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cAMP Inhibits Transepithelial Chloride Secretion across Bovine Ciliary Body/Epithelium

¹From the Laboratory of Experimental Optometry, Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hung Hom, Hong Kong; and the ²Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.

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 (I_sc) 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 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.

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.

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