cAMP inhibits transepithelial chloride secretion across bovine ciliary body/epithelium

To investigate the potential significance of cAMP in the regulation of Cl(-) transport across the bovine ciliary body/epithelium (CBE).

Fresh native bovine CBE preparation was mounted in a modified Ussing chamber. The effects of cAMP-stimulating agents on short-circuit current (I(sc)) and net (36)Cl(-) secretion were determined.

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 I(sc) 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 I(sc) 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 I(sc) inhibition, suggesting that cAMP may reduce Cl(-) transport by uncoupling the intercellular gap junctions.

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.