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Effect of cAMP on Porcine Ciliary Transepithelial Short-Circuit Current, Sodium Transport, and Chloride Transport

¹From the Laboratory of Ocular Pharmacology and Physiology, University Eye Hospital Basel, Basel, Switzerland; ²The Second Affiliated Hospital, Medical College of Zhejiang University, Hangzhou, China; and the ³Division of Radiological Chemistry, University Hospital Basel, Basel, Switzerland.

PURPOSE. To investigate the effects of 8-bromo-cAMP (cAMP) on porcine ciliary transepithelial short-circuit current (Isc) and transport of chloride (Cl^–) and sodium (Na⁺).

METHODS. With Ussing-type chambers, cAMP-induced changes in Isc, electrical resistance (ER), and transepithelial ³⁶Cl^– and ²²Na⁺ fluxes were measured. Drugs were applied to the nonpigmented epithelium (NPE) and/or pigmented epithelium (PE) side(s). The effect of IBMX (1, 5, or 10 µM; 3-isobutyl-1-methylxanthine) on Isc-increase induced by 8-bromo-cAMP on the PE side was also tested.

RESULTS. On the NPE side, a single concentration (10 µM, 100 µM, or 1 mM) of 8-bromo-cAMP induced a biphasic (transient peak followed by sustained plateau) Isc increase. On the PE side, 8-bromo-cAMP induced a similar but delayed biphasic Isc increase at 1 mM, a slight plateau-Isc increase at 100 µM, and no Isc increase at 10 µM. In the concentration–response curve, the cAMP-induced peak-Isc increase became significant at a concentration 10,000 times lower on the NPE than on the PE side. At 10 µM, the cumulative cAMP-induced Isc-increase reached its maximum on the NPE side, but was virtually nonexistent on the PE side. IBMX (a phosphodiesterase inhibitor) but not 8-CPT-6-Phe-cAMP (higher permeability than 8-bromo-cAMP) significantly increased the peak-Isc concentration–response curve induced by 8-bromo-cAMP (10 nM–1 mM) on the PE side. On the NPE but not the PE side, 10 µM 8-bromo-cAMP induced a significant but transient increase in net PE-to-NPE ³⁶Cl^– flux (1.03 ± 0.18 µEq/min per square centimeter; P < 0.001). Neither ER nor transepithelial ²²Na⁺ flux was changed after cAMP exposure.

CONCLUSIONS. In porcine ciliary processes, apparently on the NPE side, cAMP triggers a biphasic (transient peak followed by a sustained plateau) Isc increase. Only the peak-Isc increase involves an increase in net PE-to-NPE Cl^– transport.