-circuit current (Isc) and net 36Cl- secretion were determined. RESULTS. Addition of cAMP-stimulating agents inhibited net Cl- secretion. Forskolin, when added bilaterally, reduced Cl- secretion by 60%. Similarly, bilateral isoproterenol or vasoactive intestinal peptide inhibited Cl- transport by 15% and 37%, respectively, suggesting a cAMP-sensitive Cl- transport across the ciliary epithelium. This notion was supported by the exogenous application of 8-bromo-cAMP (8-Br-cAMP) or 3-isobutyl-1-methylxanthine (IBMX), which reduced the net Cl- secretion by 49% and 85%, respectively. In unstimulated preparations, addition of 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) to the blood side had no effects on Isc and net Cl- transport, indicating that Cl- reabsorption was negligible under baseline conditions. Also, pretreatment with NPPB from the blood side did not prevent forskolin-induced Isc inhibition, suggesting that the inhibition of Cl- transport did not result from the facilitation of Cl- reabsorption. However, pretreatment with heptanol from both sides completely blocked the forskolin-induced Isc inhibition, suggesting that cAMP may reduce Cl- transport by uncoupling the intercellular gap junctions. CONCLUSIONS. The results suggest that cAMP plays a crucial role in modulating Cl- secretion across the ciliary epithelium. The effect is possibly mediated, at least in part, by the regulation of the permeability of gap junctions between pigmented and nonpigmented ciliary epithelial cells.