POLAR SOLUTE TRANSPORT ACROSS THE PIGMENTED RABBIT CONJUNCTIVA - SIZEDEPENDENCE AND THE INFLUENCE OF 8-BROMO CYCLIC ADENOSINE-MONOPHOSPHATE

Purpose. To characterize the conjunctival permeability to polar solute s ranging from 182 to 167,000 daltons in molecular weight (m.w.). Meth ods. Solute transport across the excised pigmented rabbit conjunctiva with a baseline transepithelial electrical resistance (TEER) of 1,285 +/- 46 ohm.cm(2) was evaluated in the modified Ussing chamber under op en-circuit conditions. The model solutes were mannitol (m.w. 182), 6-c arboxyfluorescein (m.w. 376), and fluorescein isothiocyanate-labeled d extrans (FD4, m.w. 4,400-FD150, m.w. 167,000). Results. For a given so lute, the apparent permeability coefficient (Papp) was independent of solute concentration and direction of transport. As expected, the Papp decreased with solute size, from 27.7 x 10(-8) cm/sec for mannitol to 0.31 x 10(-8) cm/sec for FD150. When the experimental temperature was lowered from 37 degrees C to 4 degrees C, Papp decreased by similar t o 50% for FD4 through FD40 and by >80% for both FD70 and FD150. Equiva lent pore analysis, assuming restricted solute diffusion via cylindric al, water-filled pores across the isolated tissue, revealed a radius o f 5.5 nm at a pore density of 1.9 x 10(8) pores per cm(2). The additio n of 1 mM 8-bromo cyclic adenosine monophosphate (8-BrcAMP), known to stimulate Cl- secretion and decrease TEER, to the mucosal side of the conjunctiva increased the transport of mannitol, FD4, and FD40 by 28%, while not affecting FD150 transport. Conclusions. Our findings sugges t that polar solutes up to FD40 traverse the conjunctival epithelial b arrier primarily by restricted diffusion through equivalent pores of 5 .5 nm radius and that solute movement is affected by reduction of TEER . On the other hand, polar solutes of the FD70 or larger may cross the barrier primarily via non-diffusional pathways such as non-specific e ndocytosis.