Polar solute transport across the pigmented rabbit conjunctiva: size dependence and the influence of 8-bromo cyclic adenosine monophosphate.
OBJECTIVE
To characterize the conjunctival permeability to polar solutes ranging from 182 to 167,000 daltons in molecular weight (m.w.).


METHODS
Solute transport across the excised pigmented rabbit conjunctiva with a baseline transepithelial electrical resistance (TEER) of 1,285 +/- 46 ohm.cm2 was evaluated in the modified Ussing chamber under open-circuit conditions. The model solutes were mannitol (m.w. 182), 6-carboxyfluorescein (m.w. 376), and fluorescein isothiocyanate-labeled dextrans (FD4, m.w. 4,400-FD150, m.w. 167,000).


RESULTS
For a given solute, 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 approximately 50% for FD4 through FD40 and by > 80% for both FD70 and FD150. Equivalent pore analysis, assuming restricted solute diffusion via cylindrical, water-filled pores across the isolated tissue, revealed a radius of 5.5 nm at a pore density of 1.9 x 10(8) pores per cm2. The addition 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 suggest that polar solutes up to FD40 traverse the conjunctival epithelial barrier 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 endocytosis.
Horibe
Y
Y
Department of Pharmaceutical Sciences, School of Pharmacy, University of Southern California, Los Angeles 90033, USA.
Hosoya
K
K
Kim
K J
KJ
Ogiso
T
T
Lee
V H
VH
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D003228:Conjunctiva / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism*; D003911:Dextrans / Q000737:chemistry*; D016503:Drug Delivery Systems; D005452:Fluoresceins / Q000737:chemistry*; D005456:Fluorescent Dyes / Q000737:chemistry*; D008297:Male; D008353:Mannitol / Q000737:chemistry*; D008970:Molecular Weight; D010539:Permeability; D010952:Plasma Substitutes / Q000737:chemistry; D011817:Rabbits
D003911:Dextrans; D005452:Fluoresceins; D005456:Fluorescent Dyes; D010952:Plasma Substitutes; C024098:6-carboxyfluorescein; D008353:Mannitol; D000242:Cyclic AMP