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Functional Characterization of Organic Cation Drug Transport in the Pigmented Rabbit Conjunctiva

From the Departments of ¹ Pharmaceutical Sciences, ³ Medicine, ⁴ Ophthalmology, ⁵ Physiology and Biophysics, ⁶ Biomedical Engineering, and ⁷ Molecular Pharmacology and Toxicology, ⁸ Will Rogers Institute Pulmonary Research Center, Schools of Pharmacy, Medicine, and Engineering, University of Southern California, Los Angeles, California.

PURPOSE. To characterize carrier-mediated organic cation drug transport in the rabbit conjunctiva.

METHODS. The transport of [¹⁴C]guanidine, the model substrate, in the excised pigmented rabbit conjunctiva was evaluated in the modified Ussing chamber. Tetraethylammonium (TEA) transport also was investigated to determine substrate specificity.

RESULTS. The apparent permeability coefficient for guanidine and TEA in the mucosal-to-serosal (ms) direction was 5.4 and 49.6 times greater than that in the serosal-to-mucosal (sm) direction, respectively. Guanidine transport in the ms (but not sm) direction revealed temperature and concentration dependency over 0.02 to 10 mM with an apparent Michaelis–Menten constant of 3.1 mM and a maximal flux of 11.4 nmol/(cm² · h). Net guanidine transport measured at 0.1 mM across the conjunctiva was decreased by 71% or 82%, respectively, on the addition of 1 µM valinomycin (a K⁺ ionophore) in both bathing fluids or in a high K⁺ buffer in the mucosal fluid. Interestingly, net guanidine transport was reduced, rather than enhanced, by 63% upon acidifying the mucosal bathing fluid. By contrast, net guanidine transport was not affected by the serosal presence of 0.5 mM ouabain (a Na⁺,K⁺-ATPase inhibitor), by the mucosal and serosal presence of 0.1 µM monensin (a Na⁺ ionophore) or 0.3 µM carbonyl cyanide p-(trifluoromethoxy)phenyl-hydrazone (FCCP, a H⁺ ionophore). Guanidine transport in the ms direction was polyspecific, as indicated by the 48% to 82% inhibition by structurally diverse amines. In particular, guanidine ms transport was inhibited by the antiglaucoma drugs dipivefrine (72%), brimonidine (70%), and carbachol (78%).

CONCLUSIONS. A carrier-mediated organic cation transport process appears to exist in the conjunctiva, mediating the absorption of organic amines, including certain amine-type ophthalmic drugs. This process may be driven by an inside-negative apical membrane potential difference.

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